Stress-Response Compound Widespread in Animals Is Found in Plants

molecule made famous by its association with human heart disease and marine animals’ ability to survive high-pressure conditions turns out to be made by plants too, researchers report this week (May 19) in Science Advances. As it does in animals, trimethylamine N-oxide (TMAO) helps plants cope with stressful conditions, according to the study. The authors have already licensed the discovery to a company that is working to commercialize TMAO as a way to boost yields in agriculture.

“Nobody has published before that plants have TMAO in the tissues,” says study coauthor Rafael Catalá of the Centro de Investigaciones Biológicas (CIB) Margarita Salas in Madrid.

The new study grew out of earlier work in which Catalá and his colleagues looked for genes in the model plant Arabidopsis thaliana whose expression was changed by exposure to cold. One gene they found turned out to code for a type of enzyme called a flavin-containing monooxygenase (FMO) called FMOGS-OX5. In further analyses, reported in the current study, the team found that the expression of several other FMOgenes is also dialed up in Arabidopsis in response to cold.

FMOs are known to make TMAO in animals in response a variety of stressors. Wondering what the connection was between the FMOs and the plant’s cold response, the team used nuclear magnetic resonance to look for TMAO in wildtype Arabidopsis. They found it, and confirmed its presence with liquid chromatography–tandem mass spectrometry. The team also verified that FMOGS-OX5 can generate TMAO from its precursor, TMA, in vitro.

In animals, TMAO functions as an osmolyte, a type of molecule cells use to maintain the properties of their fluid and prevent proteins from becoming misfolded when confronted with conditions such as high salt concentrations. To see whether it plays a similar role in plants, Catalá and his colleagues treated Arabidopsisroots with tunicamycin, a compound that makes proteins unfold, as can happen under abiotic stress conditions such as cold or lack of water. The tunicamycin made the roots grow more slowly, but this effect was mitigated if the roots were grown in medium supplemented with TMAO, the researchers report.

When the researchers engineered Arabidopsis to overexpress FMOGS-OX5, the plant also increased the expression of 184 other genes, many of which had been previously linked to responses to abiotic stressors, the authors report. Applying TMAO to wildtype plants had a similar effect on gene expression, although it did not change FMOGS-OX5’s expression level, suggesting that TMAO acts downstream of FMO to enhance the expression of stress-response genes.

To find out whether TMAO is widespread in plant species, the team also looked for it in tomato, maize, barley, and a relative of tobacco, and found it was present in all of them. Moreover, their TMAO content rose when the plants were subjected to conditions of low water, high salt, or low temperatures (except barley, in which TMAO did not increase in the high-salt test but did in the other conditions). Spraying or watering tomato plants with a TMAO-containing solution made them visibly healthier, with more leaves, when they were exposed to each of the three stress conditions.

Catalá says externally applied TMAO has the potential to be “a very powerful tool for agriculture.” He and the paper’s senior author, Julio Salinas, also of the CIB Margarita Salas, have filed patents on the agricultural use of TMAO, which is being commercialized by the company Plant Response. The company’s field tests have had good results, Catalá adds.

Paul Verslues, who studies plant drought response at the Academia Sinica in Taipei, Taiwan, questions whether TMAO will be useful agriculturally. “TMAO protection of protein folding may be relevant to plant survival of severe stress but it is unknown whether it is also beneficial to protecting plant growth under less severe drought or salinity stress,” he writes in an email to The Scientist. The stresses the researchers subjected the plants to were too harsh to be reflective of agricultural conditions, and more experiments would be needed to determine whether TMAO also helps plants cope with milder stress conditions.

Verslues also notes other reservations about the study’s findings, including that Arabidopsis made to overexpress FMOGS-OX5 had greater stress tolerance than did wildtype plants but did not accumulate more TMAO, which he says suggests that FMOs may “also produce some other compound that promotes stress tolerance” apart from TMAO. Additionally, the authors did not take the step of knocking out all of a plant’s FMO genes to test whether those genes are truly required for TMAO production in plants.

Catalá argues that the study’s main finding, that TMAO exists in plants and has “a key role in plant tolerance to abiotic stress,” stands without testing such mutants. And he says it’s likely that FMOs do indeed produce other compounds involved in the stress response, but that the paper shows they are involved in making TMAO and that TMAO enhances stress tolerance.

Aleksandra Skirycz, a plant biologist at the Boyce Thompson Institute who was not involved in the study, calls it “a very nicely designed story.” For her, the “really exciting aspect of this work is that you have a molecule that would work as an osmolyte for protection [and] at the same time would probably have other signaling functions,” a phenomenon she calls “moonlighting.” It’s not yet clear how TMAO influences gene expression, Catalá says, and that will be an avenue for the group to pursue in the future.

In the biomedical literature, TMAO tends to come up in a negative context rather than a positive one, as high levels of it in patients’ blood have been linked to an elevated risk for blood clots. Studies have suggested that gut microbes break down choline, a nutrient present in high levels in meat, to generate TMAO and related compounds, providing a mechanistic link between a meat-heavy diet and risk of heart attack and stroke. Catalá says it’s not at all clear what implications, if any, the finding of TMAO in plants could have for human diet and health.

Story by: Shawna Williams for The Scientist