Peptides from plants and microorganisms: a defense strategy for biotic and abiotic stresses

Abstract

Antimicrobial peptides (AMPs) from plants and microorganisms have emerged as promising tools due to their multifunctional roles in plant defense. These small, bioactive molecules, such as thionins, systemins, defensins, cyclotides, hevein-like peptides, and cyclic dipeptides, exhibit broad-spectrum activity against fungal pathogens, bacteria, and insect pests. Recent studies have further elucidated their supportive roles in conferring tolerance to abiotic stresses, including salinity, drought, and heavy metals exposure, thus expanding their potential applications. Previous studies demonstrated that the integration of AMPs genes into transgenic crops has shown significant potential in improving plant resistance to both biotic and abiotic stresses. Importantly, in our recent study, a cyclic dipeptide cyclo(L-Ala-Gly) from Priestia megaterium BP01R2 enables salinity stress alleviation in plants. The latest finding revealed that cyclo(His-Pro) in Arabidopsis navigated carbon flux from glycolysis to the pentose phosphate pathway and its supplementation increased NADPH levels and the NADPH/NADP⁺ ratio in plants. This review explores the latest advances in the application of plant- and microorganisms-derived AMPs, with a focus on their functional mechanisms and their roles in the development of stress-resilient crops. It also provides an overview of ongoing efforts to harness peptides in sustainable agricultural practices.

1 Introduction

The remarkable diversity, ubiquity, and versatility of antimicrobial peptides (AMPs) make them an abundant source of novel metabolites with potential applications in medicine, agriculture, and the food industry (Campos et al., 2018; Bakare et al., 2022; Simonsen et al., 2005). Various strategies have been implemented to alleviate the adverse effects of biotic and abiotic stresses on plants, including plant breeding (Hill et al., 1998), acclimation (Pandolfi et al., 2016), seed biopriming (Corbineau et al., 2023; Aizaz et al., 2023), plant growth-promoting rhizobacteria (PGPR) (Vaishnav et al., 2019), and the use of non-protein amino acids (NPAAs) (e.g., 5-hydroxynorvaline, meta-tyrosine, GABA, BABA, Canavanine, L-DOPA, Mimosine, etc.) (Silva Rodrigues-Corrêa and Fett-Neto, 2019). Research has also investigated the roles of various AMPs in plant defense systems to enhance resistance to phytopathogens (Maximiano et al., 2022) and to bolster adaptive mechanisms against abiotic stresses (Kulaeva et al., 2020). These studies show that AMPs can act directly as insecticidal molecules, inhibiting the growth of insect larvae (Mulla and Tamhane, 2023), or as inhibitors that suppress the growth of pathogenic bacteria or fungi (Huang et al., 2021).

Despite extensive investigation into their roles in suppressing bacteria and fungi (Leannec-Rialland et al., 2022; McLaughlin et al., 2021; Nawrot et al., 2014), the contributions of AMPs to mitigating the adverse effects of abiotic stresses remain less explored. Specifically, relatively few studies have examined the functions of AMPs in improving plant tolerance to salinity and drought stress (Li et al., 2021; Kumar et al., 2019; Hung et al., 2024), heavy metal toxicity (Mirakhorli et al., 2019), or wound stress (Rawat et al., 2017). This disparity in research emphasis can be attributed to several factors. Historically, AMPs were primarily identified and characterized for their antimicrobial properties, leading to a predominant focus on biotic stress responses, where their inhibitory effects on pathogens are straightforward to assay and quantify (Tang et al., 2023). In contrast, abiotic stresses, such as salinity and drought, involve complex physiological and molecular pathways (e.g., osmotic regulation or ion homeostasis), making it less intuitive and more challenging to evaluate the indirect effects or multifunctional roles of AMPs (Kulaeva et al., 2020). For instance, elucidating mechanisms of membrane permeability modulation and hormone signaling under abiotic conditions requires interdisciplinary approaches using transcriptomics, proteomics, and functional genomics, which demand specialized expertise and significant facilities (Deshmukh et al., 2025). Furthermore, abiotic stress experiments often necessitate controlled environments, which are more complicated than in vitro antimicrobial assays. While funding priorities have shifted toward abiotic stress research in recent years, driven by climate change concerns and the need for resilient crops, the focus remains on broader molecular mechanisms, leaving AMP-specific studies underexplored (Oyebamiji et al., 2024). This knowledge gap is critical, as emerging evidence suggests AMPs could enhance plant resilience through mechanisms like reactive oxygen species scavenging and hormone crosstalk, yet comprehensive studies remain sparse (Kaya and Adamakis 2025; González Ortega-Villaizán et al., 2024). Addressing this gap through targeted research could unlock novel applications for AMPs in sustainable agriculture. Our recent research, which demonstrated that the exogenous application of the cyclic dipeptide cyclo(L-Ala-Gly) contributes to salinity stress tolerance in plants, further highlights the potential of small bioactive molecules to enhance crop resilience against escalating global abiotic stresses (Hung et al., 2024). Beyond their direct application, AMP-encoding genes have also been widely incorporated into transgenic crops to confer new agronomic traits, including resistance to both biotic and abiotic stresses (Su et al., 2020; Parisi et al., 2020). To address this critical gap, this review synthesizes recent advances in the development of transgenic plants engineered with AMP-encoding genes from diverse plant and microbial sources. Furthermore, we explore the emerging potential of cyclic dipeptides to enhance the resistance of major crops to both biotic and abiotic stresses. By consolidating these findings, this manuscript offers valuable insights for researchers and agricultural scientists, guiding the development of novel, sustainable strategies to improve crop resilience and productivity in the face of diverse environmental challenges.

2 Classification and mechanisms of AMPs

2.1 Thionins

2.1.1 Family and structure

Thionins are a major class of plant-derived antimicrobial peptides (AMPs), alongside defensins, non-specific lipid transfer proteins, hevein-like peptides, knottin-like peptides, and cyclotides, distinguished by their cysteine spacing motifs and three-dimensional (3D) structures (Odintsova et al., 2018). As the first plant AMPs identified with in vitro antipathogenic activity, thionins are small peptides of approximately 5 kDa and 45–47 amino acids (aa), found predominantly in certain angiosperm families, including Poaceae (e.g., barley, wheat), Brassicaceae (e.g., Arabidopsis), and Ranunculaceae. Their compact structure, characterized by 6 or 8 cysteine residues forming 3 or 4 disulfide bridges, confers exceptional stability under diverse environmental conditions (Odintsova et al., 2018). Some common thionin structures were predicted using AlphaFold3 (Abramson et al., 2024) and are shown in Figure 1.

Figure 1

All depicted thionins share a common structural motif, including at least one α-helix and one β-sheet, which are critical for their antimicrobial function. These elements are stabilized by disulfide bonds, a hallmark of thionins, enabling them to disrupt microbial membranes. Differences in loop lengths and helix-sheet arrangements reflects species-specific variations, yet the conserved core structure underscores their shared evolutionary origin and functional role in plant defense. The diversity of plant species (Crambe abyssinica, Triticum aestivum, Hordeum vulgare, and Oryza sativa) highlights the widespread occurrence and adaptation of thionins across monocot and dicot plants.

2.1.2 Mechanisms of antimicrobial action

Thionins primarily target pathogen cell membranes by interacting with negatively charged phospholipids, such as phosphatidic acid or phosphatidylserine, forming proteolipid complexes that disrupt membrane integrity through solubilization and lysis (Ji et al., 2015). This interaction also involves the extraction of phospholipids, reducing membrane fluidity and causing irreparable damage (Stec et al., 2004). Beyond their direct antimicrobial activity, thionins contribute to plant defense by activating signal transduction pathways mediated by phytohormones, including salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) (Künstler et al., 2020). Previous studies further suggest that thionins enhance plant immunity by modulating reactive oxygen species (ROS) production and synergizing with other AMPs to bolster resistance against fungal and bacterial pathogens (Chan et al., 2005; Hao et al., 2020).

2.1.3 Roles of thionins in biotic stress mitigation

2.1.3.1 Antimicrobial activities

Thionins exhibit broad-spectrum activity against diverse biotic stressors, including bacteria, fungi, insects, and nematodes. A thionin-like peptide, CaThi, from Capsicum annuum, inhibited Candida spp. with half-maximal inhibitory concentrations (IC₅₀) ranging from 10 to 40 µg/mL (Taveira et al., 2016). CaThi was shown to permeabilize the plasma membrane of all tested Candida spp. and induce oxidative stress in Candida tropicalis, with synergistic interactions with fluconazole significantly enhancing candidacidal activity (Taveira et al., 2016). Similarly, a thionin (Thi2.1) from Arabidopsis thaliana, when expressed in the BVE-E6E7 bovine endothelial cell line, reduced the viability of Escherichia coli, Staphylococcus aureus, and Candida albicans (Loeza-Ángeles et al., 2008). A cowpea thionin II expressed antifungal activity against Fusarium culmorum at minimum inhibitory concentration (MIC) ~ 50 µg mL⁻¹, but Aspergillus niger and Penecillium expansum at MIC greater than 500 µg mL⁻¹ (Schmidt et al., 2019). The thionin II activity remained at elevated temperature (100°C, 15 min) but lost its antifungal property in the presence of cations such as Na⁺, K⁺, Ca²⁺, and Mg²⁺ (Schmidt et al., 2019). A study by Moghadam et al. (2023) evaluated the antifungal efficacy of aqueous and alcoholic extracts from three varieties of C. annuum against various Candida species (Moghadam et al., 2023). The results, however, indicated that these extracts exhibited minimal inhibitory effects, with MIC exceeding 512 µg mL⁻¹, suggesting limited antifungal activity in their crude form (Moghadam et al., 2023).

