Genetic dissection of aroma and other fruit quality traits in melon

Melon is a valuable species mainly appreciated by its fruit, which can present different colors, shapes, flavor and ripening behavior. The focus of melon breeding is currently on improving shelf life and flavor, which depend on the ripening behavior and mainly on volatile organic compounds (VOCs), respectively. The origin and the chemical structure of VOCs can be diverse and is related to its final flavor. Some enzymes involved in VOCs biosynthesis have been already described, but many more remain unknown. Our objective was to dissect the genetics behind VOCs production in melon, and observe the effect of different ripening QTLs in the volatile profile. For that reason, mapping populations generated from a cross between the non-climacteric non-aromatic ‘Piel de Sapo’ (PS) cultivar and the climacteric aromatic ‘Védrantais’ (VED) cultivar were analyzed by Gas Chromatography-Mass Spectrometry (GC-MS) in search for QTLs associated to aroma. A Recombinant Inbred Line (RIL) population and two reciprocal Introgression Line (IL) collections were analyzed. In the RILs population, 166 QTLs were identified forming clusters in specific genomic regions, highlighting chromosome 8 as a main player in both ripening and aroma regulation. On the other hand, more than 1,000 QTLs were detected in the two IL collections. Some regions linked ripening and aroma; however, ripening-independent aroma QTLs were also identified. Two interesting regions on chromosomes 6 and 11 were selected for fine mapping. On chromosome 6, two enoyl-CoA isomerases 2 are candidates for the degradation of 3Z-monounsaturated lipid-derived VOCs. On chromosome 11, a complex regulation of ester formation was revealed, finding 3 sub-QTLs in the region of study, with some candidate genes related to the primary metabolism. In addition, a functional validation of an aldehyde oxidase on chromosome 5 was attempted. The preliminary results showed that this enzyme is putatively involved in benzaldehyde conversion into benzoic acid.

A secondary project involved the use of CRISPR-Cas9 to validate the effect of CmOFP13 in melon, a gene reported to modulate fruit shape and previously validated by overexpression in Arabidopsis. Edited knockout melon plants were obtained, producing more elongated ovaries and fruits, and validating our hypothesis.

Overall, this work has shed light on the many genomic regions responsible for VOCs production, establishing the basis for future fine mapping works. This knowledge will contribute to improve melon fruit quality and could be transferred to breeding programs.