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Genetic and epigenetic regulation of vitamin C accumulation in apple fruits

Final Report Summary - VITAFRUIT (Genetic and epigenetic regulation of vitamin C accumulation in apple fruits)

To date, fruit-breeding programmes have mostly focused on easily identifiable phenotypic characteristics such as fruit color or fruit size. However, the availability of high-throughput analytical and molecular methodologies now allows us to pursue more challenging quality traits such as nutritional contents like the increase in vitamin concentrations. The main goal of this proposal is to identify the genetic and molecular factors that control vitamin C (L-ascorbic acid, AsA) and carotenoid accumulation in apple fruits.
Apple, is one of the most widely cultivated tree fruits in the world, and understanding the mechanisms that regulate AsA and carotenoid contents, would lead to the identification of novel genes and alleles that can be used for plant breeding for AsA- and carotenoid-enriched apple fruits. This in turn can contribute to improved apple fruit nutritional and postharvest quality and consequently, add extra value to the crop. AsA and carotenoids are also involved in plant defense systems against biotic and abiotic stresses and are therefore of high biological importance.
One of the objectives of this project (VITAFRUIT) is to exploit natural variation to find genes/alleles that are present in Malus x domestica genotypes that have a crucial effect on the accumulation of AsA and carotenoids in apple fruits. To this end, a screening of a collection of apple genotypes that come from different geographic origins and include cultivars with red, green and yellow skin color, old and elite cultivars, and also early- and late-ripening cultivars has been carried out. The quantification of AsA and total carotenoids contents in the pulp of these fruits at commercial maturity has revealed that there is a striking difference in carotenoids and AsA concentrations between genotypes of the collection being up to 28-fold and 11-fold for total carotenoids and AsA, respectively. This collection is consequently an appropriate working material for the study of the regulation of the two metabolic pathways. Genotypes with differing AsA and carotenoids contents have been selected for further RNA-seq analysis which will reveal key regulatory genes from the already known metabolic pathways and also putative upstream regulators, namely transcription factors.
The storage of apple fruits is a common practice in the apple industry and in fact, consumers do not normally eat the fruits right after harvest. Thus, the nutritional composition of the fruits when they are actually consumed can greatly differ from the composition at harvest time. In the VITAFRUIT project postharvest storage has been carried out using fruits from commercial varieties with differing carotenoids and AsA concentrations at harvest time. The analysis of the AsA and carotenoids concentrations during postharvest storage and shelf-life has revealed that the evolution of the concentrations differs between varieties, and the comparison of the expression levels of genes of the AsA and carotenoids metabolism suggests bottlenecks in the regulation of the contents of AsA and carotenoids in stored fruits. The results indicated that the transcriptional regulation of genes, involved in the degradation of carotenoids, seems to be a key factor on the regulation of the carotenoid concentration rather than the steps leading to carotenoid biosynthesis during postharvest storage. Moreover, the transcriptional analysis of genes which have been previously described to be key in the regulation of vitamin C in apple fruits at harvest time, showed that the AsA concentration during postharvest conditions seemed to be regulated by different genes. Since major decreases in AsA levels are, in most of the apple varieties, dramatically produced during postharvest, the identification of the genes involved in the regulation of AsA concentrations in postharvest storage is highly relevant.
The knowledge generated by the VITAFRUIT project will have a positive impact in several sectors of society. Firstly, by increasing food quality we can improve human health and nutrition. Secondly, there will be also economic benefits for the apple (and fruit) industry, breeders and farmers and the development of new AsA and carotenoids-rich cultivars will lead to renewal and stimulation across the entire production chain. The increase in antioxidant contents can lead to improved stress resistance, less input requirements and a sustainable production. Finally, the results developed here will also contribute to the pool of scientific knowledge on AsA and carotenoids metabolism and can deliver biotechnological tools necessary for academic research and innovation in food crop production.