Stomatal Integration of Guard Cell: Hormonal Crosstalk, Molecular Mechanisms and Translation strategies for Climate- Resilient Crops

Together, guard cells balance two conflicting pressures: water retention during drought and pathogen defence by means of pattern-triggered immunity, achieved through common molecular components: OST1 kinase, SLAC1 anion channels and WRKY transcription factors. In this review, the perception of signals at the membrane level, to the reprogramming of transcription at the nucleus, in guard cells is summarized and discussed, particularly in light of the fact that these cells are faced with multiple signals coming from different parts of the plant during simultaneous biotic and abiotic stress. We discuss antagonism and synergy between ABA-, jasmonic acid and ethylene-dependent regulation of stomatal closure, the PYR/PYL–PP2C–SnRK2 module involved in ABA-dependent regulation of stomatal closure and the PRR–MAPK cascade involved in immune-triggered stomatal closure. ROS and calcium function as convergence nodes and their amplitude, kinetics and subcellular localization define the identity of stress. Only ~40% of guard cells contain ABA-responsive programmes in individual cells, highlighting the presence of heterogeneity, which was not detected by population-based measurements. Pathogens exploit structural weaknesses like the targeting of OST1 by HopM1 for degradation and the stabilization of JAZ by coronatine, and guard cells metabolic constraints affect the metabolic cost of multiple responses to stress to measurable degree. We then examine translational approaches – genome editing specific to guard cells, tuning of hormone-sensitivity, and stress priming – as methods to produce climate-resilient crops with yield stability. Literature was collected from PubMed, Web of Science and Scopus by combining the keywords stomatal guard cells, ABA signaling, stomatal immunity, PAMP-triggered immunity, hormonal crosstalk and drought–disease trade-offs and prioritizing post-2015 publications and single-cell, genome-scale and quantitative studies. We finish with a list of three priority areas: crop-specific signaling architecture, translation of mechanistic insight into field performance, and quantification of metabolic flux trade-offs; each specifying potential concrete directions for future research.

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