Copper exposure interferes with the regulation of the uptake, distribution and metabolism of sulfate in Chinese cabbage

Exposure of Chinese cabbage (Brassica pekinensis) to enhanced Cu2+ concentrations (1–10 μM) resulted in leaf chlorosis, a loss of photosynthetic capacity and lower biomass production at ≥5 μM. The decrease in pigment content was likely not the consequence of degradation, but due to hindered chloroplast development upon Cu exposure. The Cu content of the root increased with the Cu2+ concentration (up to 40-fold), though only a minor proportion (4%) was transferred to the shoot. The nitrate uptake by the root was substantially reduced at ≥5 μM Cu2+. The nitrogen content of the root was affected little at lower Cu2+ levels, whereas that in the shoot was decreased at ≥5 μM Cu2+. Cu affected the uptake, distribution and metabolism of sulfate in Chinese cabbage. The total sulfur content of the shoot was increased at ≥2 μM Cu2+, which could be attributed mainly to an increase in sulfate content. Moreover, there was a strong increase in water-soluble non-protein thiol content in the root and, to a lesser extent, in the shoot at ≥1 μM, which could only partially be ascribed to a Cu-induced enhancement of the phytochelatin content. The nitrate uptake by the root was substantially reduced at ≥5 μM Cu2+, coinciding with a decrease in biomass production. However, the activity of the sulfate transporters in the root was slightly enhanced at 2 and 5 μM Cu2+, accompanied by enhanced expression of the Group 1 high affinity transporter Sultr1;2, and the Group 4 transporters Sultr4;1 and Sultr4;2. In the shoot, there was an induction of expression of Sultr4;2 at 5 and 10 μM Cu2+. The expression of APS reductase was affected little in the root and shoot up to 10 μM Cu2+. The upregulation of the sulfate transporters may be due not only to greater sulfur demand at higher Cu levels, but also the consequence of interference by Cu with the signal transduction pathway regulating the expression and activity of the sulfate transporters.