ABP1-TMK auxin perception mediates rapid phosphorylation for regeneration and auxin canalization

The phytohormone auxin triggers transcriptional reprograming utilizing a well-characterized nuclear perception machinery. In contrast, mechanisms underlying other auxin effects, such as auxin feed-back on its transport, rapid regulation of ion fluxes or ultrafast global phospho-response, remain enigmatic. Auxin Binding Protein 1 (ABP1) has been contested as an auxin receptor candidate since decades. Here we show that a fraction of Arabidopsis thaliana ABP1 is secreted and binds auxin specifically at the acidic pH typical for the apoplast. ABP1 and its plasma membrane-localized partner, Transmembrane Kinase 1 (TMK1) are required for auxin-induced phosphorylation of about thousand proteins. Loss-of-function abp1 alleles but not complemented lines show defects in in vitro shoot regeneration and formation of auxin-transporting channels for vasculature formation and regeneration. These results support a role of ABP1-TMK1 in cell surface auxin perception mediating global auxin phospho-response and regenerative development.