Alleviation of cadmium toxicity by silicon or sulfate supply in plants
Plants growing in Cd-polluted soils can inevitably take up and accumulate Cd, which will affect plant growth and even threat human health after consuming Cd-containing edible organs of plants. In this thesis, effects of Si and sulfate on Cd toxicity were studied. Although Si can decrease Cd accumulation in plants, the underlying mechanisms are still poorly understood. Hydroponic experiments under short- and long-term Cd treatment in the presence and absence of Si showed that exogenous Si supply decreased Cd in wheat plants. After short-term exposure, Si considerably decreased Cd in the apoplastic fluid of roots. Si neither affected gene expression related to Cd uptake and transport nor cell wall properties, whereas Si delayed suberin deposition in roots. We reason that delayed suberization by Si enlarged longitudinal apoplastic space, thereby decreasing Cd concentrations in apoplastic fluid as a `dilution` effect. After long-term Cd exposure, cell wall properties and the expression of genes related to Cd influx and transport were unaffected. Intriguingly, Si up-regulated Cd efflux-related gene expression and enhanced root oxalate exudation, which might contribute to decrease Cd after long-term Cd exposure. Taken together, our 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. Products of sulfate assimilation, glutathion (GSH) and phytochelatin (PC) are chelators of Cd and can detoxify Cd toxicity. To date, it is still unclear whether excess sulfate supply could alleviate Cd toxicity. We found that sulfate supply reversed detrimental effects of Cd on biomass and oxidative stress, but also increased Cd concentrations in leaves, suggesting that sulfate enhances Cd tolerance in faba bean. We reason that sulfate accelerated Cd accumulation in cell walls of leaves is related to enhanced Cd tolerance.
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