2.1.3.2 Antifungal defense

A 2004 study by Oard et al. showed that, of 12 natural and synthetic AMPs, purothionin, a type I thionins from wheat seed, exhibited the strongest inhibitory activity against Rhizoctonia solani LR172, causal agent of rice sheath blight and aerial blight of soyabeans in the USA (Oard et al., 2004). Also in Oard et al. study, a strong correlation between membrane permeabilization and antifungal activity of the tested peptides was found with the most significant changes in membrane integrity at ≥ 0.5 µmol L⁻¹ of purothionin, followed by 2 µmol L⁻¹ of cecropin B, a natural peptide from cecropia moth (Oard et al., 2004). A 15 kDa thionin protein, Thi2.4, from A. thaliana was shown to interact with the fungal virulence factor fruit body lectin (FFBL), thereby reducing the toxicity of Fusarium graminearum. In addition to this protein–protein interaction, Thi2.4 exhibits direct antifungal activity by disrupting fungal cell membranes (Asano et al., 2013).

2.1.3.3 Insect and nematode resistance

In barley (Hordeum vulgare), the thionin genes THIO1567 and THIO1570 exhibited significantly higher transcript levels in the bird cherry–oat aphid (Rhopalosiphum padi L.)-resistant genotype Hsp5 compared to the susceptible genotype Lina, suggesting a role in aphid resistance (Mehrabi et al., 2014). Similarly, in rice, thionin gene expression was more strongly suppressed in the susceptible genotype Nipponbare (Oryza sativa) than in the resistant accession TOG5681U (Oryza glaberrima) following nematode infection, indicating a potential contribution of thionins to nematode resistance (Petitot et al., 2017).

2.1.4 Roles of thionins in abiotic stress mitigation

Under drought stress conditions, the susceptible barley cultivar Concerto showed increased expression of the HvTHIO1 gene, suggesting a potential protective role (Leybourne et al., 2022). In rice, the defensin-dissimilar thionin gene OsThi9 was strongly expressed in roots and stems under cadmium (Cd) exposure (0.1 µM CdCl₂). Overexpression of OsThi9 reduced Cd translocation to shoots, lowering Cd levels in leaves and grains (Liu et al., 2023). A combination of osmotic and heat stress caused higher susceptibility of Arabidopsis plants to Botrytis cinerea, possibly due to the reduced expression of defense genes, including PLANT DEFENSIN 1.3 (PDF1.3), BOTRYTIS SUSCEPTIBLE 1 (BOS1), THIONIN2.2 (THI2.2), and cell wall-related genes (Sewelam et al., 2021). Recently, Yan et al. (2025) found that OsTHION15 was significantly upregulated under drought stress, while the Osthion15 mutant led to higher sensitivity to drought and abscisic acid (ABA) stress, indicating its key roles in abiotic stress resistance (Yan et al., 2025). While these studies indicate altered expression of thionin genes under abiotic stress conditions, such changes do not directly confirm the functional roles of thionins in stress tolerance. Therefore, further research is needed to elucidate the direct involvement of the thionins themselves in abiotic stress tolerance.

2.1.5 Applications of thionin genes in transgenic plant development

Thi2.1, encoding 5 kDa cysteine-rich antimicrobial peptides, was upregulated in the resistant Arabidopsis ecotype UK-4 upon Fusarium oxysporum f. sp. matthiolae infection. Its overexpression in susceptible Col-2 seedlings delayed chlorophyll loss, inhibited fungal growth, and triggered severe fungal phenotype abnormalities (Epple et al., 1997). When Thi2.1 was introduced into the tomato genome, the resulting transgenic plants showed enhanced resistance to Fusarium wilt (FW) (F. oxysporum f. sp. lycopersici) and bacterial wilt (BW) (Ralstonia solanacearum) (Chan et al., 2005). These Thi2.1 transgenic lines matched the BW-resistant variety H7996 in disease incidence. Similarly, Thi2.1 transgenic lines R7 and R11 showed a disease resistance comparable to the FW resistant variety MH1 and significantly lower disease incidence than WT plants (Chan et al., 2005). Meanwhile, β-purothionin, a 45-aa wheat endosperm thionin with 4 disulfide bonds and a strong cationic charge, offers environmental stability (Oard and Enright, 2006; Kaas et al., 2010; Stec, 2006). Transgenic Arabidopsis expressing β-purothionin gene driven by a leaf-specific chloroplast carbonic anhydrase promoter showed top-tier resistance to Pseudomonas syringae strain DC3000, with no leaf infection symptoms. In vitro tests against F. oxysporum revealed transgenic seedlings surviving 12–15 days with minimal necrosis and discoloration, while control died within 6–8 days (Oard and Enright, 2006). Thio-60 extracted from transgenic onion (Allium cepa L.) outperformed its non-transgenic counterpart, inhibiting A. niger spore germination by 52% versus 37% (Tawfik et al., 2022). A barley α-hordothionin (α-HT) gene with 384 bp in length, driven by the constitutive El2Ω or β-amylase (β-Amy) promoter (the 5’-UTR region), was transformed into sweet potato cultivar Kokei No. 14, to tackle Ceratocystis fimbriata, the top postharvest disease of sweet potato (Muramoto et al., 2012). C. fimbriata causes the most damaging postharvest disease of sweet potato (Ipomoea batatas (L.) Lam.) worldwide (Zhang et al., 2018). Wounds on storage roots were inoculated with a suspension of C. fimbriata spores to examine the resistance. Transgenic lines El2Ω: α-HT No. 1 and β-Amy: α-HT No. 060201 had much smaller black lesions than non-transgenic Kokei No. 14, with lesion areas of 119 mm² and 111 mm² compared to 283 mm², respectively, indicating that the transgenic lines acquired solid resistance (Muramoto et al., 2012). In non-transgenic rice, OsTHI7 normally exhibited root tip-specific expression, but its promoter activity was not detectable inside Meloidogyne graminicola-induced galls (Ji et al., 2015). This down-regulation might be a strategy by M. graminicola to suppress plant defense, thereby preventing the thionins from reaching toxic levels against the nematode. In contrast, transgenic rice lines that overexpressed OsTHI7 showed decreased susceptibility to M. graminicola infection. These lines exhibited a significant reduction, approximately 39.2%, in the number of females and total nematodes per plant compared to control plants (Ji et al., 2015). This suggests a protective effect of OsTHI7 against the nematode, likely due to its direct toxic effect, as it possesses characteristics to alter cell membrane permeability and contribute to toxicity. Similarly, overexpression of OsTHI7 in rice decreased susceptibility to Pythium graminicola colonization (Ji et al., 2015). Transgenic line OX22 displayed significantly healthier shoots with a lower disease rate and greater shoot length compared to control plants. Quantitative PCR analysis revealed that the quantity of P. graminicola DNA was significantly lower in the roots of OsTHI7-overexpressing lines (70% of total DNA) compared to wild-type and empty vector controls (97% and 98% respectively) (Ji et al., 2015).

Huanglongbing (HLB), the deadliest threat to global citrus production, has spurred extensive resistance engineering (Hao et al., 2017). A modified citrus thionin gene (Mthionin) gene, derived from an endogenous citrus thionin, was inserted into the Carrizo citrus genome. The transgenic Carrizo plants resisted HLB disease (Candidatus Liberibacter asiaticus) (Las) and citrus canker disease (Xanthomonas citri) (Hao et al., 2016). Leaf infiltration assays showed Mthionin transgenic leaves with little to no canker at X. citri concentration of 10^4–10⁷ CFU mL⁻¹. Grafted plants with transgenic Carrizo rootstock had significantly lower Las titers in young leaves and roots than controls, indicating that Mthionin is a promising HLB fighter (Hao et al., 2016). In another study, Mthionin gene, driven by a double 35S promoter, was inserted into the A. thaliana genome (Hao et al., 2020). Lines A24 and A52, with peak Mthionin expression, showed reduced water soaking, lesions, and fungal biomass in detached leaf assays. Sprayed with F. graminearum (5 x 10⁵ conidia mL⁻¹), GUS transgenic plants suffered severe symptoms, such as dry flowers, dry siliques, and dead branches, while Mthionin plants remained symptom-free. Furthermore, F. graminearum conidia barely germinated on Mthionin leaves. At 48 h post-inoculation, the DEFENSIN1.2 expression in Mthionin plants was significantly higher than in GUS plants, hinting that Mthionin bolsters resistance to F. graminearum via defense genes and phytohormone signaling. Ectopic expression of two barley thionin genes AK252675.1 and AK359149 in Nicotiana benthamiana reduced host susceptibility to Myzus persicae, underscoring the important role of thionin genes against aphids (Escudero-Martinez et al., 2017). The Thio-60 gene from A. thaliana was transformed into date palm (Phoenix dactylifera L.) cultivars Barhy, Sakkoti, and Shamia, yielding transgenic plants with higher resistance to F. oxysporum, confirmed by detached leaf pathogenicity test (Allah et al., 2023). In a more recent advancement, thionin genes (e.g., thio-60 and thio-63) were introduced into Paulownia trees using chitosan nanoparticles as a delivery system for genetic transformation. The resulting transgenic lines exhibited enhanced resistance to fungal pathogens. Specifically, thio-60 transgenic lines increased resistance to Fusarium equiseti, while thio-63 transgenic lines enhanced resistance to A. niger (Bouqellah et al., 2024).

