Salt stress creates an unfavorable environment for plant growth through osmotic and ionic stresses. Higher concentrations of soluble ions in the rhizosphere reduce the soil water potential hindering water uptake by plant roots concurrently creating osmotic stress or dehydration stress. Eventually, the gradual accumulation of salt ions inside the plant tissue causes ionic stress to the plant. The salt overlay sensitive (SOS) pathway is a well-established Ca²⁺-dependent signaling cascade for Na⁺ ion homeostasis. Na⁺ ion probably enters into the cell through anatomical leaks or embroiling hyperpolarization-activated NSCC to eventuate a rapid spike in the calcium-gated channel (Ji et al., 2013). The spike in the Ca²⁺-gated channel activates calcium-binding protein (CBP) SOS3 through myristoylation, a post-translational modification of SOS3, to mediate subsequent binding and activation of a serine/threonine kinase protein, SOS2 (Mahajan et al., 2008). Finally, the SOS2-SOS3 complex activates SOS1 available in the cell membrane to remove excess Na⁺ ions from the cell (Mahajan et al., 2008). In Arabidopsis, calcineurin B-like 4 (CBL4) acts as a SOS3 protein and CBL-interacting protein kinase 24 (CIPK24), a serine/threonine protein kinase, performs as SOS2 (Yang et al., 2019). Further, the study identified a SOS3-like calcium-binding protein 8 (SCaBP8), later named calcineurin B-like 10 (CBL10), that can bind with the SOS2 protein and intercedes phosphorylation for subsequent downstream signaling (Quan et al., 2007; Lin et al., 2009). CBL4 is mostly functional in root tissue, while CBL10 is mostly linked with the shoot and critically associated with the reproductive tissue development of different plants and the signaling of both CBL4 and CBL10 under salt stress work independently (Yang et al., 2019). CBL10 directly acts on translocon of the outer membrane of chloroplasts 34 (TOC34) protein and reduces the GTPase activity of TOC34 (Cho et al., 2016). For TOC34 protein translocation in the plastid, GTP binding and hydrolysis are highly essential while transportation of several proteins inside the chloroplast is mediated by Ca²⁺/CaM signaling (Chigri et al., 2005). Since CBL10 and TOC34 are expressed in different types of plastids in plants (Cho et al., 2016), it can be speculated that salt stress signaling modulates the functioning of various proteins in the cytoplasm and other organelles. Moreover, another finding reveals that the Ca²⁺ dynamics in the cytosol and nucleus are differentially regulated to control overall stress signaling in Arabidopsis (Huang et al., 2017). CBL10 also interacts with CIPK24 in addition to various other CIPKs and activates several transporters in vacuoles like H⁺/Ca²⁺ antiporter (also known as CAX1), H⁺-ATPase transporter, and Na⁺/H⁺ transporter (NHX1) in Arabidopsis (Yang et al., 2019) for maintaining Na⁺ and K⁺ ion homeostasis in the plant cell. Besides, Annexin1 has been found to regulate Ca²⁺ influx in the root epidermal tissue of Arabidopsis upon sensing salt stress (Laohavisit et al., 2013).

Another crosstalk has been detected in Arabidopsis and other plants to combat salt stress through phosphatidic acid (PA)-mediated signaling cascade (Yu et al., 2010; McLoughlin et al., 2012). Some membrane phospholipids, namely, phosphatidylcholine and phosphatidylethanolamine, are hydrolyzed by phospholipase D (PLD) to produce PA and the residual head group. Moreover, the hydrolysis of phosphatidylinositol lipid is carried out by phospholipase C to produce diacylglycerol (DAG) that is eventually phosphorylated by DAG kinase to produce PA (Yao and Xue, 2018). Various stresses animate transient increase in PA which eventually bind with mitogen-activated protein kinase 6 (MPK6) to trigger phosphorylation of SOS1 protein, facilitating Na⁺ extrusion from the cell and other TFs, having a direct effect on salt-stress alleviation (Yu et al., 2010). Additionally, PA binds with two sucrose non-fermenting-1 (SNF1)-related protein kinase 2 (SnRK2.4 and SnRK2.10) proteins for shaping root architecture under salt stress (McLoughlin et al., 2012; Julkowska et al., 2015). A comprehensive study unravels that the binding of mitogen-activated protein kinase kinase 7 (MKK7) and MKK9 with PA is crucial for their translocation to the membrane. These interactions trigger MPK6 to activate SOS1 in the membrane (Shen et al., 2019).

