Brassinosteroid Induces Phosphorylation of the Plasma Membrane H+-ATPase during Hypocotyl Elongation in Arabidopsis thaliana

Brassinosteroids (BRs) are steroid phytohormones that regulate plant growth and development, and promote cell elongation at least in part via the acid-growth process. BRs have been suggested to induce cell elongation by the activating plasma membrane (PM) H⁺-ATPase. However, the mechanism by which BRs activate PM H⁺-ATPase has not been clarified. In this study, we investigated the effects of BR on hypocotyl elongation and the phosphorylation status of a penultimate residue, threonine, of PM H⁺-ATPase, which affects the activation, in the etiolated seedlings of Arabidopsis thaliana. Brassinolide (BL), an active endogenous BR, induced hypocotyl elongation, phosphorylation of the penultimate, threonine residue of PM H⁺-ATPase, and binding of the 14-3-3 protein to PM H⁺-ATPase in the endogenous BR-depleted seedlings. Changes in both BL-induced elongation and phosphorylation of PM H⁺-ATPase showed similar concentration dependency. BL did not induce phosphorylation of PM H⁺-ATPase in the BR receptor mutant bri1-6. In contrast, bikinin, a specific inhibitor of BIN2 that acts as a negative regulator of BR signaling, induced its phosphorylation. Furthermore, BL accumulated the transcripts of SMALL AUXIN UP RNA 9 (SAUR9) and SAUR19, which suppress dephosphorylation of the PM H⁺-ATPase penultimate residue by inhibiting D-clade type 2C protein phosphatase in the hypocotyls of etiolated seedlings. From these results, we conclude that BL-induced phosphorylation of PM H⁺-ATPase penultimate residue is mediated via the BRI1-BIN2 signaling pathway, together with the accumulation of SAURs during hypocotyl elongation.