Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2002
Publication Date: 3/1/2003
Citation: DORSCH, J.A., COOK, A., YOUN, K.A., ANDERSON, J.M., BAUMAN, A.T., VOLKMANN, C.J., MURTHY, P.N., RABOY, V. Seed Phosphorus and Inositol phosphate phenotype of barley low phytic acid genotypes. PHYTOCHEMISTRY. 2003. Interpretive Summary: "Low phytic acid" genotypes of grains crops like barley and corn have great potential value for use in crop breeding. They can improve the nutritional value of seed crops in terms of phosphorus and mineral availability. However, before decisions can be made as to which low phytic acid genotypes or mutations to use in plant breeding, a careful chemical analysis of their seed phosphorus and "inositol phosphate" (phytic acid is a kind of inositol phosphate) characteristics must be conducted. This chemical description or "phenotype" of these genotypes also represents the first step in understanding the biology of these low phytic acid crop types. This study found that one of the barley low phytic acid genotypes, barley low phytic acid 2-1, is very similar to a corn genotype referred to as corn low phytic acid 2-1. Both lpa2 genotypes are probably mutations in "inositol phosphate" metabolism, and the genes they represent may prove highly valuable in engineering optimal low phytate lines. Three other barley low phytic acid genotypes were also studied. One of these, "M 955", produces seed that has more than 90% less phytic acid than does normal seed, and M 955 lines have normal germination and viability in the field. This work therefore provides a first step in understanding the seed chemistry and biology of these first four barley low phytic acid genotypes.
Technical Abstract: myo-Inositol-1,2,3,4,5,6-hexakisphosphate (Ins P6 or "phytic acid") typically represents ~75% of the total phosphorus and >80% of soluble myo-inositol (Ins) phosphates in seeds. The seed phosphorus and Ins phosphate phenotypes of four non-lethal barley (Hordeum vulgare L.) low phytic acid mutations are described. In seeds homozygous for M 635 and M 955 reductions in Ins P6, ~75 and 90% respectively, are accompanied by reductions in other Ins phosphates and molar-equivalent increases in Pi. This phenotype suggests a block in supply of substrate Ins. In seeds homozygous for barley low phytic acid 1-1 (lpa1-1), a 45% decrease in Ins P6 is mostly matched by an increase in Pi but also accompanied by small increases in Ins(1,2,3,4,6)P5 . In seeds homozygous for barley lpa2-1, reductions in seed Ins P6 are accompanied by increases in both Pi and in several Ins phosphates, a phenotype that suggests a lesion in Ins phosphate metabolism, rather than Ins supply. The increased Ins phosphates in barley lpa2-1 seed are: Ins(1,2,3,4,6)P5; Ins(1,2,4,6)P4 and/or its enantiomer Ins(2,3,4,6)P4; Ins(1,2,3,4)P4 and/or its enantiomer Ins(1,2,3,6)P4; Ins(1,2,6)P3 and/or its enantiomer Ins(2,3,4)P3; Ins(1,5,6)P3 and/or its enantiomer Ins(3,4,5)P3 (the methods used here cannot distinguish between enantiomers). This primarily "5-OH" series of Ins phosphates differs from the "1-/3-OH" series observed at elevated levels in seed of the maize lpa2 genotype, but previous chromosomal mapping data indicated that the maize and barley lpa2 loci might be orthologs of a single ancestral gene. Therefore one hypothesis that might explain the differing lpa2 phenotypes is that their common ancestral gene encodes a multi-functional, Ins phosphate kinase with both "1-/-3-" and "5-kinase" activities. A putative pyrophosphate-containing Ins phosphate, possibly an Ins P7, was also observed in the mature seed of all barley genotypes except lpa2-1. Barley M 955 indicates that at least for this species, the ability to accumulate Ins P6 can be nearly abolished while retaining at least short-term (~1.0 years) viability.