Isolation and Characterization of a Novel Rice Gene Required for Wild-Type Levels of Seed Phytic Acid.

Isolation and characterization of a novel rice gene required for wild-type levels of seed phytic acid Phytic acid (InsP6) is the primary storage form of phosphorus found in plant seeds. InsP6 has a major impact on animal and human nutrition as it cannot be digested by non-ruminants and its ability to bind minerals reduces the bioavailability of these essential micronutrients. As such, InsP6 is generally considered to be an anti-nutrient and developing seed crops that have reduced levels of InsP6 while maintaining their agronomic performance is of significant interest. InsP6 and other inositol polyphosphates are also involved in a number of important biological processes. We have employed both forward and reverse genetics approaches in rice and Arabidopsis to identify and characterize genes involved in InsP6 metabolism in seeds. The rice OsLpa1 encodes a novel protein required for wild-type levels of seed InsP6 and was identified from a low phytic acid (lpa) mutant exhibiting a 45-50% reduction in seed InsP6 with a corresponding increase in inorganic phosphate. The predicted OsLPA1 protein contains a region of weak homology to a 2-phosphoglycerate kinase from hyperthermophilic methanogens including Walker A and B motifs characteristic of P-loop domains. OsLpa1 is highly conserved in plants and Arabidopsis contains two OsLpa1-like genes, At3g45090 and At5g60760. Analysis of homozygous T-DNA insertion mutants of At5g60760 revealed significantly reduced levels of seed InsP6 while no changes were observed in At3g45090 mutants. A double knockout mutant was created and its seed InsP6 content was similar to that of the At5g60760 mutant indicating that At3g45090 does not provide functional redundancy. OsLpa1 was confirmed to be the ortholog of At5g60760 by complementation with a cDNA clone corresponding to the largest of three alternative transcripts of OsLpa1.. The spatial and temporal expression of At5g60760 during seed/siliques development is consistent with its involvement in seed InsP6 biosynthesis. Our complementary studies in rice and Arabidopsis will facilitate rapid progress in determining the function of OsLpa1/At5g60760 gene as well as enabling us to compare seed InsP6 metabolism in monocotyledons and dicotyledonous species.