Improved tomato taste and shelf life through heterosis at the RIN locus ? identification of new target genes through transcriptomic and metabolic profiling
01 / 2011 - unknown
Improving the shelf life of tomato is a long sought-after goal for breeding companies, in order to increase the time available for transport between harvest and marketing and to even out discrepancies between supply and demand. Since the availability of mutants that delay or inhibit ripening, their applicability in breeding programs has been subject of research. Never ripe (Nr), non ripening (nor), green-ripe (gr), alcobaca (alc), and ripening inhibitor (rin) are such non-allelic mutants in which ripening processes are slowed down, and which have been tested for breeding ((Kopeliovitch et al., 1979; Farkas, 1995; Garg et al., 2008a). Whereas the homozygous mutants show inhibited ripening to an extent that makes them unfit for consumption, it was found that plants heterozygous for the mutation have extended storage life, while having color development, lycopene content and flavor that are deemed acceptable for marketing, albeit intermediate between homozygous mutants and wild-type plants. In practical breeding programs, the rin mutation appears to give the best results in heterozygous plants, and is therefore the most widely used (Markoviæ et al., 2008; Scott, 2008). The gene underlying the ripening inhibitor (rin) mutation encodes a MADS-box transcription factor that plays a crucial role in ripening processes in tomato (Vrebalov et al., 2002). Fruits from plants homozygous for the mutant rin allele fail to ripen and show defects in all processes of ripening, such as autocatalytic ethylene production, increased respiration, carotenoid synthesis, softening, and flavor development (Tigchelaar et al., 1978). Whereas, like other homozygous ripening mutants, rin/rin fruits do not develop color or flavor that is acceptable to consumers, it was recognized early on that hybrid or heterozygous Rin/rin-fruits show delayed ripening with acceptable flavor and color, combined with a considerably enhanced shelf-life due to slower softening (Nahum, 1989). Rin/rin-hybrids have now taken a prominent share in many markets, primarily due to their prolonged shelf-life. However, depending on the genotype, flavor and color do not develop completely in hybrid fruits (Kopeliovitch et al., 1982; Garg et al., 2008a; Garg et al., 2008b; Markoviæ et al., 2008). Targeted analysis of expression of genes involved in ripening processes have shown that genes involved in lycopene synthesis and in fruit softening are expressed at a lower level in hybrid fruit compared to wild type fruit. In addition it was shown that the climacteric rise in ethylene production was not observed in hybrid fruit, despite the significant expression of ethylene biosynthetic enzyme encoding genes (Knapp et al., 1989; Kitagawa et al., 2005). This observation suggests that not all heterosis effects of Rin/rin are the result of a simple additive gene dosage effect or are still not fully understood.. Therefore, a more detailed study and comparison of gene expression and metabolite content for wild type, rin/rin, and hybrid fruits will provide us with new insights in regulatory processes during ripening, and that this may identify novel target genes for improving tomato fruit shelf life while minimizing the impact on colour and taste.