As noted earlier, it is suggested that whilst sulfate, cysteine and glutathione may be candidates for negative metabolic regulators of high-affinity sulfate transporter gene expression, this regulation may be overridden by O-acetylserine acting as a positive regulator (Smith et al, 1997).

The sulfate transporter (AST68) is de-repressed along with APS reductase (APR1), and serine acetyltransferase (SAT1) by sulfate starvation in Arabidopsis thaliana (Takahashi et al, 1997). In sulfur-starved cells of spinach only small increases in mRNA levels of CysA (cytoplasmic) and CysB (chloroplastic/plastidic) isoforms of cysteine synthase (O-acetylserine sulfhydrylase; OASS [EC 4.2.99.8]) were observed (Takahashi and Saito, 1996). However, under nitrogen and nitrogen/sulfur double-deficient stress conditions (but not under sulfur starvation alone), mRNA levels of CysC (mitochondrial) increased 500% within 72 h (Takahashi and Saito, 1996).

ATP sulfurylase activity and sulfate uptake are induced in roots in response to S starvation. This is blocked by providing cysteine or glutathione. When buthionine sulfoximine (an inhibitor of GSH synthesis) is supplied, this reverses the inhibitory effect of cysteine on ATP sulfurylase, suggesting that GSH is responsible (Lappartient and Touraine, 1996).

ATP sulfurylase mRNA of Chlamydomonas reinhardtii increases in abundance in response to sulfur starvation. Sulfur-stress induced accumulation of ATS1 transcript is reduced in a strain defective in SAC1, a gene that is critical for acclimation to sulfur-limited growth (Yildiz et al, 1996). As noted earlier, ATP sulfurylase is derepressed by S starvation in tobacco cells overexpressing the Arabidopsis ATP sulfurylase encoded by APS2 (Hatzfeld et al, 1998).

In maize, ATP sulfurylase and APS sulfotransferase are located exclusively in the bundle-sheath cells, whereas gluthathione synthetase is located exclusively in the mesophyll cells (Burgener et al, 1998). Feeding experiments with ³⁵S-sulfate indicate that reduction of sulfate to cyst(e)ine occurs exclusively in the bundle-sheath, and that cyst(e)ine is then transported from the bundle-sheath to mesophyll for glutathione biosynthesis. Mesophyll cells are the main site of assimilation of nitrate, as nitrate reductase is a marker for mesophyll cells (Burgener et al, 1998).

Burgener M, Suter M, Jones S, Brunold C 1998 Cyst(e)ine is the transport metabolite of assimilated sulfur from bundle-sheath to mesophyll cells in maize leaves. Plant Physiol. 116: 1315-1322.

Hatzfeld Y, Cathala N, Grignon C, Davidian JC 1998 Effect of ATP sulfurylase overexpression in bight yellow 2 tobacco cells. Regulation of ATP sulfurylase and SO₄^2- transport activities. Plant Physiol. 116: 1307-1313.

Lappartient AG, Touraine B 1996 Demand-driven control of root ATP sulfurylase activity and SO₄^2- uptake in intact Canola: The role of phloem-translocated glutathione. Plant Physiol. 111: 147-157.

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, Saito K 1996 Subcellular localization of spinach cysteine synthase isoforms and regulation of their gene expression by nitrogen and sulfur. Plant Physiol. 112: 273-280.

Yildiz FH, Davies JP, Grossman A 1996 Sulfur availability and the SAC1 gene control of adenosine triphosphate sulfurylase gene expression in Chlamydomonas reinhardtii. Plant Physiol. 112: 669-675.