PREBIOTICS TO KEEP YOUR MICROBIOTA HEALTHY

PREBIOTICS TO KEEP YOUR MICROBIOTA HEALTHY

• wHAT ARE PREBIOTICS?

Substances in the diet that stimulate the growth of gut-dwelling micro-organisms causing great direct and indirect health benefits.

We have all heard about the importance of increasing the daily intake of fibre in the diet (hence the outbound boom of multiple fibre-rich products populating supermarket shelves), as well as the fact that a wide variety of fruits are rich in pectins, among many other examples. But did you know that “feeding” prebiotics to the soil can help plants in the same way that our microbiota helps us?

• BIODIVERSITY IN THE SOIL

Soil has an incredible capacity to harbour life; we only have to think that one gram of soil is estimated to be home to at least 10,000 different species of microorganisms, including bacteria, actinomycetes, cyanobacteria, fungi, algae, protozoa and viruses, and it is estimated that they could account for approximately 25% of the planet’s biodiversity. Furthermore, more than 40% of living organisms in terrestrial ecosystems are associated with soils during their biological cycle. This gives us an idea of the important reservoir of life that resides in our soils.

• ROLE OF BIODIVERSITY

In both natural and agricultural ecosystems, soil myco-organisms play vital roles that are directly related to biological, atmospheric and hydrological systems. Soil organisms are key elements of nutrient cycles, regulating soil organic matter dynamics; carbon sequestration and greenhouse gas emissions; modifying soil physical structure and water regimes; increasing the volume and efficiency of nutrient uptake by vegetation through mutually beneficial relationships; and improving plant health.

These activities are essential for the proper functioning of natural ecosystems and constitute an important resource for the sustainable management of agricultural systems. This soil biodiversity is susceptible to erosion, and the fundamental services it performs for natural ecosystems may be seriously threatened by human activities, climate change and natural disasters.

• CONSEQUENCES OF SOIL IMPOVERISHMENT

An impoverished soil leads to a loss of biodiversity, as well as the disruption of the microbial balance present in the soil, thus causing a partial or total loss of the vital functions described above, which are necessary for the maintenance of the ecosystem. For agriculture, this means a loss in the return of organic matter and nutrients, a deficiency in the absorption of nutrients, an increase in pathogenic micro-organisms and a loss of soil structure. In short, a drastic reduction of the soil’s potential for sustainable agriculture and increased food security.

• cAN THIS PROCESS OF IMPOVERISHMENT BE PREVENTED OR REVERSED?

In the same way that soil can be impoverished and broken down, it is also possible to enrich and strengthen it, making microbial diversity more abundant and respecting the existing microbial balance. One way of helping our soils to achieve this is through prebiotics, products whose objective is to stimulate the microbial communities that live in it, reinforcing their balance and therefore increasing their capacity to generate beneficial effects for the plant: mobilisation of nutrients, stimulation of growth, defence against pathogens, and the ecosystem in general.

• NATURE’S USE OF PREBIOTICS AS A STRATEGY TO SUPPORT SURVIVAL

The concept of “feeding” the soil with prebiotics seems novel, but the use of prebiotics is a strategy that has been used by nature for millions of years. Plants are able to generate a rich micro-ecosystem around their roots, the rhizosphere, through a plant-micro-organism interaction that has evolved over time.

The rhizosphere is considered to be the largest terrestrial ecosystem, it is the part of the soil close to the roots of the plant, which extends concretely between 1 and 3 mm from the surface of the roots into the soil and it is in this niche that a fascinating diversity of micro-organisms are harboured in and around their roots, generating a specific microbiome. This microbiome is richer than the microbiomes found in regions of the soil further away from the roots and may consist mainly of fungi, bacteria, oomycetes and archaea that may be pathogenic or beneficial to the health and fitness of the plant. To grow healthy, it is important for the plant to monitor its rhizosphere for pathogenic micro-organisms and, at the same time, maximise the services of beneficial micro-organisms in nutrient uptake and growth promotion. To this end, plants employ different strategies to modulate their rhizosphere, including structural modifications, the coordinated action of different defence responses, as well as the exudation of a large number of compounds. All of them are important, but let’s take a closer look at the last of these strategies.

• EXUDATES AND THEIR SIMILARITY TO PREBIOTICS

Root exudates are organic substances released by roots that include sugars, amino acids, organic acids, enzymes, hormones, vitamins, among others, and are estimated to be between 20-48% of their photosynthetic products. These compounds act as “prebiotics” as they are used by the microorganisms present in the rhizosphere to their advantage, increasing the abundance and diversity of these populations. In other words, the plants that we grow are themselves capable of “cultivating” and agglutinating a whole series of populations of micro-organisms around their roots. Plants are not satisfied with simply increasing the abundance of their rhizosphere indiscriminately. Plants are demanding creatures and tend to modulate the composition of these populations, favouring certain organisms over others.

• MODULATING ROLE OF SOIL THROUGH EXUDATES

Throughout their life cycle(germination, seedling, vegetative and flowering), plants need a series of nutritional, physical-chemical and growth stimulation requirements (not very well understood), among many others, in order to develop, and these vary according to the development cycle in which they find themselves. Part of these requirements can be obtained through interaction with the micro-ecosystem found in the environment around its roots, by means of the different exudates and/or rhizodeposits that the plant makes available to the rhizosphere. It is by varying the compounds that make up these exudates that modulation of the microbial populations can be achieved.

It is assumed that the different micro-organisms present in the soil are organisms that have different nutritional preferences to other organisms. What plants achieve by varying the composition of their exudates is that the growth of certain populations is favoured to the detriment of others. And, therefore, to achieve abundant populations enriched with certain organisms that have positive effects on plant growth, such as the well-known PGPR (plantgrowth promoting rhizobacteria), which have numerous advantages that favour crop productivity, both directly and indirectly.

• IMPORTANCE OF THE PGPR

They are micro-organisms capable of colonising the rhizosphere and promoting crop growth by increasing the availability of nutrients such as increased nitrogen fixation or phosphate solubilisation, both of which are essential for plant growth.

PGPR are capable of producing growth-promoting hormones such as auxins, gibberellins, cytokinins and abscisic acid. This phenomenon stimulates aerial plant growth and root formation.

By producing antibiotics and antifungal metabolites, PGPRs have the ability to control pathogenic fungi and bacteria affecting plants by acting as antagonists. In addition, they have the potential to produce lytic enzymes, cyanide or induction of resistance mechanisms, allowing them to inhibit the growth of pathogenic microorganisms that affect development.

• USE OF PREBIOTICS AS A STRATEGY TO INCREASE BIODIVERSITY

Just as plants use their exudates as prebiotics to modulate the enrichment of rhizosphere microbial populations to meet their needs. Using soil prebiotics based on non-synthetic products is a solution based on strategies already used by nature, which leads to an increase and reinforcement of soil biodiversity and has considerable potential to reverse the effects of degraded and/or impoverished and polluted soils, as well as to increase and reinforce beneficial rhizosphere-plant synergies, to obtain high yields, making agricultural management more sustainable and providing greater food security for all.