## 2 Functional Attributes of PGPR and Their Mechanisms
### Bioprotection  
Several enzymes synthesized by PGPR have a critical function in protecting a plant from stress and pathogens [60]. Some of them, including chitinases, cellulases, and glucanases, could be labeled as biopesticides since they hinder plant pathogen growth by hydrolyzing polysaccharides and fibrillar materials of the cell wall of pathogenic fungi [60; 61]. In this regard, [PERSON] et al. [62] reported that enzyme synthesis (e.g., proteases, chitinase, and \(\beta\)-1,3-glucanase) by PGPR strains can be considered an important strategy to control soil-borne pathogens through enzymatic degradation or deformation of their cell wall. Another effective mechanism of PGPR is the synthesis of volatile organic compounds (VOCs), which makes them able to interact with plants and other soil microorganisms by causing systemic resistance to disease and pathogens and promoting plant development [63; 64]. In fact, some characteristics of these secondary metabolites, such as low molecular weight (<300 g mol\({}^{-1}\)), high vapor pressure (>0.01 kPa), and low boiling point, enable them to volatilize and act as the agents of cell signaling [63; 65]. Nearly 846 different potential VOCs produced by soil bacteria have been identified [58], the most important of which is N,N-dimethylhexadecylamine (DMHDA), a plant protector against pathogens (e.g., _Botrytis cinerea_ and _Pytophthora cinnamomi_), and dimethyl disulfide (DMSD), an elicitor of plant defense as well as a plant growth stimulator by increasing the status of sulfur nutrition in plants [49; 63]. Plants can usually produce enough vitamins (e.g., biotin, riboflavin, niacin, thiamin, and pantotheane) for their development and provide them to soil microorganisms through root exudates as main nutritive compounds for their survival and development; on the other hand, unhealthy and stressed plants may suffer from vitamin deficiency [5; 66]. In this context, some PGPR, especially _Bacillus_ sp. and _Rhizobium_ sp., have great potential to synthesize vitamins, such as pantothenic acid, thiamine, riboflavin, pyrroloquinoline quinone, and biotin, and can contribute to their supply to plants [66; 67]. The main functions of vitamins are (i) to act as cofactors in various metabolic pathways, (ii) to facilitate the synthesis of vital metabolites for plants and microbes, (iii) to induce resistance to pathogens, (iv) to promote plant growth and productivity, and (v) to participate in energy transformation in the plant from reserved compounds [67]. Harnessing the antagonistic activity of PGPR has already been suggested as an effective approach to control plant pathogens and inhibit the metabolic activities of various microorganisms through antibiotics [66; 68]. In addition to the direct antipathogenic potential of PGPR, they also act as the determinative agents to trigger induced systemic resistance (ISR) in plants and promote plant growth through antifeedant, anthelmithic, phytotoxic, antioxidant, cytotoxic, and antitumor activities in insects and mammals [69]. Among them, diacetylphloroglucinol (2,4-DAPG) synthesized by _Pseudomonas_ sp. [70; 71], phenazine by _Pseudomonas_ sp. [72], lipoproteins (e.g., iturin, fengycin, and bacillomycin) and polyketide by _Bacillus_ sp. [73], phenazine-1-carboxylic acid (PCA) by _Pseudomonas fluorescens_[74], and circulin and colistin by _Bacillus subtilis_[75] are the most efficient low molecular weight extracellular metabolites that have been extensively studied. One of the potent biological approaches of PGPR strains is the ability to synthesize 1-aminocyclopropane-1-carboxylate deaminase (ACCD), which can regulate plant growth and induce stress tolerance by decreasing ethylene levels [76]. ACCD, as one of the major enzymes in the intermediate precursor of ethylene production in plants, is responsible for the conversion of ACC to a-ketobutyrate and ammonium [77; 78].