The expression level of threonine synthase and cystathionine-γ-synthase is influenced by the level of both threonine and methionine in Arabidopsis plants

Abstract

The biosynthesis pathways of the essential amino acids methionine and threonine diverge from O-phosphohomoserine, an intermediate metabolite in the aspartate family of amino acids. Thus, the enzymes cystathionine-gamma-synthase (CGS) in the methionine pathway and threonine synthase (TS), the last enzyme in the threonine pathway, compete for this common substrate. To study this branching point, we overexpressed TS in sense and antisense orientation in Arabidopsis plants with the aim to study its effect on the level of threonine but more importantly on the methionine content. Positive correlation was found not only between TS expression level and threonine content, but also between TS/threonine and CGS expression level. Plants expressing the sense orientation of TS showed a higher level of threonine, increased expression level of CGS, and a significantly higher level of S-methylmethionine, the transport form of methionine. By contrast, plants expressing the antisense form of TS showed lower levels of threonine and of CGS expression level. In these antisense plants, the methionine level increased up to 47-fold compared to wild-type plants. To study further the effect of threonine on CGS expression level, wild-type plants were irrigated with threonine and control plants were irrigated with methionine or water. While threonine increased the expression level of CGS but reduced that of TS, methionine reduced the expression level of CGS but increased that of TS. This data demonstrate that both methionine and threonine affect the two enzymes at the branching point, thus controlling not only their own level, but also the level of each other. This mechanism probably aids in keeping the levels of these two essential amino acids sufficiently high to support plant growth.

Fortifying plants with the essential amino acids lysine and methionine to improve nutritional quality

“…Similarly, transgenic and mutant Arabidopsis having low TS expression level have 50‐fold and 15‐fold more soluble Met in their leaves, respectively (Avraham and Amir, ; Bartlem et al ., ). However, the increased Met content was at the expense of Thr, whose level was significantly reduced, questioning the biotechnological suitability of this genotype.…”

Proteomic identification of differentially expressed proteins in Arabidopsis in response to methyl jasmonate

“…1). Gamma-glutamylcysteine synthetase catalyzes the first and rate-limiting step in glutathione biosynthesis, while threonine synthase plays an essential role in controlling the branching point of biosynthesis pathways of methionine and threonine (Goto et al, 2002;Avraham and Amir, 2005).…”

Toward the Construction of a Kinetic Model of Methionine and Thereonine Biosynthesis to Increase Seed Nutritional Value: Characterization of Threonine Synthase.

“…For example, transgenic Medicago sativa (alfalfa) expressing AtCGS downstream of the rubisco small subunit promoter accumulated up to 32-fold more L-Met than wild-type plants while expressing the same gene under the CaMV-35S promoter in potato did not alter L-Met accumulation (Kim et al 2002;Gakiere et al 2002). Reduction in the expression of TS in A. thaliana and potato resulted in a wide range of increased L-Met accumulation, 17-to 47-fold and 2-to 240-fold, respectively, beyond the required 3-fold increase (Avraham and Amir 2005;Zeh et al 2001). Deleterious phenotypes, including reduced growth and yield, have also been reported, particularly when employing the CaMV-35S promoter, as reported for the transgenic A. thaliana and potato with reduced TS expression (Avraham and Amir 2005;Zeh et al 2001).…”

“…and L-Thr pools and plant phenotype in different species, as exemplified by studies in A. thaliana and S. tuberosum Avraham and Amir, 2005 . These results highlight the necessity of characterizing TS and mapping metabolic flux in the target species prior to engaging in genetic engineering endeavours in vivo, since the degree to which TS affects OPHS flux distribution may vary across species.…”

“…The pursuit of increased L-Met production has involved experiments using strong constitutive promoters, such as the CaMV-35S promoter, to increase expression of CGS or decrease expression of TS (Gakiere et al 2002;Zeh et al 2001). The resulting 15-to 240-fold increases in L-Met levels are higher than necessary to meet human dietary requirements, and are often associated with phenotypic effects, such as retarded growth, leaf chlorosis, and reduction in fresh weight Gakiere et al 2002;Avraham and Amir, 2005). A 3-fold increase in seed L-Met is sufficient to achieve adequate nutritional quality, and is unlikely to cause perturbations to metabolic flux in other pathways, thus avoiding undesirable phenotypic effects.…”