Silicon (Si) can alleviate cadmium (Cd) toxicity in many plants, but mechanisms underlying this beneficial effect are still lacking. In this study, the roles of Si in time-dependent apoplastic and symplastic Cd absorption by roots of wheat plants were investigated. Results showed that, during short-term Cd exposure, the symplastic pathway of Cd in roots was not significantly affected by Si. Cell wall properties and cell wall-bound Cd regarding the apoplastic pathway were unaffected by Si either. Nevertheless, Cd concentrations in the apoplastic fluid of roots were decreased by Si. The reason could be that Si delayed endodermal suberization of roots resulting in promoted apoplastic Cd translocation to shoots, thus decreasing Cd in the apoplastic fluid of roots after short-term Cd stress. By contrast, after long-term Cd stress, cell wall properties and the expression of genes related to Cd influx and transport were unaffected. Intriguingly, Si up-regulated the expression of the Cd efflux-related gene TaTM20 and repressed apoplastic Cd translocation to shoots, which might contribute to decrease of Cd after long-term Cd exposure. Taken together, these results indicate that Si-dependent decrease in root Cd concentrations during short-term Cd exposure helps plants to mitigate Cd toxicity in the long-term.
Keywords: Apoplast; Cadmium; Short-term; Silicon; Suberization.
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