Brassinosteroid reduces ABA accumulation leading to the inhibition of ABA-induced stomatal closure.

Proper regulation of stomatal movement in response to various environmental stresses or developmental status is critical for the adaptation of many plant species to land. In plants, abscisic acid (ABA)-induced stomatal closure is a well-adapted method of regulating water status. In addition to ABA, we previously showed that plant-specific steroidal hormone, brassinosteroid (BR), also induces stomatal closure; however, BR modulates ABA-induced stomatal closure negatively at high concentrations. In this study, we further investigated the cross-talk between ABA and BR in relation to stomatal movement. In contrast to previous reports that ABA-induced stomatal closure was inhibited by brassinolide (BL), the most active BR, we showed that BL-induced stomatal closure was enhanced by ABA, indicating that the sequence of ABA or BL treatments led to different results. We found that this phenomenon occurred because the guard cells still had the capacity to be closed further by ABA, as the degree of stomatal closure by BL was always less than that by ABA. We also found that BL-induced stomatal closure required Open Stomata 1 (OST1) activity and the induced expression of OST1 was indifferent to the sequence of ABA and/or BL treatments. In addition, we examined the underlying mechanism by which inhibition of ABA-induced stomatal closure by BL occurred. We revealed that the downregulation of ABA-biosynthetic genes by BL resulted in a lower accumulation of ABA. These results suggested that the regulation of stomatal movement is finely controlled by the combined effects of plant hormones, ABA and BR.