For about 70 years, breeders have selected tomato varieties with uniformly light green fruit before ripening. These tomatoes then turn red evenly as they ripen, and they look nice in a supermarket display. Researchers now have pinpointed the molecular changes responsible for this “uniform ripening” trait of many modern tomatoes. But these changes, they show, also reduce the fruit's sugar content.
Ann Powell of the University of California, Davis and colleagues report that the gene at the heart of uniform ripening encodes a protein called GLK2. This protein increases the fruit's photosynthetic capacity, helping along the production of sugars and lycopene, the pigment that gives a ripe tomato its brilliant color. Breeding the tomatoes to contain the “uniform ripening” mutation disables GLK2, however. This change has the unintended effect of impairing the development of chloroplasts, the structures in plant cells that enable plants to photosynthesize. Impairing their development decreases the production of key ingredients that give tomatoes their sweetness.