Growth regulation tailors development in plants to their environment. A prominent example of this is the response to gravity, in which shoots bend up and roots bend down¹. This paradox is based on opposite effects of the phytohormone auxin, which promotes cell expansion in shoots while inhibiting it in roots via a yet unknown cellular mechanism². Here, by combining microfluidics, live imaging, genetic engineering and phosphoproteomics in Arabidopsis thaliana, we advance understanding of how auxin inhibits root growth. We show that auxin activates two distinct, antagonistically acting signalling pathways that converge on rapid regulation of apoplastic pH, a causative determinant of growth. Cell surface-based TRANSMEMBRANE KINASE1 (TMK1) interacts with and mediates phosphorylation and activation of plasma membrane H⁺-ATPases for apoplast acidification, while intracellular canonical auxin signalling promotes net cellular H⁺ influx, causing apoplast alkalinization. Simultaneous activation of these two counteracting mechanisms poises roots for rapid, fine-tuned growth modulation in navigating complex soil environments.

生长调节使植物的发育适应其环境。一个突出的例子是对重力的响应，其中芽向上弯曲，根向下弯曲¹ 。这一悖论是基于植物激素生长素的相反作用，植物激素生长素促进芽中的细胞扩张，同时通过一种尚不清楚的细胞机制抑制根部的细胞扩张² 。在这里，通过在拟南芥中结合微流体、实时成像、基因工程和磷酸蛋白质组学，我们加深了对生长素如何抑制根部生长的理解。我们发现，生长素激活两种不同的、拮抗作用的信号通路，这些通路集中于质外体 pH 值的快速调节，而 pH 值是生长的决定因素。细胞表面的跨膜激酶 1 (TMK1) 与质膜 H ⁺ -ATP 酶相互作用并介导质膜 H + -ATP 酶的磷酸化和激活，从而实现质外体酸化，而细胞内典型的生长素信号传导则促进细胞内 H ⁺净流入，导致质外体碱化。这两种抵消机制的同时激活，可以在复杂的土壤环境中快速、微调生长调节。