Abstract
PtdIns 3, 5-kinase, Fab1/PIKfyve produces PtdIns (3,5) P2 from PtdIns 3-P, and functions in vacuole/lysosome homeostasis. We show that expression of Arabidopsis FAB1A/B in S. pombe fab1 knockout cells fully complements the vacuole morphology phenotype. Subcellular localizations of FAB1A and FAB1B fused with green fluorescent protein revealed that FAB1A/B-GFPs localize to the endosomes in root epidermal cells of Arabidopsis. Reduction in the expression levels of FAB1A/B impairs vacuolar acidification and endocytosis. Conditional knockdown mutant shows various phenotypes including root growth inhibition, hyposensitivity to exogenous auxin, and disturbance of root gravitropism. These phenotypes are observed also in the overproducing mutants of FAB1A and FAB1B. The overproducing mutants reveal additional morphological phenotypes including dwarfism, male-gametophyte sterility, and abnormal floral organs. Taken together, this evidence indicates that imbalanced expression of FAB1A/B impairs endomembrane homeostasis including endocytosis, vacuole formation, and vacuolar acidification, which causes pleiotropic developmental phenotypes mostly related to the auxin signaling in Arabidopsis.
