Endophytic microorganisms and fertilization

Various types of microorganisms live, permanently or for part of their life cycle, in the tissues of all plants. Because of this habit of life, they are called endophytes.

These are bacteria, fungi and other microorganisms that take advantage of this “protected” environment and that often positively influence the physiology of the plant, in particular by facilitating its supply of nutrients and promoting its growth. The influence of endophytic microorganisms on plant behaviour is so important that it has generated the concept of MAP, Microbiome-Associated Phenotype. They also contribute to improving resistance to various stresses, to the point that sometimes an increase can be observed in the number and variety of endophytic microorganisms present in plants subjected to conditions of particular stress. In some cases, thanks to the phenomena of long-term coevolution of plants and endophytes, the latter have acquired the ability to synthesise chemical compounds initially produced by the host plant.

Endophytic microrganisms have a positive effect on plant growth and development, primarily improving the supply of nutrients, especially nitrogen and phosphorus, through their conversion into bioavailable forms.

The fact that endophytes produce vitamins and phytohormones explains why plants inhabited by such microorganisms generally tend to give higher yields: for example, the endophytes Rahnella aquatilis and Pseudomonas putiida, capable of synthesising

indoleacetic acid, have a positive effect on the growth and development of some cereals. The endophyte Bacillus subtilis, on the other hand, is capable of synthesising gibberellins.

Certain endophytic microorganisms have been shown to increase plant tolerance to stresses caused by drought, water stagnation, salinity, the presence of heavy metals, toxic organic compounds and pathogens by modulating the levels of ethylene, a stress hormone whose synthesis is induced by numerous biotic and environmental stress factors. Some endophytes are able to exercise control over ethylene synthesis by producing an enzyme that degrades its precursor.

Plants growing on soil contaminated by xenobiotics, which are toxic substances of various types foreign to the environment in which they are found, usually contain microorganisms that are not only resistant to such compounds but are also able to degrade them. Types of bacteria such as Achromobacter, Pseudomonas, Alcaligenes, Enterobacter, Acinetobacter and Bacillus isolated from Cannabis sativa have shown the ability to degrade phenol and benzene. A particularly interesting observation in the field of bioremediation is the ability of these endophytes to degrade xenobiotics within the plant, thus reducing damage from phytotoxicity.

Endophytes obtain food in a situation often characterised by less competition with other living beings, but sometimes, they occupy the same ecological niches as phytopathogenic microorganisms. This makes it likely that endophytes have a role in the biocontrol of various plant diseases. Endophytic bacteria can also produce antibiotics and bacteriocins, specific proteins that suppress the cellular activity of other strains of the same species or related bacterial species.

Endophytes have been shown to produce siderophores (which chelate iron ions) and vitamins, the presence of which can increase plant resistance to pathogens. Other endophytes produce antitumor and antiviral compounds, regulate osmotic pressure and stomatal opening, and modulate root development, triggering a system of induced resistance in plants.

The taxonomic study of the populations of microorganisms present in the rhizosphere, in the phyllosphere and inside the plant has revealed very strong overlaps in the composition of these populations, demonstrating a strong link between life “outside” and “inside” the plant itself.

This evidence underlines the importance of careful soil management aimed at increasing its biological fertility and, in particular, at enriching the rhizosphere, from where endophytes frequently originate, with useful microrganisms.

In this context, fertilization, especially organic and organo-mineral fertilization, plays a fundamental role. Maintaining or restoring an adequate balance among the different forms of organic matter in the soil is an essential condition for promoting microbial biodiversity in soils. At the same time, the use of fertilizers inoculated with useful microorganisms can act directly on the composition of the microbial populations of the rhizosphere.

For this reason, Agribios has studied specific formulations of microbial inoculums inserted into the organic substance intended for specific functions such as the stimulation of the root system of cultivated plants, the solubilization of phosphorus, nitrogen fixation and the induction of resistance to various stresses (drought, parasitic attacks, etc.) in order to improve the vegetative state of the plants as well as production yields.