Mechanisms of Arsenic Uptake, Transport, and in planta Metabolism in Rice

Abstract

Arsenic (As) in food is a threat for human health, and among all cereal rice is the most important source of the metalloid through diet. The dynamic of the element and the natural ability of rice to uptake, transport, and accumulate the metalloid at grains have motivated important research to be carried out in this regard. Thus, the aim of this chapter is to draw a synthesis on the main factors which ultimately determines As content in rice, from its uptake from the soil solution to its transport, metabolism, and final accumulation in grain. The element is of natural occurrence, varying in concentration, and its chemical species are modified due to a range of factors, such as the soil redox state. Among all As species, arsenite [As (III)] is the most common in anoxic conditions, such as the ones found in flooded paddy rice fields, which form is preferred for rice root uptake. Rice is naturally efficient in As(III) uptake, compared to most crops already studied, which is mainly due to its improved ability to uptake and transport silicon, especially with the aid of aquaporins, as silicic acid and arsenite are chemically analogous. Similarly, the second most important As form, arsenate [As(V)], is mainly uptaken and transported through the phosphate way. After being uptaken, As is transported either via xylem or phloem and is loaded to the grain. Within rice tissues, the element can be metabolized, being reduced, biomethylated, complexed to other elements, or even sequestered into vacuoles. The knowledge regarding the mechanisms of As uptake, transport, and metabolization in rice allows one to draw strategies in order to mitigate the content of this element in the grains, either via management practices or also via breeding and biotechnological approaches.