Regulation of Expression of Sulfate Transporters and APS Reductase in Leaf Tissue of Chinese Cabbage (Brassica pekinensis)

Abstract

Leaf discs from sulfur-sufficient and sulfate-deprived Chinese cabbage plants were incubated at various levels of sulfate (ranging from 0 to 40 mM) for 24 h in the light. Sultr4;1 was the sole constitutively expressed sulfate transporter present in sulfur-sufficient leaf discs and its expression was enhanced upon incubation at ≤8 mM and decreased at higher sulfate concentrations. Leaf discs from sulfate-deprived plants were characterized by a high expression of Sultr1;2, Sultr4;1, Sultr4;2 and APS reductase. The high expression was down-regulated upon incubation of the leaf discs at >2 mM sulfate, whereas at 20–30 mM their expression was quite similar to that of sulfur-sufficient leaf discs. In both sulfur-sufficient and sulfur-deprived leaf discs there was an accumulation of sulfate upon incubation nearly linear with the sulfate concentration. The thiol content of leaf discs was slightly enhanced upon incubation at high sulfate concentrations. Evidently, the expression of the sulfate transporters and APS reductase were strictly controlled by the in situ sulfate concentration in the leaf tissue possibly via an interaction with the sulfate reduction in the chloroplasts. It is suggested that H2S might function as endogenous gaseous transmitter in cross-talk between the sulfate reduction pathway in the chloroplasts and the transcription of sulfate transporters/sulfate reducing enzymes in the nucleus.