The Sulfate Assimilation Pathway in Higher Plants: Recent Progress Regarding Multiple Isoforms and Regulatory Mechanisms

The sulfur atom is an essential nutrient for living organisms because it plays a central role in protein folding, enzyme catalysis and maintenance of the redox status of cells. Microorganisms and plants can synthesize organic sulfur compounds, including cysteine, methionine and glutathione, from inorganic sulfur compounds such as sulfates. In contrast, animals utilize organic sulfur compounds that are mainly synthesized by plants. In the last decade, many genes whose products are involved in sulfate assimilation have been isolated from higher plants, and it has been revealed that there exist multiple isoforms for each step. The different properties of isoforms has been examined for sulfate transporters, which serve at the first step of sulfate assimilation, and for O-acetylserine(thiol)lyases, which catalyze the incorporation of sulfide into cysteine. Currently, however, it is not clear why plants developed multiple forms with similar catalytic properties. Mechanisms of regulation of the sulfate assimilation pathway have also attracted much attention, since metabolites such as cysteine and glutathione affect related enzymatic activity or corresponding gene expression. The precursor of cysteine, O-acetylserine, is a key compound in maintaining the balance between sulfate and nitrate assimilation in higher plants. In addition to nutrition, sulfate assimilation is important for conferring tolerance against environmental stresses including heavy metals. Genetic engineering of components of the sulfate assimilation pathway is a useful approach to generate useful plants for agriculture and phytoremediation.