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PGPM has been studied as biofertilizer which could enhance the supply of macro and micronutrients, promote plant growth and reduce the need of chemical fertilization. Nitrogen, phosphorus and iron are essential nutrients for plants. Hence, in PGPM selection test, the nitrogen fixation, phosphate solubilization and siderophore production capacity are usually investigated.

Nitrogen is an essential macronutrient for synthesize of proteins and nucleic acids. The microbes viz., Azospirillum, Azotobacter, Achromobacter, Bradyrhizobium, Beijerinckia, Rhizobium, Clostridium, Klebsiella., Anabaena., Nostoc, Frankia are biological nitrogen fixers through reduction of nitrogen gas (N₂) to ammonia (NH₃) (Souza et al., 2015; Bhat et al., 2019).

Phosphorus is an essential macronutrient for production of phospholipids, adenosine triphosphate (ATP), and increase the photosynthesis. Nonetheless, a large proportion of P in the soil is in insoluble forms, making it unavailable for the plants. PGPM changes the pH of the soil to solubilize inorganic phosphates. In alkaline soils, PGPM reduces pH by excretion of organic acids, such as gluconate, citrate, lactate and succinate, solubilizing Ca₃(PO₄)₂. In acid soils, PGPM increases the pH by production of protons, during the assimilation of ammonium (NH4+), solubilizing AlPO₄ and FePO₄ (Martínez-Viveros et al., 2010). Bacillus, Pseudomonas, Rhizobium, Achromobacter, Burkholderia, Microccocus, Agrobacterium, Erwinia sp., Penicillium sp. and Aspergillus sp. are capable of solubilizing inorganic phosphorus, transforming into forms capable of being absorbed by plants, such as monobasic (H2PO4-) or dibasic phosphate (HPO4⁻²).

Iron is a micronutrient required to chlorophyll biosynthesis, photosynthesis and respiration. Burkholderia, Enterobacter, Grimontella and Pseudomonas are siderophore producers. Siderophores are chelator agents, with high specificity for binding iron, followed by the transportation and deposition of Fe³⁺ within bacterial cells. In this way, the excretion of siderophores improve plant nutrition and inhibit phytopathogens through iron sequestration from the environment (Souza et al., 2015; Varma et al., 2019).

Sulfur is an essential macronutrient found in cysteine and methionine. These amino acids are important in maintaining of enzymes and protein synthesis. Cysteine is important in cell division, and methionine is a precursor of ethylene, responsible for fruit ripening (Taiz and Zeiger, 2017). Bacillus are producers of volatile compounds, such as dimethyl disulfide that provides sulfur for plants. In addition, Bacillus and Aspergillus produces organic and inorganic acids, acidolysis, chelation and exchange reactions which are capable of solubilize potassium (Varma et al., 2019).

Phytohormones are organic compounds responsible for the development of plants. There are PGPM capable of modulating phytohormones. The effects of stress on plants are mitigated by microbial inoculants, through the production of auxin, cytokinin, gibberellin, ACC deaminase, abscisic acid, jasmonates, brassinosteroids, and strigolactones (Saravanakumar, 2012; Oosten et al., 2017; Arora et al., 2020; Khan et al., 2020).

Auxin and ACC-deaminase are usually investigated in PGPM selection tests. This is because, auxin produced by microorganisms will increase auxin in the plant, and promote plant growth by enhancing nutrient and water uptake. Microbial auxins also beneficial in the regulation of cell division, shoot growth, differentiation of vascular tissue, adventitious and lateral root, elongation and surface area of root. ACC-deaminase produced by microorganisms is a beneficial enzyme for reducing ethylene levels, mitigating stress in plants. High ethylene levels cause leaf chlorosis, necrosis, senescence, reduction in fruit yield, root development, leaf expansion, and photosynthesis (Souza et al., 2015). Pseudomonas sp. and Bacillus promote plants growth by increase auxin and ACC-deaminase (Samaddar et al., 2019; Danish et al., 2020; Khoshru et al., 2020).

