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

Abstract

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.Phosphoinositides (PIs) are a family of eight molecules in which the hydroxyl groups on the inositol moiety can be phosphorylated in a variety of combinations (Stevenson et al., 2000;Meijer and Munnik, 2003;van Leeuwen et al., 2004). PIs undergo cycles of phosphorylation and dephosphorylation through organelle-specific PI kinases and phosphatases, leading to distinct subcellular distributions of PI species (De Matteis and Godi, 2004). PIs control the timing and location of many cellular events including vesicle targeting, interactions between the membrane and the cytoskeleton, membrane budding and fusing, nuclear and cytoplasmic signal transduction, and activity of membrane channels (Hilgemann and Ball, 1996;Martin, 1998;Czech, 2000;Odorizzi et al., 2000;Stevenson et al., 2000; Simonsen et al., 2001;Hardie, 2003;Meijer and Munnik, 2003;Oliver et al., 2004;van Leeuwen et al., 2004). Specific PI-binding sites have been found on a variety of effector proteins including protein kinases, actin-binding proteins, GTPases, and membrane trafficking proteins, and it is thought that binding to PIs can target effector proteins to specific membrane locations (Martin, 1998;Hu et al., 1999;Yao et al., 1999;Dowler et al., 2000;Tall et al., 2000;Ellson et al., 2002;Itoh and Takenawa, 2002).Unraveling the specific functions of the PI species and the enzymes that modify them is challenging for several reasons. Enzyme specificities do not always correlate ...

Inositol Polyphosphate 5-Phosphatases 1 and 2 Are Required for Regulating Seedling Growth

“…In contrast, fra3 mutants are not ABA hypersensitive, most likely because the Ins(1,4,5)P 3 increase noted in these mutants occurs only within the inflorescence stems . Similarly, alterations in the sac9 mutant, which result in elevated Ins(1,4,5)P 3 and PtdInsP 2 levels in the root only, do not result in ABA hypersensitivity (Williams et al, 2005). Thus, it appears that Ins(1,4,5)P 3 increases may only be associated with ABA hypersensitivity when the second-messenger increase occurs within the majority of cells within the seedling.…”

TheArabidopsis thaliana Myo-Inositol 1-Phosphate Synthase1 Gene Is Required forMyo-inositol Synthesis and Suppression of Cell Death

“…Since we found no differences in basal Ins(1,4,5)P 3 levels in mips1 mutants, this suggests that signaling via phospholipase C hydrolysis of PtdIns (4,5)P 2 is not impaired. In addition, the increased sensitivity of mips1 mutants to ABA, salt, and sorbitol is not consistent with a lack of phospholipase C signaling, since mutants in phospholipase C signaling with increased sensitivity to these stimuli most often contain elevated Ins(1,4,5)P 3 (Xiong et al, 2001;Williams et al, 2005;Gunesekera et al, 2007). So while we cannot rule out a complex alteration in phospholipase C signaling in mips1 mutants, we feel it is more likely that the reduced capacity to handle ROS (described above) in mips1 mutants results in increased sensitivity to oxidative stress, ABA, salt, and sorbitol.…”

Does diacylglycerol serve as a signaling molecule in plants?

“…7 Nevertheless, IP3 accumulates in plant cells following their exposure to environmental stimuli, and its accumulation has been correlated with the mobilization of intracellular calcium. 3,8,9 Perhaps, therefore, IP3 receptors differing from ligand-gated calcium channels have evolved in plants.…”