The nuclear Ca²⁺ signature is mediated in plants by CaM and CDPK which are basically EF-hand motif-containing Ca²⁺ sensors (Kim et al., 2009). In response to salt stress in common ice plants (Mesembryanthemum crystallinum), a CDPK (McCDPK1) has been isolated containing a nuclear localization signal (NLS) for translocation of McCDPK1 into the nucleus to mitigate salt stress (Patharkar and Cushman, 2000). Several Ca²⁺ binding TFs viz., AtNIG1 (Arabidopsis thaliana NaCl-inducible gene 1), have been detected for decoding of Ca²⁺ signaling in both cytosol and the nucleus (Kim and Kim, 2006), which is an integral part of the promoter region of various salinity stress-responsive genes (Chinnusamy et al., 2003). CaM isoforms have also been discovered in soybeans that modulated the salt-induced Ca²⁺ signaling by activating an MYB-type TF, which is an upstream regulator of several salt and dehydration responsive genes (Yoo et al., 2005). The overall Ca²⁺ signaling under salt stress is depicted in Figure 1.

Drought is one of the most challenging abiotic stresses that impart severe detrimental effects on growth and crop production. Plants encounter drought stress if the plant-available water is limited in the soil or when plants undergo higher transpiration losses compared to the root water uptake (Salehi-Lisar and Bakhshayeshan-Agdam, 2016). Roots are the first organs to sense drought stress and several sensory proteins, such as histidine kinases, receptor-like kinases, and G-protein-coupled receptors receive the external signal and amplify it through secondary messengers, such as Ca²⁺, reactive oxygen species (ROS), and IP3 (Jain et al., 2018). These secondary messengers will subsequently activate complex regulatory network controlling root growth under drought stress by regulating hormonal pathways and the expression of several TFs, such as APETALA 2/ethylene-responsive element binding factor (AP2/ERF), myeloblastosis viral oncogene homolog TF (MYB), basic leucine zipper (bZIP), and NAM/ATAF1/CUC2 (NAC) TF (Valliyodan and Nguyen, 2006; Jung et al., 2017). Similar to SOS signaling during salt stress, some CBL proteins (SOS3 homolog) interact with CIPK (SOS2 homolog) to confer abscisic acid (ABA) and drought stress signaling. An earlier study in Arabidopsis explains that a CBP SCaBP5 (CBL1) interacts with a protein kinase, PKS3 (CIPK15), to regulate ABA-dependent seed germination, stomatal opening, and subsequent gene expression (Guo et al., 2002). Additionally, PKS3 physically binds with ABI1 (ABA-insensitive 1) and ABI2, which are protein phosphatase 2C (PP2C) protein and are supposed to negatively regulate certain features of the ABA cascade, such as stomatal closure (Guo et al., 2002). In Arabidopsis, ABA-dependent stomatal closure is positively regulated by SnRK2 protein kinase (SRK2E/OST1/SnRK2.6) containing two C-terminal domains where domain I work in an ABA-independent manner and domain II is activated in presence of ABA (Yoshida et al., 2006). It has been found that SRK2E/OST1 physically interacts with ABI1 through domain II and is involved in ABA and osmotic stress signaling to control stomatal closure in Arabidopsis. Further studies identify that CBL9 interacts with CIPK3 and forms a module for ABA signaling and in addition to that abscisic acid repressor 1 (ABR1) acts as a downstream component of CIPK3 (Pandey et al., 2008; Sanyal et al., 2017). Another study in Arabidopsis identifies CML20 as a negative regulator of ABA-induced stomatal opening while the mutant Arabidopsis lines (cml20) show relatively less water loss from leaves and more drought tolerance (Wu et al., 2017). Moreover, drought and ABA stress upregulate transcript expressions of several stress-responsive genes, such as MYB2, RAB18, ERD10, COR47, and RD29A in the cml20 lines. On contrary, another CBP (CBP60g) acts as a positive regulator of drought stress in Arabidopsis (Wan et al., 2012). Classical model on ABA-mediated drought stress signaling depicts that upon binding with ABA, pyrabactin resistance 1/PYR1-Like/Regulatory components of ABA receptors (PYR/PYL/RCAR) inactivate PP2C which eventually activates SnRK2s, other ion channels, and TFs to turn on ABA-responsive genes (Hubbard et al., 2010). The mechanistic action of different environmental cues controlling the ABA signaling and other CBPs is nicely reviewed by Kumar et al. (2019). Hypo-methylation of CaM might be changing their binding partners and subsequent downstream signaling to depict salt and dehydration tolerance (Banerjee et al., 2013). Several CDPKs are also found to be involved in drought stress signaling (Xu et al., 2010; Ho et al., 2013; Pandey et al., 2013). Functional characterization of OsCDPK1 in rice (Oryza sativa) reveals negative regulation of gibberellic acid biosynthesis while it activates a 14-3-3 protein (GF14c) and improves drought tolerance in transgenic seedlings during constitutive expression (Ho et al., 2013). Another study in Arabidopsis states that CAMTA1 mutant (camta1) plants are highly susceptible to drought stress, and CAMTA1 further regulates the expression of several stress-responsive genes and TFs, such as RD26, ERD7, RAB18, LTPs, COR78, CBF1, heat shock proteins (HSPs), and AP2-EREBP (Pandey et al., 2013). An interesting study in maize (Zea mays) establishes that ZmCCaMK (calcium/CaMs-dependent protein kinase) phosphorylates a NAC TF (ZmNAC84) and induces antioxidant defense by activating downstream genes to enhance drought tolerance (Zhu et al., 2016). The overall Ca²⁺ signaling under drought stress is portrayed in Figure 2.

