Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard

Se can be accumulated by plants and volatilized to dimethylselenide, providing an attractive technology for Se phytoremediation. To determine the rate-limiting steps in Se volatilization from selenate and selenite, time- and concentration-dependent kinetics of Se accumulation and volatilization were studied in Indian mustard (Brassica juncea). Time-dependent kinetic studies showed that selenate was taken up 2-fold faster than selenite. Selenate was rapidly translocated to the shoot, away from the root, the site of volatilization, whereas only approximately 10% of the selenite was translocated. For both selenate- and selenite-supplied plants, Se accumulation and volatilization increased linearly with external Se concentration up to 20 μm; volatilization rates were also linearly correlated with root Se concentrations. Se-volatilization rates were 2- to 3-fold higher from plants supplied with selenite compared with selenate. Se speciation by x-ray absorption spectroscopy revealed that selenite-supplied plants accumulated organic Se, most likely selenomethionine, whereas selenate-supplied plants accumulated selenate. Our data suggest that Se volatilization from selenate is limited by the rate of selenate reduction, as well as by the availability of Se in roots, as influenced by uptake and translocation. Se volatilization from selenite may be limited by selenite uptake and by the conversion of selenomethionine to dimethylselenide.

硒（Se）可被植物富集并挥发为二甲基硒化物（dimethylselenide），为硒植物修复技术提供了极具应用价值的解决方案。为明确硒酸盐（selenate）与亚硒酸盐（selenite）介导的硒挥发过程的限速步骤，本研究以印度芥菜（Brassica juncea）为实验材料，探究了硒富集与挥发过程的时间依赖型及浓度依赖型动力学特征。时间依赖型动力学研究结果显示，植物对硒酸盐的吸收速率为亚硒酸盐的2倍。硒酸盐可快速向地上部转运（远离挥发过程发生的根系位点），而仅有约10%的亚硒酸盐发生了转运。在施加硒酸盐或亚硒酸盐的植株中，当外源硒浓度不超过20 μM时，硒的富集量与挥发速率均随外界硒浓度呈线性增长；且挥发速率与根系硒浓度亦呈线性相关。施加亚硒酸盐的植株其硒挥发速率较施加硒酸盐的植株高2~3倍。通过X射线吸收光谱（X-ray absorption spectroscopy）对硒形态进行分析发现，施加亚硒酸盐的植株体内富集的硒为有机硒，最可能为硒代蛋氨酸（selenomethionine）；而施加硒酸盐的植株体内仅富集硒酸盐。本研究数据表明，硒酸盐介导的硒挥发过程受硒酸盐还原速率以及根系硒可利用性（受吸收与转运过程调控）的共同限制；而亚硒酸盐介导的硒挥发过程可能受亚硒酸盐吸收速率以及硒代蛋氨酸向二甲基硒化物的转化过程限制。