The integration of thionin genes into transgenic plant systems reveals consistent patterns of enhanced resistance across diverse biotic stressors and crop species. In response to fungal pathogens such as Fusarium spp., A. niger, C. fimbriata, and F. graminearum, overexpression of thionin variants (e.g., Thi2.1, β-purothionin, α-HT, Mthionin, Thio-60) has been shown to significantly reduce lesion sizes in sweet potato, inhibit spore germination in onion, delay seedling mortality A. thaliana, and produce symptom-free phenotypes. These effects are primarily attributed to direct antimicrobial activity and the upregulation of defense-related genes.

Comparable efficacy is observed under bacterial stress conditions, including infections by R. solanacearum, P. syringae, X. citri, and Ca. L. asiaticus. Transgenic lines exhibit reduced pathogen titers, absence of visible symptoms, and resistance levels on par with commercial cultivars. Although less frequently reported, insect resistance has also been demonstrated, notably against M. persicae in N. benthamiana, where ectopic thionin expression decreases host plant susceptibility. While model species such as A. thaliana and N. benthamiana continue to serve as valuable platforms for mechanistic studies, economically important crops—tomato, sweet potato, citrus, onion, date palm, and Paulownia spp.—exhibit promising translational outcomes, including postharvest protection and reduced disease incidence under laboratory- and field-relevant conditions. Collectively, these findings underscore the potential of thionin genes to confer broad-spectrum biotic resistance through membrane disruption and activation of defense pathways, supporting their utility in multi-pathogen engineering strategies across diverse plant systems.

2.2 Defensins

2.2.1 Family and structure

Like thionins, defensins are a key family of small, cysteine-rich, cationic peptides, but they are distinguished by a different structural architecture. They typically range from 45 to 54 aa, and feature eight conserved cysteine residues forming four disulfide bridges, which confer stability against protease degradation, extreme pH, and high temperatures (Azmi and Hussain, 2021; Kovaleva et al., 2020). The disulfide bonds cysteine residues in plant defensins possess the same pattern: Cys1-Cys8, Cys2-Cys5, Cys3-Cys6, and Cys4-Cys7 (Leannec-Rialland et al., 2022) with few exceptions (Janssen et al., 2003; Lay et al., 2003). Found across plant families such as Brassicaceae, Fabaceae, and Solanaceae families, plant defensins belong to the cis-defensin superfamily, distinct from the trans-defensin superfamily of mammals (Parisi et al., 2019). GMA4CG_V6, derived from MtDef4—a 47-aa defensin from Medicago truncatula—exhibited a multifaceted mode of action and antifungal activity against B. cinerea. Its 17-aa frame, containing the gamma-core motif of MtDef4, retains potent antifungal activity even at elevated salt concentrations (Tetorya et al., 2023). A characteristic γ-core motif (GXCX3–9C) is critical for their antimicrobial activity (Slezina et al., 2022; 2023; Sonderegger et al., 2018). The γ-core motif also plays role in mediating the entry of defensins into fungal cells (Sagaram et al., 2013; Tetorya et al., 2023). The Scots pine (Pinus sylvestris L.) defensin PsDef5.1 features a cysteine-rich α-motif (CX5CX3CX7CX9CXC) and a γ-core motif (GXCX9C), underscoring its structural complexity (Shalovylo et al., 2021). PsDef2 (RMCKTPSAKFKGYCVSSTNCKNVCRTEGFPTGSCDFHITSRKCYCYKPCP) and PsDef1 (RMCKTPSGKFKGYCVNNTNCKNVCRTEGFPTGSCDFHVAGRKCYCYKPCP), both defensins derived from P. sylvestris, differ by six aa in their primary sequences (Bukhteeva et al., 2022). Although this significant divergence is notable, with approximately 10–13% of the sequence, did not strongly affect their biological activities, including antimicrobial, antibacterial, and insect α-amylase inhibitory effects, implying the structural core, likely the cysteine-stabilized α-β (CSαβ) fold typical of plant defensins, is preserved (Bukhteeva et al., 2022). The CSαβ fold is likely critical for structural stability, functional scaffold, and dynamic flexibility of plant defensins. Some common defensin structures were predicted using AlphaFold3 (Abramson et al., 2024) and are shown in Figure 2.

Figure 2

All the defensins share a common structural motif, including at least one α-helix and one β-sheet, which are stabilized by disulfide bonds typical of defensins. These elements are critical for their ability to disrupt microbial membranes. The variation in loop lengths and helix-sheet arrangements reflects species-specific adaptations, while the conserved core structure underscores their shared evolutionary role in plant defense. The diversity of plant species such as Beta vulgaris, A. thaliana, Arabidopsis helleri, Dahlia merckii, Heliophila coronopifolia, and Raphanus sativus highlights the widespread occurrence of these defensins across different plant families.

2.2.2 Mechanism of action

Similar to thionins, defensins and other AMPs deploy one or both primary antimicrobial strategies: membrane disruption and inhibition of cellular machinery (Figure 3). Their amphipathic nature enables them to target pathogen membranes, while specific residues enhance their efficacy. DefSm2-D (KLCEKPSKTWFGNCGNPRHCGDQCK-SWEGVHGACHVRNGKHMCFCYFNCPQAE), an antifungal defensin from wild thistle (Silybum marianum): truncated versions like SmAPγ27-44 (WEGAVHGACHVRNGKHMC), with its Arg38 residue in the γ-core domain, exhibited potent antagonism against F. graminearum (MIC₅₀ of 20 μM), outperforms SmAPα1-21 (KLCEKPSKTWFGNCGNPRHCG) (MIC₅₀ of 32 μM) and SmAPα10-21 (WFGNCGNPRHCG) (MIC₅₀ of 70 μM) (Sagaram et al., 2013). The presence of three cationic lysine (Lys) residues, one anionic glutamic acid (Glu), and a tryptophan (Trp) in SmAPα1–21 likely enhances its membrane-binding capacity. Moreover, its selective action—targeting fungal cell walls over host cells—hinges on compositional differences between pathogen and plant membranes (Fernández et al., 2021).

Figure 3

[A] In the barrel-stave model, the attached AMPs insert perpendicularly into the bacterial membrane. Their hydrophobic surfaces (shown in orange) bind to the lipid core of the bilayer, and their hydrophilic surfaces (shown in blue) form cylindrical pores, disrupting the membrane integrity. In the toroidal model, AMPs insert themselves into the lipid bilayer and induced the membrane to bend inward, creating a pore lined by both AMPs and the lipid headgroups. This results in a toroidal structure, where the lipids are significantly distorted. In the barrel-stave model, AMPs alone form a rigid pore structure, while the toroidal model triggers the interaction between the peptides and the lipids. In the carpet model, AMPs cover the bacterial membrane surface in a manner similar to a carpet. Initially, the AMPs do not form distinct pores. As the concentration of AMPs increases, they align parallel to the membrane surface and interact with the lipid headgroups through electrostatic and hydrophobic forces. This accumulation disrupts the membrane through a detergent-like mechanism involving partial micellization. Finally, the membrane undergoes disintegration and fragmentation, causing the release of cellular components and subsequent bacterial cell death. [B] AMPs can pass through cellular barriers to attach to nucleic acids (DNA, mRNA). AMPs exhibit high binding affinity to single- or double-stranded nucleic acids, allowing them to break down DNA molecules and inhibit mRNA synthesis. The attachment of AMPs on the double-helix DNA structure inhibits mRNA synthesis and disrupts DNA integrity. Also, AMPs can cause the blockage of protein translation by targeting ribosomes and protein misfolding by forming a complex with DnaK, a critical protein functioning in stabilizing de novo proteins. Figure 3 is reproduced and adapted from earlier studies (Le et al., 2017; Matsuzaki, 2019).

