On Thursday, I wrote about a way of genetically modifying carrots to turn them into rich sources of calcium. The method could be more widely used in vegetables to help reduce nutritional deficiencies, but it risks raising the ire of the anti-GM environmentalist camp. But there is another way of altering the genes of crop plants that avoids such controversy, and it’s a traditional one – selective breeding.
By cross-breeding individuals with desirable qualities, farmers have been tinkering with the genes of both animals and plants for centuries. Traditionally, the process has been a bit messy. Genes don’t always easily translate into physical characteristics, so there is a certain amount of trial-and-error involved.
Now, Carlos Harjes from Cornell University had developed a way of using modern genetic techniques to make selective breeding even more selective. For his first trick, he has developed a variety of maize to combat vitamin A deficiencies. Best of all, no genes were added, tweaked or subtracted in the making of this vegetable – he only used the natural genetic variation within the world’s maize strains.
Maize is a good choice for smart breeding. It’s one of the most genetically diverse food crops on the planet and it’s the dominant domesticated plant of the Americas and sub-Saharan Africa. In these regions, up to a third of children under the age of 5 suffer from vitamin A deficiencies. If untreated, their eyes start to fail, followed shortly by their immune systems.
Eating varied diets can help, but that is a luxury unavailable to poorer countries with few options for importing non-seasonal crops. Vitamin supplements are another option, but the same problems persist along with issues over their safety when compared to vitamins gleaned directly from food. Genetic modification is cheaper but faces strong cultural backlash and developing countries in particular have been largely reticent to accept these crops.
Harjes’s super-maize could be the answer to all of these complaints – it’s a long-term, inexpensive solution that sweeps aside the objections against GM crops. Its secret is a higher level of carotenoids, a group of chemicals collectively known as provitamin A that act as precursors to vitamin A itself.
Vitamins and provitamins
There are several types of carotenoid and they are converted from one to the other by a suite of metabolising enzymes. At one point, this road of transformation develops a fork – down one path lie chemicals like alpha-carotene, and down the other lies beta-carotene, the king of carotenoids. Harjes’s technique relies on finding genes that favour the beta-carotene arm of the fork. Simply drive the reaction down that path and your plant is a richer source of vitamin A.
The amount of beta-carotene in the world’s maize strains varies by almost a factor of thirty. Among the many varieties that Harjes tested, some had 14ug of beta-carotene for every gram, but most of the world’s maize only has about 0.5ug/g. The key to successfully fortifying a maize lineage with vitamin A is telling the plentiful strains from the lacking ones.
Colour is an obvious place to start. Beta-carotene gives colour to orange and yellow vegetables like maize and carrots, but Harjes found that a maize cob’s colour was a surprisingly poor predictor of its beta-carotene content. Instead, he searched for genes that directly influence the production of beta-carotene.
Gene of choice
The top candidate turned out to be lycE (or lycopene epsilon cyclase) a gene that controls the proportion of carotenoids that go down the beta-carotene path. Just four naturally occurring changes in the gene explained the majority of the variation in the two paths. By choosing the right combination, the team managed to triple the amount of beta-carotene in the final plant.
By using Harjes’s results, and screening the relevant part of the maize genome, breeders should be able to accurately hand-pick the best plants for cross-breeding – a technique called ‘marker-assisted selection’.
Previously, farmers could assess the levels of carotenoids in their plants through an expensive technique called HPLC. With this new information, they could use genetic screening methods that are a thousand times cheaper, an enormous boon for farmers whose finances are in as much need as their vitamin A levels.
Reference: Harjes, C.E., Rocheford, T.R., Bai, L., Brutnell, T.P., Kandianis, C.B., Sowinski, S.G., Stapleton, A.E., Vallabhaneni, R., Williams, M., Wurtzel, E.T., Yan, J., Buckler, E.S. (2008). Natural Genetic Variation in Lycopene Epsilon Cyclase Tapped for Maize Biofortification. Science, 319(5861), 330-333. DOI: 10.1126/science.1150255