A Hydrogen-Sulfide-Repressed Methionine Synthase Acts as a Positive Regulator for Fruit Ripening in Tomato.

Ethylene is a key phytohormone that regulates the ripening of climacteric fruits, and methionine is an indirect precursor of ethylene. However, whether methionine synthase plays a role in fruit ripening in (tomato) is still unknown. In this study, we find that a tomato methionine synthase (named ), which could be repressed at the transcriptional level by hydrogen sulfide (HS), acts as a positive regulator for tomato fruit ripening. By a bioinformatics analysis, it is found that SlMS1 and SlMS2 in tomato are highly homologous to methionine synthases in . The expression pattern of and is analyzed in tomato, and SlMS1 expression is up-regulated during fruit ripening, suggesting its potential role in regulating fruit ripening. A potential bipartite nuclear localization signal is found in the amino acid sequence of SlMS1; thus, SlMS1 is tagged with GFP and observed in the leaves of . Consistently, SlMS1-GFP shows strong nuclear localization and also cytoplasmic localization. The role of in regulating fruit ripening is investigated in tomato fruit by transient silencing (virus-induced gene silencing, VIGS) and transient overexpression. The results show that silencing causes delayed fruit ripening, evidenced by more chlorophyll and less carotenoid accumulation, while overexpression accelerates fruit ripening significantly compared with control. Further investigation shows that overexpression could up-regulate the expression of carotenoid-synthesis-related genes (, , ), chlorophyll-degradation-related genes (, , , ), cell-wall-metabolism-related genes (, , , , ) and ethylene-synthesis-pathway-related genes (, , ), while silencing causes the opposite results. The correlation analysis indicates that expression is negatively correlated with chlorophyll content and positively correlated with carotenoid and ripening-related gene expressions. Taken together, our data suggest that SlMS1 is a positive regulator of tomato fruit ripening and a possible target gene for the ripening-delaying effect of HS.