The dual effects of methyl salicylate on ripening and expression of ethylene biosynthetic genes in tomato fruit

Tomato fruit (Lycopersicon esculentum Mill. cv. Sun Bright) at three ripening stages (mature green, breaker and turning) were treated with three different concentrations of methyl salicylate (MeSA) vapor to investigate the impact on ripening and ethylene production. The tomato ripening process, including the development of red color, ethylene production and respiration rate, was enhanced by 0.1 mM of MeSA during the mature green stage and 0.01 mM of MeSA during the breaker stage. But in fruit at the turning stage, even a low concentration of MeSA (0.01 mM) retarded the ripening process. High concentration (0.5 mM) of MeSA prevented red color development, ethylene production and respiration in all maturity stages. Northern hybridization experiments on mature green fruit, involving four cDNAs encoding ACC synthase and ACC oxidase, showed that the abundance of LE-ACS2 and LE-ACS4 mRNAs increased during storage concomitant with a burst in ethylene production. These increases in mRNAs of LE-ACS2 and LE-ACS4 with ripening were suppressed by treatment with 0.5 mM of MeSA. But in 0.1 mM MeSA treated fruit, the transcript of LE-ACS2 had a large increase at day 1 and day 3 compared with untreated fruit. The abundance of LE-ACS4 was undetectable in untreated fruit at day 0, but accumulated with 0.1 mM MeSA treated fruit. Transcripts for the LE-ACS6 gene were undetectable after day 1 in the mature green stage. The transcripts for the LE-ACO1 gene, which were present at harvest in mature green fruit, increased greatly during ripening in storage. These increases in LE-ACO1 mRNA with ripening were delayed by treatment with both 0.1 and 0.5 mM MeSA. The results suggest that increased ethylene production by 0.1 mM MeSA in tomato fruit is possibly mediated by depressing the negative feedback regulation of the LE-ACS6 genes and increasing the expression of LE-ACS2 and LE-ACS4 through positive feedback regulation. Published by Elsevier Science Ireland Ltd.