12 - Sulfur Amino Acids in Plants

Abstract

This chapter discusses sulfur amino acids in plants. Most of the inorganic sulfate assimilated and reduced by plants appears ultimately in cysteine and methionine. These amino acids contain about 90% of the total sulfur in most plants. Nearly all of the cysteine and methionine is in protein. The chapter highlights the typical dominance of protein cysteine and protein methionine in the total organic sulfur. In the nonprotein fraction, reduced glutathione (GSH) is ubiquitous and is commonly a major constituent. The soluble fraction of plants also includes a variety of other sulfur-containing compounds that are normally present in relatively small amounts: (1) intermediates on the route to protein cysteine and protein methionine, such as cysteine, cystathionine, homocysteine, and methionine and (2) compounds involved in methyl transfer reactions and polyamine synthesis such as AdoMet, AdoHcy, and 5'-methyl-thioadenosine. Cysteine is the precursor of protein cysteine, GSH, and the sulfur moiety of methionine and is, therefore, the major portal for organic reduced sulfur. Serine is the source of the C3 moiety of cysteine in plants.