2.2.3 Roles of defensins in biotic stress

Defensins play important roles in plant immunity, with expression often triggered by the mitogen-activated protein kinase (MAPK) cascade via effector-triggered immunity (ETI) and MAMP-triggered immunity (MTI) in response to fungal and bacterial assaults (Contreras et al., 2020). In pepper, the defensin gene CADEF1 remained silent in healthy leaves but surged under infection by Xanthomonas campestris pv. vesicatoria (Mee Do et al., 2004). In rice, OsDEF7 and OsDEF8 ramped up expression following Xanthomonas oryzae pv. oryzae infection (Weerawanich et al., 2018). Arabidopsis defensin-like (DEFL) genes spiked in leaves challenged by Alternaria brassicicola or P. syringae, likely via JA signaling (Tesfaye et al., 2013). The transcription factor WRKY75 further amplifies this response, upregulating JA pathway genes like ORA59 and PDF1.2 in overexpressing plants, while their expression drops in wrky75 mutants (Chen et al., 2021). Defensins also exhibit direct antimicrobial activity, for example, cowpea Cp-thionin II inhibited S. aureus (MIC ~ 128 μg mL⁻¹), E. coli (MIC ~ 64 μg mL⁻¹), and P. syringae (MIC ~ 42 μg mL⁻¹) (Franco et al., 2006), while Scots pine PsDef5.1, fused with thioredoxin, suppressed pathogens like Fusarium sporotrichiella and Bacillus pumilus (Shalovylo et al., 2021). In resistant wild chickpea (ICC17160), six defensin genes (CaDEF1.1B, CaDEF2.4, CaDEF2.5a, CaDEF3, CaDEF5, and CaDEFL2) showed heightened expression against F. oxysporum and Rhizoctonia bataticola compared to the susceptible JG62 variety (Nitnavare et al., 2023). In a recent study by Kalunke et al. (2025), the bi-domain defensin MtDef5 from Medicago truncatula demonstrated antifungal properties at very low concentrations (Kalunke et al., 2025). It consists of two single-domains, MtDef5A and MtDef5B, linked by a short peptide APKKVEP. Between these two domains, MtDef5B showed greater antifungal potency compared with MtDef5A, exhibiting potent broad-spectrum inhibitory activity against Botrytis cinerea (Bc), Fusarium graminearum (Fg), Fusarium virguliforme (Fv), and the oomycete Phytophthora capsici (Pc), with MICs ranging from 0.75 µM to 1.5 µM for these pathogens. In contrast, a variant called GMA5AC_V2, derived from carboxy-terminal γ-core motif of MtDef5A (GMA5AC), exhibited two-fold higher activity (MIC ~ 1.5 µM) than GMA5AC (MIC ~ 3 µM) against Bc. It was at least four-fold more potent than GMA5AC against Colletotrichum gloeosporioides (Cg), Fg, and Fv. Notably, while GMA5AC had a MIC value of >12 µM against Cg, GMA5AC_V2 had a MIC value of 3 µM against this pathogen (Kalunke et al., 2025). The GMA5AC_V2 potent antifungal activity could be attributed to the replacement of two cysteine residues (C12 and C14) with phenylalanine in its structure (Kalunke et al., 2025). Furthermore, GMA5AC_V2 also displayed a multi-faceted mode of action similar to MtDef5B as it disrupted the fungal plasma membrane, induced the production of ROS in both Bc and Cg germlings, and inhibited protein translation in vitro (Kalunke et al., 2025). In summary, plant defensins contribute to biotic stress mitigation by acting as both immune response regulators and direct antimicrobial agents, enhancing resistance against a broad spectrum of fungal and bacterial pathogens.

2.2.4 Roles of defensins in abiotic stresses

2.2.4.1 Defensin-mediated mechanisms for heavy metal tolerance

A primary mechanism for heavy metal tolerance is the chelation of metal ions by the cysteine-rich domains of defensins, followed by the efflux of the metal-defensin complex from the cytoplasm to less sensitive extracellular compartments like cell walls or the apoplast, or into vascular tissues for transport (Luo et al., 2018; Luo et al., 2019; Li et al., 2025; Luo et al., 2019). Defensins mitigate abiotic stresses through these mechanisms, which include metal chelation, regulation of metal transport, and modulation of physiological responses. Many defensins (e.g., CAL1, DEF8, AtPDF1.5, AtPDF2.5, NtCAL1) are secreted to the cell wall or extracellular spaces (Luo et al., 2018; Gu et al., 2023; Liu et al., 2021; Luo et al., 2020, 2019; Gu et al., 2023; Jin et al., 2024), while others, like AtPDF2.6 and SpPDF, are localized in the cytoplasm, where they directly chelate metals to mitigate toxicity (Luo et al., 2019; Li et al., 2025).

2.2.4.2 Specific roles of defensins in Cd stress mitigation

Cd is a major heavy metal pollutant posing significant threats to food security and human health, making Cd detoxification and accumulation a critical area of study (Luo et al., 2018; Luo et al., 2019; Li et al., 2024; Luo et al., 2020). Several defensins play key roles in managing Cd stress by regulating its chelation, transport, and accumulation in different plant tissues. Some defensins are regulators of root-to-shoot translocation, acting as secreted proteins that primarily control the movement of Cd from roots to shoots. For example, CAL1 (Cd Accumulation in Leaf 1) in rice is a cell wall-localized defensin-like protein (Luo et al., 2018) that positively regulates Cd accumulation in leaves (Luo et al., 2018; 2020). It chelated Cd in the cytosol and drove its secretion to extracellular spaces, promoting its loading into xylem vessels for root-to-shoot translocation and accumulation in leaves and straws, but not in rice grains (Jin et al., 2024). Similarly, NtCAL1 in tobacco functions as a positive factor in plant Cd accumulation and resistance (Jin et al., 2024), with its overexpression enhancing Cd translocation to the shoot. Overexpression of NtCAL1 enhanced the content of ascorbic acid and the activities of antioxidant enzymes such as catalase (CAT) and ascorbate peroxidase (APX), helping to scavenge oxidative stress induced by Cd (Jin et al., 2024). It also positively regulated the expression of metal transport-related genes such as NtRAMP3, NtHMAα, NtHMAβ (Jin et al., 2024). AtPDF2.5 in Arabidopsis mediated Cd tolerance and accumulation (Luo et al., 2019), and its expression, which is induced by Cd, was primarily expressed in root xylem vascular bundles (Luo et al., 2020). This defensin has Cd-chelating activity (Luo et al., 2019) and promotes cytoplasmic Cd efflux and subsequent accumulation in the apoplast of cell walls, thereby enhancing detoxification and apoplastic accumulation (Luo et al., 2020). Additionally, OsThi9, a defensin-dissimilar thionin, alleviates Cd toxicity by sequestering Cd in cell walls and reducing its translocation to upper parts, which diminishes Cd accumulation in stems and brown rice (Liu et al., 2023). Other defensins act as modulators of shoot and grain accumulation, controlling Cd allocation within the plant. CAL2, a close homolog of CAL1, is a cell wall-localized protein with Cd chelation activity. Overexpression of CAL2 has been shown to increase Cd accumulation in both rice shoots and grains (Luo et al., 2020). Its expression is unaffected by Cd stress but is positively regulated by the endoplasmic reticulum stress response regulator OsbZIP39 (Li et al., 2024). DEFENSIN 8 (DEF8) in rice is a dual-function protein that mediates both xylem Cd loading and phloem Cd unloading (Gu et al., 2023). It was highly expressed in rice grains and was induced by Cd exposure (Gu et al., 2023). While DEF8 facilitates Cd transport from roots to shoots, it prevents Cd loading from the phloem into the grains during filling, making it an ideal target for breeding low-Cd rice varieties (Kazachkova 2023). AtPDF1.5 enhanced adaptation to low nitrogen levels and Cd stress in Arabidopsis (Wu et al., 2021). Its mechanism for Cd tolerance may be attributed to chelation and signal transmission, as it regulates the expression of metal transporter genes such as AtHMP07, AtNRAMP4, AtNRAMP1, AtHIPP3, resulting in higher Cd accumulation in shoots and promoting Cd transport (Wu et al., 2021). Some defensins are cytoplasmic chelators that act within the cytoplasm to directly sequester metals. AtPDF2.6 in A. thaliana is a non-secreted, cytoplasm-localized protein that actively chelates cytoplasmic Cd to mitigate its toxicity (Luo et al., 2019). Similarly, SpPDF in Sedum plumbizincicola, a hyperaccumulator plant, is another cytoplasm-localized defensin that confers Cd accumulation via its chelation (Li et al., 2025). Its chelation activity leads to increased Cd accumulation in the roots and reduced translocation to the shoots, helping the plant compartmentalize metals away from sensitive aerial parts (Li et al., 2025).

2.2.4.3 Roles of defensins in other abiotic stress mitigation

Apart from heavy metal responses, defensins actively contribute to abiotic stress responses, including drought, salinity, and heavy metal tolerance. The pepper CADEF1 gene, for example, was activated by abiotic elicitors (e.g., H₂O₂, wounding, salinity, drought) and stress hormones like SA, Methyl Jasmonate (MeJA), JA, abscisic acid (ABA), and ET (Contreras et al., 2020). Similarly, rice OsDEF7 and OsDEF8 responded to imbibition, anoxia, drought, and cold (Weerawanich et al., 2018). In Arabidopsis halleri ssp. halleri, a zinc (Zn)-tolerant species, defensin proteins accumulated in shoots and gene expression increased under Zn exposure (Mirouze et al., 2006). Arabidopsis AtPDF1.1 is upregulated by iron overload and Pectobacterium carotovorum infection, enhancing tolerance by chelating iron and disrupting pathogen homeostasis (Hsiao et al., 2017). These previous studies, however, did not directly confirm the functional roles of defensin genes in plant stress tolerance under various abiotic stresses, underscoring the need for further mechanistic studies to validate their contributions.

