Seed Total Phosphate and Phytic Acid

Both the total amount and composition of seed phosphorus (P) are important to maize end-use quality. Seed total P represents a major pool in the flux of P through the agricultural ecology. The phosphate stored in seeds of major crops represents a sum equivalent to more than 50% of phosphate fertilizer applied annually worldwide (Lott et al. 2000). Clearly this pool has value as a target in efforts to enhance the management of P in agricultural production. The most abundant form of P in mature crop seeds is phytic acid (myo-inositol-1,2,3,4,5,6-P₆ or InsP₆). In maize it often represents about 80% of seed total P (Lott et al. 2000; Raboy 2006). Seed-derived dietary phytic acid binds tightly to nutritionally important minerals such as calcium, iron and zinc, and is not efficiently digested by humans and non-ruminants such as poultry, swine and fish (Brinch-Pedersen et al. 2002). As a result most seed-derived phytic acid P consumed by non-ruminants is excreted. Perhaps 40% of maize produced in the USA is used in non-ruminant feeds, and non-ruminant waste P has the potential of contributing to water pollution, an environmental problem requiring additional management (EPA 2002; Sharpley et al. 1994). In the context of human nutrition, excretion of phytic acid can contribute significantly to mineral depletion and deficiency, such as iron and zinc deficiency, in populations that rely on whole grains and legumes as staple foods (Brown and Solomons 1991). It is estimated that more than a billion people suffer iron deficiency, and hundreds of millions suffer zinc and other mineral deficiencies.

There has been substantial progress in the genetics of seed P composition. In the case of forward genetics, a number of low phytic acid (lpa) mutations have been isolated in maize and other crop species, using mutagenized wild-type germplasm and screens for either reduced seed phytic acid or increased seed inorganic P (screen nos 1 and 2, respectively; Fig. 4.1; reviewed in Raboy 2006). These mutations block the synthesis of phytic acid during seed development, but in nearly all cases have little effect on seed total P or the distribution of P in the mature seed. Instead, a far greater proportion of seed total P is found as inorganic P (Pi), resulting in the “high inorganic P” phenotype of lpa genotypes (Fig. 4.1). Thus lpa alleles alter the chemistry of seed total P, but with one possible exception (barley lpa1-1; see below), have not been shown to substantially alter total P concentration. Advances in the molecular biology of phytic acid metabolism and storage have also resulted in reverse genetics approaches to engineering seed P composition (Shi et al. 2007; Stevenson-Paulik et al. 2005).