Following on from a previous post, Can plant science help alleviate vitamin deficiencies?
A complete understanding of the biosynthesis and regulation of vitamin pathways will be vital for the manipulation of the nutritional quality of foodstuffs in the years to come.
While engineering vitamin pathways or vitamin-related metabolism is one avenue that is worthwhile to pursue, the exploitation of natural variation is another approach for the development of crops containing adequate, balanced levels of vitamins.
Strategies to exploit natural genetic variability of a given trait using molecular markers are now well established in plants and have been applied to some vitamins (A, C, and E) in a variety of species.
In the case of vitamin C, quantitative trait loci (QTL) – stretches of DNA containing or linked to the genes that underlie a quantitative trait – have been located on genetic maps using ‘introgression line, advanced backcrossing, and recombinant inbred line populations’, derived from crosses between a cultivated variety and other cultivars or wild related species in tomato, strawberry, and apple (Malus domestica; Davey, M et al., 2006, ‘Genetic Control of Fruit Vitamin C Contents’, Plant Physiology).
This approach can be applied to most species for which parental lines with differential vitamin content can be identified. In their article, ‘Candidate Genes and Quantitative Trait Loci Affecting Fruit Ascorbic Acid Content in Three Tomato Populations’, published in Plant Physiology, Stevens et al., argue that a candidate gene approach or positional cloning can then be used to identify the polymorphic locus responsible for the vitamin variation.
Once vitamin QTL have been located, their stability in various genetic backgrounds and environmental conditions can be assessed and molecular markers can be used to help their transfer to elite varieties.
As an alternative and complement to conventional QTL mapping, the association mapping approach that scans the genome for significant association between genetic markers and the trait studied – reviewed by Myles et al. in The Plant Cell) might be used to discover new alleles – one of a number of alternative forms of the same gene or same genetic locus – controlling vitamin content.
This requires a population of known structure, displaying wide natural genetic variation in the particular vitamin, as well as high-density molecular marker maps and well-annotated genomes. The introgression of useful traits from wild species into cultivated varieties may, however, pose specific problems, such as strong unbreakable linkages with many other undesirable traits.
Mutant collections, in which a very large genetic variability beyond the natural variation found in domesticated species can be induced in a cultivated variety, could provide a useful source of genotypes with enhanced vitamin content, such as in the case of vitamin C.
Alternatively, as put forward by Austin et al., ‘Next-generation mapping of Arabidopsis genes’, published in The Plant Journal, in crop species with available mutant genetic resources, the development of next-generation mapping of unknown mutations, which is based on whole-genome sequencing and has been already developed in model species, becomes increasingly attractive for the discovery of new genes and allelic series enhancing vitamin content of food products.
When plant fitness is not affected by the mutation, this approach is one of the most promising for the rapid generation of high-quality elite varieties with enhanced vitamin content.
Indeed, in all cases, the consequences of manipulating the production of these compounds in the plant itself will need to be taken into account, and a compromise between a nutritionally rich and environmentally robust plants will need to be found.
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