Calcium ion and CaM are important components in mediating the heat shock (HS) response in plants. Similar to other stresses, the increase in [Ca²⁺]_cyt is the first step in the HS signal transduction pathway. Transgenic tobacco (Nicotiana tabacum) plants expressing aequorin, a luminescent protein, exhibit an increase in [Ca²⁺]_cyt upon heat treatment. Additionally, exogenous application of Ca²⁺ results in improved thermo-tolerance, whereas the effect is diminished in the presence of Ca²⁺ chelators and inhibitors (Gong et al., 1998). Plants cells when exposed to high temperatures mobilize HSPs or molecular chaperones to aid in proper protein folding for mitigating the damage caused by heat stress. Laser confocal and fluorescence microscopic studies in wheat demonstrated that intracellular Ca²⁺ levels were increased within 1 min upon exposure to heat stress at 37°C. Furthermore, HS at 37°C upregulates CaM 1-2 expression after 10 min, while two HSP genes, hsp26 and hsp70, are induced later on. The expression of these HSP genes is upregulated after exogenous Ca²⁺ application, and their expressions are downregulated in the presence of CaM antagonists, Ca²⁺ chelators, and Ca²⁺ channel blockers (Liu et al., 2003). Similarly, maize seedlings pre-treated with CaCl₂ showed enhanced thermotolerance coupled with a remarkable increase in CaM accumulation upon heat stress exposures (Zeng et al., 2015). The purified heat-inducible homolog of Hsp70 (DgHsp70) from orchard grass (Dactylis glomerata) possesses ATPase, holdase, and ATP-dependent foldase activity along with a CaM-binding domain (CaMBD). Moreover, binding to AtCaM2 in the presence of Ca²⁺ affects the ATPase and foldase activities of DgHsp70 protein (Cha et al., 2012). AtCaM3 plays a pivotal role in heat stress signaling by regulating the activity of CaM-binding protein kinase (AtCBK3) which eventually controls HS TFs (AtHSFA1a) and HS protein genes through phosphorylation or dephosphorylation modulating heat stress response in Arabidopsis (Zhang et al., 2009). Furthermore, AtCaM3 acts as a downstream factor in nitric oxide (NO) signaling resulting in the activation of heat shock factors (HSFs), accumulation of HSPs, and development of thermotolerance (Xuan et al., 2010). In a similar manner, Ca²⁺/CaM signaling is involved in HS response and an increase in the expression of Ca²⁺/HS-related proteins in rice (Wu et al., 2012). In Arabidopsis, AtPP7, a Ser/Thr phosphatase, is found to be induced under heat stress and known to interact with CaM in a Ca²⁺-dependent manner (Zeng et al., 2015). Moreover, AtPP7 is also found to interact with AtHSF1, an HS TF, thereby, suggesting its possible involvement in the regulation of the HSP genes (Liu et al., 2007). Vitis amurensis (V. amurensis) is a wild species of grapevine having high resistance to cold and disease. Overexpression of VaCPK29 improves heat tolerance in Arabidopsis (Dubrovina et al., 2017). Another finding in V. amurensis depicts the differential regulations of various VaCaMs and VaCMLs in response to an array of abiotic stresses, such as salt, cold, heat, and osmotic stress (Dubrovina et al., 2019). Altogether, 79 CML genes have been identified in the Chinese cabbage (Brassica rapa ssp. pekinensis), and expressions of many BrCMLs, particularly BrCML21–1, are upregulated under heat stress (Nie et al., 2017). However, comprehensive research is warranted to elucidate the specific mechanisms by which these genes are regulated in different plants.

