Tomatoes used to taste better too…

Geneticists have found a way to return tomatoes to their former flavor

Alexander Ershov, N+1

Geneticists and chemists from China and the USA have experimentally confirmed the widespread opinion that modern tomato breeding varieties are significantly inferior in taste and aroma to old cultivars and even their wild ancestors. In an article published in Science (Tieman et al., A chemical genetic roadmap to improved tomato flavor), scientists not only identified the genetic changes that accompanied this degradation, but also proposed several point interventions that could restore tomatoes to their former aroma.

In the last few years, thanks to the strong reduction in the cost of DNA reading technology, scientists for the first time managed to begin to really understand the consequences of traditional breeding in agriculture. Breeding is a blind process by its very nature: breeders make random changes to the DNA of the original varieties, and then select the best (according to some pre-selected criteria) samples for subsequent breeding. What background changes occur in cultivars – both at the genome level and at the phenotype level – breeders almost never know. To establish this, it is necessary to conduct a separate study, which involves sequencing large volumes of DNA and generally goes beyond the selection approach. It has only recently become possible to conduct such studies.

For example, the work of geneticists from the USA and Spain, published in 2012, showed that in the case of breeding according to criteria convenient for producers, simultaneous degradation of the taste of the product is possible, which the breeder may not even pay attention to. We are talking about such a feature of modern cultivated tomatoes as uniform ripening. The fruits of modern varieties at each stage of maturation usually have a uniform color, unlike the wild ancestors of tomatoes, which ripen from tip to petiole and may even begin to crack even before they turn completely red. This is very inconvenient for the producer, so evenly ripening varieties are significantly more popular than the ancestral form.

Tomatoes with unevenly and evenly spiced fruit (Powell et al., Science 2012)

However, such a seemingly good breeding feature, as it turned out, simultaneously carries with it a significant decrease in the taste qualities of the product. In the 2012 paper under discussion, scientists for the first time established that the uniform ripening of tomatoes is associated with a point mutation that turns off one specific gene – GLK2, it stimulates the development of chloroplasts in the immature fruits of many plants. In breeding tomatoes, this gene was turned off by a random mutation, as a result of which they acquired a nice uniform color without green spots in the cuttings. However, the decrease in the number of chloroplasts at the same time significantly affected the amount of nutrients produced in the fruits. As a result, such varieties have become more watery and tasteless than their ancestors.

A new article by scientists from the USA and China continues the line of research set by the work of 2012, but brings it to a completely different scale. In this case, scientists investigated not just the consequences of mutations of one particular gene, but conducted a mass analysis of most modern and many old traditional tomato varieties by dozens of different parameters. Scientists sequenced 328 varieties of tomatoes and their wild ancestors, conducted their chemical analysis, and a third of the studied varieties were further studied in experiments with volunteers.

The study began by identifying a set of chemical compounds whose presence is of paramount importance for the taste of tomatoes. To do this, the scientists asked the participants of the experiment to evaluate the organoleptic qualities of a hundred different varieties and analyzed the correlation of the obtained score with the concentration of volatile and non-volatile substances in the fruits (it was determined on a chromatograph). As a result, we managed to obtain a set of 27 compounds that are most important for the taste of tomatoes (the composition of this list was determined in an experiment, but its length, of course, is purely subjective). Further, the concentration of all 27 substances in each of the studied varieties was used as a key feature for the search for genetic polymorphisms: scientists were looking for such genetic features that would be associated with an increased concentration of any of these substances. The data obtained are presented in the form of so–called Manhattan graphs, in which the influence of individual sections of the genome on the taste of tomatoes is presented in the form of "skyscrapers" - the higher they are, the more significant the relationship between a specific gene and a trait.

Manhattan graph on the content of all dry matter in fruits (orange). Two genes are visible that affect this trait to the greatest extent – Lin5 and SSC11.1. Below, a strong drop in diversity in the loci of these genes is visible (the blue graph drops compared to the yellow and green), which reflects the selection sweep (Denise Tieman et al., Science, 2017).

