Arabidopsis phosphatidylinositol 4‐phosphate 5‐kinase genes PIP5K7, PIP5K8, and PIP5K9 are redundantly involved in root growth adaptation to osmotic stress. (5th April 2021) - Kuroda, Ryo
Summary: Phosphatidylinositol 4‐phosphate 5‐kinase (PIP5K) produces phosphatidylinositol (4, 5)‐bisphosphate (PtdIns(4, 5) P 2 ), a signaling phospholipid critical for various cellular processes in eukaryotes. The Arabidopsis thaliana genome encodes 11 PIP5K genes. Of these, three type B PIP5K genes, PIP5K7, PIP5K8, and PIP5K9, constitute a subgroup highly conserved in land plants, suggesting that they retain a critical function shared by land plants. In this study, we comprehensively investigated the biological functions of the PIP5K7–9 subgroup genes. Reporter gene analyses revealed their preferential expression in meristematic and vascular tissues. Their YFP‐fusion proteins localized primarily to the plasma membrane in root meristem epidermal cells. We selected a mutant line that was considered to be null for each gene. Under normal growth conditions, neither single mutants nor multiple mutants of any combination exhibited noticeable phenotypic changes. However, stress conditions with mannitol or NaCl suppressed main root growth and reduced proximal root meristem size to a greater extent in the pip5k7pip5k8pip5k9 triple mutant than in the wild type. In root meristem epidermal cells of the triple mutant, where plasma membrane localization of the PtdIns(4, 5) P 2 marker P24Y is impaired to a large extent, brefeldin A body formation is retarded compared with the wild type under hyperosmotic stress. These results indicate that PIP5K7, PIP5K8, and PIP5K9 are not required under normal growth conditions, but are redundantly involved in root growth adaptation to hyperosmotic conditions, possibly through the PtdIns(4, 5) P 2 function promoting plasma membrane recycling in root meristem cells. Significance Statement: PIP5K is a key enzyme producing PtdIns(4, 5) P 2, a typical signaling phospholipid in eukaryotes. Although an Arabidopsis triple null mutant of the PIP5K7, PIP5K8, and PIP5K9 genes, the subgroup of which is highly conserved in land plants, exhibited no phenotypes under normal growth conditions, its main root growth was hypersensitive to hyperosmotic conditions, suggesting a conserved function of the subgroup for adaptation to adverse environments but not for plant growth and development under favorable growth conditions.