AGD1, a class 1 ARF-GAP, acts in common signaling pathways with phosphoinositide metabolism and the actin cytoskeleton in controlling Arabidopsis root hair polarity

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SUMMARYTheArabidopsis thaliana AGD1gene encodes a class 1 adenosine diphosphate ribosylation factor-gtpase-activating protein (ARF-GAP). Previously, we found thatagd1mutants have root hairs that exhibit wavy growth and have two tips that originate from a single initiation point. To gain new insights into how AGD1 modulates root hair polarity we analyzed double mutants ofagd1and other loci involved in root hair development, and evaluated dynamics of various components of root hair tip growth inagd1by live cell microscopy. Because AGD1 contains a phosphoinositide (PI) binding pleckstrin homology (PH) domain, we focused on genetic interactions betweenagd1and root hair mutants altered in PI metabolism.Rhd4,which is knocked-out in a gene encoding a phosphatidylinositol-4-phosphate (PI-4P) phosphatase, was epistatic toagd1.In contrast, mutations toPIP5K3andCOW1,which encode a type B phosphatidylinositol-4-phosphate 5-kinase 3 and a phosphatidylinositol transfer protein, respectively, enhanced the root hair defects ofagd1.Enhanced root hair defects were also observed in double mutants toAGD1andACT2,a root hair-expressed vegetative actin isoform. Consistent with our double-mutant studies, targeting of tip growth components involved in PI signaling (PI-4P), secretion (RABA4b) and actin regulation (ROP2), were altered inagd1root hairs. Furthermore, tip cytosolic calcium ([Ca2+]cyt) oscillations were disrupted in root hairs ofagd1.Taken together, our results indicate that AGD1 links PI signaling to cytoskeletal-, [Ca2+]cyt−, ROP2-, and RABA4b-mediated root hair development.

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