Characterization of RIN Isoforms and Their Expression in Tomato Fruit Ripening

Ripening of tomato fleshy fruit is coordinated by transcription factor RIN, which triggers ethylene and carotenoid biosynthesis, sugar accumulation, and cell wall modifications. In this study, we identified and characterized complete sequences of the RIN chromosomal locus in two tomato Solanum lycopersicum cultivars, its rin/RIN genotype, and three wild green-fruited species differing in fruit color and composition. The results reveal that S. lycopersicum cultivars and some wild species (S. pennellii, S. habrochaites, and S. huaylasense) had a 3'-splicing site enabling the transcription of RIN1i and RIN2i isoforms. The other wild species (S. arcanum, S. chmielewskii, S. neorickii, and S. peruvianum) had a 3'-splicing site only for RIN2i, which was consistent with RIN1i and RIN2i expression patterns. The genotype rin/RIN, which had an extended 3'-terminal deletion in the rin allele, mainly expressed the chimeric RIN-MC transcript, which was also found in cultivars (RIN/RIN). The RIN1, but not RIN2, protein is able to induce the transcription of the reporter gene in the Y2H system, which positively correlated with the transcription profile of RIN1i and RIN target genes. We suggest that during fruit ripening, RIN1 activates ripening-related genes, whereas RIN2 and RIN-MC act as modulators by competing for RIN-binding sites in gene promoters, which should be confirmed by further studies on the association between RIN-splicing mechanisms and tomato fruit ripening.