Q J Med 2002; 95: 553-554
© 2002 Association of Physicians
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Worried about your diet?
Fruit is supposed to be enormously important in terms of our (ill-defined) general health. Although no-one knows what five portions are, and the data for specific disease risks are few, the public health message to eat fruit and vegetables is clear—rather like ‘Go to work on an Egg’ or ‘Drink a Pint of Milk a Day’. But this sceptical view does not conceal the fact that ripe fruits are an important part of the human diet.
The maturation and ripening of fleshy fruits varies considerably in style among plant species and involves changes in colour, texture, flavour and smell, all of which must be co-ordinated. These changes are also accompanied by changes in nutritional values. Those accustomed to human crises and the use of words in a particular way must adopt a Tweedeldumic point of view here, for there are two types of ripening: climacteric and non-climacteric.
In climacteric ripening, there is a burst of respiration at the beginning of the process that is missing in the non-climacteric type. This respiratory burst increases the production of ethylene, which is needed for the ripening of fruits such as tomatoes, bananas, apples, and most stone fruit, and which has been used as a gaseous hormone to promote these changes. In non-climacteric ripening (strawberries, grapes, lemons, oranges), ethylene is not important.
Now, the tomato is as useful as the tobacco plant in experimental botany, and a great deal of pre-existing data make it the experimental plant of choice for some types of work (like the Beagle dog in pre-clinical toxicology). There are lots of tomato-ripening mutants and some recessive forms will inhibit all ripening phenomena including the respiratory climacteric, carotenoid accumulation, softening and the acquisition of flavour—supermarket tomatoes do not all have these mutations and the smell of tomatoes is meant to come from the stem.
The recessive ripening-inhibitor (rin) mutation stops all of the desirable activities described above and the plant does not respond to exogenous ethylene. In some beautiful work, Vrebalov and her colleagues3 have shown that the gene belongs to a family of transcription factors connected to floral development and is probably phylogenetically ancient (repression of the gene in one context cause homeotic conversion of sepals to leaf-like structures). Similar sequences have been found in other Solanaceae (peppers and petunias—the fruits of petunias are not fleshy).
In one sense I told a lie about the supermarket, for the rin mutation is used to yield fruit with a long shelf life and acceptable quality. Heterozygotes stay firm and ripen slowly, allowing handling packaging and transportation. It might be possible to add convenient genes to tomatoes, for the effects of these transcription factors are additive.
It is odd that people worry about this sort of thing. Did you know there are two sorts of photosynthesis? C3 and C4. The terminology refers to the number of carbon atoms in the first molecule created when atmospheric CO2 is assimilated by the plant. In C3 plants, CO2 reacts with ribulose 1,5-bisphosphate (RuBP), catalysed by an enzyme called ribulose 1,5-bisphosphate carboxylase–oxygenase (Rubisco). The product is phosphoglycerate, an organic molecule with a backbone of three carbon atoms, which is then used to build sugars and more complex carbohydrates
In C4 plants, CO2 is assimilated using a different enzyme—phosphoenolpyruvate carboxylase (PEPC). This attaches CO2 to the three-carbon compound phospho-enolpyruvate (PEP) to produce oxaloacetate, a molecule that contains four carbon atoms. Oxaloacetate can be converted into a series of other four-carbon products, including malate, citrate and aspartate. These four-carbon molecules are broken back down into three-carbon compounds and CO2, which is then fed to Rubisco.
This two-stage process is more efficient, because it allows C4 plants to boost the levels of CO2 reaching Rubisco. PEPC has a stronger affinity for CO2 than Rubisco does, and so can work better at low concentrations of the gas. In addition, Rubisco is inhibited by oxygen—an unavoidable product of the light-harvesting stage of photosynthesis—whereas PEPC is unaffected.
On at least 30 separate occasions, different plant lineages have evolved to use the Sun's energy in this more efficient way, starting from about 10 million years ago, when falling concentrations of CO2 in the atmosphere gave plants using C4 photosynthesis an important selective advantage. The ancestors of maize were among these plants. But rice, wheat and most other cereals all use conventional C3 photosynthesis. The major play in plant genetic engineering is to convert rice to C4 photosynthesis—that's what the fuss about the genome of Oryza sativa is about.
Now if genetic engineering of plants bothers you, you've got something to worry about. On the other hand, it will make the first ‘green revolution’ look tame, in terms of yield, and will reduce fertilizer inputs to crops enormously. Those who need rice, need science.
References
1. See the review by Surrige C. Nature2002; 416:576–8.[Medline]
2. Evolutionary options. Raven JA. Nature2002; 416:375–7.[Medline]
3. A MADS-box gene necessary for fruit ripening at the Tomato Ripening Inhibitor (Rin) locus. Vrebalov J, Ruezinsky D, Padmanabhan V, White R, Medrano D, Drake R, Schuch W and Giovannoni J. Science2002; 296:343–6.
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