Spring 2010

Know How

Illustrations by Diane Doering

In the hands of a careful plant breeder, a vegetable can become like an artist’s canvas, willing to take on any color in the rainbow. Breeders have managed to turn cauliflowers orange, asparagus purple and tomatoes black. And while consumers are drawn to the unusual hues, the primary goal of these transformations is usually to boost human health. Orange, for example, is typically the mark of beta-carotene, which turns into vitamin A in the body. Here’s how a carrot breeder would add a splash of purple—the color associated with cancer-fighting anthocyanin compounds—to a typical orange carrot variety.

1 Head for the wilds. A purple carrot might look odd by today’s standards, but it’s actually a lot closer to what nature intended. Most cultivated carrots were purple until a few hundred years ago, when orange mutants began to catch the fancy of European diners. Wild, purple-skinned carrots can still be found in parts of central Asia, and these ancient relatives hold the genes a breeder needs to return a garden-variety carrot to its original hue.

2 Let the breeding begin. Back at the greenhouse, the wild plants must be coaxed to grow. Once they flower, the breeder uses a small paintbrush to collect pollen and transfer it to the blossoms of conventional orange carrot plants. The seeds created through this assisted fertilization are true hybrids, containing approximately equal parts purple carrot DNA and orange carrot DNA.

3 Thin the patch. Invariably, these first hybrids are far from ideal. They contain too much wild carrot DNA and it shows; the vast majority are too small, too bitter or too twisted—or all of these things—to succeed in the commercial market. The trick is to weed out any other influence from the wild parent except its color.

4 Cross again… and again…and again. Even in the age of biotechnology, most plant breeders tease out favorable traits the same way farmers have for several millennia, through a painstaking cycle of trial-and-error. Promising hybrids are crossed with other promising hybrids—and in some cases with the parent strains again—through multiple generations in a process that can require years to produce desired results. For carrots, the wait is particularly frustrating as the plants flower only once every two years. But with patience comes perfection, as repeated selection slowly hones in on the right mix of genes to produce the ideal set of traits.

5 Lock it in. As soon as a breeder comes up with a perfect purple carrot, it’s time to stop cross-breeding and start self-pollinating the plant. This halts the process of DNA mixing and allows this sought-after plant—with its desirable assortment of genes that were so painstakingly assembled—to be propagated intact. Voilà—a purple carrot for the commercial market.

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