Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/2008
Publication Date: 5/21/2008
Citation: Fu, J., Peterson, K., Guttieri, M., Souza, E.J., Raboy, V. 2008. Barley (Hordeum vulgare L.) inositol monophosphatase: gene structure and enzyme characteristics. Plant Molecular Biology. 67:629-642. Interpretive Summary: Understanding the fundamental metabolic pathways in plant cells provides the foundation of knowledge necessary to develop new science and technology to deal with today's agricultural problems. A compound called "myo-inositol" is central to many processes and pathways important to how plants respond to stress such as heat and drought. It is also central to metabolic pathways in seeds that lead to the synthesis of compounds important to seed nutritional quality, such as phytic acid. The synthesis of myo-inositol requires two enzymes; "myo-inositol phosphate synthase" and "inositol monophosphatase" or "IMP". In this research the gene in barley which encodes the IMP enzyme was identified, and the characteristics and actions of the IMP enzyme itself were studied. This work will increase our understanding of how this basic part of plant cell machinery works, which will enhance our ability to deal with real agricultural problems in the future.
Technical Abstract: The de novo synthesis of myo-inositol (Ins) is catalyzed by two enzymatic activities; Ins(3)P1 synthase (MIPS; EC. 22.214.171.124) and Ins monophosphatase (IMPase; EC 126.96.36.199). The barley IMP-1 gene and gene products were studied to facilitate research into the regulation of Ins synthesis and supply. In many organisms IMPase is encoded by small multigene families, but the barley (Hordeum vulgare L.) genome (~5,000 Mb), like the Arabidopsis genome (157 Mb), contains only one copy. Thus IMP copy number has no relation to genome size. The 1.4 kb barley IMP-1 promoter contains one low temperature response element (RE), two heat shock REs, one gibberellin and two auxin REs, and five sugar REs. Barley IMP-1 is expressed in all tissues assayed and expression levels are not greatly altered by abiotic stress treatments. Reduced use of Ins for Ins P6 synthesis in developing seed of barley low phytic acid (lpa) mutants can result in Ins accumulation, and IMP-1 expression is reduced in proportion to the increase in Ins level. The barley recombinant enzyme's activity was suppressed by Li, as are other IMPases. It had a relatively high affinity for D/L-myo-inositol-3-P1 (Km=9.7 µM), similar to that observed for a prokaryotic (Synechocystis sp PCC 6803) IMPase (Km=3.4 µM), both much lower than was reported for human (Km=43 µM) or bovine (Km=100 µM) recombinant IMPases. However, barley IMPase had a much lower kcat (1.29 s-1) as compared with values observed for the human and bovine enzyme (24 and 26 s-1, respectively). Thus the barley enzyme had higher substrate affinity but a lower catalytic efficiency. These data suggest that barley IMPase is regulated by cellular Ins levels and is primarily responsive to metabolic Ins needs. The prokaryotic and higher plant IMPase enzymes probably differ in this regard from their mammalian counterparts, whose catalytic properties would be relatively more efficient in signal-termination and recycling of Ins from Ins(1,4,5)P3 produced during signal transduction.