Research Progress on Reducing Cadmium and Arsenic Accumulation in Rice Grains

Heavy metals pollution in farmland soils induced by mining, waste discharge and sewage irrigation can be concentrated and transported via the food chain, thereby posing great risks to human health. Rice is an important staple food, which however, is readily to uptake and accumulate cadmium (Cd) and arsenic (As) from contaminated soils, rendering it to be a main dietary source of As and Cd of humans. Therefore, exploring strategies to reduce Cd and As content in rice grains has practical importance to ensure food safety and human health. Ways and strategies to reduce Cd and As in rice grains are reviewed, including: (1) Passivate Cd and As by adding biochar, iron salts and fertilizer in paddy soils, decrease their bioavailability and thus the uptake by rice; (2) Eliminate membrane lipid peroxidation stress and promote the formation of non-protein thiol in rice via external application of sulfur (S), fix Cd in vacuoles of rice tissues and reduce its transport to grains, thus reduce Cd accumulation in grains; (3) External application of silicon (Si) to compete with arsenite (As³⁺) for rice uptake of As; (4) Manipulate redox state in rhizosphere by changing water content in rice field, thereby affect the uptake and accumulation of Cd and As in rice. Root oxygen secretion can promote the formation of iron film on rice root surface, facilitate As oxidation into arsenate (As⁵⁺) by changing soil redox state, thereby enhance the adsorption and fixation of As⁵⁺ on root surface to reduce its uptake by rice; (5) Regulate the expression of transporters, i. e., knock out Mn transporter OsNRAMP5 or stem/leaf cytoplasmic transporter OsLCT1, silence Si efflux transporter Lsi2, and overexpress OsHMA3n, OsHMA2, and ScAcr3p transporters can reduce Cd and As accumulation in rice grains by promoting root As³⁺ efflux and reducing Cd/As³⁺ translocation to xylem and grain; (6) Screen and cultivate low Cd/As accumulation species. Through the combination of agronomic measures, genetic engineering, and germplasm resource screening, rice species with low Cd/As accumulation and high yield can be selected. Besides, reasonable planning of planting patterns can help to decrease Cd/As uptake and accumulation in rice. The information provide supports for safe production of rice in slight-moderate contaminated soils.