Mutations in the arabidopsis phosphoinositide phosphatase gene SAC9 lead to overaccumulation of PtdIns(4,5)P2 and constitutive expression of the stress-response pathway

Phosphoinositides ( PIs) are signaling molecules that regulate cellular events including vesicle targeting and interactions between membrane and cytoskeleton. Phosphatidylinositol ( PtdIns)( 4,5) P 2 is one of the best characterized PIs; studies in which PtdIns( 4,5) P 2 localization or concentration is altered lead to defects in the actin cytoskeleton and exocytosis. PtdIns( 4,5) P 2 and its derivative Ins( 1,4,5) P 3 accumulate in salt, cold, and osmotically stressed plants. PtdIns( 4,5) P 2 signaling is terminated through the action of inositol polyphosphate phosphatases and PI phosphatases including supressor of actin mutation ( SAC) domain phosphatases. In some cases, these phosphatases also act on Ins( 1,4,5) P-3. We have characterized the Arabidopsis ( Arabidopsis thaliana) sac9 mutants. The SAC9 protein is different from other SAC domain proteins in several ways including the presence of a WW protein interaction domain within the SAC domain. The rice ( Oryza sativa) and Arabidopsis SAC9 protein sequences are similar, but no apparent homologs are found in nonplant genomes. High- performance liquid chromatography studies show that unstressed sac9 mutants accumulate elevated levels of PtdIns( 4,5) P 2 and Ins( 1,4,5) P 3 as compared to wildtype plants. The sac9 mutants have characteristics of a constitutive stress response, including dwarfism, closed stomata, and anthocyanin accumulation, and they overexpress stress- induced genes and overaccumulate reactive- oxygen species. These results suggest that the SAC9 phosphatase is involved in modulating phosphoinsitide signals during the stress response.