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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.
To find out, Dr Powell and her colleagues looked in the part of a tomato’s genome that Mendelian genetics shows is where u is found. This has been worked out over years of intensive study of the process by which genes are mixed up during fertilisation. Such mixing shows approximately where on a chromosome a gene is located. When they sequenced the DNA of this region, the team found eight genes, any one of which might, in principle, have been u. But they discovered that in all cases where the version of u in the plant was recessive, there was one gene out of the ten that was broken. An extra genetic letter inserted into its DNA caused the genetic equivalent of a full stop in the message, meaning that the protein produced from the gene was too short, and did not work properly.
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.
Whether this discovery actually helps is moot. Any tinkering that brought back the flavour by manipulating the transcription factor would probably also bring back the original uneven colouring. But at least you now know that when your grandmother tells you that tomatoes tasted better when she was a girl, science will back her up.
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