Impact of Sulfate Salinity on the Uptake and Metabolism of Sulfur in Chinese Cabbage

Increasing soil salinity is a major threat to crop production in many agricultural areas throughout the world. Although sodium chloride (NaCl) is one of the most abundant salts in soils, others viz. sulfate salts may also be present in high concentrations in some soil types. Sulfate salts, e.g. Na2SO4, are still widely under-represented amongst salt stress studies and the mechanism of its toxicity is poorly understood. Exposure of Chinese cabbage to Na2SO4 already reduced growth at levels ≥20 mM, accompanied by an increase in the total sulfur content of both roots and shoots, which in the shoot for a greater part could be ascribed to an accumulation of sulfate. Moreover, there was an increase in the total water-soluble non-protein thiol content (glutathione) in roots and shoots. Enhanced sulfur metabolite levels (sulfate, glutathione) would down-regulate the expression and activity of the sulfate transporters and APS reductase (glutathione). Indeed, Na2SO4 exposure resulted in a down-regulation of the sulfate uptake capacity of the roots at ≥5 mM, whereas the transcript level of the sulfate transporters Sultr1;2 and Sultr4;1 and APS reductase in the roots was reduced at ≥20 mM. Apparently in the shoot this regulatory signal transduction pathway was overruled by the toxic effects of Na2SO4, since in contrast to the roots, the transcript levels of Sultr4;1 and APS reductase were enhanced in the shoot at ≥30 mM and ≥5 mM Na2SO4, respectively.