Diverse Regulatory Mechanisms of Amino acid Biosynthesis in Plants

Amino acids play a variety of essential roles in higher plants in addition to protein synthesis. Amide amino acids are directly involved in nitrogen assimilation, transport and storage of reduced nitrogen (1, 2). Amino acids or their biosynthetic intermediates also serve as precursors for plant hormones and defense products. For example, methionine is involved in the synthesis of ethylene and auxin is produced from tryptophan or an indolic precursor (3). Phenylalanine is the starting point for the synthesis of hundreds of structurally diverse, and often abundant compounds, including antimicrobial phytoalexins, lignin and UV-protective flavonoids (4–6). Phytoalexins and anti-herbivory compounds are also synthesized from the tryptophan pathway, as are indolic alkaloids, many of which are important pharmaceutical agents (7). The polyamines spermidine and spermine are produced from methionine and the amino acid proline is proposed to confer tolerance to osmotic stress arising through either salinity or drought (8, 9).