The phosphatidylinositol signaling pathway utilizes inositol-containing second messengers to mediate signaling events. The enzymes that metabolize phosphoinositides can in some cases serve to terminate the signaling actions of phosphoinositides. The inositol polyphosphate 5-phosphatases (5PTases) comprise a large protein family that hydrolyzes 5-phosphates from a variety of inositol phosphate and phosphoinositide substrates. I have examined the substrate specificity of the At5PTase11 protein from the model plant, Arabidopsis thaliana. The At5PTase11 gene (At1g47510) encodes an active 5PTase enzyme that can dephosphorylate the phosphoinositide substrates phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P2], and phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3]. In addition, the At5PTase11 gene is regulated by abscisic acid, jasmonic acid, and auxin, suggesting a role for phosphoinositide action in these signal transduction pathways. To further delineate the function of At5PTase11 in Arabidopsis thaliana, two independent T-DNA insertion mutant lines were isolated (At5ptase11-1 and At5ptase11-2). Analysis of At5ptase11 mutant lines revealed that At5ptase11 mutant seeds germinate slower compared to wild-type seeds. Moreover, At5ptase11 mutant seedlings demonstrated less hypocotyl growth when grown in the dark. These results indicate that At5PTase11 is required for the early stages of seed germination and seedling growth. Since there are 15 predicted 5PTases in Arabidopsis thaliana, a group of 5PTases have been analyzed to identify the 5PTases with similar substrate selectivity. At5PTase1 (At1g34120), At5PTase2 (At4g18010) and At5PTase3 (At1g71710) have been found to hydrolyze all four potential substrates, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4], PtdIns(4,5)P2, and PtdIns(3,4,5)P3. At5PTase7 (At2g32010) hydrolyzed PtdIns(4,5)P2, and PtdIns(3,4,5)P3 which is similar to the substrate selectivity of At5PTase11. In addition, At5PTase4 (At3g63240), and At5PTase9 (At2g01900) hydrolyzed only PtdIns(4,5)P2. These results indicate that there are different groups of Arabidopsis thaliana 5PTases based on the substrate selectivity. These results suggest that Arabidopsis thaliana 5PTases with similar substrate selectivity may have overlapping functions. In summary, the findings that At5PTase11 is a phospholipid-specific 5PTase and At5PTase11 functions in the early stages of seed germination and seedling growth indicate that 5PTases play important roles in plant growth and development. / Ph. D.