Inositol Polyphosphate 5-Phosphatases 1 and 2 Are Required for Regulating Seedling Growth
2007
Gunesekera, Bhadra | Torabinejad, Javad | Robinson, Jamille | Gillaspy, Glenda E.
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)P₃]. 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)P₃ 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 wild-type 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)P₃, 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)P₃ 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)P₃ levels is important during germination and early seedling development.
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