Effect of brassinolide, alone and in concert with abscisic acid, on control of stomatal aperture and potassium currents of Vicia faba guard cell protoplasts

The essential role of brassinosteroids (BRs) in normal plant growth, development and physiology has been established by the analysis of biosynthesis and signal transduction mutants. Some of the BR-related mutants also display altered sensitivity to the phytohormone abscisic acid (ABA) suggesting that BRs normally counteract the effects of ABA on root growth, seed germination, and possibly stomatal movement. In this study, the effect of a specific BR, brassinolide (BL), on guard cell function of Vicia faba was examined alone and in conjunction with ABA. Unlike other described plant responses, BL did not oppose the effect of ABA in regulation of stomatal movement. On the contrary, BL modulated stomatal aperture by promoting stomatal closure and inhibiting stomatal opening, functions of this hormone that were previously undescribed. This study also demonstrated a role for plant steroidal hormones in ion channel regulation: BL inhibited inwardly rectifying K⁺ currents of V. faba guard cell protoplasts in a manner similar to ABA. In both stomatal movement assays and whole-cell patch clamp experiments, the effects of BL and ABA applied together were not additive, suggesting that these two hormones may function in interacting pathways to regulate stomatal apertures and guard cell physiology.