Heat stress causes a rise in intracellular levels of [Ca²⁺]_cyt which then combines with CaM and activates L-glutamate decarboxylase (GAD) for subsequent decarboxylation of glutamate (Glu) into γ-amino butyrate (GABA). However, calcium and CaM inhibitors block the GABA accumulation in Arabidopsis seedlings (Locy et al., 2000). CaM binding to GAD plays an important role in the normal development of plants while GABA is found to impart partial protection against heat stress by improving the leaf turgor, increasing the concentration of protective osmolytes, and reduction in the oxidative damage (Nayyar et al., 2014). Furthermore, transcriptomic analyses of rice reveal that OsMSR2 (multi-stress-responsive gene 2), a novel CML is strongly upregulated under various abiotic stresses, such as cold, heat, and drought stress (Xu et al., 2011).

Plant responses to cold stress involve complex crosstalk through activation of several signaling pathways that could improve the chilling and freezing tolerance of plants. Cold temperature is perceived by various cold sensors localized in the plasma membrane which eventually led to a transient increase in the cytosolic or nuclear Ca²⁺ levels followed by activation of the downstream signaling cascade (Yuan et al., 2018). The rate of cooling also plays an important role in temperature sensing, a faster rate of cooling results in increased sensitivity along with increased Ca²⁺ response, whereas prolonged or repeated exposure to low temperatures attenuates the response (Knight and Knight, 2012). An earlier study revealed that CAX1 might have played a vital role in the cold stress response of Arabidopsis (Dodd et al., 2010). Additionally, two calcium-permeable mechano-sensitive channels have been detected as a regulator of cold-induced increment of cytosolic Ca²⁺ levels (Mori et al., 2018). In Arabidopsis, Yang et al. (2010) reported CRLK1 as a novel plasma membrane-localized calcium/CaM-regulated receptor-like kinase 1 containing two CaM-binding sites, which is stimulated in response to cold and oxidative stress and induces various cold-responsive (COR) genes, such as CBF1, RD29A, COR15a, and KIN1. Further research studies affirmed that an increase in cellular Ca²⁺ level caused CRLK1 to phosphorylate a specific mitogen-activated protein kinase kinase kinase (MEKK1), followed by the activation of protein kinase kinase 2 (MKK2) and MAPK signaling cascade leading to the activation of cold response genes (Furuya et al., 2013, 2014). Several Ca²⁺/CaM-binding proteins, such as PsCCaMK in pea (Pisum sativum), CaM-binding membrane transporter-like proteins (MCamb1 and MCamb2) in Physcomitrella patens, OsCPK24 in rice and TCH, and a CaM-related protein, in Arabidopsis, show upregulation upon cold stress (Takezawa and Minami, 2004; Zeng et al., 2015; Liu et al., 2018). In common bean (Phaseolus vulgaris), the Ca²⁺-CaM-dependent NAD kinase (NADK) activity has increased after 60 min and 120 min of cold-shock (Ruiz et al., 2002). Further study speculates that CPKs induces a signaling cascade involving ROS, NO, and MPKs for ABA-dependent cold adaptation (Liu et al., 2018; Lv et al., 2018). The mechanistic action of CPKs under cold