Arabidopsis Methionineγ-Lyase Is Regulated According to Isoleucine Biosynthesis Needs But Plays a Subordinate Role to Threonine Deaminase

Abstract

The canonical pathway for isoleucine biosynthesis in plants begins with the conversion of threonine to 2-ketobutyrate by threonine deaminase (OMR1). However, demonstration of methionine g-lyase (MGL) activity in Arabidopsis (Arabidopsis thaliana) suggested that production of 2-ketobutyrate from methionine can also lead to isoleucine biosynthesis. Rescue of the isoleucine deficit in a threonine deaminase mutant by MGL overexpression, as well as decreased transcription of endogenous Arabidopsis MGL in a feedback-insensitive threonine deaminase mutant background, shows that these two enzymes have overlapping functions in amino acid biosynthesis. In mgl mutant flowers and seeds, methionine levels are significantly increased and incorporation of [13 C]Met into isoleucine is decreased, but isoleucine levels are unaffected. Accumulation of free isoleucine and other branched-chain amino acids is greatly elevated in response to drought stress in Arabidopsis. Gene expression analyses, amino acid phenotypes, and labeled precursor feeding experiments demonstrate that MGL activity is upregulated by osmotic stress but likely plays a less prominent role in isoleucine biosynthesis than threonine deaminase. The observation that MGL makes a significant contribution to methionine degradation, particularly in reproductive tissue, suggests practical applications for silencing the expression of MGL in crop plants and thereby increasing the abundance of methionine, a limiting essential amino acid.

Molecular Characterization of Barley Methionine γ-Lyase and Gene Expression by Abiotic Stress and Aspartate Family Amino Acids

“…It has been reported that MGL plays a role in protecting plant cells from abiotic stress because of production of α-ketobutyrate, a precursor of isoleucine accumulating in dehydrated plant cells as well as leucine, valine, and proline as osmolytes [31], and because of upregulation of the gene expression by drought, osmotic, and salt stresses in Arabidopsis [18][19][20][21]. Our results demonstrated that HvMGL was up-regulated 4.7-fold, especially by drought stress.…”

“…AtMGL was induced by drought and salt stresses, suggesting that MGL plays an important role in plant tolerance against osmotic stress by promoting isoleucine biosynthesis as an osmolyte [18][19][20][21]. The silencing of potato MGL gene (StMGL1) caused methionine accumulation in transgenic potato tubers [22,23], showing that StMGL1 encodes a functional MGL.…”

The draft genome of tropical fruit durian (Durio zibethinus)

“…3a and Supplementary Table 8). Enrichment in gene sets related to sulfur comprised five clusters, of which acidthiol ligase enzymes (containing a number of key enzymes in carbon-sulfur reactions and flavonoid production) and methionine metabolic pathways (of which MGL is a key member) 28 contained the most significantly upregulated leading-edge genes. Ripening-related gene sets included genes regulated by the MADS-BOX transcription factor family 29,30 , SEPALLATA transcription factor family, and ethylene-related genes such as ACS (aminocyclopropane-1-carboxylic acid synthase), a key ethylene-production enzyme involved in ripening 31 .…”

Proline Metabolism and Its Implications for Plant-Environment Interaction

“…Note that such reasons have been shown not to explain low water potential-induced proline accumulation (see for example Voetberg and Sharp, 1994). In addition to proline, other amino acids accumulate upon drought stress including leucine, isoleucine, valine, lysine, methionine and phenylalanine (Charlton et al, 2008;Joshi and Jander, 2009). Several of these amino acids, particularly the branched chain amino acids (leucine, isoleucine and valine) and threonine, are present at higher levels than other amino acids and have synthesis pathways that consume NADPH in the plastids and catabolism pathways that release reductant in the mitochondria (pathway information is available from AraCyc).…”