2.2.5 Applications of defensin genes in transgenic plant development

Defensin genes have been harnessed to engineer stress-resistant crops, demonstrating their versatile applications against both biotic and abiotic challenges. The first attempt to transform defensin genes into plant genomes to increase resistance towards phytopathogenic fungi was made by Bondt et al. (1998) (Bondt et al., 1998). Subsequently, the expression of alfalfa antifungal peptide (alfAFP) gene, a defensin isolated from Medicago sativa seeds, in transgenic potato plants, rendered high levels of field resistance against Verticillium dahliae, the causative agent of “early dying” disease of potato (Gao et al., 2000). In vitro assays showed that alfAFP inhibited hyphal elongation of V. dahliae, Alternaria solani and Fusarium culmorum, but not Phytophthora infestans. The secretion of alfAFP in the intercellular spaces of stem tissues protected transgenic potato in both greenhouse and multi-year trials (Gao et al., 2000). Transgenic wheat (Bobwhite and Xin Chun 9 genotypes) expressing an apoplast-targeted antifungal plant defensin, MtDef4.2, from Medicago truncatula displayed significant resistance to the leaf rust pathogen Puccinia triticina (Pt) (Kaur et al., 2017). This resistance was observed in growth chamber bioassays, where transgenic lines showed highly resistant infection types compared to non-transgenic controls. Interestingly, while MtDef4.2 reduced the proliferation of infection hyphae and minimized lesion sizes on infected wheat leaves, it did not negatively affect symbiotic relationship between the host plant and the beneficial arbuscular mycorrhizal fungus Rhizophagus irregularis (Kaur et al., 2017). This is a significant advantage for its agricultural application.

While early efforts focused on introducing defensins to combat phytopathogenic fungi (Bondt et al., 1998; Wang et al., 1999; Lebedev et al., 2002; Cary et al., 2000; Park et al., 2002; Swathi Anuradha et al., 2008; Abdallah et al., 2010), recent applications have expanded to include environmental stressors. Overexpression of the NaD1 gene from Nicotiana alata in tobacco (Nicotiana tabacum) enhanced drought tolerance by maintaining photosynthetic pigments and boosting antioxidant enzyme activity, which reduces oxidative damage (Royan et al., 2023). In the context of heavy metal tolerance, the AhPDF1.1 defensin from a Zn-tolerant species, when transformed into Arabidopsis, conferred increased Zn tolerance (Mirouze et al., 2006). Further findings highlight the multifaceted roles of defensins in heavy metal mitigation. Overexpression of AtPDF2.5 significantly enhanced tolerance and accumulation of Cd in both the host Arabidopsis and in rice through heterologous expression. This suggests that AtPDF2.5 promotes cytoplasmic Cd efflux via chelation, thereby enhancing detoxification and apoplastic accumulation (Luo et al., 2019). Similarly, AtPDF2.6 overexpression also enhanced Cd tolerance, while its knockout increased Cd sensitivity, emphasizing its role in detoxification via chelation and efflux (Luo et al., 2019). Beyond direct chelation, defensins can be regulated by stress-signaling pathways. The OsbZIP39 transcription factor, a key regulator of the unfolded protein response in the endoplasmic reticulum, positively regulates Cd accumulation in rice. Upon Cd exposure, OsbZIP39 is activated and enhances the expression of the defensin-like protein OsCAL2, which in turn facilitates Cd accumulation in the plant (Li et al., 2024). These findings show that defensin-based strategies can be leveraged to develop crops with resilience to various environmental stresses through distinct mechanisms, including antioxidant activity, metal chelation, and ion efflux.

2.3 Cyclotides

2.3.1 Family and structure

Cyclotides represent another fascinating class of AMPs with a unique, highly stable structure, which enables them to resist harsh environmental conditions. This is a salient group of plant-derived macrocyclic peptides, typically containing 28–37 aa and featuring an embedded cystine knot. These peptides have been identified in five major plant families, including Rubiaceae, Violaceae, Solanaceae, Cucurbitaceae, and Fabaceae (Slazak et al., 2020). Cyclotide-like genes have also been identified in members of the Poaceae family, including O. sativa, Zea mays, T. aestivum, Agrostis stolonifera, Schedonorus arundinaceus, Pennisetum glaucum, Sorghum bicolor, Hordeum vulgare, Saccharum officinarum, and Setaria italica. Cyclotides are distinguished by their unique structural characteristics, which include a head-to-tail cyclic peptide backbone and a cystine knot motif, where two disulfide bonds are interlinked by a third. This structural conformation confers exceptional stability, enabling cyclotides to remain functional under extreme temperatures, pH variations, and enzymatic degradation while maintaining solubility in both organic and aqueous environments (Veer et al., 2019).

One of the most well-characterized cyclotides, kalata B1, was initially isolated from Oldenlandia affinis and has since been identified in several Viola species, including Viola tricolor, Viola yedoensis, Viola philippica, Viola baoshanensis, and Viola odorata. It was the first cyclotide to have its structure elucidated (Saether et al., 1995; Veer et al., 2019). The exceptional stability of cyclotides is attributed to their cyclic peptide backbone and cystine knot motif, which enable them to withstand extreme conditions such as high temperatures and enzymatic degradation within the human digestive system. The stability of kalata B1 has been linked to these structural features, which historically contributed to the use of O. affinis as a medicinal tea in Congolese tribes, where it was known as “kalata-kalata” in the Tsjiluba language and used to facilitate childbirth (Gran et al., 2000). Some cyclotides structures were predicted using AlphaFold3 (Abramson et al., 2024) and shown in Figure 4.

Figure 4

The figure illustrates the 3D-structures of six common cyclotides, which are characterized by one or more β-sheets, with and without α-helix. These representations highlight structural diversity across different plant species.

2.3.2 Mechanism of action

As with other AMP classes, cyclotides exert their bioactivity through multiple mechanisms, including membrane disruption and targeting of insect physiology. Cycloviolacin-O2 (CyO2) from V. odorata exhibits antimicrobial properties by disrupting cellular membranes and interfering with intracellular targets, with MICs ranging from 0.8 to 100 µM against fungal and bacterial pathogens such as F. oxysporum, Botrytis cinerea, and P. syringae (Oguis et al., 2019). Additionally, insecticidal cyclotides such as kalata B1 and kalata B2 target the physiological integrity of insect midgut cells. Kalata B1, for example, induces rupture of epithelial cells in the midgut of Helicoverpa armigera larvae, a mode of action similar to that of Bacillus thuringiensis δ-endotoxins (English and Slatin, 1992; Barbeta et al., 2008). Furthermore, the high metal-binding affinity of cyclotides suggests their potential involvement in detoxification processes, though the underlying mechanisms require further investigation (Zhang et al., 2015). The paper by Miszczak et al. (2025) tested whether differences in cyclotide production between metallicolous (M) and non-metallicolous (NM) populations under Zn and lead (Pb) stress and their functions in alleviating metal stress (Miszczak et al., 2025). In M population, the decrease in cyclotide production under heavy metal stress may be explained the reallocation of resources towards other tolerance mechanisms (e.g., metal sequestration or callose production), while in NM population, the increase in cyclotide production suggests an inducible defense response to mitigate oxidative stress or cellular damage caused by heavy metals. In these conditions, cyclotides may act as metal chelators or membrane stabilizers to protect metal-sensible plants (Miszczak et al., 2025). However, the paper did not provide direct evidence of cyclotide function, such as metal binding, detoxification, or cellular protection, leaving their mechanistic role unclear.

2.3.3 Roles of cyclotides in biotic stress

Cyclotides, like thionins and defensins, serve as potent natural defenses against biotic stressors. Kalata B1 and B2, derived from O. affinis, exhibited significant insecticidal activity. Kalata B1 effectively inhibited the growth of Helicoverpa punctigera and H. armigera larvae at a concentration of 0.13% (w/v), while reducing nutrient uptake at 0.24% (w/v) (Jennings et al., 2001; 2005). Kalata B2 demonstrated superior molluscicidal efficacy compared to metaldehyde in controlling the golden apple snail (Pomacea canaliculata), a major pest in rice cultivation, with an LC₅₀ of 53 μM, whereas metaldehyde exhibited an LC₅₀ of 133 μM (Plan et al., 2008). Unlike metaldehyde, which poses toxicity risks to non-target mammals, kalata B2 presents an environmentally safer alternative (Tan et al., 2022). Furthermore, the cyclotide Cter M, isolated from Clitoria ternatea, demonstrated efficacy against H. armigera, contributing to the development of Sero-X^®, a bioinsecticide approved for commercial use in Australia in 2017 (Oguis et al., 2019). In terms of antimicrobial properties, cycloviolacin-O2 (CyO2) from V. odorata exhibits antifungal and antibacterial activity against pathogens such as Colletotrichum utrechtense, Alternaria alternata, and Dickeya dadantii (Oguis et al., 2019). Additionally, cyclotide-like genes in Z. mays (Zmcyc1 and Zmcyc5) were reported to contribute to plant defense against fungal pathogens such as F. graminearum and insect pests like Rhopalosiphum maydis, suggesting a broader protective function in plant immunity (Salehi et al., 2017).

