Auxin induces the TIR1/AFB-dependent expression of SAUR proteins that act as inhibitors of PP2C.D phosphatases, which dephosphorylate the penultimate Thr residue 17. previously demonstrated that auxin-induced apoplastic acidification and growth are mediated by TIR1/AFB–Aux/IAA nuclear auxin perception in hypocotyls 16. Phosphorylation of the conserved penultimate Thr residue (Thr948 in AHA1, Thr947 in AHA2) has been proposed to release the autoinhibition of the ATPase pump activity by the cytoplasmic C-terminal region 9, 10, 11, 12, 13, 14, 15. In Arabidopsis, PM H +-ATPase is encoded by an autoinhibited H +-ATPase (AHA) gene family comprising 11 members 8. Auxin triggers the efflux of protons, resulting in apoplastic acidification by activating the plasma membrane (PM)-localized P-type H +-ATPase 6, 7. Auxin induces rapid cell expansion by acidifying the cell-wall space (apoplast), leading to the activation of cell-wall-localized proteins for wall loosening 3, 4, a growth mechanism that has been known as the acid growth theory for half of a century 5. Thus, our findings reveal a crucial connection between auxin and plasma membrane H +-ATPase activation in regulating apoplastic pH changes and cell expansion through TMK-based cell surface auxin signalling.Įmbedded in a rigid cell wall, the plant cell must modify its wall to gain the adjustable elasticity to regulate cell expansion in space and time. Our genetic, biochemical and molecular evidence demonstrates that TMKs directly phosphorylate plasma membrane H +-ATPase and are required for auxin-induced H +-ATPase activation, apoplastic acidification and cell expansion. Auxin induced interactions between TMKs and H +-ATPases in the plasma membrane within seconds, as well as TMK-dependent phosphorylation of the penultimate threonine residue on the H+-ATPases. Here we show that the transmembrane kinase (TMK) auxin-signalling proteins interact with plasma membrane H +-ATPases, inducing their phosphorylation, and thereby promoting cell-wall acidification and hypocotyl cell elongation in Arabidopsis. Auxin induces the phosphorylation and activation of the plasma membrane H +-ATPase that pumps protons into the apoplast 2, yet how auxin activates its phosphorylation remains unclear. The acid growth hypothesis has been proposed to explain auxin-stimulated cell expansion for five decades, but the mechanism that underlies auxin-induced cell-wall acidification is poorly characterized. The phytohormone auxin controls many processes in plants, at least in part through its regulation of cell expansion 1.
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