Mutations in the Arabidopsis Phosphoinositide Phosphatase Gene SAC9 Lead to Overaccumulation of PtdIns(4,5)P₂ and Constitutive Expression of the Stress-Response Pathway

• Published in: Plant physiology (Bethesda) 2005-06, Vol.138 (2), p.686-700
• Main Authors: Williams, Mary Elizabeth, Torabinejad, Javad, Cohick, Evan, Parker, Katherine, Drake, Elizabeth J, Thompson, James E, Hortter, Michelle, DeWald, Daryll B
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; amino acid sequences; Arabidopsis - genetics; Arabidopsis - metabolism; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; Arabidopsis thaliana; Biological and medical sciences; Cell Biology and Signal Transduction; Chromosome Mapping; Chromosomes, Plant; Fundamental and applied biological sciences. Psychology; gene expression regulation; Gene Expression Regulation, Plant; Genes; Genes. Genome; Genetic mutation; Guard cells; Inositol 1,4,5-triphosphate; Inositols; Molecular and cellular biology; Molecular genetics; Molecular Sequence Data; Mutation; nucleotide sequences; Phenotype; Phenotypes; phenotypic variation; Phosphatases; phosphatidylinositol 4,5-biphosphate; Phosphatidylinositol 4,5-Diphosphate; Phosphatidylinositol Phosphates - metabolism; Phosphatidylinositols; phosphoinositide phosphatase; phosphoinositides; phosphoric monoester hydrolases; Phosphoric Monoester Hydrolases - genetics; Phosphoric Monoester Hydrolases - metabolism; plant biochemistry; Plant cells; plant genetics; plant proteins; Plant roots; plant stress; Plants; reactive oxygen species; Sequence Alignment; Sequence Homology, Amino Acid; Signal Transduction
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.105.061317

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, these 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 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)P₂ and Ins(1,4,5)P₃ as compared to wild-type 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.