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Enhancement of cadmium accumulation in sweet sorghum as affected by nitrate.

The pollution of cadmium (Cd) in soils is increasingly concerned worldwide. Phytoextraction of Cd pollutants by high biomass plants such as sweet sorghum has been regarded as an environmentally-friendly, cost-effective and sustainable strategy for remediating this problem. Nitrogen (N) is a macronutrient which correlated with plant growth, development and stress resistance. Nevertheless, how nitrate, as an important form of N, affecting Cd uptake, translocation and accumulation in sweet sorghum is still unclear. In the present study, a series of nitrate levels (N1, 0.5 mM; N2, 2 mM; N3, 4 mM; N4, 8 mM and N5, 16 mM) with or without 5 μM CdCl2 treatment in sweet sorghum was investigated hydroponically. Results indicated that Cd accumulation in the aboveground of sweet sorghum was enhanced by optimum nitrate supply resulting from both increased dry weight and Cd concentrations. Although roots-to-shoots Cd translocation was not enhanced by the increase of nitrate, it was prone to transfer Cd from cell walls to vacuoles in leaves to some extent. Intriguingly, expression levels of Cd uptake and transport genes, SbNramp1, SbNramp5 and SbHMA3 were not closely related to increased Cd as affected by nitrate supply. The expression of SbNRT1.1B regarding nitrate transport showed inverted "U" shape as the increase of nitrate levels under Cd stress which was in agreement with the trends of Cd concentrations changes in the aboveground. Based on aforementioned results, nitrate might regulate Cd uptake and accumulation through the expression of SbNRT1.1B rather than SbNramp1, SbNramp5 and SbHMA3, the well-documented genes related to Cd uptake and transport in sweet sorghum.