Fresh vegetables out of a garden almost always taste better than what you get in a grocery store, but nowhere is this difference more stark than with tomatoes. Those hard, red-orange-ish, tasteless orbs at the grocery store hardly seem lie the same substance at all as those incredibly sweet, juicy ones that come out of a garden. Here’s why:
WOULD you rather have tomatoes that look good, or taste good? Most people, no doubt, would swear that they prefer taste to looks when it comes to buying fruit and vegetables. But that is not how they behave. Years of retailing experience have shown that what actually gets bought is what looks good. And, unfortunately, for tomatoes at least, that is not well correlated with taste. A uniformly red skin – the sort preferred by consumers – is associated with a “cardboardy” flavour. But until now, nobody knew why.
The answer is provided by a paper in Science, written by by Ann Powell of the University of California, Davis, and her colleagues. The reason turns out to lie deep in the genetic regulation of photosynthesis. For 70 years, tomato breeders have sought fruit that ripen evenly. For that to happen, they need to start from a state of uniform light-greenness. Older varieties of tomato, by contrast, are dark green over the part of the fruit nearest the stem.
Those decades of selective breeding have done what was required. Traditional genetics identified a gene known as u (for “uniform ripening”). This, in classic Mendelian fashion, came in two forms, a dominant and a recessive. Dominant versions of a gene always trump recessive ones, so the recessive characteristic emerges only when both of a plant’s parents contribute a recessive version of the gene to their offspring. Identifying strains with the relevant recessives, and then cross-fertilising them, is the sort of thing that plant breeder are good at. But what they did not know was exactly what sort of gene u actually is…
The gene in question was for a type of protein known as a transcription factor. Transcription factors are molecules that regulate the expression of other genes and the factor in question is one that is known, in other plants, to regulate chlorophyll distribution, and thus photosynthesis.
Since about 10% of the sugars in an old-fashioned tomato are produced by photosynthesis in the fruit itself, rather than being transported in from elsewhere, and since making those sugars also results in other flavoursome molecules derived from them, Dr Powell thinks she has found the explanation for cardboard tomatoes.
So now we know why those nasty things in the local grocery store that they call tomatoes are so inedible.
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