Brassinosteroids regulate vacuolar morphology in root meristem cells of Arabidopsis thaliana.

Brassinosteroids (BRs) are plant hormones that regulate plant development and environmental response. Brz-insensitive-long hypocotyl4 (BIL4) was identified as a positive regulator of BR signaling that interacts with the BR receptor, BRASSINOSTEROID INSENSITIVE 1 (BRI1), and inhibits vacuolar degradation of BRI1 in Arabidopsis thaliana. Although BIL4 also localizes to the vacuolar membrane, the possible vacuolar function of BIL4 remains unknown. Here, we studied the effect of BIL4 and BR signaling on vacuole shape in root meristem cells using genetic and pharmacological approaches. In BIL4-deficient plants, vacuoles assumed a smaller luminal structure. Treatment with brassinolide (BL), the most active BR, resulted in visibly larger vacuoles, whereas treatment with the BR biosynthesis inhibitor Brz resulted in substantially smaller luminal vacuolar structures. In the bri1 mutant, vacuolar shapes exhibited small and fragmented structures. Our results suggest that BR signaling impacts vacuolar shape.