2.3.4 Roles of cyclotides in abiotic stress

Emerging evidence suggests that cyclotides also contribute to abiotic stress tolerance, although their exact mechanisms remain underexplored. V. baoshanensis, a known Cd hyperaccumulator, accumulates Cd concentrations of 1168 mg kg⁻¹ in shoots and 981 mg/kg in roots when grown in Pb-Zn mining areas (Liu et al., 2004). In response to Cd exposure, cyclotide precursor mRNAs VbCP7S and its spliced variant VbCP6S were upregulated in the leaves, indicating a potential role in metal binding and detoxification (Zhang et al., 2009). In Z. mays, cyclotide-like genes Zmcyc1 and Zmcyc5 are induced under abiotic stress conditions, including drought, salinity, and mechanical wounding. Their expression is also modulated by phytohormones such as SA and MeJA, suggesting involvement in stress response pathways linked to plant defense (Salehi et al., 2017). Additionally, studies have highlighted the strong metal-binding affinity of cyclotides, raising the possibility that they contribute to heavy metal detoxification, though further research is required to elucidate their precise function in abiotic stress adaptation (Zhang et al., 2015).

2.3.5 Applications of cyclotide genes in transgenic plant development

Cyclotides present promising applications in transgenic plant development, demonstrating a dual role in conferring resistance to both biotic and abiotic stresses. In the context of abiotic stress, cyclotides show potential for heavy metal detoxification. For example, five cyclotide precursor genes (VbCP1–VbCP5) from V. baoshanensis enhanced copper (Cu) tolerance when expressed in Saccharomyces cerevisiae (Zhang et al., 2009), demonstrating their metal-binding capabilities. This approach is similar to the use of a chicken-derived metallothionein gene, which significantly improved Cd sequestration in Chlamydomonas reinhardtii (Cai et al., 1999). This highlights that cyclotides, like other metal-binding proteins, can be leveraged to increase tolerance to heavy metal contamination. In parallel, cyclotides have been successfully engineered for biotic stress resistance. The kalata B1 (Oak1) gene from O. affinis was introduced into rice, resulting in transgenic plants that provided protection against a specific agricultural pest, the golden apple snail (Lim and Lai, 2017). These findings collectively underscore the exceptional versatility of cyclotides as valuable molecular tools for developing crops with improved resilience to both herbivorous pests and heavy metal contamination, using distinct mechanisms for each stress type. More recently, a study by Qu et al. (2020) successfully produced a structurally validated cyclotide in rice suspension cells in the presence of a supporting biosynthetic enzyme asparaginyl endopeptidase (AEP), which is derived from the O. affinis plant (Qu et al., 2020). This enzyme is crucial in performing the final, essential step in cyclotide biosynthesis—the head-to-tail cyclization of the linear precursor peptide (Tang and Luk 2021). Without it, the plant would only produce a linear peptide, not the highly stable, circular cyclotide. The study’s main finding was that co-expressing this enzyme with the cyclotide precursor gene was necessary to achieve the correct, functional structure of the kalata B1 cyclotide in the rice suspension cell (Qu et al., 2020). This research confirms that the biological machinery of a plant can be harnessed to correctly produce complex cyclotides, validating their potential for crop engineering.

2.4 Hevein-like antimicrobial peptides

2.4.1 Family and structure

Hevein-like AMPs are a family of structurally related peptides characterized by their similarity to hevein, a peptide originally identified in the latex of the rubber tree (Hevea brasiliensis) (Van Parijs et al., 1991). These peptides are cysteine- and glycine-rich, typically containing 6–8 cysteine residues that form disulfide bonds, contributing to their stable, compact structures. They are found across various plant families, including both monocots and dicots, indicating their widespread distribution in the plant kingdom (Slavokhotova et al., 2017). A key structural feature of hevein-like AMPs is the chitin-binding domain, which facilitates interaction with chitin and related oligosaccharides, a critical component of fungal cell walls and insect exoskeletons. For instance, the 40-amino-acid Pn-AMP2 and the 8C-hevein-like ginkgotide gB5 from Ginkgo biloba exemplify this structural motif, which underpins their biological activities (Koo et al., 1998; Wong et al., 2016).

2.4.2 Mechanisms of antimicrobial action

Hevein-like AMPs exhibit broad-spectrum antimicrobial activity through mechanisms primarily based on their chitin-binding capability. The chitin-binding domain allows these peptides to target chitin-containing structures in fungal cell walls, disrupting membrane integrity and inhibiting fungal growth. Pn-AMP2 demonstrated antifungal activity against both chitin-containing and non-chitin-containing fungi, suggesting additional mechanisms such as membrane permeabilization or interference with cellular processes (Koo et al., 1998). Similarly, the PMAPI protein from paper mulberry (Broussonetia papyrifera) inhibited Trichoderma viride with an IC50 of 0.1 μg/μL (Zhao et al., 2011). Additionally, some hevein-like AMPs, such as WAMP-1a from Triticum kiharae, exhibited antibacterial activity against Gram-positive and Gram-negative bacteria by disrupting bacterial cell membranes (Odintsova et al., 2009). These peptides also counteract fungal proteases, such as fungalysin from Fusarium verticillioides, which degrades plant chitinases, thereby preserving plant defense mechanisms (Slavokhotova et al., 2014).

2.4.3 Roles in biotic stress mitigation

Hevein-like AMPs play a critical role in plant defense against biotic stresses, including fungal, bacterial, and insect challenges. Their antifungal properties are well-documented, with peptides like WAMP1 and WAMP2 from Triticum kiharae showing upregulated expression in response to Fusarium oxysporum infection and inhibiting fungal growth (Andreev et al., 2012). Ginkgotide gB5 from Ginkgo biloba inhibited fungi such as A. niger (IC₅₀ ~ 6.8 μg mL⁻¹) and R. solani (IC₅₀ ~ 20 μg mL⁻¹) (Wong et al., 2016). Beyond fungi, WjAMP1 from wasabi (Wasabia japonica) suppressed bacterial growth, including E. coli (IC₅₀ ~ 27.5 μg mL⁻¹) and Pseudomonas cichorii (IC₅₀ ~ 13.8 μg mL⁻¹) (Kiba et al., 2003). Hevein-like AMPs also contributed to insect resistance, as seen with the MLX56 protein from mulberry, which exhibited antagonistic activity against herbivores like Spodoptera litura larvae and Henosepilachna vigintioctopunctata (Murata et al., 2021). Additionally, WAMP2-derived peptides enhanced the efficacy of commercial fungicides like Folicur^® EC 250, achieving synergistic suppression of fungal spore germination (≥ 90%) (Odintsova et al., 2020).

2.4.4 Roles of hevein-like AMPs in abiotic stress alleviation

In addition to biotic stress, hevein-like AMPs contribute to abiotic stress responses, particularly salinity stress. The expression of wamp genes encoding WAMP1 and WAMP2 in T. kiharae is significantly induced by elevated NaCl levels (100–200 mM), suggesting a role in mitigating salinity stress (Andreev et al., 2012). Furthermore, transgenic asparagus plants expressing a hevein-like gene show enhanced resistance to stem wilt, accompanied by increased activities of antioxidant enzymes such as CAT, superoxide dismutase (SOD), and phenylalanine ammonia lyase (PAL), and reduced malondialdehyde levels, indicating a broader role in oxidative stress alleviation (Chen et al., 2019).

2.4.5 Applications of hevein-like AMPs genes in transgenic plant development

Hevein-like AMP genes have been extensively utilized in transgenic plant development to enhance resistance to a wide range of biotic stresses, particularly against fungal pathogens and herbivorous insects. As the primary mechanism of hevein-like AMPs is their ability to bind to chitin, a major component of fungal cell walls and insect skeletons, the core function of these peptides does not directly contribute to abiotic stress tolerance. Their application across different crop species demonstrates their broad potential. Several hevein-like genes have been successfully used to combat fungal diseases. The pnAMP-h2 gene from Pharbitis nil conferred improved resistance to Phytophthora parasitica in tobacco (Koo et al., 2002), while the amp1 and amp2 genes from T. kiharae significantly reduced lesion areas caused by Phytophthora infestans in tomatoes (Khaliluev et al., 2011). Similarly, the SmAMP2 gene from Stellaria media and a hevein-like gene in asparagus both enhanced resistance to fungal pathogens, including Phoma asponaqi and the agent of stem wilt, respectively (Odintsova et al., 2009; Chen et al., 2019). Beyond fungi, hevein-like AMPs also offer protection against insect pests. The MLX56 gene in transgenic tomatoes provided resistance to herbivorous insects, highlighting its potential as a biological alternative to traditional crop protection methods (Murata et al., 2021). These applications collectively underscore the critical role of hevein-like AMPs in developing resilient, disease-resistant crops by targeting distinct biotic threats.