stress has been studied in a wide range of crops. The ZmCPK1 in maize acts as a negative regulator of the cold stress response (Weckwerth et al., 2015); while PeCPK10 in Populus euphratica (Chen et al., 2013), OsCPK7, OsCPK13, and OsCPK17 in rice (Saijo et al., 2000; Komatsu et al., 2007; Almadanim et al., 2017), and AtCPK1 in Arabidopsis act as positive regulator conferring cold tolerance (Almadanim et al., 2017). Furthermore, CaM3, an isoform of CaM, mediate reduced expression of the COR genes, such as KIN1/2 and LT178, which affirmed that CaM3 exerted a negative effect on the COR expression probably by the activation of the Ca²⁺-ATPase (Townley and Knight, 2002). Finally, Ca²⁺-binding TF like CAMTA3 could bind to the promoter region of the CBF2/DREB1c gene and positively regulates their expression to confer freezing tolerance in plants (Doherty et al., 2009). The well-documented basic helix-loop-helix (bHLH) TF inducer of CBF expression 1 (ICE1) is the modulator of CBF genes during cold stress (Chinnusamy et al., 2003; Lindemose et al., 2013). When MYB15 and ICE1 interacted, they bound to the DREB1A/CBF3 promoter to regulate cold stress tolerance. Another plausible role of ICE1 is to act as a co-receptor for the cooling induction of Bon1-associated protein 1 (BAP1), which is responsive to cold stress (Zhu et al., 2011). The overall Ca²⁺ signaling under heat and cold stress is depicted in Figure 3.

Ever-increasing anthropogenic activities in recent years, such as industrialization and urbanization along with non-judicious use of inorganic fertilizer and pesticide containing heavy metals in agricultural fields, have led to severe heavy metal/metalloid pollution in soil and environment, which is malicious to the local ecosystem. The most common heavy metals/metalloid are lead (Pb), chromium (Cr), iron (Fe), arsenite (AsIII), arsenate (AsV), zinc (Zn), cadmium (Cd), copper (Cu), mercury (Hg), aluminum (Al), and nickel (Ni) (Wuana and Okieimen, 2011).

Lead ions (Pb²⁺) are capable of binding to the Ca-binding sites of CaM, eventually gaining entry into plant cells through Ca²⁺ permeable channels (Gorkhali et al., 2016). Arabidopsis CNGCs and tobacco NtCBP4 are involved in the transport of Ca²⁺ ions during signal transduction and overexpressing NtCBP4 confers Ni⁺ tolerance, but Pb²⁺ hypersensitivity (Moon et al., 2019). Interestingly, the expression of a truncated NtCBP4 gene, which lacked a major part of the C-terminal half containing the cyclic nucleotide-binding domain and CaMBD in transgenic tobacco plants resulted in increased Pb²⁺ tolerance associated with reduced Pb²⁺ accumulation, while overexpression of full-length NtCBP4 led to Pb hypersensitivity along with overaccumulation of Pb. In rice roots, Pb²⁺ treatment stimulates ROS production, Ca²⁺ accumulation, and activation of MAPKs in addition to cell death and stunted root growth. Conversely, pre-treatment of the roots with an antioxidant, CDPK-inhibitor (W7), and an nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor effectively reduce the Pb²⁺-induced cell death and MAPK activity depicting the interplay between CDPK and ROS signaling under Pb²⁺ stress (Huang and Huang, 2008).

