ABSTRACT
Cysteine (Cys) and methionine (Met) are two sulfur-containing amino acids found in proteins. In the biosynthetic pathway in plants, the sulfur atom, one of the essential elements required for growth of plants, is first incorporated into cysteine then cysteine is converted into methionine (see Chap. 6). Cysteine biosynthesis in plants plays a key role in the sulfur cycle in nature. The inorganic sulfur in the environment (e.g., sulfate ion in the soil and sulfur dioxide gas in the air) is fixed into cysteine mainly by the cysteine biosynthetic pathway in plants. Animals, however, do not have the assimilatory mechanism of inorganic sulfur; they require methionine as an essential amino acid for the source of sulfur nutrient. Cysteine is incorporated into proteins and glutathione (GSH) or serves as the sulfur donor of methionine and sulfur-containing secondary products in plants, for instance, allyl cysteine sulfoxides in Allium and glucosinolates in Cruciferae.
