# The Role of Plant Growth-Promoting Rhizobacteria (PGPR) in Mitigating Plan's Environmental Stresses
## 3 Effectiveness of PGPR in Hydrocarbons and Heavy Metals Contaminated Soils  
PGPR can have wildly different levels of effectiveness in aiding remediation depending on many of the biological and chemical properties of the contaminated soil, ranging from presence and ratio of nutrients (e.g., N and P, competition and predation, mutation, horizontal genes transfer), to metal ions availability, quantity, and type of contaminant, and moisture. High oxygen concentration can generally help decompose organic pollutants since most degrading pathways oxidase these chemicals; it is, however, mandatory to remember that many microorganisms cannot tolerate even low oxygen levels. The addition of this nutrient could potentially kill beneficial organisms. It is also hard to overestimate the effect of temperature and pH in determining an organism survival and reproductive rates, enzymes activity, and metabolic processes, as well as the bioavailability of contaminants and their chemical form [77]. For this reason, many sites polluted by hydrocarbon are added with nutrients (nitrogen, phosphorus, potassium, etc.) and high-energy electron acceptors (especially oxygen). Some of these chemicals can be provided by plants through roots exudates. This sets up a positive feedback loop, where plants allow the presence and growth of PGPR and bacteria render the soil less toxic and aid plant growth through several mechanisms [43]. It is important to remember that petroleum hydrocarbons (PHC) are not degraded with the same efficiency. The microorganisms begin to degrade the simpler hydrocarbons (n-alkanes) first, followed by the branched ones, cyclic alkanes, to finally arrive at the most complex hydrocarbons (polyaromatics hydrocarbons PAH) [43; 78]. This means that molecules, such as naphthalene, phenanthrene, and pyrene are not easy to get rid of through biodegradation. For such molecules, the presence of plants (or phytoremediation) might provide a significant difference in degradation rates. For example, in a study conducted to observe the phytoremediation capability of _Trigonella foenum-gracum_ and _Brassica juncea_ in the presence of PGPR on saline soil contaminated with 400 mg/kg of phenanthrene, it was possible to reach a 99% of dissipation rate in 60 days [79]. However, many organic compounds can be highly toxic for plants, so much so that it could undermine the success of the phytoremediation. Bacteria can significantly decrease the harmful effect of many such compounds. They achieve this effect through nitrogen fixation and mobilization of nutrients, prevention of ethylene production (ACC deaminase activity), and the direct production of phytohormones. This is why it is important to create interaction between PGPR and the host plant [80] since they can suppress the inhibition of germination caused by contaminants [81]. It is, however, important to distinguish between two types of root-associated bacteria: those who remain close to or on the root surface are referred to as rhizospheric bacteria, while those that manage to enter the root tissue are called endophytes. Since the latter are protected by the root, they can influence the host plant more directly while living in a homeostatic environment, generally providing better support for degrading contaminants [82; 83]. Organic compounds are not the only toxic pollutants for plants; heavy metals can cause massive changes in soil's chemical-physical proprieties. Plants need to produce a homeostatic network to control absorption, accumulation, and the oxidative stress that derives from heavy metals presence [84]; to do this, plants trigger many physiological and molecular mechanisms, such as active transport of ions into cell vacuoles, sequestration of metal-siderophore complexes, and production of root exudates to solubilize mineral nutrient and stimulate microbial growth [57; 85].