Spatio-temporal distribution of micronutrients in rice grains and its regulation.

Rice has been a staple food for more than half of the global population. Different parts of rice grains contain different amounts of macro- and micro-nutrients. Polished white rice, which is the main form of rice consumption, mainly contains starch, however, the bran and germ, which are removed during polishing, contain large amounts of micronutrients and bioactive compounds. To engineer nutritionally superior rice varieties, it is imperative to understand the spatial and temporal distribution of different nutrients in different parts of the rice grain. Keeping this in mind, in this review, we have performed a comprehensive literature review to put together all the recent findings regarding the spatio-temporal distribution of all the important micronutrients in different cell-layers/tissues of developing seeds and mature seed grains. Furthermore, we have overviewed the underlying cell-layer specific possible regulatory mechanism responsible for the loading/partitioning for each of the micronutrients into specific tissue types. Most of the nutrient filling occurs between 7 and 18 days after fertilization (DAF) through the dorsal vascular bundle and the aleurone layer. During the last few years, spatio-temporal distribution of various minerals and the role of their transporters has been studied in great detail. However, with regard to vitamins and other bioactive compounds, such studies are still very limited. Distribution of minerals in the grain is mainly regulated by the distribution of their ligands and transporters, whereas the accumulation of various vitamins is mainly metabolic enzyme activity. Collective knowledge discussed here in this niche area would help to design new studies to improve the micronutrient content located in the inner part of the seed.