Approaches to improving the bioavailability and level of iron in rice seeds

Iron deficiency is estimated to affect about 30% of the world population, making iron by far the most deficient nutrient worldwide. Iron supplementation in the form of tablets and food fortification has not been successful in developing countries, and iron deficiency is still the most important deficiency related to malnutrition. Here we present experiments that aim to increase the iron content in rice endosperm and to improve its absorption in the human intestine by means of genetic engineering. We first introduced a ferritin gene from Phaseolus vulgaris into rice grains, increasing their iron content up to twofold. To increase iron bioavailability, we introduced a thermotolerant phytase from Aspergillus fumigatus into the rice endosperm. In addition, as cysteine peptides are considered a major enhancer of iron absorption, we overexpressed the endogenous cysteine‐rich metallothionein‐like protein. The content of cysteine residues increased about sevenfold and the phytase level in the grains about 130‐fold, giving a phytase activity sufficient to completely degrade phytic acid in a simulated digestion experiment. However, the fungal protein did not retain its activity after rice cooking. Therefore a further attempt to specifically reduce the phytic acid in the inner part of the rice endosperm, the tissue eaten after the milling of rice grains, is discussed and biochemical analyses of the transgenic rice seeds are now in progress.