It is difficult to draw all possible relevant conclusions from the obtained data set. Obviously, this will be done not only by the authors of the work, but also by other scientists. However, the authors themselves discuss three separate stories that reflect how selection influenced the taste of modern tomatoes.

Firstly, scientists have found a fairly strong negative correlation between the size of fruits and the concentration of sugars (glucose and fructose) in them, as well as any soluble substances in general. As it turned out, the saturation of the taste of tomatoes was closely related to at least two genes (in addition to the already mentioned GLK2), these are the Lin5 and SSC11.1 genes. Ancestral forms of these genes, represented in the wild type and old "family" varieties, lead to the growth of smaller, but at the same time more saturated with organic substances fruits. Interestingly, the fixation of alternative forms of these genes in modern varieties – those that lead to the formation of more watery tomatoes – clearly did not happen by chance. This is evidenced by the analysis of the genetic environment of the Lin5 and SSC11 genes, where traces of so-called selective sweeping, i.e. directed selection to preserve only the current variants, are visible. Obviously, this picture is explained by the fact that breeders for many generations tried to breed varieties with as large fruits as possible, which led to the consolidation of "watery" variants of the Lin5 and SSC11 genes.

Secondly, scientists have found that the weak aroma of modern tomatoes is closely related to the increased activity of a certain protein, which for some reason has become entrenched in modern varieties. We are talking about the E8 protein, which is involved in the regulation of the production of the "maturation hormone" ethylene. In varieties where it is active – and this is the vast majority of modern varieties – odorous substances with an unpleasant smell for consumers are produced in significant concentrations (methyl salicylate and guaiacol are substances with a "medical" smell, their concentration in modern varieties is increased fivefold). At the same time, substances that people associate with fragrant tomatoes in such varieties are contained in a lower concentration. Fortunately, this problem is solved quite simply – by suppressing the synthesis of the E8 protein, for example, using RNA interference. Thus, the new work points a direct path to the creation of more flavorful GMO tomatoes.

Thirdly, the authors of the article showed that the subjective selection of convenient selection criteria by breeders can lead to an imperceptible change in the usual smell of vegetables and fruits. It is known that a significant role in the flavor of tomatoes is played by the oxidation products of carotenoid pigments (the same ones that make carrots orange, and tomatoes and cranberries red). Moreover, the amount of odorous substances produced is directly related to the amount of pigment: the more the starting substance, the more volatile reaction products. However, different odorous substances are formed from different carotenoids, and not all of the pigments exist in fruits in such concentrations as to be noticeable to the eye. As a result, a situation may arise when the selection for the amount of visible pigment, without taking into account those that are not visible, leads to a strong change in the aroma of the fruit – after all, even insignificant amounts of volatile substances, whose precursors are not visible to the eye at all, are enough for the aroma.

As the analysis of scientists showed, this is exactly the situation with the odorous substances geranylacetone and 6-methyl-5-heptene-2-one (or MHO). The first of them is synthesized from minor carotenoids, such as phytoene, and the second is produced directly from lycopene, the main red pigment of tomatoes. As a result of breeding in modern varieties, the concentration of the first substance was greatly reduced compared to ancestral varieties, and the concentration of the second increased. Since the changes occurred smoothly and it was difficult to fix them without special analysis, the aroma of modern tomatoes became a victim of the breeders' craving for bright red fruits.

The good news, according to scientists, is that most of the negative changes in the aroma and taste of fruits are easily correctable, since they do not require a sharp reduction in yield. Volatile substances are contained in fruits in trace amounts, so that strengthening their production will not require significant costs from the plant. It can be achieved by introducing "fragrant" gene variants into the genome, which were discovered in this work. This will potentially make tomatoes much tastier than all currently existing varieties – both old and new. 

It should be noted that the aroma and taste of tomatoes is determined not only by the nature of the variety and genetic characteristics. It also strongly depends on the conditions of transportation. So, recently scientists have shown that storing tomatoes in the refrigerator irreversibly spoils their taste. It turned out that lowering the temperature stops the maturation processes associated with the production of ethylene, which leads to a significant decrease in the aroma of products.

Portal "Eternal youth" http://vechnayamolodost.ru  30.01.2017