Regulation of the hvst1 gene encoding a high-affinity sulfate transporter from Hordeum vulgare

A cDNA, hvst1, was isolated from Hordeum vulgare by heterologous complementation in Escherichia coli. This cDNA encodes a high-affinity sulfate transporter that is 2442 bp in length and consists of 660 amino acids. Under steady-state conditions of sulfate supply during culture, sulfate influx (measured at 100 microM external sulfate concentration) and hvst1 transcript level were inversely correlated with sulfate concentrations in the culture solution. Glutathione (GSH) concentrations increased as external sulfate was increased from 2.5 to 250 microM. A time-course study, designed to investigate effects of sulfate withdrawal on the abundance of hvst1 transcript, showed a 5-fold increase of the latter within the first two hours. This was followed by a further slight increase during the next 46 h. These changes were accompanied by a parallel increase in sulfate influx and a decrease of root GSH concentrations. When plants that had been deprived of sulfate for 24 h were exposed to L-cysteine (Cys) or GSH for 3 h, GSH was the more effective down-regulator, reducing hvst1 transcript level to below that of unstarved controls. The decrease in transcript abundance induced by sulfate or Cys was partially relieved by the addition of buthionine sulfoximine (BSO), an inhibitor of GSH synthesis. Both hvst1 transcripts and sulfate influx increased as a function of N supply to N-starved plants. Amino oxyacetate acid (AOA), an aminotransferase inhibitor, when supplied with NO3-, increased transcript abundance of hvst1, while tungstate, methionine sulfoximine (MSO) and azaserine (AZA), inhibitors of nitrate reductase, glutamine synthetase and glutamate synthase (GOGAT), respectively, were without effect. AOA decreased root concentrations of aspartate (Asp), Cys and GSH; in contrast, glutamate (Glu) concentrations remained unchanged.