Submitted to: Weed Biology and Management
Publication Type: Review article
Publication Acceptance Date: 4/3/2003
Publication Date: 7/30/2003
Citation: Hirase, K., Molin, W.T. 2003. Sulfur assimilation in plants and weed control. Weed Biology and Management. 3:147-157. Interpretive Summary:
Technical Abstract: Sulfur is an indispensable element for plants. It is found in sulfur-containing amino acids, cysteine and methionine, and in various other important biochemical components and processes. Inhibitors of sulfur assimilation, or cysteine and methionine synthesis, could be potential herbicides. The sulfur assimilation pathway in plants is described here followed by the introduction of several compounds (inhibitors and safeners) acting on this pathway. Uptake of inorganic sulfate through the root is the first step of sulfur assimilation in plants. Sulfate is reduced mainly in chloroplasts to sulfide by a multistep process, and sulfide is then incorporated into cysteine. Cysteine is then converted to cystathionine, homocysteine and methionine. Cysteine is incorporated into GSH by gamma-glutamylcysteine synthetase and GSH synthetase. Three enzymes involved in cysteine and methionine biosynthesis, cysteine synthase, cystathionine gamma-synthase and cystathionine beta-lyase, have been investigated as target sites of herbicides. Several inhibitors of these enzymes (e.g. rhizobitoxine and propargylglycine) were also phytotoxic, suggesting that the synthetic pathway of sulfur-containing amino acids could be a new target site of herbicides. Some safeners for herbicides were found to act on the sulfur assimilation pathway and on glutathione (GSH) synthesis to increase GSH, which can be involved in herbicide metabolism and detoxification. Several safeners increase GSH levels by increasing the activity of enzymes involved in sulfur assimilation and GSH synthesis. For example, dichlormid increased the activities of ATP sulfurylase, adenosine 5'-phosphosulfate reductase and GSH synthetase. The GSH conjugate of flurazole and fenchlorazole-ethyl may bind to gamma-glutamylcysteine synthetase, thereby circumventing the feed back regulation of the GSH biosynthesis pathway. Further studies on plant sulfur metabolism may lead to the discovery of new herbicides and to the comprehensive understanding of mode of action of safeners.