PGPM can also promote plant growth by increasing gibberellin, improving seed germination, and the development of stem, leaves, flower and fruit. In addition, PGPM-induced cytokinin result in increased roots development, activity of vascular cambium, cell differentiation, and apical dominance (Gouda et al., 2018; Khan et al., 2020). Under stress conditions, PGPM inoculants are able to increase abscisic acid, jasmonates and brassinosteroids concentrations in plant. Under drought, cytokinin increase the abscisic acid, causing stomatal closure to reduce foliar water loss (Arora et al., 2020). Under drought or low temperature, jasmonates and brassinosteroids increase Ca₂₊ concentration in plant, acting intracellularly as secondary messenger under stress conditions (Oosten et al., 2017; Gouda et al., 2018; Khan et al., 2020).

Microbes are known to produce exopolysaccharides, forming a protective biofilm on root surface. This mechanism enhances water retention in soil particles and maintaining soil moisture in the root zone. In this way, it protects root cell against osmotic and ionic stress, regulating osmotic balance, under changing pH, saline stress, drought and temperature extremes.

To mitigate stress PGPM produce exopolysaccharides. This mechanism acts to stabilize the soil ionic balance, immobilizing Na+ under salinity stress. Exopolysaccharides are produced by Bacillus to increase its antimicrobial activity in the soil (Hashem et al., 2019).

PGPM inoculants also promote plant growth and tolerance to abiotic stresses by increasing antioxidants levels, reducing reactive oxygen species (ROS) and oxidative stress. Temperature, pH, heavy metal, water availability and UV-B radiation cause disruption of cellular homeostasis, increasing of reactive oxygen species, such as superoxide anion, hydroxyl radical, hydrogen peroxide and singlet oxygen. High concentrations of ROS is a primary result of abiotic stress, and are extremely harmful, causing oxidative stress in cell. Moreover, in chloroplasts, mitochondria, and peroxisomes, ROS induce oxidative damage to lipids, proteins, nucleic acid, enzyme inhibition and activation of programmed cell death (Sharma et al., 2012; Khoshru et al., 2020).

Microbial inoculants reduce the damaging effects of ROS, thus securing the cell, membranes, and biomolecules by increasing the production of antioxidants such as catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (AsA), glutathione (GSH), carotenoids, tocopherols and phenolics (Gouda et al., 2018; Arora et al., 2020). Environmental stresses cause either reduction of CAT activity and increase the hydrogen peroxide (H₂O₂).

Catalase is also produced by microbes. Although it is relatively simple to examine whether microbes are catalase producers, this investigation is usually not performed on screening tests (Romeiro, 2007; Varma et al., 2019). The production of catalase by microorganisms must be carried out in screening tests, mainly with the objective of increasing the plant tolerance against abiotic stresses. This enzyme is efficient in H₂O₂ degradation, reducing ROS and oxidative stress, consequently increase the plant tolerance to abiotic stress. This is the simple analyses in PGPM, which should be routine in the selection tests.

Under stress conditions, microbial inoculants induce production of osmoregulants, such as carbohydrates, proteins, amino acids, lipids, proline, glycinebetaine, and trehalose (Oosten et al., 2017; Gouda et al., 2018; Khoshru et al., 2020). Thus, osmoregulation maintains the homeostasis, preventing membrane plasmolysis, increasing synthesis of heat shock proteins (HSPs) and regulating biological enzymatic mechanisms (Oosten et al., 2017; Khoshru et al., 2020). Under salinity, osmoregulators stabilize the osmotic balance across the membrane, maintain the turgor pressure, and ensure the correct folding of proteins (Sharma et al., 2012; Khoshru et al., 2020). Burkholderia sp. increases plant tolerance against low temperature by modifying carbohydrate metabolism (Fernandez et al., 2012). Under water stress, Pseudomonas fluorescens promote plant tolerance by increasing the activity of catalase and peroxidase, and the accumulation of proline (Saravanakumar et al., 2011). Beneficial microbes improve tolerance in plants, increasing the accumulation of osmolytes in the plant cell cytoplasm. This maintains the cell turgor and contributes to improved stress tolerance in plants (Khoshru et al., 2020). The osmoregulation mechanism is important for plants to survive and improve tolerance under extreme conditions by reducing cellular damage caused by abiotic stress (Fernandez et al., 2012; Khoshru et al., 2020).

