背景回顾：The phytohormone auxin controls many processes in plants, at least in part through its regulation of cell expansion. 

提出问题：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. 

研究落脚点：Auxin induces the phosphorylation and activation of the plasma membrane H+-ATPase that pumps protons into the apoplast, yet how auxin activates its phosphorylation remains unclear. 

主要发现：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. 

结果1-生长素诱导的TMK和H+-ATPases互作：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. 

结果2-TMK直接磷酸化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. 

结论：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.

 摘 要 

植物激素生长素控制植物生长发育的多个进程，部分是通过调控细胞扩展来实现的。“酸生长理论”已经被提出来五十年左右的时间，用以解释生长素刺激的细胞扩展，但是生长素诱导的细胞壁酸化机理还不清楚。生长素会诱导质膜上H+-ATP酶的磷酸化，并激活该酶的活性，从而将质子泵入质外体中，但是生长素如何激活H+-ATP酶的磷酸化还不清楚。本文中，作者发现拟南芥跨膜激酶TMK生长素信号转导蛋白能够与质膜H+-ATP酶互作，诱导H+-ATP酶的磷酸化，从而促进细胞壁酸化以及下胚轴细胞的伸长。生长素能够在数秒内诱导TMKs和H+-ATP酶在质膜上的互作，同时还有依赖于TMK的H+-ATP酶倒数第二位苏氨酸残基磷酸化。作者进一步通过遗传学、生物化学和分子证据证实，TMKs能够直接磷酸化质膜上的H+-ATP酶，并且该过程是生长素诱导的H+-ATP酶激活、质外体酸化以及细胞扩展所必须的。因此，本文的研究结果揭示了生长素通过基于TMK的细胞表面生长素信号转导，磷酸化并激活质膜H+-ATP酶，从而调控质外体pH，最终作用于细胞扩展。