Mechanisms Of Selenium Uptake, Translocation And Speciation Transformation In Rice And Wheat

Selenium (Se) is an essential trace element for humans and animals. Globally, an estimate of0.5to1billion people may have an insufficient intake of Se. Plants, especially cereal food, are the main source of Se in most diets. Therefore, biofortification of food crops has been proposed as a safe and effective method of increasing human Se intake. Rice (Oryza sativa) and wheat (Triticum aestivum), the staple cereal food in many parts of the world, play important roles in satisfying the nutritional Se requirements for human. In the present study, rice and wheat were chosen as our test plant species. Solution culture experiments and compartmented soil-quartz sand culture systems were applied to investigate the effects of iron plaque on Se uptake by rice. Solution culture experiments and X-ray absorption near edge spectroscopy (XANES) techniques were used to investigate the effect of sulfate on Se uptake and assimilation and the effect of different nitrogen form on Se uptake in wheat. The main findings were as follows:(1) The results of selenite and selenate uptake kinetics showed that the present of iron plaque on rice root surfaces strongly promoted the influx of selenite, but suppressed the influx of selenate. Selenite influx into rice roots with iron plaque was1.00-2.00times greater than without iron plaque; but selenate influx into roots without iron plaque was2.00-4.00times higher than with iron plaque. Furthermore, our results showed that iron plaque has the different affinity for selenite and selenate, and iron plaque sequestered much more selenite than selenate.(2) After3d induction in pretreatment Fe solutions, visible reddish coating (iron plaque) on root rice root surfaces could be observed and Fe amounts in the iron plaque increased with Fe+concentrations in pretreatment solutions. In the+selenite treatment, Se concentrations in roots increased significantly with increasing Fe2+concentrations in the pretreatment solutions, and there was positive correlations between Se concentrations in roots and in iron plaque. However, Fe2+concentrations in the pretreatment solutions had no significant effect on root-Se concentrations in the+selenate treatment. Besides, Fe2+concentrations in the pretreatment solutions had also no significant effect on shoot-Se concentrations in both+selenite treatment and+selenate treatment.(3) Under flooding conditions, the concentrations of total soluble Se,selenite, and selenate in soil solutions varied dramatically during the entire period of rice growth. The addition of selenite or selenate, particularly for selenate, significantly increased the initial Se concentrations in soil solution. Selenite and selenate could be detected in the soil solutions, and selenate was the dominant species in the soil solutions and accounted for90-99%of the total soluble Se in the soil solutions in all samplings.(4) Under flooding conditions, visible reddish coating on root surfaces was observed. Fe amounts in iron plaque were different between rice cultivars due to differences in the oxidation capacities of roots. Iron plaque formed on the rice root surfaces greatly influenced Se uptake, and iron plaque might act as a pool to Se uptake during rice growth. In addition, selenate or selenite addition to flooded paddy soil showed the same results in the increasing Se concentrations in rice grain. (5) Under flooding conditions, sulphur starvation had a positive effect on selenate uptake but had no significant effect on selenite uptake.The results of selenium speciation showed that forms of organoselenium, including Selenocystine (SeCys2), Se-methyl-selenocysteine (MeSeCys) and (Selenomethionine-Se-oxide) SeOMet, were the dominant species in the selenite-treated plants and accounted for approximately90%of the total Se. Additionally, selenate was still the dominant species in the roots and shoots in the selenate-treated plants, with little selenate converted to selenite and MeSeCys. S starvation inhibited the assimilative processes of inorganic Se into organic Se in plants supplied with selenate, but did not promote those processes in plants supplied with selenite. In addition, sulphur starvation had a positive effect on selenate uptake but had no significant effect on selenite uptake. Various organic Se species differ in their mobility within plants. Selenite taken up by roots was readily converted to other forms and tended to accumulate in roots; selenate was readily transported from roots to shoots through xylem. SeOMet and SeCys2were detected in higher proportion only in root tissues of selenite-treated plants, while MeSeCys was detected in higher proportions in shoots and roots of selenate-treated plants than in selenite-treated plants, indicating that MeSeCys was readily translocated from roots to shoots.(6) Under solution culture, nitrogen forms significantly influenced pH values of nutrient solution. With the addition of NH4+-N in nutrient solution, pH values increased with culture time, while the pH value decreased with culture time in the NO3--N treatments. Nitrogen forms and pH regulation could influence Se uptake by wheat seedlings. In the+selenite treatments, NH4+-N application could be favourable for selenite taken up by wheat seedlings. While, in the+selenate treatments, NO3--N application could be favourable for selenate uptake. Besides, pH regulation of nutrient solution had a certain effect on selenite uptake, but no effect for selenate uptake.