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Manabu Sugimoto, Hidehiko Tanaka and Nobutada Murakami
The concentrations of free proline, leucine, and isoleucine increase significantly in plant cells under osmotic stress. Methionine γ-lyase (MGL) catalyzes methionine to produce α-ketobutyrate, which is a precursor for isoleucine biosynthesis. Arabidopsis MGL gene is induced by drought and salt stresses, suggesting that plant MGL plays a role in isoleucine biosynthesis for abiotic stress tolerance in plants. However, enzymatic characterization and gene expression of plant MGL are described mostly based on results of Arabidopsis MGL. This study identified putative MGL genes from barley and some monocot plants that encode amino acid sequences showing homology with that of Arabidopsis MGL. Plant MGLs were distinguishable between monocots and dicots from their gene and amino acid sequences. Barley MGL catalyzed α, γ-elimination reaction more than α, β-elimination reaction, as did Arabidopsis MGL. Barley MGL gene was up-regulated considerably by drought stress, but down-regulated by the aspartate family amino acids, cystathionine, homocysteine, methionine, threonine, and isoleucine, with especially considerable reduction by methionine. These results suggest that barley MGL gene should be induced by drought stress specifically and be reduced by methionine to reserve methionine for the production of S-adenosylmethionine and the following metabolites, which are precursors for ethylene and polyamines playing a role in abiotic stress tolerance.