9.B.443. The Plant Auxin Binding Protein Receptor (P-ABP-R) Family
ABP1-TMK auxin perception signals are used for global phosphorylation and auxin canalization (Friml et al. 2022). The phytohormone auxin triggers transcriptional reprogramming through a well-characterized perception machinery in the nucleus, but mechanisms that underlie fast effects of auxin, such as the regulation of ion fluxes, rapid phosphorylation of proteins and auxin feedback on its transport, have been studied only more recently (Friml et al. 2022). A fraction of A. thaliana ABP1 is secreted and binds auxin specifically at an acidic pH that is typical of the apoplast. ABP1 and its plasma-membrane-localized partner, transmembrane kinase 1 (TMK1; TC# 1.A.87.2.13), are required for the auxin-induced ultrafast global phospho-response and for downstream processes that include the activation of H+-ATPase and accelerated cytoplasmic streaming. abp1 and tmk mutants cannot establish auxin-transporting channels and show defective auxin-induced vasculature formation and regeneration. An ABP1(M2X) variant that lacks the capacity to bind auxin is unable to complement these defects in abp1 mutants. Thus, ABP1 is the auxin receptor for TMK1-based cell-surface signalling, which mediates the global phospho-response and auxin canalization (Friml et al. 2022). This family and TC family 9.B.75 both include cupin domain-containing proteins and are therefore homologous, comprising a superfamily.