Arabidopsis OST1 homologs of barley are involved in stomatal regulation.

Stomata on leaves determine water loss from 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 ABA spraying and rising vapor pressure deficit (VPD). These barley mutants showed wider stomatal apertures, higher stomatal conductances and decreased VPD- and ABA-sensitivities compared with wild-type ('Golden Promise') in gas exchange experiments, whereas their net assimilation rates and stomatal densities were only mildly affected. No statistically significant difference between double mutants and wild type 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 VPD in barley. The effect of modified ABA signaling pathway on stress tolerance and yield is discussed in the context of breeding plants that are productive under various conditions of water availability.