# The Role of Plant Growth-Promoting Rhizobacteria (PGPR) in Mitigating Plan's Environmental Stresses
## 1 Introduction
Recent studies show, in _Zea mays_ and _Solanum Lycopersicum_, the secretion of specific molecules that attract beneficial bacteria active against phytopathogens [30; 31; 32]. These exudates secreted by plants show different specificities depending on the root portion. Micro-environments have different chemical-physical characteristics associated with distinct microbial communities [33]. These and numerous other factors, including the state of the plant and the presence of other vegetable species [34], make the rhizosphere one of the most complex ecosystems. Meta-transcriptomic analyses revealed the presence of many specific catabolic genes [35] in the rhizosphere microbiome of contaminated soils, suggesting that the plant selects the most suitable microorganisms for that particular environmental condition. However, how this selection takes place is still unclear, as these various shreds of experimental evidence do not explain the mechanism by which the host plant can \"choose\" and select the most suitable degrading microbiome among the many possible options. Some studies attribute this selection to the co-evolution of the latter with the host plant through vertical gene transfer or degradative pathways acquired from seeds [36]. However, this would not explain how plants of the same species grown in different contaminated soils exhibit different microbial communities with different degrading activities. Indeed, plants select most of the degradative and growth-promoting microorganisms (PGPR) from the rhizosphere soil [23; 37] by producing specific exudates in the presence of contaminating molecules to favor the microbiome having the necessary enzymes [38]. Another recent study concerning a remediation intervention of sediment contaminated by hydrocarbons shows how the plants have exerted a species-specific pressure on the structure of the rhizosphere's microbial communities, highlighting its selectivity. These results also show that bioaugmentation caused a significant change in microbial communities. Still, the presence of plants in contaminated sediments certainly had a much stronger influence on the structure of the microbiome [39]. An intense competition takes place in the rhizosphere of contaminated soil, where microorganisms able to use the specific radical exudates and metabolize contaminating molecules will prevail [40]. The concept of competition as a driving force [23] refers to original studies of [PERSON], which hypothesizes that the evolution of plants is associated with three primary strategies, including competition [41]. In an attempt to explain the formation of a beneficial microbiome at the level of the rhizosphere of contaminated soil, [PERSON] and collaborators [23], also according to previous studies [40], propose a model that relates this development to the abundance of root exudates, and the efficiency of the microbiota formed to the amount of degrading genes at the beginning of plant development or to the establishment of a sort of vertical transmission. More specifically, the bacteria associated with plants are not limited to the rhizospheric, but include endophytic (inside the roots) and phyllospheric (on the surface of the leaves) bacteria [42]. Plants need to establish intimate relationships with the surrounding environment due to their sessile nature, so they have co-evolved with microorganisms to exploit this symbiosis, which allows for greater plant survival and treatment results in contaminated sites [43] by creating a symbiotic system in response to abiotic stress (Figure 1), defined as a holobient [44]. The principal abiotic stresses that plants undergo are schematized in Figure 1: organic and inorganic contaminants, salinity, and drought, which are often associated. The reaction of the plants is assisted by the rhizospheric microorganisms that put in place protective mechanisms. Furthermore, plants under prolonged stress conditions become much more susceptible to attack by pathogens, thus adding biotic stress.