The phosphoinositide phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P 2 ] is a key signaling molecule in animal cells. It can be hydrolyzed to release 1,2-diacyglycerol and inositol 1,4,5-trisphosphate (IP 3 ), which in animal cells lead to protein kinase C activation and cellular calcium mobilization, respectively. In addition to its critical roles in constitutive and regulated secretion of proteins, PtdIns(4,5)P 2 binds to proteins that modify cytoskeletal architecture and phospholipid constituents. Herein, we report that Arabidopsis plants grown in liquid media rapidly increase PtdIns(4,5)P 2 synthesis in response to treatment with sodium chloride, potassium chloride, and sorbitol. These results demonstrate that when challenged with salinity and osmotic stress, terrestrial plants respond differently than algae, yeasts, and animal cells that accumulate different species of phosphoinositides. We also show data demonstrating that whole-plant IP 3 levels increase significantly within 1 min of stress initiation, and that IP 3 levels continue to increase for more than 30 min during stress application. Furthermore, using the calcium indicators Fura-2 and Fluo-3 we show that root intracellular calcium concentrations increase in response to stress treatments. Taken together, these results suggest that in response to salt and osmotic stress, Arabidopsis uses a signaling pathway in which a small but significant portion of PtdIns(4,5)P 2 is hydrolyzed to IP 3 . The accumulation of IP 3 occurs during a time frame similar to that observed for stress-induced calcium mobilization. These data also suggest that the majority of the PtdIns(4,5)P 2 synthesized in response to salt and osmotic stress may be utilized for cellular signaling events distinct from the canonical IP 3 signaling pathway.Phosphoinositides are a class of membrane phospholipids that serve numerous roles in eukaryotic cellular processes. The family of phosphoinositides includes phosphatidylinositol monophosphate species phosphatidylinositol 3-phosphate [PtdIns(3)P] and phosphatidylinositol 4-phosphate [PtdIns(4)P], phosphatidylinositol bisphosphate species phosphatidylinositol 3,4-bisphosphate, phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P 2 ], and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P 2 ], and the phosphatidylinositol trisphosphate species phosphatidylinositol 3,4,5-trisphosphate. PtdIns(3)P and PtdIns(4)P regulate vesicle-mediated protein transport to the vacuole/lysosome and protein secretion,

L-myo-inositol 1-phosphate synthase (MIPS; EC 5.5.1.4) catalyzes the rate-limiting step in the synthesis of myo-inositol, a critical compound in the cell. Plants contain multiple MIPS genes, which encode highly similar enzymes. We characterized the expression patterns of the three MIPS genes in Arabidopsis thaliana and found that MIPS1 is expressed in most cell types and developmental stages, while MIPS2 and MIPS3 are mainly restricted to vascular or related tissues. MIPS1, but not MIPS2 or MIPS3, is required for seed development, for physiological responses to salt and abscisic acid, and to suppress cell death. Specifically, a loss in MIPS1 resulted in smaller plants with curly leaves and spontaneous production of lesions. The mips1 mutants have lower myo-inositol, ascorbic acid, and phosphatidylinositol levels, while basal levels of inositol (1,4,5)P 3 are not altered in mips1 mutants. Furthermore, mips1 mutants exhibited elevated levels of ceramides, sphingolipid precursors associated with cell death, and were complemented by a MIPS1-green fluorescent protein (GFP) fusion construct. MIPS1-, MIPS2-, and MIPS3-GFP each localized to the cytoplasm. Thus, MIPS1 has a significant impact on myo-inositol levels that is critical for maintaining levels of ascorbic acid, phosphatidylinositol, and ceramides that regulate growth, development, and cell death.

Myoinositol synthesis and catabolism are crucial in many multiceullar eukaryotes for the production of phosphatidylinositol signaling molecules, glycerophosphoinositide membrane anchors, cell wall pectic noncellulosic polysaccharides, and several other molecules including ascorbate. Myoinositol monophosphatase (IMP) is a major enzyme required for the synthesis of myoinositol and the breakdown of myoinositol (1,4,5)trisphosphate, a potent second messenger involved in many biological activities. It has been shown that the VTC4 enzyme from kiwifruit (Actinidia deliciosa) has similarity to IMP and can hydrolyze L-galactose 1-phosphate (L-Gal 1-P), suggesting that this enzyme may be bifunctional and linked with two potential pathways of plant ascorbate synthesis. We describe here the kinetic comparison of the Arabidopsis (Arabidopsis thaliana) recombinant VTC4 with D-myoinositol 3-phosphate (D-Ins 3-P) and L-Gal 1-P. Purified VTC4 has only a small difference in the V max /K m for L-Gal 1-P as compared with D-Ins 3-P and can utilize other related substrates. Inhibition by either Ca 2+ or Li + , known to disrupt cell signaling, was the same with both L-Gal 1-P and D-Ins 3-P. To determine whether the VTC4 gene impacts myoinositol synthesis in Arabidopsis, we isolated T-DNA knockout lines of VTC4 that exhibit small perturbations in abscisic acid, salt, and cold responses. Analysis of metabolite levels in vtc4 mutants showed that less myoinositol and ascorbate accumulate in these mutants. Therefore, VTC4 is a bifunctional enzyme that impacts both myoinositol and ascorbate synthesis pathways.

