Abstract
Rice (Oryza sativa L.) is one of the most important food crops, feeding half of the world’s population. However, rice production is affected by cadmium (Cd) toxicity. Due to an increase in Cd-contaminated soil and rice grains, and the serious harm to human health from Cd, research on Cd uptake, transport and resistance in rice has been widely conducted, and many important advances have been made. Rice plants absorb Cd mainly from soil through roots, which is mediated by Cd absorption-related transporters, including OsNramp5, OsNramp1, OsCd1, OsZIP3, OsHIR1,OsIRT1 and OsIRT2. Cd uptake is affected by soil’s environmental factors, such as the concentrations of Cd and some other ions in soil, soil properties, and other factors can affect the bioavailability of Cd in soil. Then, Cd is transported within rice plants mediated by OsZIP6, OsZIP7, OsLCD, OsHMA2, CAL1, OsCCX2, OsLCT1 and OsMTP1, from roots to shoots and from shoots to grains. To resist Cd toxicity, rice has evolved many resistance strategies, including the deposition of Cd in cell walls, vacuolar Cd sequestration, Cd chelation, antioxidation and Cd efflux. In addition, some unresolved scientific questions surrounding Cd uptake, transport and resistance in rice are proposed for further study.
Highlights
In recent years, the Cd content of farmland soils has continuously increased due to increasing discharges of industrial wastewater, waste gas and other residues, excessive use of metal-containing pesticides and fertilizers
Previous study demonstrated that when double-strand RNA interference plants of OsMTP1 were under Cd stress, the Cd content in the root was higher than that in the wild type, while Cd content in the shoot was lower
In the diffuse vascular bundle of the stem node, OsHMA2 can load Cd from the phloem parenchyma cell to the phloem sieve [28]. These results indicate that OsLCT1, OsZIP7 and OsHMA2 may act synergistically to promote the transfer of Cd from enlarged to diffuse vascular bundles in stem nodes
Summary
The Cd content of farmland soils has continuously increased due to increasing discharges of industrial wastewater, waste gas and other residues, excessive use of metal-containing pesticides and fertilizers. The concentration of Cd in rice grains grown on farmland with heavy Cd contamination has exceeded the global safety standard level (≤0.4 mg/kg), rendering the grains inedible [3]. It is important to investigate the mechanisms of Cd uptake, transport and resistance in rice. Scientists have made many important advances in understanding Cd uptake, transport and resistance in rice. We explore unresolved scientific questions regarding Cd uptake, transport and resistance in rice that demand further study in the future. The research progress in this field can be used to reduce the Cd-caused yield loss by enhancing rice resistance to Cd toxicity, produce safe rice grains by reducing Cd accumulation in rice grains and repair the Cd-contaminated paddy fields by cultivating and planting rice varieties with strong Cd enrichment
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