Four Amino Acids Explain Why Wild Tomatoes Outsmart Their Deadliest Pest

Why can a wild tomato shrug off an attack that devastates commercial fields? An international team of researchers from China and Brazil has traced the answer down to a surprisingly small molecular detail: the three-dimensional handedness of a single plant defense chemical, and the four amino acids in one enzyme that decide which version the plant makes. The work, supported by FAPESP and published in Nature Communications, reveals that swapping just four amino acids is enough to change the spatial structure of the defense molecule and increase its toxicity against the tomato leafminer (Phthorimaea absoluta), the pest that causes the greatest damage to tomato crops worldwide. The finding points toward a new route to resistant tomatoes through precise gene editing, without introducing any foreign genes.

The study was coordinated by Jiancai Li, a professor at the Shanghai Institute of Plant Physiology and Ecology and at the University of the Chinese Academy of Sciences in Beijing. José Maurício Simões Bento, a professor at the Luiz de Queiroz College of Agriculture at the University of São Paulo (ESALQ-USP), also took part. Li’s team specializes in the genetics, biochemistry and biosynthesis of specialized plant metabolites, while Bento’s group studies chemical ecology and plant-insect interactions. The collaboration grew out of a joint call for proposals issued in 2024 by FAPESP and the National Natural Science Foundation of China (NSFC) following FAPESP Week China. “Integrating these fields was crucial. Once the genetic and biochemical mechanisms responsible for the different molecular forms were identified, it became crucial to grasp the biological ramifications of those differences and evaluate whether a slight alteration in the three-dimensional structure of a molecule could impact a plant’s capacity to defend itself against a pest,” Bento explains.

The researchers focused on steroidal glycoalkaloids, or SGAs, natural compounds produced by plants of the genus Solanum, which includes tomatoes, potatoes and eggplants. These substances form part of the plants’ chemical armor against herbivorous insects and pathogens. In the leaves of cultivated tomatoes (Solanum lycopersicum), the dominant SGA is α-tomatine. In the leaves of the wild tomato Solanum pennellii, however, a related compound called filotomatine predominates. The two molecules carry exactly the same sugar content, yet the three-dimensional structure of their molecular cores differs. That subtle difference in architecture turned out to carry real ecological weight.

The key concept is chirality. A molecule is chiral when its structure cannot be perfectly superimposed on its mirror image, much like a pair of hands: the same parts, arranged in mirror-image fashion, yet impossible to fit one over the other. Chiral molecules can therefore exist as a pair of enantiomers, right-handed and left-handed versions of the same structure. “Life itself is a highly chiral environment. Amino acids and sugars, for example, predominantly exist in one of their enantiomeric forms. Therefore, when faced with a specific biological target, only one configuration can properly fit into the cavity or groove of a protein while the other can’t,” Li explains. In other words, in biology, shape is not a cosmetic detail; it determines whether a molecule can interact with its target at all.

Experiments with insects demonstrated this principle directly. Filotomatine, predominant in wild tomato leaves, provided greater protection against the tomato leafminer than α-tomatine, the compound that predominates in cultivated varieties. The result appeared both in experiments on plants and in bioassays using an artificial diet, in which insects were fed a laboratory-prepared diet with the test compounds added separately. That setup allowed the researchers to compare the effects of the two molecules on the insects without interference from other plant characteristics. “One of the most interesting aspects of the study is that it shows that in plant defense, it’s not only the presence or quantity of a substance that matters but also how its molecules are organized in space,” says Bento. “α-tomatine and filotomatine are extremely similar molecules, but they differ in their three-dimensional configuration. The study showed that that difference is biologically relevant.” The exact molecular target of these glycoalkaloids inside the insect is not yet known, but Li believes stereospecific recognition, the ability of a biological target to distinguish between different spatial configurations, likely underlies the observed difference.

To find where the chemical difference originates, the team investigated the GAME8 gene, which encodes an enzyme involved in the biosynthesis of these glycoalkaloids. Wild tomatoes carry two very similar versions, GAME8a and GAME8b. The enzyme produced by GAME8a predominantly directs the formation of the configuration associated with filotomatine, while the enzyme from GAME8b favors the configuration associated with α-tomatine. In cultivated tomatoes, the two GAME8 genes have identical coding sequences, and their enzymes predominantly favor the α-tomatine configuration. The enzymes act on a glycoalkaloid precursor molecule and determine which spatial configuration will be formed at a decisive stage of the reaction chain. In wild tomatoes, the GAME8a enzyme favors the R configuration, leading to filotomatine, while GAME8b favors the S configuration, leading to α-tomatine. The letters R and S come from the Latin rectus (right) and sinister (left), the standard labels for the two possible spatial configurations around a chiral center.