Laccases, EC 1.10.3.2 or p-diphenol:dioxygen oxidoreductases, are multi-copper containing glycoproteins. Despite many years of research, genetic evidence for the roles of laccases in plants is mostly lacking. In this study, a reverse genetics approach was taken to identify T-DNA insertional mutants (the SALK collection) available for genes in the Arabidopsis laccase family. Twenty true null mutants were confirmed for 12 laccase genes of the 17 total laccase genes (AtLAC1 to AtLAC17) in the family. By examining the mutants identified, it was found that four mutants, representing mutations in three laccase genes, showed altered phenotypes. Mutants for AtLAC2, lac2, showed compromised root elongation under PEG-induced dehydration conditions; lac8 flowered earlier than wild-type plants, and lac15 showed an altered seed colour. The diverse phenotypes suggest that laccases perform different functions in plants and are not as genetically redundant as previously thought. These mutants will prove to be valuable resources for understanding laccase functions in vivo.

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 ...

Signals can be perceived and amplified at the cell membrane by receptors coupled to the production of a variety of second messengers, including myoinositol 1,4,5-trisphosphate [Ins(1,4,5)P 3 ]. The myoinositol polyphosphate 5-phosphatases (5PTases; EC 3.1.3.56) comprise a large protein family that hydrolyzes 5-phosphates from a variety of myoinositol phosphate (InsP) and phosphoinositide phosphate (PtdInsP) substrates. Arabidopsis thaliana has 15 genes encoding 5PTases. Biochemical analyses of a subgroup of 5PTase enzymes suggest that these enzymes have both overlapping and unique substrate preferences. Ectopic expression of these genes in transgenic plants can reduce Ins(1,4,5)P 3 levels and alter abscisic acid (ABA) signaling. To further explore the function of 5PTases in signaling, we have identified and characterized T-DNA insertional mutants for 5PTase1 and 5PTase2 and produced a double mutant. When grown in the dark, the seeds from these mutants germinate faster than wildtype seeds and the mutant seedlings have longer hypocotyls than wild-type seedlings. Seeds from these mutant lines also demonstrate an increase in sensitivity to ABA. These changes in early seedling growth are accompanied by mass increases in Ins(1,4,5)P 3 , but not by changes in endogenous ABA content. By labeling the endogenous myoinositol pool in 5ptase1 and 5ptase2 mutants, we detected increases in Ins(1,4,5)P 3 and a decrease in PtdIns, PtdIns(4)P, and phosphatidylinositol (4,5) bisphosphate. Taken together, these data indicate that the At5PTase1 and At5PTase2 genes have nonredundant roles in hydrolyzing inositol second-messenger substrates and that regulation of Ins(1,4,5)P 3 levels is important during germination and early seedling development.

No abstract

Yeast TGN resident proteins that frequently cycle between the TGN and endosomes are much more slowly transported to the prevacuolar/late endosomal compartment (PVC) than other proteins. However, TGN protein transport to the PVC is accelerated in mutants lacking function of Inp53p. Inp53p contains a SacI polyphosphoinositide phosphatase domain, a 5-phosphatase domain, and a proline-rich domain. Here we show that all three domains are required to mediate "slow delivery" of TGN proteins into the PVC. Although deletion of the proline-rich domain did not affect general membrane association, it caused localization to become less specific. The proline-rich domain was shown to bind to two proteins, including clathrin heavy chain, Chc1p. Unlike chc1 mutants, inp53 mutants do not mislocalize TGN proteins to the cell surface, consistent with the idea that Chc1p and Inp53p act at a common vesicular trafficking step but that Chc1p is used at other steps also. Like mutations in the AP-1 adaptor complex, mutations in INP53 exhibit synthetic growth and transport defects when combined with mutations in the GGA proteins. Taken together with other recent studies, our results suggest that Inp53p and AP-1/clathrin act together in a TGN-to-early endosome pathway distinct from the direct TGN-to-PVC pathway mediated by GGA/clathrin.