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Nramp5 expression and functionality likely explain higher cadmium uptake in rice than in wheat and maize

Sui, Fu-Qing, Chang, Jia-Dong, Tang, Zhong, Liu, Wen-Ju, Huang, Xin-Yuan, Zhao, Fang-Jie
Plant and soil 2018 v.433 no.1-2 pp. 377-389
cadmium, corn, crops, cultivars, genes, hydroponics, plasma membrane, proteins, rice, roots, wheat, yeasts
BACKGROUND AND AIMS: Cereals are the main dietary source of cadmium (Cd). Rice grains often contain higher levels of Cd than other cereals, but the reasons are unknown. The aims of this study were to compare Cd uptake, translocation and influx kinetics between rice, wheat and maize and to investigate whether the expression and functionality of Nramp5 genes differ between the three crop species. METHODS: Two cultivars each of rice, wheat and maize were grown hydroponically and exposed to a range of Cd concentrations. Nramp5 genes were cloned from the three plants and their expression levels determined. The Cd transport activities of Nramp5 proteins were tested in yeast. RESULTS: Under hydroponic conditions, Cd uptake in rice was 2.5–8.1 and 1.1–3.6 times that of wheat and maize, respectively. The maximum Cd influx velocity of rice was 6.5 and 2.2 times that in wheat and maize, respectively. Wheat showed the lowest Cd uptake but the highest Cd root-to-shoot translocation. The absolute expression level of OsNramp5 in rice roots was 4–5 times that of TaNramp5A and TaNramp5D in wheat or ZmNramp5 in maize. All Nramp5 proteins were localized to the plasma membrane. When expressed in yeast, OsNramp5 showed a greater Cd transport activity than wheat or maize Nramp5. CONCLUSIONS: Rice has a greater Cd uptake ability than wheat or maize, likely because OsNramp5 is more highly expressed and the protein has a higher Cd transport activity than wheat or maize Nramp5.