Mutations in the Arabidopsis Phosphoinositide Phosphatase Gene SAC9 Lead to Overaccumulation of PtdIns4,5)P2 and Constitutive Expression of the Stress-Response Pathway1

Phosphoinositides (PIs)are signaling molecules that regulate cellular events including vesicle targeting and interactions between membrane and cytoskeleton. Phosphatidylinositol (PtdIns)(4,5)P2 is one of the best characterized PIs; studies in which PtdIns(4,5)P2 localization or concentration is altered lead to defects in the actin cytoskeleton and exocytosis. PtdIns(4,5)P2 and its derivative Ins(1,4,5)P3 accumulate in salt, cold, and osmotically stressed plants. PtdIns(4,5)P2 signaling is terminated through the action of inositol polyphosphate phosphatases and PI phosphatases including supressor of actin mutation (SAC)domain phosphatases. In some cases, hese phosphatases also act on Ins(1,4,5)P3. 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 aWWprotein interaction domain within the SAC domain. The rice (Oryza sativa)and Arabidopsis SAC9 rotein 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)P2 and Ins(1,4,5)P3 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.