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 F1 hybrid lines of the rin mutant from various crosses between the lines of the rin mutant and wild type. In the fruit of these F1 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 F1 hybrids and investigated the physiological and transcriptional properties of the fruit. Compared with the wild‐type parent, this F1 line showed about half the lycopene content, lower fruit softening and lower mRNA accumulation of the genes that encode phytoene synthase (Psy), polygalacturonase (PG), β‐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 F1 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 F1 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 F1 hybrids affects the gene transcription and activation of ripening‐related factors, resulting in changes in fruit properties, including the extension of the shelf‐life.