Characterization of tomato fruit ripening and analysis of gene expression in F1 hybrids of the ripening inhibitor (rin) mutant

The ripening inhibitor (rin) mutation of tomato yields non-ripening fruit, and the gene corresponding to RIN, LeMADS-RIN, is known to encode a transcriptional factor that controls ripening-related genes. In this study, to evaluate the heterozygosity effect of rin on fruit ripening, we developed eight F-1 hybrid lines of the rin mutant from various crosses between the lines of the rin mutant and wild type. In the fruit of these F-1 hybrid lines, the shelf-life was improved, but both the shelf-life and colouring varied between the lines. We then chose one line of the F-1 hybrids and investigated the physiological and transcriptional properties of the fruit. Compared with the wild-type parent, this F-1 line showed about half the lycopene content, lower fruit softening and lower mRNA accumulation of the genes that encode phytoene synthase (Psy), polygalacturonase (PG), beta-galactosidase (TBG4) and expansin (LeEXP1). The characteristic climacteric rise in ethylene production typically observed in the wild-type parent during fruit maturation was not observed in the fruit of this F-1 line. The genes that encode ethylene biosynthetic enzymes, namely 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS2 and ACS4) and ACC oxidase (ACO1), were, however, significantly expressed in the F-1 hybrid, suggesting that the post-transcriptional regulator for activating these enzymes is affected by LeMADS-RIN. These results suggest that the heterologous effect of LeMADS-RIN in F-1 hybrids affects the gene transcription and activation of ripening-related factors, resulting in changes in fruit properties, including the extension of the shelf-life.