Integrative analysis of plant responses to a combination of water deficit, heat stress and eCO2 reveals a role for OST1 and SLAH3 in regulating stomatal responses

The frequency, intensity and duration of global change factors and/or environmental stressors, such as droughts, heat waves, floods, and pollution, are increasing due to anthropogenic activities, subjecting plants to compound conditions of Global Change Factor combination (GCFc), or stress combination. These can have contrasting or additive effects on plant physiological performance and impact different ecosystems and agro-ecosystems worldwide. Using an integrative physiological, genetic, hormonal, and transcriptomic analysis, we studied the response of Arabidopsis thaliana (L.) Heynh to a combination of water deficit (WD), heat stress (HS) and/or elevated levels of CO2 (eCO2). Our findings reveal high specificity in plant responses to GCFc. We further reveal that stomatal aperture regulation under conditions of GCFc is controlled by a blend of unique and shared regulators, that together determine the specific aperture size under each different set of GCFc conditions. Under a combination of WD, HS, and eCO2 (WD+HS+eCO2), for example, stomatal closure required the function of nitric oxide, OPEN STOMATA 1 and the S-type anion channel SLAH3, but not the S-type anion channel SLAC1, or many other regulators typically found to control stomatal aperture under less complex conditions, including HS+WD. Our findings highlight the complexity of plant physiological and molecular responses to conditions of stress combination and open the way for future studies of plant responses to stress combination, crucial to our understanding of how GCFc impacts different ecosystems and agro-ecosystems worldwide. In addition, our study provides an initial definition for a stomatal hierarchical stress code that could apply to future studies.