Cadmium is extremely phytotoxic even in low concentrations resulting in reduced photosynthetic rate, impeded stomatal conductance, and adverse effects on several biochemical and physiological activities in plants (Guo et al., 2019). Cd-stress depicted a detrimental effect in root growth of white clover (Trifolium repens) and Arabidopsis seedlings (Brunetti et al., 2011; Stravinskiene and Račaite, 2014). Arabidopsis root hair growth is also inhibited by Cd²⁺ as it affects the cytosolic Ca²⁺ gradient required for root growth (Fan et al., 2011). Cd²⁺ can also displace Ca²⁺ from CaM proteins, thus affecting calcium-dependent signaling (Baliardini et al., 2015). An increase in [Ca²⁺]_cyt levels occurs after Cd²⁺ exposure, which in turn activates the NADPH oxidase enzyme and transient H₂O₂ accumulation followed by superoxide (O2-) accumulation in the mitochondria causing oxidative damage (Garnier et al., 2006). Baliardini et al. (2015) identified the probable role of the CAX1 gene in Cd²⁺ tolerance. Several reports revealed that the application of Ca²⁺ played a crucial role in alleviating the damages caused by Cd²⁺ uptake in plants (Zorrig, 2012; Moon et al., 2019).

In a similar manner, pre-treatment with Ca²⁺ alone or in combination with other compounds can ameliorate the toxicity of Al, Cr, and Ni in different plants (Mozafari, 2013; Fang et al., 2014; Hossain et al., 2014). Ca²⁺ oscillation in rice roots was monitored using a Ca²⁺ indicator, and ROS-specific dyes revealed a significant increase in Ca²⁺ after exposure to arsenate [As(V)]. Further, transcriptome analysis of rice roots upon As(V) exposure reveals overexpression of various calcium-related proteins and kinases, such as CaMs, CBLs, CDPKs, and CIPKs, coupled with several TFs, MAPKs, NADPH oxidases along with genes involved in abiotic stresses (Huang et al., 2012). Additionally, transcript analyses depict upregulation of different Ca²⁺/CaM-binding partners, such as CDPKs in rice upon Cr⁶⁺ stress, CaM in basidiomycetous yeast species, Cryptococcus humicola, consequent to Al stress, and another CaM in zucchini (Cucurbita pepo) upon Ni stress (Huang et al., 2014; Zhang et al., 2016; Valivand et al., 2019). Transcriptomic analysis of Tibetan Plateau annual wild barley (Hordeum spontaneum) affirms that multiple CIPKs are playing a pivotal role to mitigate heavy metal toxicity (Hg, Cd, Cr, Pb, and Cu) and other climatic vagaries such as salt, drought, ABA, cold, and heat stress (Pan et al., 2018). Lanthanum (La^III), a rare earth element, is reported to trigger an increase in Ca²⁺ levels in the cell, besides interacting with CaM, consequently affecting the expression and conformation of CaM (Wang et al., 2016; Zhang et al., 2016). At low concentrations, La^III could bind with the two Ca-binding sites of the extracellular CaM by electrostatic attraction to improve CaM function and vice versa (Wang et al., 2016).

Most of our current understanding of symbiosis signaling is based on the work done mainly on the association of rhizobia and mycorrhiza with Lotus japonicus and Medicago truncatula. Besides rhizobia and mycorrhiza, beneficial interactions also occur with endophytic fungi and plant-growth-promoting bacteria which colonize at the root of the host and result in better plant performance (Khare et al., 2018). Symbiotic interactions start through sequential cytoplasmic and nuclear Ca²⁺ elevations in the plant cell, and the responses depend on the amplitude, duration, frequency, and location of Ca²⁺ signature and selective activation of specific calcium channels in cellular membranes (Whalley and Knight, 2013; Yuan et al., 2017).