From the structural stability and direct antimicrobial activity of hevein-like AMPs—which utilize disulfide bonds and chitin-binding domains for pathogen recognition—attention shifts to systemins, a class of peptide hormones that amplify plant defense responses through signaling cascades, notably via the JA pathway. This progression from direct antimicrobial effectors to systemic signaling molecules underscores the functional diversification of AMPs in plant immunity, contributing not only to resistance against biotic stressors but also to enhanced tolerance of abiotic challenges.

2.5 Systemins

2.5.1 Family and structure

Systemin (SYS) is an 18-amino-acid peptide hormone derived from a 200-amino-acid precursor, prosystemin (PS), in plants, notably within the Solanaceae family (Zhang et al., 2020). SYS is processed from PS and functions as a long-distance signaling molecule, capable of traveling 40 cm from the injection site to the plant apex at a rate of 2.5 cm h⁻¹ in tomato plants (Mucha et al., 2019). Hydroxyproline-rich systemins (HypSYS) are related glycopeptides, first identified in Solanaceae species such as tobacco (N. tabacum) and tomato (Solanum lycopersicum) (Pearce et al., 1991). HypSYS peptides are small, defense-signaling molecules characterized by hydroxyproline modifications, which enhance their stability and activity. In petunia (Petunia hybrida), HypSYS peptides, termed PhHypSYS, exhibited distinct functional roles compared to their tobacco and tomato counterparts (Pearce et al., 2007).

2.5.2 Mechanisms of antimicrobial and defense action of systemin and prosystemin

SYS and HypSYS peptides activate plant defense responses primarily through the JA signaling pathway. In tomato, SYS enhances resistance to necrotrophic fungi like Plectosphaerella cucumerina by upregulating JA-biosynthesis gene LOX2 and JA-responsive genes such as JASMONATE ZIM-DOMAIN PROTEIN 10 (JAZ10), DEFENSIN1.2 (PDF1.2), and DEFENSIN1.3 (PDF1.3) (Zhang et al., 2018; Pastor-Fernández et al., 2022). SYS also interacts with membrane receptors BAK1 and BIK1, which are markers of pattern-triggered immunity (PTI), showing elevated expression only under pathogen challenge, suggesting a unique perception mechanism distinct from classical damage-associated molecular patterns (Lin et al., 2014). Additionally, SYS-treated plants exhibited increased hydrogen peroxide (H₂O₂) production following flagellin 22 (flg22) treatment, amplifying defense responses (Zhang et al., 2018). HypSYS peptides from tobacco and tomato, induced defense genes against herbivores, while PhHypSys in petunia upregulated defensin I, enhancing pathogen resistance (Pearce and Ryan, 2003; Pearce et al., 2007).

2.5.3 Roles in biotic stress mitigation

SYS and HypSYS peptides are pivotal in mitigating biotic stresses, including fungal, bacterial, and insect challenges. SYS enhanced resistance to necrotrophic fungi such as B. cinerea in Arabidopsis and tomato by activating JA-responsive defenses, reducing fungal colonization (Coppola et al., 2019). Exogenous SYS application to tomato plants restricts Spodoptera littoralis larval growth across generations and attracts natural enemies through volatile compound emission. Similarly, PS application increases S. littoralis mortality and suppresses B. cinerea colonization (Molisso et al., 2022). HypSYS peptides from tobacco and tomato induce anti-herbivore defenses, such as protease inhibitors, while PhHypSYS in petunia targets pathogen defenses via defensin I upregulation, demonstrating functional diversity within the HypSYS family (Pearce and Ryan, 2003; Pearce et al., 2007). SYS treatment increased the expression of pathogenesis-related (PR) gene PR1 and PR10, as well as the PTI market genes Pti5 and GRAS2. It also activated the MAPK signaling pathway, with peak MAPK observed at 3 hours after treatment. In addition, the H₂O₂ content in SYS-treated tomato fruits reached the highest level at 12 hours (Wang et al., 2024).

2.5.4 Roles of SYS and PS in abiotic stress alleviation

While the primary role of SYS and HypSys is in biotic stress response, their involvement in abiotic stress alleviation is less direct but notable. The activation of JA-signaling pathways by SYS and HypSys can indirectly enhance resilience to oxidative stress, as JA-responsive genes like glutathione S-transferase (GST) contribute to detoxification and attenuation of oxidative damage (Gullner et al., 2018). Additionally, the upregulation of heat shock proteins (HSPs) in SYS- or PS-treated plants under biotic stress conditions suggests a potential overlap with abiotic stress responses, as HSPs stabilize proteins under various stresses (Corrado et al., 2016; ul Haq et al., 2019). However, specific roles in abiotic stresses like salinity or drought require further investigation.

2.5.5 Applications of SYS and PS genes in transgenic plant development

The genes encoding SYS and PS have been leveraged in transgenic plant development to enhance resistance to a range of biotic stresses, particularly against insects and fungal pathogens. Overexpression of the tomato PS gene in Arabidopsis conferred higher resistance to the necrotrophic fungus B. cinerea by modulating JA-responsive genes (Zhang et al., 2018). Further research on a truncated PS (tPS) cDNA, which lacks the SYS sequence, showed its remarkable effectiveness in two different host plants. In tomato, it resulted in reduced larval growth and higher mortality of the herbivorous insect S. littoralis and suppressed necrosis from B. cinerea (Molisso et al., 2022). A similar transformation in tobacco demonstrated that tPS enhances resistance to B. cinerea by upregulating a suite of defense genes, including heat shock proteins (HSP), glutathione S-transferases (GST), and proteinase inhibitor II (PI II) (Corrado et al., 2016). These findings show that both the full PS protein and its truncated version act as potent signaling molecules that fortify plant defenses through different pathways. Extending the theme of peptide-mediated signaling seen in SYSs, which amplify defense through JA pathways, cyclic dipeptides (CDPs) represent an even more compact and stable class of AMPs. These small molecules provide direct antimicrobial effects while also inducing resistance mechanisms, akin to those seen in hevein-like AMPs and SYSs. This dual functionality reinforces the interconnected roles of diverse AMPs in holistic plant stress management and transgenic innovation. Table 1 summarizes the application of SYS and PS genes, along with other AMP genes, in transgenic engineering strategies aimed at improving plant tolerance to both biotic and abiotic stresses.

2.6 Cyclic dipeptides in plant defense and stress mitigation

2.6.1 Family and structure

CDPs, also referred to as 2,5-diketopiperazines (DKPs), are the smallest cyclic peptides, characterized by a stable 2,5-DKP ring structure (Mishra et al., 2017). These naturally occurring compounds are found in bacteria, yeast, fungi, plants, and animals. Synthesized as secondary metabolites or protein metabolism byproducts, CDPs are produced via CDP synthases and non-ribosomal peptide synthetases (NRPSs), utilizing aminoacyl-tRNAs as substrates, with additional modifications by tailoring enzymes (Fizza et al., 2025). Approximately 100 CDP synthases have been identified, generating 75 of the 210 possible natural CDPs. The cyclization process, involving macrolactomization and macrolactamization, enhances their resistance to peptidase degradation, rendering CDPs highly stable and promising for applications in medicine, industry, and agriculture (Bechtler and Lamers, 2021).

2.6.2 Mechanisms of CDPs in biotic stress mitigation

CDPs enhance plant resistance to biotic stresses through direct pathogen inhibition, insect and nematode deterrence, and induction of systemic resistance. CDPs such as cyclo(L-Phe-L-Pro) and cyclo(L-Phe-trans-4-OH-L-Pro) from Lactobacillus plantarum MiLAB 393 inhibited fungi including Fusarium sporotrichioides, Aspergillus fumigatus, and Kluyveromyces marxianus (Ström et al., 2002). Similarly, Bacillus sp. strain N produced CDPs like cyclo(L-Pro-L-Leu), cyclo(D-Pro-L-Leu), and cyclo(D-Pro-L-Tyr), which suppressed Penicillium expansum (MIC ~ 4 µg mL⁻¹), outperforming the fungicide Bavistin, and bacteria such as Bacillus subtilis and Escherichia coli (Kumar et al., 2012). Cyclo(L-Pro-L-Asp) exhibited potent antifungal activity against Aspergillus flavus (MIC ~ 16 µg mL⁻¹) and Trichophyton rubrum (MIC ~ 2 µg mL⁻¹), surpassing standard fungicides (Challa et al., 2014). Cyclo(L-Val-L-Pro) and cyclo(L-Phe-L-Pro) from Lactobacillus plantarum LBP-K10 inhibited Ganoderma boninense, a major oil palm pathogen (Kwak et al., 2014). Additionally, cyclo(L-Leu-L-Pro) from Achromobacter xylosoxidans suppressed aflatoxin production in Aspergillus species by targeting glutathione S-transferase (Yan et al., 2004).

