Auxin Activates the Plasma Membrane H+-ATPase by Phosphorylation during Hypocotyl Elongation in Arabidopsis    

Abstract

The phytohormone auxin is a major regulator of diverse aspects of plant growth and development. The ubiquitin-ligase complex SCF TIR1/AFB (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 H + -ATPase 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 H + -ATPase 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.

Effective diffusion rates and cross-correlation analysis of “acid growth” data

“…Curve maximum corresponding to the maximum of growth rate after addition of IAA that increase the catalytic activity of the H ? -ATPase (Takahashi et al 2012). Since the similar mechanisms was described for FC, that also elevated H ?…”

Cellular responses to naphthoquinones: juglone as a case study

“…At least in maize coleoptile cells, auxin-induced medium acidification is (Hager et al 1991;Frias et al 1996;Philippar et al 1999). Interestingly, it has been only recently shown (Takahashi et al 2012) that application of IAA to endogenous auxin-depleted hypocotyl sections of Arabidopsis seedlings induced phosphorylation of the penultimate threonine of H ? -ATPase and increased H ?…”

Constitutive Expression of Arabidopsis SMALL AUXIN UP RNA19 (SAUR19) in Tomato Confers Auxin-Independent Hypocotyl Elongation

“…Briefly, coleoptile, hypocotyl, or stem segments from the growing region are decapitated and incubated briefly in hormone-free media to deplete endogenous auxin (Takahashi et al, 2012). Segments are then transferred to media containing auxin, and elongation growth is measured over time.…”

“…However, this model lacked both strong genetic support and an understanding of the molecular components linking auxin to PM H + -ATPase activation. Crucial insight into this process was provided when Takahashi et al (2012) (Fuglsang et al, 1999;Kinoshita and Shimazaki, 1999;Jelich-Ottmann et al, 2001). Furthermore, the kinetics of auxin-induced Thr-947 phosphorylation correlated closely with the elongation kinetics of auxin-treated hypocotyl segments, strongly suggesting that auxin promotes cell expansion by posttranslationally activating PM H + -ATPases.…”