It’s no secret that the mass-produced tomatoes we buy at a typical grocery store tend to taste like cardboard. Now researchers have discovered one reason why: a genetic mutation, common in store-bought tomatoes, that reduces the amount of sugar and other tasty compounds in the fruit.
Mass-produced tomato varieties carrying this genetic change are light green all over before they ripen. Tomatoes without the mutation — including heirloom and most small-farm tomatoes — have dark-green tops before they ripen. There is also a significant difference in flavor between the two types of tomatoes, but researchers had not previously known the two traits had the same root cause.
The study authors set out to pin down the genetic change that makes tomatoes lose their dark-green top. They focused their attention on two genes — GLK1 and GLK2 — both known to be crucial for harvesting energy from sunlight in plant leaves.
They found that GLK2 is active in fruit as well as leaves — but that in uniformly colored tomatoes, it is inactivated.
Adding back an active GLK2 gene to bland, commercial-style tomatoes through genetic engineering created tomatoes that had the heirloom-style dark-green hue. The darker green comes from greater numbers of structures called chloroplasts that harvest energy from sunlight.
The harvested energy is stored as starches, which are converted to sugars when the tomatoes ripen.
The vast majority — 70% to 80% — of the sugar in tomatoes travels to the fruit from the leaves of the plant. But the remaining amount of sugar is produced in the fruit. This contribution is largely wiped out in uniform, commercial-style tomatoes — and thus they won’t be as sweet.
For the science nerds, here is the paper’s abstract:
Modern tomato (Solanum lycopersicum) varieties are bred for uniform ripening (u) light green fruit phenotypes to facilitate harvests of evenly ripened fruit. U encodes a Golden 2-like (GLK) transcription factor,SlGLK2, which determines chlorophyll accumulation and distribution in developing fruit. In tomato, two GLKs—SlGLK1 and SlGLK2—are expressed in leaves, but only SlGLK2 is expressed in fruit. Expressing GLKsincreased the chlorophyll content of fruit, whereas SlGLK2 suppression recapitulated the u mutant phenotype. GLK overexpression enhanced fruit photosynthesis gene expression and chloroplast development, leading to elevated carbohydrates and carotenoids in ripe fruit. SlGLK2 influences photosynthesis in developing fruit, contributing to mature fruit characteristics and suggesting that selection of u inadvertently compromised ripe fruit quality in exchange for desirable production traits.
It’s no wonder that tomatoes you can grow in your backyard taste that much better.
(via Los Angeles Times)