Plant root-secreted flavonoids and strigolactones are sensed by Rhizobium sp. and mycorrhizal fungi, respectively. Upon recognizing these compounds, symbionts produce modified lipo-chitooligosaccharides, such as nodulation (Nod) factors in the case of rhizobial symbiosis, and mycorrhizal (Myc) factors in the case of arbuscular mycorrhizal fungi symbiosis (Oldroyd, 2013). Upon recognition and binding with the Nod factor, LysM receptor-like kinases form heterocomplex on the cell membrane and activate the cytosolic kinase domain of LysM, triggering cytosolic Ca²⁺-oscillation within seconds to minutes. Nuclear Ca²⁺ oscillation is generated through some yet unidentified secondary messenger activated by LysM receptor-like kinases (Oldroyd, 2013). Successful formation of both root nodule and arbuscular mycorrhizal symbiosis requires activation of a common symbiotic pathway composed of eight components, SYMRK (in L. japonicus)/DMI2 (in M. truncatula), POLLUX/DMI1, CASTOR, NENA, NUP85, NUP133, CCaMK, and CYCLOPS/IPD3 (Singh and Parniske, 2012; Oldroyd, 2013). Nuclear membrane-localized Ca²⁺-regulated cation channels, such as CASTOR and POLLUX in L. japonicus (Charpentier et al., 2008) and DMI1 in M. truncatula (Ané et al., 2004), have been identified as crucial for symbiotic Ca²⁺ oscillations in the root cell nucleus (Singh et al., 2014). Besides these, three components of the nuclear pore complex, i.e., NUP133, NUP85, and NENA, are also essential for symbiotic signaling (Binder and Parniske, 2014). Nucleus-localized CCaMK, calcium- and CaM-dependent serine/threonine protein kinase, directly interacts with CYCLOPS and phosphorylates it, which is critical for decoding the calcium oscillations (Singh et al., 2014). The mechanism by which CCaMK-CYCLOPS activates specific downstream signaling for nodulation or mycorrhiza formation yet remains unresolved.

Transcription factors, such as nodulation signaling pathway 1 (NSP1) and NSP2, are critical for rhizobium-specific gene expression as it interacts with the promoters of rhizobium-induced genes, such as nodule inception (NIN) and ERN1 (Cerri et al., 2012). Both the CCaMK–CYCLOPS complex and the NSP1–NSP2 complex are vital for driving specific genes expression associated with successful nodule formation. In the case of mycorrhizal signaling, expression of mycorrhiza-specific RAM2 (required for arbuscular mycorrhization 2) gene is controlled by the same signaling complexes with a substitution of NSP1 by RAM1 (Gobbato et al., 2013).

Plant's first line of defense is activated by interaction either between plasma membrane bound pattern-recognition receptors (PRRs) with several elicitors, such as pathogen-associated molecular patterns (PAMPs), microbe-associated molecular patterns (MAMPs), endogenous damage-associated molecular patterns (DAMPs), or compounds produced from cell damage due to pathogenic activity (Dodds and Rathjen, 2010). PRR initiates an overall defense response through activating PAMP-triggered immunity (PTI), followed by rapid activation of MAPKs, oxidative burst, and Ca²⁺ influx at the plasma membrane (Ranf et al., 2011) and finally augmenting biosynthesis of plant defense hormones, namely, salicylic acid (SA), jasmonic acid (JA), and their derivatives (Vlot et al., 2009). Microbe-induced transient increase of Ca²⁺ flux is the most important upstream signaling event leading to SA biosynthesis (Du et al., 2009). CaM-binding CBP60 family TFs, CBP60g, and systemic-acquired resistance deficient 1 (SARD1) act as a positive regulator of isochorismate synthase 1 (ICS1) expression, a key enzyme in the SA biosynthesis necessary for inducing systemic-acquired resistance (SAR) (Zhang et al., 2010). The importance of CaM binding of CBP60g and other related proteins (CBP60a) in defense signaling has been well-studied in Arabidopsis (Wang et al., 2009; Truman et al., 2013). The crucial role of CAMTA as an early integration node of both PTI and ETI signaling pathways came from a transcriptomics study by comparative analysis of different CAMTA conditional mutant transgenic lines of Arabidopsis (Jacob et al., 2018). CAMTA3 negatively regulates expression of enhanced disease susceptibility 1 (EDS1) and brassinosteroid insensitive 1-associated kinase 1 (BAK1) interfering with SA and JA signaling, respectively (Du et al., 2009; Rahman et al., 2016). Plants with CAMTA3 mutation exhibit greater resistance to pathogenic bacteria (Pseudomonas syringae, Botrytis cinereal, and Sclerotinia sclerotiorum) but become more susceptible to insect attack (Trichoplusia ni) (Galon et al., 2008; Rahman et al., 2016). Similarly, AtCAMTA3 negatively regulates plant immunity by conferring non-host resistance to Xanthomonas oryzae (Rahman et al., 2016).

