| Plants take in carbon dioxide (CO2) for photosynthesis and lose water through stomatal pores, specialized structures found on the leaf epidermis. Each stomatal pore is defined and controlled by a pair of guard cells that open and close the pore in response to environmental signals such as light, humidity, and CO2 levels, and to internal signals such as plant hormones. For a plant to survive, grow, and reproduce, it must maintain the optimum balance between having the stomatal pores wide enough for CO 2 uptake, but not so open that the plant dries out from water loss. The effect of the plant hormone abscisic acid (ABA) on guard cell physiology has been especially studied.; Brassinolide (BL) represents the most potent compound from the only class of steroid hormones in plants, brassinosteroids (BRs). The essential role of 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 ABA, suggesting that BL normally counteracts the effects of ABA, e.g. as in root growth and seed germination. Overall the results of this study indicate that BL can modulate stomatal aperture by promoting stomatal closure and inhibiting stomatal opening, functions of this hormone that were previously undescribed. Unlike as in other plant responses, BL did not modulate the effect ABA in regulation of stomatal movement. Inhibition of K+ currents by BL, with and without ABA, was also examined. |
