Interdependence of threonine, methionine and isoleucine metabolism in plants: accumulation and transcriptional regulation under abiotic stress

Abstract

Pathways regulating threonine, methionine and isoleucine metabolism are very efficiently interconnected in plants. As both threonine and methionine serve as substrates for isoleucine synthesis, their synthesis and catabolism under different developmental and environmental conditions also influence isoleucine availability. Together, methionine gamma-lyase and threonine deaminase maintain the isoleucine equilibrium in plants under varied substrate availabilities. Isoleucine and the two other branched-chain amino acids (BCAAs) (leucine and valine) share four common enzymes in their biosynthesis pathways and thus are coordinately regulated. Induction of free amino acids as osmolytes in response to abiotic stress is thought to play a role in plant stress tolerance. In particular, the accumulation of BCAAs is induced many-fold during osmotic stress. However, unlike in the case of proline, not much research has been focused on understanding the function of the response involving BCAAs. This review describes pathways influencing branched-chain amino acid metabolism and what is known about the biological significance of their accumulation under abiotic stress. A bioinformatics approach to understanding the transcriptional regulation of the genes involved in amino acid metabolism under abiotic stress is also presented.

Nitric oxide affects salt-induced changes in free amino acid levels in maize

“…In the case of Ile this change may be the result of a simultaneous increase in the level of its precursor Thr after the DETA/NO + NaCl treatment. A recent study emphasized the importance of branched-chain amino acids as osmolytes which function might contribute to the improved salt tolerance in the present experiment (Joshi et al, 2010).…”

Seed-Specific Expression of a Feedback-Insensitive Form of CYSTATHIONINE-γ-SYNTHASE in Arabidopsis Stimulates Metabolic and Transcriptomic Responses Associated with Desiccation Stress

“…S4). In addition, the expression level of Thr dehydratase, the main enzyme of BCAA synthesis (Joshi et al, 2010), did not exhibit any altered expression. Tyr is the only amino acid that exhibited reduced contents in its soluble and protein-bound forms, up to 3-and 2-fold reductions, respectively (Fig.…”

“…S3). These four amino acids are known to be involved in plant defense mechanisms against abiotic stresses and to play active roles in drought, salt, and osmotic stresses (Joshi et al, 2010). BCAAs are related to Met metabolism, since they can be produced from Met via METHIONINEg-LYASE (MGL), a catabolic enzyme of Met (Joshi et al, 2010).…”

“…In addition, the higher levels of BCAAs that also can be produced from Met suggest that Met catabolism contributes to their accumulation. Since the expression levels of MGL and Thr dehydratase, the main enzymes that produce the BCAAs (Joshi et al, 2010), were not altered significantly in SSE seeds, the accumulation of BCAAs most probably occurred via the enhanced activities of enzymes in their biosynthetic pathways or from higher flux toward their biosynthesis pathways.…”

The ZmASR1 Protein Influences Branched-Chain Amino Acid Biosynthesis and Maintains Kernel Yield in Maize under Water-Limited Conditions

“…Of high relevance are 12 standard or related amino acids and three sugars: (1) Ala, which is synthesized from pyruvate; (2 and 3) Asn and Gln, which are the central regulators of carbon/nitrogen metabolism; (4) Gly, whose metabolism occurs by several different pathways, including Thr catabolism; (5-7) Ile (Thr-derived pathway) and the two other BCAAs, Leu and Val (pyruvate-derived pathway), that show the highest hydrophobicity among the proteinogenic amino acids and are accordingly the major constituents of transmembrane regions of membrane proteins; (8 and 9) Phe and Trp, whose synthesis initiates from chorismate and that serve in plants as precursors for a wide range of secondary metabolites having multiple functions; (10) Pro, which is synthesized from either Glu or Orn and has multiple functions in stress adaptation, recovery, and signaling; (11) benzoate, which derives from Phe and is a precursor to several important benzenoid compounds, including the defense signaling compound salicylic acid; (12) citramalate, which may serve as a precursor for Ile biosynthesis in plants as in microorganisms (de Kraker et al, 2007;Joshi et al, 2010;Less et al, 2010;Szabados and Savouré, 2010); (13) Suc, which is the major end product of photosynthesis and the major transport form of carbon from source and sink; (14) its cleavage product Glc, which enhances the expression of genes implied in carbohydrate metabolism (Koch, 1996;Trouverie et al, 2004); (15) galactinol (raffinose family oligosaccharide [RFO]), which plays a novel role in the protection of cellular metabolism, in particular, the photosynthesis of chloroplast, from oxidative damage caused by several types of abiotic stresses (Nishizawa et al, 2008). Our finding that ZmASR1-OE triggers an adjustment in the Glc level is in good agreement with the observation that a QTL for hexose content colocates with the ZmASR1 locus (Pelleschi et al, 2006) and a previous report in potato in which SlASR1 was overexpressed .…”