Crosstalk between ABA and ethylene in regulating stomatal behavior in tomato under high CO2 and progressive soil drying

Increasing atmospheric CO2 concentrations accompanied by intensifying drought markedly impact plant growth and physiology. This study aimed to explore the role of abscisic acid (ABA) in mediating the response of stomata to elevated CO2 (e[CO2]) and drought. Tomato plants with different endogenous ABA concentrations [Ailsa Craig (AC), the ABA-deficient mutant flacca, and ABA-overproducing transgenic tomato SP5] were grown in ambient (a[CO2], 400 μmol mol-1) and elevated (e[CO2],800 μmol mol-1) CO2 environments and subjected to progressive soil drying. Compared with a[CO2] plants, e[CO2] plants had significantly lower stomatal conductance in AC and SP5 but not in flacca. Under drought, e[CO2] plants had better water status and higher water use efficiency. e[CO2] promoted the accumulation of ABA in leaves of plants subjected to drought, which coincided with the up-regulation of ABA biosynthetic genes and down-regulation of ABA metabolic genes. Although the increase of ABA induced by drought in flacca was much less than in AC and SP5, flacca accumulated large amounts of ethylene, suggesting that in plants with ABA deficiency, ethylene might play a compensatory role in inducing stomatal closure during soil drying. Collectively, these findings improve our understanding of plant performance in a future drier and higher-CO2 environment.