Regulatory Role of Cystathionine-γ-Synthase and de novo Synthesis of Methionine in Ethylene Production during tomato Fruit Ripening

Abstract

The essential amino acid methionine is a substrate for the synthesis of S-adenosyl-methionine (SAM), that donates its methyl group to numerous methylation reactions, and from which polyamines and ethylene are generated. To study the regulatory role of methionine synthesis in tomato fruit ripening, which requires a sharp increase in ethylene production, we cloned a cDNA encoding cystathionine gamma-synthase (CGS) from tomato and analysed its mRNA and protein levels during tomato fruit ripening. CGS mRNA and protein levels peaked at the "turning" stage and declined as the fruit ripened. Notably, the tomato CGS mRNA level in both leaves and fruit was negatively affected by methionine feeding, a regulation that Arabidopsis, but not potato CGS mRNA is subject to. A positive correlation was found between elevated ethylene production and increased CGS mRNA levels during the ethylene burst of the climacteric ripening of tomato fruit. In addition, wounding of pericarp from tomato fruit at the mature green stage stimulated both ethylene production and CGS mRNA level. Application of exogenous methionine to pericarp of mature green fruit increased ethylene evolution, suggesting that soluble methionine may be a rate limiting metabolite for ethylene synthesis. Moreover, treatment of mature green tomato fruit with the ethylene-releasing reagent Ethephon caused an induction of CGS mRNA level, indicating that CGS gene expression is regulated by ethylene. Taken together, these results imply that in addition to recycling of the methionine moieties via the Yang pathway, operating during synthesis of ethylene, de novo synthesis of methionine may be required when high rates of ethylene production are induced.

Dimethyl Disulfide Produced by the Naturally Associated Bacterium Bacillus sp B55 Promotes Nicotiana attenuata Growth by Enhancing Sulfur Nutrition

“…22 -limiting conditions, efficient recycling of Met (in the Yang cycle) becomes vital in addition to a de novo synthesis (Kushad et al, 1985(Kushad et al, , 1988Sauter et al, 2005;Katz et al, 2006;Bürstenbinder et al, 2007). Compared with N. attenuata wild-type seedlings, control 35S-etr1 seedlings accumulated higher basal transcript levels of genes involved in Met biosynthesis (Na-CGS542 and Na-MS90), SAM synthesis (Na-SAMS113), and Met recycling in the Yang cycle (Na-MTK3797).…”

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

“…Although we were not able to detect ET levels in the developing seeds, we suspect that its levels increased, since we found that several key genes involved in its synthesis and downstream targets were up-regulated. We previously showed a strong correlation between higher Met content and higher ET levels in tomato (Solanum lycopersicum; Katz et al, 2006), suggesting that this also might occur in SSE seeds. The higher germination rates of SSE seeds in the presence of ACC, the substrate for ET synthesis, also supports this assumption.…”

Cross-talk between sulfur assimilation and ethylene signaling in plants

“…However, additional evidence suggests that in addition to recycling the Met moieties via the Yang cycle, the de novo synthesis of Met is required when high rates of ethylene production are induced. 79 Moreover, S availability and ethylene have been shown to regulate GSH synthesis and stress tolerance to ozone 80 and Cd 81,82 stress. Glutathione acts as a storage and transport form for Cys; otherwise the excess Cys present in the cell becomes toxic.…”