Enriched potato may help fight malnutrition

by Beth Skwarecki, December 2007

A new type of potato, rich in beta-carotene, may one day fight Vitamin A deficiency in the developing world.

In a recent issue of the American Journal for Potato Research (September 2007), researchers from the Boyce Thompson Institute and the US Department of Agriculture report that they have turned a zeaxanthin-rich potato into a beta-carotene rich one by silencing a single gene.

The researchers began with several potatoes including Yema de Huevo, a small, deep-yellow variety whose name means "egg yolk". It's commonly eaten in South America, in dishes like the Colombian soup ajiaco.

Yema de Huevo potatoes make beta-carotene, but an enzyme called beta-carotene hydroxylase (bch) immediately turns it into the yellow pigment zeaxanthin that gives the potatoes their characteristic color. The researchers were able to silence the bch gene, causing the potato to build up beta-carotene instead.

Author Joyce Van Eck explains that foods such as carrots have plenty of beta-carotene, but they aren't part of the local cuisine in many parts of the developing world. Potatoes, on the other hand, are an important staple crop already eaten by half a billion people in developing nations.

Vitamin A deficiency hits children the hardest, leading to blindness and then to immune deficiency. Weakened immune systems succumb easily to childhood diseases of diarrhea and measles. Van Eck says she hopes the enriched potatoes can make their way into the diets of young children in the developing world, 1-3 year olds in particular.

To increase beta-carotene in the yellow potatoes, Van Eck and her colleagues used a technique called RNA interference. The researchers started with tiny potato plants growing in test tubes. They cut the plants' stems into small pieces, treated them with hormones to grow leaves and roots, and infected them with Agrobacterium tumefaciens, a soil bacterium that is able to insert its genes into DNA in the root cells of plants. The bacteria had been engineered with a special gene that would "interfere" with the potato's own gene for beta-carotene hydroxylase, shutting it off.

The resulting potatoes showed a huge increase in their beta-carotene content, from 8 micrograms in Yema de Huevo potatoes up to 335 micrograms per 100 grams, or about 18% of a young child's daily requirement. A child could then get half of her daily Vitamin A from two to three ice cream scoop sized portions of potatoes.

The researchers are trying to enrich the potatoes further, and a gene from cauliflower may help. Van Eck collaborated with Li Li from the USDA lab to give potatoes a gene that was found in a mutant orange cauliflower. This gene also causes plant cells to accumulate beta-carotene.

The cauliflower gene isn't involved in the same pathway as bch. Instead, it seems to help form chromoplasts, structures in the plant cell that store carotenoids. The chromoplasts act as a metabolic sink, draining beta-carotene out of the pathway as fast as it can be made. The cell continues to make beta-carotene, thinking there is never enough, when really the chromoplasts are stuffed full of it.

The next step, Van Eck says, is to combine the two approaches in the same potato, hopefully increasing the beta-carotene content even further. A third approach will be to boost the action of other enzymes in the beta-carotene pathway. This work, though, is still in its very early stages.

To save lives with a beta-carotene rich potato, people in developing countries will need to grow and eat it, accepting it as part of their culture and their lives. It's important that the potato's cooking qualities allow it to fit into local recipes, for example. Good news about the zeaxanthin-blocked potatoes is that they don't have enough beta-carotene to look orange; they look like ordinary white potatoes. That might help people to accept them into their diet.

The potatoes also have to grow well. The enhanced beta-carotene has to be a "field stable" trait, meaning that farmers must be able to harvest enhanced potatoes season after season. And if the new type of plant doesn't produce as much food as a traditional variety, farmers may give up on it - after all, a crop that doesn't produce enough food is a waste of space.

If the continued experiments succeed, the researchers will work with a nutritionist to identify parts of the developing world where vitamin A deficiency is common and potatoes are a staple food in the diet. Those would be the best places to start introducing a beta-carotene rich potato.