Molecular evolution and expression divergence of three key Met biosynthetic genes in plants: , and .

Methionine (Met) is an essential sulfur-containing amino acid in animals. Cereal and legume crops with limiting levels of Met represent the major food and feed sources for animals. In plants, cystathionine gamma-synthase (CGS), methionine methyltransferase (MMT) and homocysteine methyltransferase (HMT) are committing enzymes synergistically synthesizing Met through the aspartate (Asp) family pathway and the S-methylmethionine (SMM) cycle. The biological functions of , and genes have been respectively studied, whereas their evolution patterns and their contribution to the evolution of Met biosynthetic pathway in plants are unknown. In the present study, to reveal their evolution patterns and contribution, the evolutionary relationship of , and gene families were reconstructed. The results showed that s began in the ancestor of the land plants and kept conserved during evolution, while the s and s had diverged. The genes were divided into two branches in the angiosperms, Class 1 and Class 2, of which Class 2 only contained the grasses. However, the genes diverged into Class 1 and Class 2 in all of the seed plants. Further, the gene structure analysis revealed that the s, s and s were relatively conserved except for the s in Class 2. According to the expression of , and genes in soybeans, as well as in the database of soybean, rice and , the expression patterns of the s were shown to be consistently higher in leaves than in seeds. However, the expression of s and s had diverged, either expressed higher in leaves or seeds, or showing fluctuated expression. Additionally, the functions of genes had diverged into the repair of -adenosylmethionine and SMM catabolism during the evolution. The results indicated that the and genes have experienced partial subfunctionalization. Finally, given the evolution and expression of the , and gene families, we built the evolutionary model of the Met biosynthetic pathways in plants. The model proposed that the Asp family pathway existed in all the plant lineages, while the SMM cycle began in the ancestor of land plants and then began to diverge in the ancestor of seed plants. The model suggested that the evolution of Met biosynthetic pathway is basically consistent with that of plants, which might be vital to the growth and development of different botanical lineages during evolution.