Sulfate uptake in plants is highly dependent upon temperature, and the size of the endogenous sulfate pool. Sulfate uptake is inhibited by respiratory inhibitors, uncoupling agents and ATPase inhibitors (Anderson, 1980). There appear to be two uptake systems; high-affinity and low-affinity (as for nitrate). As recently reviewed by Chrispeels et al (1999), a class of sulfate transporters have been identified in plant roots which are part of the starvation-induced, high-affinity uptake system (Smith et al, 1995b; 1997). A second class of sulfate transporter genes appear to encode low-affinity transporters that may load sulfate into the vascular tissue in roots and unload it into leaf cells, or which may transport sulfate between cellular or subcellular compartments (Chrispeels et al, 1999; Takahashi et al, 1996; 1997; 2000; Smith et al, 1995a).

A cDNA encoding a high-affinity sulfate transporter has been isolated from barley by complementation of a yeast mutant deficient in the gene encoding for the yeast SUL1 sulfate transporter (Smith et al, 1997). The cDNA (HVST1), encodes a polypeptide of 660 amino acids (Mol weight = 72,550) that is predicted to have 12 membrane-spanning domains and has extensive sequence homology with other eukaryotic sulfate transporters (Smith et al, 1997). The K_m for sulfate was 6.9 microM when the HVST1 cDNA was expressed in the yeast mutant (Smith et al, 1997). The strong pH-dependency of sulfate uptake suggests that the HVST1 polypeptide is a proton/sulfate co-transporter (Smith et al, 1997). The gene encoding HVST1 is expressed specifically in root tissues and the abundance of the mRNA is strongly derepressed by sulfur nutrition. Upon re-supply of sulfate, the abundance of the mRNA corresponding to HVST1, and the capacity of the roots to take up sulfate, decreases rapidly, concomitantly with increases in tissue sulfate, cysteine and glutathione contents (Smith et al, 1997). Addition of the cysteine precursor, O-acetylserine, to plants grown with adequate sulfur supply, leads to increases in sulfate transporter mRNA, sulfate uptake rates and tissue contents of glutathione and cysteine. It is suggested that whilst sulfate, cysteine and glutathione may be candidates for negative metabolic regulators of sulfate transporter gene expression, this regulation may be overridden by O-acetylserine acting as a positive regulator (Smith et al, 1997).

The Arabidopsis thaliana gene, AST68 is also able to functionally complement a Saccharomyces cerevisiae mutant lacking a sulfate transporter gene (Takahashi et al, 1997). AST68 gene encodes a 677-amino acid polypeptide (74.1 kDa). It is a single-copy gene that maps to the top arm of chromosome 5 (Takahashi et al, 1997). The steady-state mRNA abundance of AST68 increases specifically in roots up to 9-fold in response to sulfate starvation. AST68 transcripts are accumulated in the central cylinder of sulfate-starved roots, but not in the xylem, endodermis, cortex, and epidermis (Takahashi et al, 1997). The sulfate transporter (AST68) is de-repressed along with APS reductase (APR1), and serine acetyltransferase (SAT1) by sulfate starvation in A. thaliana (Takahashi et al, 1997). However, the sulfate transporter (AST68) exhibits the most intensive and specific response in roots, indicating that AST68 plays a central role in the regulation of sulfate assimilation in plants (Takahashi et al, 1997).

Sulfate uptake is not inhibited by large molar excesses of nitrate, chlorate or phosphate. However, it is inhibited by various group VI anions structurally related to sulfate. Sulfate in turn inhibits uptake of chromate and to a lesser extent selenate. Group VI anions may compete with sulfate for uptake by the sulfate uptake mechanism (Anderson, 1980).

Anderson JW 1980 Assimilation of inorganic sulfate into cysteine. In (BJ Miflin ed) "The Biochemistry of Plants", Vol 5, Academic Press, New York, pp. 203-223.

Chrispeels MJ, Crawford NM, Schroeder JI 1999 Proteins for transport of water and mineral nutrients across the membranes of plant cells. Plant Cell 11: 661-675.

Smith FW, Ealing PM, Hawkesford MJ, Clarkson DT 1995a Plant members of a family of sulfate transporter reveal functional subtypes. Proc. Natl. Acad. Sci. U.S.A. 92: 9373-9377.

Smith FW, Hawkesford MJ, Prosser IM, Clarkson DT 1995b Isolation of a cDNA from Saccharomyces cerevisiae that encodes a high-affinity sulphate transporter at the plasma membrane. Mol. Gen. Genet. 247: 709-714.

Smith FW, Hawkesford MJ, Ealing PM, Clarkson DT, VandenBerg PJ, Belcher AR, Warrilow AGS 1997 Regulation of expression of a cDNA from barley roots encoding a high-affinity sulphate transporter. Plant J. 11: 83-92.

Takahashi H, Sasakura N, Noji M, Saito K 1996 Isolation and characterization of a cDNA encoding a sulfate transporter from Arabidopsis thaliana. FEBS Lett. 392: 95-99.

Takahashi H, Watanabe-Takahashi A, Smith FW, Blake-Kalff M, Hawkesford MJ, Saito K 2000 The roles of three functional sulphate transporters involved in uptake and translocation of sulphate in Arabidopsis thaliana. Plant J. 23: 171-182.

Takahashi H, Yamazaki M, Sasakura N, Watanabe A, Leustek T, Engler JA, Engler G, Van Montagu M, Saito K 1997 Regulation of sulfur assimilation in higher plants: a sulfate transporter induced in sulfate-starved roots plays a central role in Arabidopsis thaliana. Proc. Natl. Acad. Sci. U.S.A. 94: 11102-11107.