Phosphate in plant seeds and grains is mostly in the form of phytic acid (inositol-hexakiphosphate or InsP6). Phytic acid is largely indigestible by nonruminants. It is an important antinutritional factor because it prevents the uptake of many important minerals, especially bivalent cations such as Fe2+, Mn2+, Mg2+, Zn2+ and Ca2+. In low-income countries, diets based on grains and oilseeds can lead to iron and zinc deficiencies, and this affects hundreds of millions of people today(Brown and Solomons 1991). Initiatives to fight iron and zinc deficiencies screen for plant cultivars with low phytate levels; indeed, intervention experiments with low phytate corn led to better iron absorption in humans (Mendoza and Alvarez-Buylla 1998). In high-income countries, animal agriculture would benefit from low phytate grains, because it would lead to a reduced discharge of phytate-derived phosphate-rich waste. Animal studies revealed that low-phytate seeds provide more available phosphorus and reduce phosphorus in waste (Brinch-Pedersen et al. 2002).
Low phytate mutants are available in maize, barley, rice, wheat, soybean and the model plant Arabidopsis. The reduction of phytate in seeds is compensated by increased inorganic phosphorus levels, so that the total seed phosphorus levels are equal to wild type (Raboy 2007). Negative outcome of these mutants is that they negatively affect plant performance by reducing key physiological processes in seeds like germination, stress tolerance and seed filling. A transgenic approach to embryo-specific reduction of gene expression in the phytate biosynthesis pathway was successful in overcoming these problems (Shi et al. 2007).
Phytic acid is a very important molecule, as it regulates many cellular functions in all eukaryotes. It is involved in development, DNA repair, RNA editing, mRNA transport, stress response, and phosphate homeostasis and sensing. Biosynthesis of phytic acid occurs largely in the cytosol and starts with the synthesis of myo-inositol, also the precursor for many other important compounds in the cell (Figure 3.7), The stepwise phosphorylation of myo-inositol leads to the final product phytate (phytic acid), which is stored as mixed phytate salts in protein storage vacuoles (Brinch-Pedersen et al. 2002; Shears 2004; Raboy 2007).
Figure 3.7: Phytic acid biosynthesis starting at myo-inositol. Apart from being phosphorylated to phytic acid, myo-inositol plays a central role in several metabolic processes as well as signal transduction in the plant cell. Abbreviations: D-glucose-6-P, D-glucose-6-phosphate; Ins, inositol; PtdIns, phosphatidylinositol. (Taken from: Brinch-Pedersen et al. 2002)
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