Control of plant exocyst function by protein phosphorylation in root hairs and pollen tubes - regulation of tip growth through AGC1 kinases and exocyst phosphoregulation

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Root hairs and pollen tubes are tip-growing cells that target cellular growth exclusively to one focal region at the cell surface. Exocyst is a conserved multiprotein complex required for the tethering of transport vesicles to the plasma membrane to sustain the secretion of membrane and cell wall material. We have characterized the three Arabidopsis protein kinases AGC1.5, AGC1.6, and AGC1.7 that are strongly expressed in root hairs and pollen tubes. Loss-of-function mutations and overexpression of the kinases lead to polarity defects in both tip growing processes, indicating that these kinases are required for proper tip growth. The kinases localize, in root hair cells, to the subapical region where they may positively or negatively regulate fusion, membrane recycling or membrane transport processes. Exo70C1 and Exo70C2, soluble subunits of exocyst, are highly abundant in both cell types and Exo70C1 is phosphorylated by the three AGC1 kinases in vitro. Here, we will test the in vivo relevance of the Exo70C1 phosphorylation by mapping phosphorylation sites in vitro, test the ability of the protein to complement mutant phenotypes and examine whether the paralogous, Exo70C2 as well as other exocyst subunits of interest are also AGC1 phosphorylation targets. Further, we will examine the role of nitrosylation in controlling AGC1.5, AGC1.6, and AGC1.7 activity, which is interesting in view of the fact that nitrosylation gradients have been described in root hair and pollen tube growth that could regulate the kinases at the subcellular level.