Industrial runoff contains heavy metals, contaminating soil and poisoning crops. Sehar et al. investigated the role of ET in overcoming arsenic (As) phytotoxicity in the presence of selenium in mustard. The higher accumulation of As in leaves than in roots was the primary cause of reduced photosynthetic performance and reduced growth in mustard. ET application alone and in the presence of Se resulted in adaptive responses to As toxicity through increased expression of ASCORBATE PEROXIDASE (APX) and GLUTATHIONE REDUCTASE (GR), accumulation of reduced glutathione (GSH) and the suppression of abscisic acid (ABA)-mediated stomatal closure. Evolutionary insights into ET action were presented by Bharadwaj et al. The mechanism of ET action in liverwort (M. polymorpha) in response to a range of abiotic stresses was found to be markedly similar to that of angiosperms against a range of abiotic stresses. Mutant lines Mpein3 and Mpctr1 defective in positive and negative regulators of ET were compared with respective wild types against a range of abiotic stresses such as heat, salinity, nutrient deficiency, and far-red light. According to a previous study (Li et al., 2020), lines defective in the EIN3 transcription factor would be insensitive to ET response, indicating that EIN3 is a positive regulator of ET signaling, whereas the CTR1 mutants would display constitutive ET responses, suggesting that it is a negative regulator of ET perception. As hypothesized, Mpctr1 mutant lines showed prominent ET response and more resilience under sublethal conditions such as 29°C, 10 mM NaCl, 1/20 Gamborg’s dilution, or 50 days of exposure to far-red light, whereas Mpein3 mutant showed lower resilience than wild type.

Two comprehensive reviews dealing with TARGET OF RAPAMYCIN (TOR) and ET interconnection under nutrient deficiency and strategic post-harvest processing for increasing the shelf life of fruits and vegetables showcased detailed reports on the ET action and control mechanisms. Garcia et al. set ground on the so-called “stop growing” and “searching for nutrient” schemes to deal with nutrient deficit through TOR and ET interaction. Authors described reports where TOR and ET share antagonistic relationships: TOR blocks ET signaling alongside CTR1 by phosphorylating EIN2 and EIN3. However, nutrient deprivation inactivates TOR reactivating ET signaling.

In their review of post-harvest practices, Cocetta and Natalini describe how apart from conventional means such as chilling treatments and antisense approaches for ACS and POLYGALACTURONASE (PG) genes, transgenic approaches have been applied to elucidate ET biochemical and metabolic pathways. Ripening-related transcription factors of the APETALA2/ETHYLENE-RESPONSE FACTOR (AP2/ERF) family have been studied in tomatoes for their role in regulating ripening-responsive genes (Fenn and Giovannoni, 2021). Genetic analysis has led to the identification of ripening mutants like ripening inhibitor (rin), non-ripening (nor), colorless non-ripening (Cnr), green-ripe (Gr), green flesh (gf), high pigmnet1 (hp1), high pigment2 (hp2), and never ripe (Nr) in tomato (Osorio et al., 2020). These mutants are used commercially to develop new hybrids with longer shelf life. The CRISPR/Cas9 approach is promising, and it has been applied to regulatory protein genes CNR and NOR and to transcription factors AP2a, FUL1, and FUL2 to achieve targeted deletion or substitution (Wang et al., 2019).

Vaughan-Hirsch et al. investigated the biological function of custom-synthesized 1- aminocyclopropane-1-carboxylic-acid dipeptide (di-ACC) molecule taken up by Arabidopsis just as ACC, in part via LYSINE HISTIDINE TRANSPORTERS (e.g., LHT1). Once taken up, the ACC dimer can evoke a triple response phenotype in dark-grown seedlings, reminiscent of ET responses induced by ACC itself, albeit less efficiently than ACC. Moreover, di-ACC operates via the known ET signaling pathway and not through ACC. Altogether, the study supported that di-ACC appears to be transported and processed into ET like ACC, and it can increase ET production levels and trigger further ET reactions in Arabidopsis.

Considering the importance of AP2/ERF in resilience responses, Ding et al. conducted a genome-based study in barley to explore the role of AP2/ERFs in starch synthesis. After re-examining the available genome database (Morex), these researchers identified 64 new genes in the HvAP2/ERF gene family and corrected some previously misannotated and duplicated genes. HvAP2-18 was identified as a promising candidate gene to shed light on the mechanism governing the production of starch in barley.

The role of the AP2/ERF gene family in resistance of N. benthamiana against P. infestans infection after INF1 (elicitin from P. infestans) treatment has been confirmed by Imano et al. The authors successfully demonstrated that NbERF-IX-33 transcription factors are responsible for the accumulation of the sesquiterpenoid phytoalexin capsidiol that serves as an important protective compound against P. infestans infection.

The articles in this collection contribute to our understanding of ET as a regulatory signal in plants. ET is again demonstrated to serve as an important mediator of stress in liverworts and angiosperms. Future studies in this exciting area will help to unravel stress response networks and how they interact with ET to determine which pathways may have existed in the ancestor of early land plants and to create strategies for reducing the adverse effects of abiotic stresses, which are becoming more common as a result of climate change. A graphical summary of the articles in the volume is presented in Figure 1 .

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.