TMK-PIN1 drives a short self-organizing circuit for auxin export and signaling
Ontology highlight
ABSTRACT:
ORGANISM(S): Arabidopsis Thaliana
SUBMITTER: 
Tongda Xu
PROVIDER: PXD068245 | iProX | Wed Sep 10 00:00:00 BST 2025
REPOSITORIES: iProX
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Project description:The phytohormone auxin controls a myriad of processes in plants, at least in part through its regulation of cell expansion. The "acid growth hypothesis" has been proposed to explain auxin-stimulated cell expansion for five decades, but the mechanism underlying auxin-induced cell wall acidification is poorly characterized. Auxin induces the phosphorylation and activation of the plasma membrane (PM) H+-ATPase that pumps protons into the apoplast, yet how auxin activates its phosphorylation remains elusive. Here, we show that the transmembrane kinase (TMK) auxin signaling proteins interact with PM H+-ATPases and activate their phosphorylation to promote cell wall acidification and hypocotyl cell elongation in Arabidopsis. Auxin induced TMK's interaction with H+-ATPase on the plasma membrane within 1-2 minutes as well as TMK-dependent phosphorylation of the penultimate Thr residue. Genetic, biochemical, and molecular evidence demonstrates that TMKs directly phosphorylate PM H+-ATPase and are required for auxin-induced PM H+-ATPase activation, apoplastic acidification, and cell expansion. Thus, our findings reveal a crucial connection between auxin and PM H+-ATPase activation in regulating apoplastic pH changes and cell expansion via TMK-based cell surface auxin signaling.
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Project description:Extracellular perception of auxin, an essential phytohormone in plants, has been debated for decades. Auxin binding protein 1 (ABP1) physically interacts with quintessential transmembrane kinases (TMKs) and was proposed to act as an extracellular auxin receptor, but its role was disputed because abp1 knockout mutants lack obvious morphological phenotypes. Here we identified two new auxin-binding proteins, ABL1 and ABL2, that are localized to the apoplast and directly interact with the extracellular domain of TMKs in an auxin-dependent manner. Furthermore, functionally redundant ABL1 and ABL2 genetically interact with TMKs and exhibit functions that are overlapping with those of ABP1 as well as independent of ABP1. Importantly, the extracellular domain of TMK1 itself binds auxin and synergizes with either ABP1 or ABL1 in auxin binding. Thus, our findings discovered new auxin receptors ABL1 and ABL2 having functions overlapping with but distinct from ABP1 and acting together with TMKs as co-receptors for extracellular auxin.
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Project description: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.
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