The brassinosteroid receptor StBRI1 promotes tuber development by enhancing plasma membrane H+-ATPase activity in potato

The brassinosteroid (BR) receptor BRASSINOSTEROID-INSENSITIVE 1 (BRI1) plays a critical role in plant growth and development. Although much is known about how BR signaling regulates growth and development in many crop species, the role of StBRI1 in regulating potato (Solanum tuberosum) tuber development is not well understood. To address this question, a series of comprehensive genetic and biochemical methods were applied in this investigation. It was determined that StBRI1 and Solanum tuberosum PLASMA MEMBRANE (PM) PROTON ATPASE2 (PHA2), a PM-localized proton ATPase, play important roles in potato tuber development. The individual overexpression of StBRI1 and PHA2 led to a 22% and 25% increase in tuber yield per plant, respectively. Consistent with the genetic evidence, in vivo interaction analysis using double transgenic lines and PM H+-ATPase activity assays indicated that StBRI1 interacts with the C-terminus of PHA2, which restrains the intramolecular interaction of the PHA2 C-terminus with the PHA2 central loop to attenuate autoinhibition of PM H+-ATPase activity, resulting in increased PHA2 activity. Furthermore, the extent of PM H+-ATPase autoinhibition involving phosphorylation-dependent mechanisms corresponds to phosphorylation of the penultimate Thr residue (Thr-951) in PHA2. These results suggest that StBRI1 phosphorylates PHA2 and enhances its activity, which subsequently promotes tuber development. Altogether, our results uncover a BR-StBRI1-PHA2 module that regulates tuber development and suggest a prospective strategy for improving tuberous crop growth and increasing yield via the cell surface-based BR signaling pathway.