The researchers then asked what within the enzymes themselves caused this divergence. Although GAME8a and GAME8b are very similar, they identified four amino acids that play the decisive role. “Those four amino acids are located around the enzyme’s catalytic cavity and determine the orientation of the precursor molecule inside it. That orientation defines which hydrogen atom will be modified and consequently the final configuration of the product,” Li explains. The mechanism can now be described at the molecular level: the precursor fits into a cavity within the enzyme, and the four amino acids help determine its position. Depending on its orientation, one or the other hydrogen atom of the precursor ends up positioned for modification, which determines whether the product carries the R or S configuration. The chirality of tomato glycoalkaloids can thus be traced through a straightforward chain: differences in the gene, differences in the enzyme, different positioning of the precursor molecule, R or S configuration, and finally filotomatine or α-tomatine. “The Chinese team managed to reveal the molecular mechanism with great precision,” Bento comments.

One of the most intriguing findings emerged when the two molecules were tested against a different insect. Filotomatine and α-tomatine showed similar defensive effects against Spodoptera litura, a generalist herbivore known as the tobacco cutworm or cotton leafworm that feeds on many plant species. The superiority of filotomatine is therefore not absolute; it depends on which insect is trying to feed on the plant. According to Li, this supports the hypothesis that each molecular configuration’s activity depends on its interaction with specific targets within the insect’s body. “Our results suggest that filotomatine isn’t simply more toxic in a general sense. If that were the case, we’d expect all wild tomato species to accumulate filotomatine,” he argues. Instead, different wild species accumulate different configurations, suggesting each chemical profile may offer specific advantages against the herbivores in a given environment. “That suggests that plants’ chemical defenses may evolve in response to very specific interactions with certain herbivores rather than simply to make plants more toxic in general,” Bento notes.

The results also raise an evolutionary question. Solanum pennellii possesses the genetic capacity to produce both isomers, yet it predominantly accumulates filotomatine in its leaves and α-tomatine in other tissues. Since leaves are among the main targets of herbivores, this distribution may not be coincidental. “Currently, our evidence is correlational. We observed that filotomatine provides greater resistance to P. absoluta, which naturally interacts with S. pennellii. Since leaves are the organs most exposed to herbivorous pests, we interpret that as a likely result of long-term coevolution between the plant and its specialized herbivores,” Li says, while cautioning that definitive proof of such an evolutionary scenario is extremely difficult to obtain.

The identification of the four amino acids that control the GAME8 enzyme’s preference moves the work from fundamental biology toward agricultural application. In principle, the cultivated tomato’s gene could be edited so the plant produces filotomatine and gains resistance to P. absoluta. “From a technical standpoint, modifying GAME8 so that the tomato plant accumulates filotomatine is perfectly feasible. We can do that by editing the four key amino acids using current biotechnologies without introducing exogenous genes,” says Li. But caution is warranted. Because a configuration more effective against one insect species is not necessarily superior against others, the modified plants would need testing against other major tomato pests such as whiteflies. Glycoalkaloids also matter beyond defense: altering their metabolism can affect agronomic and nutritional traits, so fruit quality, flavor and antinutritional properties must be evaluated before any such modification reaches cultivars intended for consumption. A long series of tests still lies ahead. Yet the study establishes a broader principle with wide appeal: improving a plant’s chemical defense does not necessarily require producing more of a toxic substance. Sometimes, changing the spatial orientation of a molecule the plant already makes is enough.

Subject of Research: How the chirality of steroidal glycoalkaloids, controlled by four amino acids in the GAME8 enzyme, determines tomato resistance to the tomato leafminer.

Article Title: Molecular architecture determines tomato plant resistance to its main pest

Article References: Molecular architecture determines tomato plant resistance to its main pest. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: tomato, tomato leafminer, Phthorimaea absoluta, steroidal glycoalkaloids, chirality, GAME8, plant defense, gene editing, Solanum pennellii, chemical ecology, Nature Communications, FAPESP