Arabidopsis <i>OST1</i> homologs of barley are involved in stomatal regulation

Stomatal pores on leaves determine water loss of plants and carbon uptake for photosynthesis. These key physiological processes affect biomass production, stress sensitivity and water use efficiency in plants and are also addressed in breeding programs. Abscisic acid (ABA) is an important plant stress hormone, whose signaling module described for Arabidopsis is conserved in land plants: it controls the activity of protein kinase OST1 (Open Stomata 1, also SnRK2.6/SRK2E), which triggers stomatal closure by activating guard cell anion channels. Here, we generated barley double mutants defective in proteins homologous to Arabidopsis OST1 and measured their stomatal conductance and responses to rising VPD and ABA spraying. These barley mutants showed wider stomatal aperture, higher stomatal conductance and decreased VPD- and ABA-sensitivities compared with wildtype (cv Golden Promise) in gas exchange experiments, whereas their net assimilation rates and stomatal densities were only mildly affected. No statistically significant difference between the lines was detected in grain yield per plant under well-watered conditions. These results show that OST1-like kinases HvSnRK2.7 and HvSnRK2.9 control stomatal conductance and sensitivity to ABA and abiotic factors in barley. The effect of alleviating ABA signalling pathway on stress tolerance and yield is discussed in the context of breeding plants productive under various water availability conditions.