# The Role of Plant Growth-Promoting Rhizobacteria (PGPR) in Mitigating Plan's Environmental Stresses
## 1 Introduction  
Although the soil is a valuable and non-renewable ecological system, it has always been subject to widespread degradation due to anthropic activities. Process industry [1], transport, urban sprawl, agriculture, and illegal dumping or landfill [2] without adequate resource recovery [3] represent just some of the many causes of direct release or indirect deposition of organic and inorganic pollutants (including heavy metals, mineral oils, and polycyclic aromatic hydrocarbons) into the soil, with hazardous effects on the environment and human health [4]. The remediation of contaminated soils and sites is, therefore, a significant step in the protection of the environment and living organisms and must be included in the broader multidisciplinary scenario of a strategic green transition. Nowadays, several approaches have been developed to deal with contaminated water [5; 6; 7; 8] and soils [9; 10], making the optimal selection a tricky yet crucial phase for the success of the remediation [11; 12]. However, remediation activities inevitably involve a certain consumption of raw materials and energy, also relying in many cases on the use of impacting chemical products or processes. This could compromise the sustainability of the intervention or even invalidate itspositive aspects when performed to remove contamination without any consideration of the side effects, thus further impacting an already compromised environmental situation [13; 14]. In order to avoid this, more and more efforts are being made to replace environmentally unsound technologies with green and sustainable solutions [15], aiming not only at eliminating or reducing contamination but also at minimizing environmental impact [16]. Phytoremediation is a solar-driven, cost-effective, and fully sustainable [17; 18] soil remediation technology based on the ability of plants to capture, extract, accumulate, or degrade a wide range of inorganic (heavy metals, radionuclides) and organic (hydrocarbons, polycyclic aromatic hydrocarbons, pesticides, pharmaceuticals) soil contaminants [19; 20; 21]. Besides, biomass from phytoremediation can be converted into bioenergy and also support biological diversity and stabilize the soil. In addition, the combined use with other solutions to further increase the overall sustainability is under investigation [22]. Nevertheless, some intrinsic factors due to the high variability of biological systems [23], including relatively long intervention times and the uncertainty about the achievable results, make phytoremediation still little appreciated. Indeed, the effectiveness of the treatment strongly depends on the selected species [24]. It is also necessary to consider numerous other aspects, such as the characteristics of the contamination, the properties of the soil, and, above all, the fundamental interaction of the plant microorganisms [25]. Increasing evidence suggests that this latter aspect strongly determines phytoremediation achievement, which goes well beyond the activity of the individual microorganism, on which many of the works on phytoremediation focus. Thus, integrated studies of the plant microbiome are essential, because understanding how the host plant organizes its beneficial microbiome--thousands of degrading microbial taxa [26]--in various stressful situations due to the presence of contaminants is the key to effectively achieving the remediation objectives. Root exudates play a crucial role in the composition of the microbial community of the rhizosphere [27]. Indeed, plants secrete specific molecules depending on different contaminants, thus attracting microorganisms that have catabolic pathways suitable for the degradation of those particular molecules [28]. It is hypothesized that the colonization of roots by specific rhizobacteria could begin through a first selection related to the characteristics of the bacterial cell wall followed by a second more stringent and specific selection for certain groups of microorganisms [29].