Abstract
Growth regulation tailors plant development to its environment. A showcase is growth adaptation to gravity, where shoots bend up and roots down. This paradox is based on different responses to the phytohormone auxin, which promotes cell expansion in shoots, while inhibiting it in roots via a yet unknown cellular mechanism. Here, by combining microfluidics, live imaging, genetic engineering and phospho-proteomics in Arabidopsis thaliana, we reveal how auxin inhibits root growth. We show that auxin activates two distinct, antagonistically acting signalling pathways that converge on the rapid regulation of the apoplastic pH, which is the direct growth-determining mechanism. Cell surface-based TRANSMEMBRANE KINASE 1 (TMK1) interacts with and mediates phosphorylation and activation of plasma membrane H+-ATPases for apoplast acidification, while intracellular TIR1/AFB-mediated signalling triggers net cellular H+-influx, causing apoplast alkalinisation. The simultaneous activation of these two counteracting mechanisms poises the root for a rapid, fine-tuned growth modulation to subtle changes in the environment.
Cell surface and intracellular auxin signalling for H+ fluxes in root growth
