Myo-inositol hexaphosphate, also known as phytic acid (PA), is the most abundant storage form of phosphorus in seeds. PA acts as a strong chelator of metal cations to form phytate and is considered an anti-nutrient as it reduces the bioavailability of important micronutrients. Although the major nutrient source for more than one-half of the global population, rice is a poor source of essential micronutrients. Therefore, biofortification and reducing the PA content of rice have arisen as new strategies for increasing micronutrient bioavailability in rice. Furthermore, global climate change effects, particularly rising atmospheric carbon dioxide concentration, are expected to increase the PA content and reduce the concentrations of most of the essential micronutrients in rice grain. Several genes involved in PA biosynthesis have been identified and characterized in rice. Proper understanding of the genes related to PA accumulation during seed development and creating the means to suppress the expression of these genes should provide a foundation for manipulating the PA content in rice grain. Low-PA rice mutants have been developed that have a significantly lower grain PA content, but these mutants also had reduced yields and poor agronomic performance, traits that challenge their effective use in breeding programs. Nevertheless, transgenic technology has been effective in developing low-PA rice without hampering plant growth or seed development. Moreover, manipulating the micronutrient distribution in rice grain, enhancing micronutrient levels and reducing the PA content in endosperm are possible strategies for increasing mineral bioavailability. Therefore, a holistic breeding approach is essential for developing successful low-PA rice lines. In this review, we focus on the key determinants for PA concentration in rice grain and discuss the possible molecular methods and approaches for manipulating the PA content to increase micronutrient bioavailability.

Phytic acid (PA) is a storage form of phosphorus in seeds. Phytase enzyme is activated at germination and hydrolyses PA into myo-inositol and inorganic phosphate. PA inhibits the absorption of minerals in the human intestine by chelation. Its degradation, therefore, is a key factor to improve mineral bioavailability in rice. Germinated brown rice (GBR) is favoured because it improves the availability of nutrients, and thus have a positive effect on health. In this study, we show the effects of soaking temperature on phytase activity and PA content in GBR. Rice phytase showed thermostability and its activity peaked at 50 °C. After 36 h of soaking, phytase activity was significantly increased at 50 °C and PA content was significantly decreased, compared to that at 30 °C. Zinc (Zn) analysis revealed that there was no significant difference in Zn content among different temperature treatments. Calculated total daily absorbed Zn (TAZ) was significantly higher in GBR compared with non-soaked seeds. Moreover, brown rice grains germinated at 50 °C showed a higher TAZ value than that at 30 °C. Seed germination and seed water soaking at high temperatures reduce PA content in brown rice showing a potentially effective way to improve mineral bioavailability in brown rice.

Phytic acid (PA) prevents the absorption of minerals in the human intestine, and it is regarded as an antinutrient. Low PA rice is beneficial because of its higher Zn bioavailability and it is suggested that the gene expression level of myo-inositol 3-phosphate synthase 1 (INO1) in developing grain is a key factor to explain the genotypic difference in PA accumulation among natural variants of rice. P fertilization is also considered to affect the PA content, but it is not clear how it affects INO1 gene expression and the PA content in different genotypes. Here, we investigated the effect of P fertilization on the PA content in two contrasting rice genotypes, with low and high PA accumulation, respectively. Based on the results of the analysis of the PA content, inorganic P content, INO1 gene expression, and xylem sap inorganic P content, we concluded that the effect of P fertilization on PA accumulation in grain differed with the genotype, and it was regulated by multiple mechanisms.