Submitted to: Journal of Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2008
Publication Date: 11/20/2008
Citation: Kim, S., Andaya, C.B., Newman, J.W., Goyal, S.S., Tai, T. 2008. ISOLATION AND CHARACTERIZATION OF A LOW PHYTIC ACID RICE MUTANT REVEALS A MUTATION IN THE RICE ORTHOLOGUE OF MAIZE MIK. Journal of Theoretical and Applied Genetics. 117(8):1291-301. Interpretive Summary: In order to identify rice plants showing reduce levels of seed phytic acid, we screened seeds from a population of plants that were chemically treated to cause gene mutations. Seeds were tested for increase inorganic phosphorus. Two different lines, designated N15-186 and N15-375, were identified and chemical analysis of seed extracts revealed that both had increased inorganic phosphorus and reduced phytic acid. N15-186 also had a 9-fold increase in myo-inositol and appeared to have reduced amounts of inositol monophosphate, both of which are compounds from which phytic acid is made. Genetic analysis of N15-186 revealed that the reduction in phytic acid was due to a mutation in a single gene. Further analysis using genetic mapping and gene sequencing resulted in the identification of the mutation in a gene encoding a rice protein that corresponds to a maize protein called myo-inositol kinase (MIK). The maize MIK gene was identified previously by other researchers studying low phytic acid in maize seeds. This work indicates that the MIK gene is also important for seed phytic acid in rice. This mutant will be useful in studying the interaction of genes involved in phytic acid metabolism. Characterization of the N15-375 plant is ongoing and results will be reported elsewhere.
Technical Abstract: Using a forward genetics approach, we isolated two independent low phytic acid (lpa) rice mutants, N15-186 and N15-375. Both mutants are caused by single gene, recessive non-lethal mutations which result in approximately 75% (N15-186) and 43% (N15-375) reductions in seed phytic acid (inositol hexakisphosphate). HPLC and GC-MS analysis of seed extracts from N15-186 indicated that, in addition to phytic acid, inositol monophosphate was significantly reduced whereas inorganic phosphorus and myo-inositol were greatly increased compared to wild-type. The changes observed in N15-186 resemble those previously described for the maize lpa3 mutant. Analysis of N15-375 revealed changes similar to those observed in previously characterized rice lpa1 mutants (i.e. significant reduction in phytic acid and corresponding increase in inorganic phosphorus with little or no change in inositol phosphate intermediates or myo-inositol). Further genetic analysis of the N15-186 mutant indicated that the mutation, designated lpa N15-186, was located in a region on chromosome 3 between the microsatellite markers RM15875 and RM15907. The rice orthologue of maize lpa3, which encodes a myo-inositol kinase, is in this interval. Sequence analysis of the N15-186 allele of this orthologue (Os03g52760) revealed a single base pair change (C/G to T/A) in the first exon of the gene which results in a nonsense mutation. Our results indicate that lpa N15-186 is a mutant allele of the rice myo-inositol kinase (OsMIK) gene. Identification and characterization of lpa mutants, such as N15-186, will facilitate studies on the regulation of phytic acid biosynthesis and accumulation and help address questions concerning the contribution of the inositol lipid-dependent and independent biosynthetic pathways to the production of seed phytic acid.