Thursday, June 28, 2012

Tomato's color uniformity: pretty or sweet

The next time you bite into a supermarket tomato and are less than impressed with the tart cardboard taste, blame aesthetics. A new study reveals that decades of breeding the fruits for uniform color have robbed them of a gene that boosts their sugar content.

Farmers pluck the fruits from the vine before they are ripe, and for about 70 years breeders have selected tomatoes that are uniformly light green at that time. This makes it easier to spot the tomatoes that are ready to be harvested and ensures that, by the time they hit supermarket shelves, the fruits glow with an even red color. Wild varieties, in contrast, have dark green shoulders, and that makes it harder to determine the right time to harvest.

In wild tomatoes, SlGLK2 (the Golden 2-like transcriptor factor behind the color change) increases the formation of chloroplasts, the compartments in plant cells that carry out photosynthesis. Chloroplasts use a green pigment, chlorophyll, to capture the sunlight plants need to grow. A higher number of chloroplasts gives wild tomatoes their darker green color.

In most tomatoes on supermarket shelves, however, SlGLK2 is inactive. While the mutation was beneficial to farmers, it's not such a sweet deal for consumers. Chloroplasts use the light energy they capture to convert carbon dioxide and water into sugars. Tomatoes with a mutated SlGLK2 gene not only have fewer chloroplasts, they also sport less sugar.

So far, this is just a hypothesis. The real culprit affecting tomato flavor could also be a production system that picks tomatoes before they are ripe.

Read the paper in Science 29 June 2012: Vol. 336 no. 6089 pp. 1711-1715 DOI: 10.1126/science.1222218.