Seed phosphorus and inositol phosphate phenotype of barley low phytic acid genotypes.

Abstract myo -Inositol-1,2,3,4,5,6-hexa kis phosphate (Ins P 6 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 P 6 , ∼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 ( lpa 1-1), a 45% decrease in Ins P 6 is mostly matched by an increase in Pi but also accompanied by small increases in Ins(1,2,3,4,6)P 5 . In seeds homozygous for barley lpa 2-1, reductions in seed Ins P 6 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 lpa 2-1 seed are: Ins(1,2,3,4,6)P 5 ; Ins(1,2,4,6)P 4 and/or its enantiomer Ins(2,3,4,6)P 4 ; Ins(1,2,3,4)P 4 and/or its enantiomer Ins(1,2,3,6)P 4 ; Ins(1,2,6)P 3 and/or its enantiomer Ins(2,3,4)P 3 ; Ins(1,5,6)P 3 and/or its enantiomer Ins(3,4,5)P 3 (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 lpa 2 genotype, but previous chromosomal mapping data indicated that the maize and barley lpa 2 loci might be orthologs of a single ancestral gene. Therefore one hypothesis that might explain the differing lpa 2 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 P 7 , was also observed in the mature seed of all barley genotypes except lpa 2-1. Barley M 955 indicates that at least for this species, the ability to accumulate Ins P 6 can be nearly abolished while retaining at least short-term (∼1.0 years) viability.