Nutritional genomics

A US research project, led by Dr Edward Buckler of Cornell University’s Institute of Genomic Diversity, has identified natural allelic variants in the lycopene epsilon cyclase (lycE) gene in maize, associated with a threefold difference in concentrations of provitamin A compounds in maize.

Researchers identified four natural polymorphisms that explained 58 per cent of the variation in levels of alpha and beta carotene, and beta cryptoxanthin in maize.

One of the lycE alleles decreases activity in one leg of a biosynthetic pathway, and directs more carbon into the other, resulting in higher levels of beta-carotene in maize kernels.

“It’s a beautiful example of the interweaving of basic science and targeted breeding,” DellaPenna says. “Some of these alleles are already present in late-stage breeding lines, so breeders can go in and select fourth-or fifth generation material to develop high provitamin A varieties.”

DellaPenna says his research group has pioneered the concept of “nutritional genomics”: as fully sequenced genomes become available for important crop species, it is apparent that many of the compounds researchers are interested in have dual functions.

Provitamin A is needed to prevent macular degeneration of the retina. “We initially worked on provitamin A compounds from the plant side – how they are made, and their role in photosynthesis.”

In plants, they are involved in oxygen production, and in limiting oxidative damage.

“The third area we are working hard is using natural variation to help us how compounds are made and modified within the plants – for example, we can take petunia lines, cross them, then select in subsequent generations for genetic variation underlying particular biosynthetic pathways.

“We can do that for metabolites as well, by cloning quantitative trait loci (QTLs) in Arabidopsis. Many of those linkages are going to be important selection targets in crops – they increase levels of vitamin E, specific carotenoids, and the bioavalability of minerals like iron.

“Iron is certainly the major limiting micronutrient in the human diet. One of the reasons for iron deficiency and anaemia in developing countries is that iron in plant tissues is much less bioavailable than in animal products.

“For example, 30 to 40 per cent of the total iron content of a hamburger might end up in the bloodstream, but if the equivalent amount were available in rice, maize or wheat, only 5 per cent would end up in the bloodstream, because other compounds in plants inhibit iron uptake.

“But there are also compounds in plants that stimulate iron uptake through the intestines. Using natural variation and QTLs to identify the genes involved in the Arabidopsis model system, we can investigate how the proteins or enzymes involved influence iron uptake by human intestinal epithelia cells in culture.

“In the longer term, we can move those genes into crops and they should have an enormous benefit on human nutrition, without changing the amount of iron in plants. If we could double or triple iron availability, it would have major benefits for people’s ability to work, to increase their red cell counts, and improve their immune responses.”