Phytic acid, myo-inositol hexakisphosphate, is the storage form of phosphorus (P) in seeds, and represents about 75% of seed total P. Non-ruminants (poultry, swine and fish) do not efficiently digest and utilize phytic acid P. Instead grain phytic acid P is mostly excreted, primarily as a mixed salt of mineral cations such as Ca, Zn and Fe. This phenomenon has several undesirable outcomes. Excreted P contributes to water pollution. The excretion of mineral cations can lead to mineral deficiencies. Also, phytic acid consumption and excretion can impair other nutritional processes such as protein utilization. One approach to this general problem in livestock production is to use an industrially produced enzyme called "phytase" as a dietary supplement. When phytase is consumed along with the feed, it will break down the phytic acid in the animal's intestinal tract, releasing the P for uptake and utilization. This approach has some advantages and disadvantages. One notable and relevant disadvantage is that even in the best of cases, phytase use can only lead to an improvement in grain phytate utilization of about 50%.

Our approach has been to use genetics to improve the availability of grain P. We have isolated more than 20 independent, sodium azide-induced low phytic acid (or lpa) mutants in barley cv. Harrington. In all lpa mutants grain total P levels are similar to wild-type, and all such mutants are recessive. They fall into two basic phenotypic classes. The first phenotypic class or lpa 1-like mutants have greatly reduced grain phytic acid P contents, accompanied by equivalent increases in inorganic P. Nearly all of the mutants we have isolated to date are lpa1-like. The first mutant, lpa1-1, causes a 50% to 60% reduction in grain phytic acid P, with a concomitant increase in inorganic P. Additional mutants of this type cause reductions in grain phytic acid P ranging up to 95%. The second or lpa 2-like mutants have greatly reduced grain phytic acid P levels, accompanied by increases in both inorganic P and "lower" inositol phosphates (compounds related to phytic acid). We have mapped lpa1-1 and lpa2-1, and a separate presentation (Larson et al.) will focus on this particular aspect of this work.

In terms of genetics research, our most important current objective is to conclude allelism tests to determine the number of loci represented by these mutants. Lpa1-1 and lpa2-1 are distinct loci, with distinct phenotypes, but do we have mutants representing additional loci? Each additional locus identified represents a valuable tool and potential target for engineering nutritionally improved feed grains.

We will also review the results of our most recent physiology experiment. Wild-type control and four mutants were grown to maturity in a greenhouse sand culture experiment where levels of nutrient P ranged from "low" to "high". As a result, plant P concentration and the resulting seed total P concentration also then varies form "low" to "high". We are primarily interested in determining the effects of the impaired ability of these mutants to store seed P as phytic acid P when seed P ranges from low to high.