Researchers from the Department of Microbiology, the University of Malaga , and the Institute of Subtropical and Mediterranean Horticulture 'La Mayora' (mixed center of the University of Malaga and the CSIC) have shown the protective effects of composted almond shells against the fungus that rot the root of avocado trees. This material modifies the microbial communities of the soil and favors the development of bacteria involved in the production of antifungals, that is, of substances that inhibit or delay the growth of the fungus. The study opens the door to the identification and characterization of new antibiotic-producing microorganisms that help reduce the current chemical treatments used in agriculture.
The almond shell is an organic amendment, that is, its effects are like those of manure, but it is of plant origin, rather than animal. Its function is to condition the soil. For example, it balances the amount of water or provides aeration, nutrients and microorganisms that can have a beneficial effect on the plant.
This organic matter cannot be used fresh because it is very reactive, with negative consequences on the soil such as a decrease in oxygen. That is why it is composted: that is, it undergoes a process where it degrades and decomposes, slowly, by the action of the microorganisms that make it up.
The key: carbon
En esta actividad de compostaje participa un grupo de hongos que se aprovechan de la cáscara de almendra para crecer. “La enmienda orgánica modifica el suelo que hay debajo. Por ejemplo, aparecen poblaciones específicas de microorganismos que se desarrollan mejor en este ambiente modificado. Entre ellas, estos hongos que degradan la cascara”, explica a la Fundación Descubre el investigador responsable de este proyecto, Francisco Cazorla, de la Universidad de Málaga.
According to experts, with the increase of these organisms, there is an increase in available carbon. The extra contribution of this element is used, in turn, by a range of bacteria to increase their number. “These organisms are the ones that are related to the biological control activity. In other words, they are involved in the production of antifungal antibiotics that protect the plant from the Rosellinia necatrix plague , the fungus that causes root rot ”, he argues.
With this study, the researchers provide a scientific basis for a common practice in the Axarquía region of Malaga. “Farmers have been using almond peel for years as an organic amendment in this area, where one of the problems of avocado cultivation is soil fungi. They noticed improvement in the tree but the reason was unknown ”, says the expert.
Soils with immunity
To carry out this research, which is published in the journal Frontiers in Microbiology , under the title 'Microbial profiling of a suppressiveness-induced agricultural soil amended with composted almond shells' , the scientists worked on an experimental avocado farm over 40 years old, grown in 'La Mayora', where they have applied composted almond shells since 2002. The results were compared with those of another group of avocado trees not treated with this organic amendment.
First, the researchers carried out physico-chemical analyzes to determine the soil components and the differences between the two. The first conclusion showed that the almond shell modified the microbiota or microorganisms developed in the field.
Subsequently, the suppressiveness of the soil, that is, its protective capacity against Rosellinia necatrix, was determined. To do this, the researchers took two types of samples: ground modified by the shell of almonds and unmodified; they planted small avocados in them and they became infected with the fungus. “The trees that grew in soil with amendment showed protection against the cause of the problem. This immunity is never absolute. It is relative. But, in our experiments, the percentage of diseased avocados was reduced by almost half ”, says Francisco Cazorla.
Analysis of the DNI
In the next phase, the experts applied a heat treatment or heat to cause the death of the microorganisms. “With the disappearance of these communities, the protective effect of the soil disappeared. However, by adding a small amount of almond shell, it would recover after a while. This confirmed that the suppressiveness or protection of the soil has a biological origin. In other words, it comes from the microorganisms that develop under the organic amendment ”, says the person in charge of the study.
The next step consisted of identifying the types of microorganisms, that is, obtaining their DNA or fingerprint. "Genomics techniques were used that revealed, on the one hand, the increase in fungi that decompose the shell and, on the other, the appearance of bacterial organisms that take advantage of the organic matter available to improve their population," continues the researcher.
The tasks of microorganisms
The last phase focused on knowing the functions performed by each of the groups of microorganisms identified in the previous stage. Again, genomic techniques revealed the genes involved in the suppressiveness process. For example, which ones participate in the metabolism of nitrogen, carbon or sulfur or which ones are producers of antibiotics.
According to the researchers, this functional analysis confirmed the relative abundance of genes involved in the carbon cycle. "We have already seen how this element facilitates the activity of groups of bacteria related to the production of antifungal antibiotics," says the expert.
The project, financed by the Ministry of Economy and Competitiveness, continues in two complementary lines of research. On the one hand, the identification and formulation of antibiotic-producing microorganisms for use in agriculture. On the other, the formation of consortia of organisms, that is, the creation of an artificial community of microbes, producers of different antibiotics that, together, can have a higher level of protection than separately. "In the end, it is a question of reducing chemical treatments and replacing them with others originating directly from the crop in order to move towards a more sustainable agriculture," concludes the expert.
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