Research uses parts of bacteria to induce resistance in tomato plants

With an unprecedented approach, scientists managed to reduce the damage caused by tomato bacterial spot, an important disease of the crop, by 45%. The Embrapa team used parts of bacteria of the genus xanthomonas, such as their cell wall and flagella, to induce resistance in plants. The results could give rise to a bioinput capable of reducing the need for chemical products used today against the disease.

In addition to low toxicity, the new product can be applied in small quantities, providing environmental and economic advantages. Embrapa is negotiating with a private partner to complete the research and bring the product to the market.

The bacteria used to induce resistance are not the ones that cause tomato disease. Even so, they stimulated the plants' immune systems. “These parts of the microorganism used, especially the cell wall, triggered rapid recognition by the plant, stimulating its defense system. We call these structures elicitors, triggers that trigger the process”, details researcher at Embrapa Meio Ambiente Bernardo Halfeld, coordinator of the study. The mechanism is analogous to the mode of action of vaccines, which also provoke the immune system through microorganisms, or their parts, without causing harm to health.

The bioinput was applied using two methods: sprinkling on the leaves (spraying) and dripping onto the soil. Both were successful. A future commercial product could have similar application methods.

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The elicitors, derived from phytopathogenic bacteria, present molecular patterns capable of being recognized by receptors present in the plant cell. When the elicitors of a xanthomonas are recognized by non-host plant receptors, mechanisms are activated that prepare the plant to trigger acquired systemic resistance.

In the research, 12 isolates of xanthomonas were  infiltrated into the plants. Ten showed hypersensitivity, a cell death reaction that occurs intensely, localized and quickly when the host cell recognizes that it is being attacked. When it dies quickly, the infection process for neighboring cells stops and the disease does not progress. 

The hypersensitivity reaction results in the rapid and localized death of a limited number of cells surrounding the site of infection when incompatible bacterial cells enter the plant, interrupting the growth and development of the pathogen in plant tissue, being one of the most important defense mechanisms of plants.

The elicitors, however, when extracted and applied to tomato plants, did not cause damage to the plants, but were able to prepare them to respond to the disease once the infection process began.

Research

According to Camila Simões, a master's student at Unesp who worked on this study, these cellular components of xanthomonas trigger rapid incompatible recognition by the tomato plant. It was based on the fact that bacteria of this genus, which are incompatible with tomato, constitute a source of elicitors capable of triggering a response that results in a reduction in the severity of bacterial spot, with prospects for practical use.

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Researcher Bernardo Halfeld explains that prospecting for elicitors is the first step towards developing a product for agricultural use. “Through partnerships with companies in the bioinput sector, it is possible to promote the obtaining of formulas with these molecules on a large scale,” he says. Furthermore, the results may offer an alternative to control other tomato diseases.

Halfeld explains that the study should boost the process of selecting biological components for the control of bacterial diseases. “This is because, in several crops of agronomic interest, there are bacteria of the genus xanthomonas who are potential providers of elicitors of this nature”, he declares. Therefore, there is the prospect that similar products could be developed for other crops and for different diseases.

In addition to its low toxicity and being able to be applied in small quantities, providing environmental and economic advantages, it can replace inputs of chemical origin currently used to combat the disease. “Today, to combat bacterial spot on tomato plants, the most commonly used input is a copper-based fungicide,” says the researcher.

In the next stage, the researcher and Unesp doctoral student Valdeir Nunes intend to expand the studies to define the effect of these elicitors on the disease and on the plant up to the productive stage. The objective is to verify the effect of the technology throughout the tomato cycle, combined with different concentrations and other modes of application of elicitors that can promote a greater range of options for the producer. 

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According to the 2017-2018 Family Budget Survey, from the Brazilian Institute of Geography and Statistics (IBGE), on average, each Brazilian consumes 4,2 kg of tomatoes per year, making it the most popular vegetable among Brazilians. Production approaches 4 million tons, placing Brazil in ninth position worldwide. Culture generates R$5,7 billion per year. However, as it is a fruit with a relatively short shelf life, it faces constant risks, which haunt the approximately 50 producers and 212 million consumers: diseases, insects and damage caused by bacterial diseases in tomato, more pronounced in the period rainy, which can reach 100%.

One of the main threats, bacterial spot, is difficult to control and requires the use of copper, whose active molecule is copper. The more you use this product, the more bacteria become resistant to it, causing a loss of efficiency over time. The continuous application of pesticides can also select resistant strains, causing certain active ingredients to lose their effectiveness.

Production

More than half of Brazilian tomato production is concentrated in two states: Goiás (29% with 1,1 million tons) and São Paulo (23%, with 918 thousand tons). Minas Gerais participates with 13% of the total: around 550 thousand tons. There is also significant production in Bahia (6%) and Paraná (5,9%), which exceed 200 thousand tons per year. Espírito Santo (4,2%), Santa Catarina (4,1%), Ceará (4%), Rio de Janeiro (3,6%) and Rio Grande do Sul (2,7%) exceed 100 thousand tons per year. The data are from IBGE, 2020.