The phytohormone auxin controls many processes in plants, at least in part through its regulation of cell expansion¹. 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. Auxin induces the phosphorylation and activation of the plasma membrane H⁺-ATPase that pumps protons into the apoplast², yet how auxin activates its phosphorylation remains unclear. 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 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. 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. 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.

植物激素生长素控制植物中的许多过程，至少部分是通过其对细胞扩张的调节¹。已经提出酸生长假说来解释生长素刺激的细胞扩张 50 年，但生长素诱导的细胞壁酸化的机制尚不清楚。生长素诱导质膜 H ⁺ -ATPase 的磷酸化和活化，该酶将质子泵入质外体²，但生长素如何激活其磷酸化仍不清楚。在这里，我们表明跨膜激酶 (TMK) 生长素信号蛋白与质膜 H ⁺ -ATP 酶相互作用，诱导它们的磷酸化，从而促进细胞壁酸化和下胚轴细胞伸长。拟南芥。^{生长素在几秒钟内诱导了质膜中 TMK 和 H +} -ATP 酶之间的相互作用，以及 H + - ATP 酶上倒数第二个苏氨酸残基的 TMK 依赖性磷酸化。我们的遗传、生化和分子证据表明，TMK 直接磷酸化质膜 H ⁺ -ATP 酶，并且是生长素诱导的 H ⁺ -ATP 酶活化、质外体酸化和细胞扩增所必需的。因此，我们的研究结果揭示了生长素和质膜 H ⁺ -ATPase 激活之间的关键联系，通过基于 TMK 的细胞表面生长素信号传导调节质外体 pH 变化和细胞扩增。