The first evidence linking CMLs with plant defense came from overexpression of soybean (Glycine max) CML1 and CML2 (previously, SCaM-4 and SCaM-5) in tobacco, Arabidopsis, and soybean causing increased resistivity against a wide range of virulent and avirulent pathogens (Heo Do et al., 1999; Park et al., 2004; Rao et al., 2014). Validation from autologous and heterologous systems highlights the role of CaM/CMLs in plant immune responses through modulating the hypersensitive response (HR) (Chiasson et al., 2005; Ma et al., 2008). Arabidopsis CML8 and CML9 are also acting as positive regulators of defense mechanisms against different strains of P. syringae through activation of SA-dependent defense responses (Zhu et al., 2017). Additionally, CML9 interacts with a plant-specific TF, pseudo-response regulator 2 (PRR2) in planta which is a player in SA-dependent immunity response in the plant (Cheval et al., 2017). Furthermore, CML37 and CML42 play contrasting roles in insect herbivory-induced resistance in positive and negative manners, respectively (Vadassery et al., 2012; Scholz et al., 2014).

Advances in functional genomics in recent times shed light on the importance of CDPKs in plant immune responses. As a family, CDPKs exert both positive and negative regulations on the immune response. Several members of Arabidopsis CDPKs, i.e., CPK4, 5, 6, and 11 are transiently activated within a few minutes after recognition of common 22-amino-acid long bacterial flagellin epitope flg22 (Boudsocq et al., 2010). These CPKs along with MAPKs positively regulate innate immune signaling perceived by PAMP receptor FLS2 (Boudsocq et al., 2010). In vitro phosphorylation studies revealed that CPKs can also phosphorylate various WRKY TF-controlling downstream immunity-related crucial gene expressions (Boudsocq et al., 2010; Gao et al., 2013). Arabidopsis mutant screening identified two CDPKs (AtCPK3 and AtCPK13) as important transcriptional regulators of plant defensing gene PDF1.2. HSF HsfB2a promotes transcriptional activation of PDF1.2, after being phosphorylated by CPK3-or CPK13 in response to wounding by herbivores, such as Spodoptera littoralis (Kanchiswamy et al., 2010). Pathogen-induced Ca²⁺ elevations in cytosol activate potato CDPKs (StCPK4 and StCPK5) which in turn stimulates respiratory burst oxidase homolog protein B (RBOHB) (Kobayashi et al., 2007). RBOH gene family encodes integral membrane protein NADPH oxidases, which are a key component of innate immunity due to its central role in ROS production and subsequent oxidative burst and CPK5-mediated in vivo phosphorylation of RBOH homolog D in Arabidopsis (Dubiella et al., 2013). Moreover, CDPKs control cell death in tobacco after recognizing Cladosporium fulvum through interaction between pathogen race-dependent elicitor Avr4/Avr9 and disease-resistant gene Cf9 (Romeis et al., 2001). In the japonica rice cultivar Nipponbare, overexpression of CDPK enhanced blast resistance by preventing Magnaporthe oryzae penetration (Bundó and Coca, 2016). In addition to enhancing plant resistance, CDPKs also perform a negative role in plant defense (Freymark et al., 2007; Asano et al., 2012; Monaghan et al., 2014). However, the function of CDPKs remains unresolved due to duplicity and functional redundancy in higher plants which further warrants comprehensive research to uncover the role of this protein toward plant defense mechanisms. The intricate Ca²⁺ signaling pathways toward regulating plant-biotic factors interaction are illustrated in Figure 4.