**1. Introduction**

Since plants colonized the land, they have developed mechanisms to respond to changing environmental conditions and to settle in extreme habitats. Recent studies indicate that several plant species require associations with microorganisms to tolerate stress and to survive [1]. The human contribution to plant breeding has not only been the development of new breeding methods, but also the acceleration of progress in the evolution of crops.

On the other hand, in recent years the interest in the use of rhizobacteria that promote plant growth has increased. The beneficial effects of these microorganisms involve the ability to act as phytohormones or biofertilizers, increasing the yield of many important crops. Ecological factors such as temperature and nutritional conditions of the soil affect the behavior of microorganisms; inoculation has a better stimulating effect on plant growth in nutrient-deficient soil conditions than in fertile soils [2].

Although most plants lack the adaptive capacity under stress conditions, this ability seems to be associated with certain microorganisms, which suggests asking the question Through what mechanisms can microorganisms and plants adapt to stress conditions? Can plant species improve their tolerance to environmental stresses when associated with certain microorganisms? The answer to these questions could change our concepts about plant breeding and could lead us to new routes towards sustainability.

If food production is to increase by 50% in the next 40 years in a scenario of scarce resources and climate change, it will require a considerable investment in capital, time, and effort. A major component of the solution will have to start from the improvement in agricultural technologies, to produce sufficient and safe food that meets the needs and preferences of the human population, without affecting the sustainability of the natural environment.

From a conceptual perspective, the effects of microorganisms on plants have long been grouped under the idea of "promoting and regulating plant growth." However, the microbiota associated with plants influences multiple regulatory cascades of plants that together define their phenotype. In addition, the effect of the modified phenotype will depend on the context, as a function of the abiotic and biotic environmental parameters, giving rise to new phenotypes through the joint modification of genomic information and the microbiota associated with plants [3].

The plant microbiome not only helps plants survive in the ecosystem but also offers critical genetic variability, hitherto little used as a strategy by plant breeders, who have traditionally exploited only the genetic variability of the host plant to develop improved varieties of high yield or with tolerance to diseases, pests and abiotic stress [4]. In the words of Walters [5], resistance induced by microorganisms has the potential to revolutionize disease control in crops, but it remains an unconventional type of crop management. This is the subject of intense research at present.
