Current understanding of the factors regulating methionine content in vegetative tissues of higher plants

Methionine is a nutritionally essential, sulfur-containing amino acid found in low levels in plants, which often limits its value as a source of dietary protein to humans and animals. Methionine is also a fundamental metabolite in plant cells since, through its first metabolite, S-adenosylmethionine (SAM), it controls the level of several key metabolites, such as ethylene, polyamines and biotin. SAM is also the primary methyl group donor that regulates different processes in plants. Despite its nutritional and regulatory significance, the factors regulating methionine content in plants are not fully known. In this review, we summarize the current knowledge and recent progress made in our understanding of the methionine metabolism. The enzymes and substrates that regulate methionine synthesis were described, as well as the influences of the catabolic pathways of methionine on its content. The current effort to tailor an improvement of methionine content in vegetative tissues with minimal interference in plant growth and productivity is described as well. The accumulated knowledge has provided new insights into the control of methionine level in plants and, in some cases, has resulted in significant improvements in the nutritional value of plants.