CDPs also confer resistance to insects and nematodes (Sun et al., 2021; Sathya et al., 2016). Cyclo(L-Leu-L-Pro) from Bacillus gaemokensis PB69 enhanced cucumber resistance to Pseudomonas syringae pv. lachrymans and increased Spodoptera litura larval mortality (Song et al., 2017). Cyclo(L-Pro-L-Ile) from Bacillus thuringiensis JCK-1233 suppressed pine wilt disease caused by Bursaphelenchus xylophilus in pine seedlings by modulating defense gene expression and preventing hypersensitive reactions (Park et al., 2020). Furthermore, CDPs act as elicitors of induced systemic resistance (ISR). Cyclo(L-Pro-L-Pro) and cyclo(D-Pro-D-Pro) induced resistance in Nicotiana benthamiana against Phytophthora nicotianae and Tobacco mosaic virus via SA-mediated pathways, triggering stomatal closure, H₂O₂ production, and nitric oxide accumulation (Wu et al., 2017). Cyclo(L-Leu-L-Pro) from Bacillus vallismortis BS07 activated SA-dependent defenses in chili peppers, reducing disease severity from Pectobacterium carotovorum and Phytophthora capsici (Noh et al., 2017).

The stability, bioactivity, and simple chemical structures of CDPs make them promising for transgenic plant development. Genes encoding CDP synthases could be integrated into crop genomes to produce antifungal CDPs like cyclo(L-Pro-L-Leu), enhancing resistance to Fusarium or Aspergillus spp (Boecker et al., 2018). The heat stability and resistance to enzymatic degradation of CDPs ensure their efficacy in field conditions, supporting their use in engineering crops with enhanced biotic stress tolerance (Zhao et al., 2021). Additionally, CDPs’ role in inducing ISR via SA or JA pathways could be leveraged to develop transgenic plants with enhanced systemic immunity (Solis-Ortiz et al., 2022). In summary, CDPs mitigate biotic stresses in crops such as cucumber, chili pepper, pine, and oil palm through direct antimicrobial action, toxin suppression, and SA-mediated ISR, offering versatile mechanisms for enhancing plant immunity against diverse pathogens and pests.

2.6.3 Roles of antimicrobial peptides in abiotic stress alleviation

While research on CDPs’ roles in abiotic stress alleviation is limited, emerging evidence suggests their involvement. A mixture of CDPs (cyclo(L-Pro-L-Tyr), cyclo(L-Pro-L-Val), cyclo(L-Pro-L-Leu), cyclo(L-Pro-L-Phe)) produced by P. aeruginosa has been shown to enhance Arabidopsis growth and root system architecture (González-López et al., 2021). This is achieved by increasing the phosphorylation of the S6 ribosomal protein kinase (S6K), a downstream substrate of the target of rapamycin (TOR) kinase (Figure 5). The TOR pathway is a central hub regulating plant growth, cell proliferation, and metabolism, and its modulation by CDPs suggests a mechanism for improving stress resilience through post-translational modifications (Shi et al., 2018; Caldana et al., 2019). Furthermore, recent studies shed light on the intricate interplay between CDPs and metabolic pathways involved in abiotic stress tolerance. For example, the apparent controversy regarding the role of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in plant salinity stress tolerance—where some studies report that its upregulation enhanced tolerance (Qiu et al., 2025; Zhang et al., 2024), while others suggest its inhibition by cyclo(His-Pro) improved tolerance (Minen et al., 2025)—can be reconciled by considering the distinct roles, isoforms, and regulatory mechanisms of GAPDH in plant metabolism, as well as the context-specific effects of salinity stress. Upregulation of plastidial GAPDH (pGAPDH) supports photosynthesis and energy production, critical for maintaining growth and osmolyte synthesis under salinity (Li et al., 2019). Conversely, inhibition of cytosolic GAPDH (cGAPDH) by cyclo(His-Pro) redirects carbon flux from glycolysis to the pentose phosphate pathway (PPP), enhancing nicotinamide adenine dinucleotide phosphate (NADPH) production and antioxidant defense, which is vital for mitigating oxidative stress (Minen et al., 2025) (Figure 6). These mechanisms are not mutually exclusive but reflect a dynamic metabolic balance tailored to specific stress conditions, tissues, and plant species. For instance, under normal conditions, the microbial inoculant Priestia megaterium BP01R2, which likely produces CDPs, induced lateral root formation, created more root hairs, increased fresh weight, and promoted main root length (Hung et al., 2024). Under salinity stress, P. megaterium BP01R2 mitigated adverse effects by reducing chlorotic and curled leaves and minimizing the decline in root development. The expression pattern of P. megaterium BP01R2 -inoculated Arabidopsis plants under salt stress resembled that of control plants rather than NaCl-treated plants, suggesting a significant stress-mitigating effect. Under stress-free conditions, genes related to root cell differentiation, root morphology, cytoskeleton, and various enzyme activities (ATPase, helicase, ligase) were upregulated, aligning with enhanced plant growth. Under stress conditions, genes involved in cell death, hypersensitive response, JA biosynthesis and response, ET response, MAPK signaling, and glutathione metabolism were also upregulated, indicating a broader activation of stress response pathways (Hung et al., 2024). Intriguingly, the exogenous application of cyclo(L-Ala-Gly), which was identified in the metabolomic data of P. megaterium BP01R2, mirrored the performance of its host bacterium BP01R2 in alleviating salinity stress in Arabidopsis (Hung et al., 2024), suggesting potential applications of these small molecules as biostimulants in agriculture. These initial studies paved the way for a partial understanding of the underlying mechanisms behind CDPs’ properties in abiotic stress mitigation. However, further investigations integrating transcriptomic, proteomic, and metabolomic approaches are needed to fully elucidate the complex mechanisms underlying CDPs’ contributions to abiotic stress tolerance, such as drought, salinity, or temperature extremes.

Figure 5

Figure 6

Figure 5 illustrates the molecular mechanisms involved in root development, with a focus on the Target of Rapamycin (TOR) signaling pathway. TOR kinase complex, consisting of TOR kinase, Raptor, and LST8, functions as the central regulator. The complex is induced by cyclic dipeptides [e.g., Cyclo(L-Pro-L-Tyr), Cyclo(L-Pro-L-Val), Cyclo(L-Pro-L-Leu), and Cyclo(L-Pro-L-Phe)], and subsequently facilitates the phosphorylation process to activate S6K1. S6K1 upregulates the expression of Cyclin B1, which leads to the formation of cyclin B1 and cyclin-dependent kinase 1 (CDK1). The Cyclin B1–CDK1 complex drives the transition from the G2 phase to the M phase (mitosis) in the cell cycle, thereby promoting root development in plants. Figure 5 was conceptualized based on the mechanisms described in (Caldana et al., 2019; González-López et al., 2021).

3 Future perspectives

From an agricultural perspective, AMPs have been extensively studied for their antifungal and antibacterial properties, with significant potential for enhancing crop resistance to biotic stresses. Recent studies also highlight salient roles of AMPs in abiotic stress responses. However, the mechanisms underlying AMPs’ involvement in abiotic stress responses remain underexplored, which necessitate comprehensive evidence on molecular features, signaling pathways, phytohormone interactions, and field-based assessments to validate their efficacy in crops. The potential of AMPs as biocontrol agents, either through direct application or transgenic approaches, is promising, given their stability and broad-spectrum activity. Nevertheless, utilizing AMPs as elicitors to induce abiotic stress tolerance requires further investigation to elucidate their modes of action and optimize their application. Bridging this knowledge gap is critical for developing resilient crop varieties to address rising environmental challenges. Additionally, the release of bioactive AMPs from genetically modified crops raises concerns for human health and ecosystems. The high stability of AMPs suggests persistence in soil and water, therefore, thorough assessments of their environmental fate, interactions with soil microbiota, and degradation dynamics, are highly recommended, to ensure safe deployment.

To advance the application of AMPs in agriculture and address current knowledge gaps, the following research directions and recommendations are proposed:

• Elucidate Molecular Mechanisms: Investigate the signaling pathways and molecular interactions of AMPs in abiotic stress responses, focusing on their crosstalk with phytohormones like JA, ABA, SA to uncover synergistic effects.

• Expand Field-Based Studies: Conduct greenhouse and in-field trials to evaluate AMP efficacy across diverse crop species under combined biotic (pathogens, insects) and abiotic (salinity, drought, heavy metals) stresses, ensuring translational relevance.

• Assess Environmental Impact: Perform comprehensive studies on the persistence, degradation, and ecological interactions of AMPs in soil and water, including their effects on non-target organisms and microbial communities, to ensure biosafety.

• Integrate Multi-Stress Resistance: Explore gene stacking strategies combining AMPs (e.g., SYS with SOS1, NaD1 with HSP90) to engineer crops with simultaneous resistance to biotic and abiotic stresses, balancing efficacy with yield stability.

These directions aim to harness AMPs potential for sustainable agriculture while addressing ecological and safety concerns, paving the way for resilient crop systems amidst global environmental challenges.

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