Auxin Activates the Plasma Membrane H+-ATPase by Phosphorylation during Hypocotyl Elongation in Arabidopsis1[W][OA]

The phytohormone auxin is a major regulator of diverse aspects of plant growth and development. The ubiquitin-ligase complex SCF (for Skp1-Cul1-F-box protein), which includes the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) auxin receptor family, has recently been demonstrated to be critical for auxin-mediated transcriptional regulation. Early-phase auxin-induced hypocotyl elongation, on the other hand, has long been explained by the acid-growth theory, for which proton extrusion by the plasma membrane H-ATPase is a functional prerequisite. However, the mechanism by which auxin mediates H-ATPase activation has yet to be elucidated. Here, we present direct evidence for HATPase activation in etiolated hypocotyls of Arabidopsis (Arabidopsis thaliana) by auxin through phosphorylation of the penultimate threonine during early-phase hypocotyl elongation. Application of the natural auxin indole-3-acetic acid (IAA) to endogenous auxin-depleted hypocotyl sections induced phosphorylation of the penultimate threonine of the H-ATPase and increased H-ATPase activity without altering the amount of the enzyme. Changes in both the phosphorylation level of HATPase and IAA-induced elongation were similarly concentration dependent. Furthermore, IAA-induced H-ATPase phosphorylation occurred in a tir1-1 afb2-3 double mutant, which is severely defective in auxin-mediated transcriptional regulation. In addition, a-(phenylethyl-2-one)-IAA, the auxin antagonist specific for the nuclear auxin receptor TIR1/AFBs, had no effect on IAA-induced H-ATPase phosphorylation. These results suggest that the TIR1/AFB auxin receptor family is not involved in auxin-induced H-ATPase phosphorylation. Our results define the activation mechanism of H-ATPase by auxin during early-phase hypocotyl elongation; this is the long-sought-after mechanism that is central to the acid-growth theory.