Seed inoculation method with PGPM is an alternative to chemical seed treatments. It consists in immersing the seed in a microorganism solution of known concentration (Romeiro, 2007; Lopes et al., 2018a). The seed germination process releases abundant carbohydrates and amino acids in the form of seed exudates (Ahemad and Kibret, 2014). In this way, these organisms introduced together with the inoculated seeds in the soil use the exudates as a nutritional source and colonize roots, as soon as they emerge (Ammor et al., 2008).

Interaction of PGPM with the plant roots modulate the level of phytohormones that are produced by plants. Phytohormones are organic compounds that modulate plant growth and are also capable of inducing tolerance in plants against various biotic and abiotic stresses (Khan et al., 2020). Microorganisms colonize plant tissues and synthesize phytohormones, such as gibberellin that improve germination (Bhat et al., 2019). PGPM is also capable of producing antimicrobial compounds that protect seeds against phytopathogens that cause seed rotting (Souza et al., 2015).

Seeds inoculation with Rhizobial and Bacillus sp. increased biomass production of Oryza sativa (Ullah et al., 2017) and Cicer arietinum L. (Khan et al., 2019), respectively. Seed inoculation with mycorrhizal fungi and plant growth-promoting rhizobacteria was more effective in promoting growth and wood production in Schizolobium parahyba, as compared to the seedling inoculation (Cely et al., 2016). In seed inoculation method, the inoculum remains dormant in the soil, until activated by the growing root tips. Under field conditions, it is often necessary to reinoculated to maintain effective cell densities (Martínez-Viveros et al., 2010). Seed inoculation method with Burkholderia phytofirmans has also been successfully used in phytoremediation of organic pollutants such as hydrocarbons (Afzal et al., 2013). Pseudomonas fluorescens was also beneficial when inoculated in the seed for increasing the vigor, biomass and resistance to water stress of Vigna radiata (Saravanakumar et al., 2011).

Root inoculation method consists of immersing roots in a microorganism solution (Romeiro, 2007). After inoculation, the seedling is planted on a proper substrate for its development. This method allows plant size standardization, as inoculation can be carried out on seedling of similar sizes. Another advantage of this inoculation method is that the inoculum is placed directly in contact with the host roots, improving root colonization (Ahemad and Kibret, 2014). This method can be preferentially used in plant species with asexual propagation, as PGPM have the ability to synthesize growth phytohormones such as auxin, that besides promoting plant growth, can also counteract phytopathogens that compromise plant survival after planting (Ahemad and Kibret, 2014; Gouda et al., 2018).

Root inoculation with Burkholderia sp. increased Vitis vinifera tolerance to low temperature, modified carbohydrate metabolism and increased plant growth and yield (Fernandez et al., 2012). In Oryza sativa, root inoculation with Rhizobial was more effective in increasing plant height and panicle length, as compared to the seed inoculation method (Ullah et al., 2017). These results also prove that inoculation method can influence the beneficial effect of microorganism in promoting plant growth.

Soil inoculation method consists of introducing PGPM directly into the soil, by drenching, soil incorporation (mixed in the substrate) or microcapsules (Romeiro, 2007; Hernández-Montiel et al., 2017; Prisa, 2020). In soil drenching, a microorganism solution is added as close as possible to the host roots (Romeiro, 2007; Lopes et al., 2018a). This is necessary because it is in the rhizosphere that the PGPM will be able to perform several critical functions for promoting plant development, such as phosphate solubilization, synthesis of siderophores and phytohormones (Gouda et al., 2018).

Inoculation of the forage grass (Brachiaria brizantha) with Burkholderia pyrrocinia and Pseudomonas fluorescens was not successful when carried out on seeds, but promote growth when inoculated by soil drenching 14 days after seedling emergence. This is because allelochemicals with negative allelopathic effects in these PGPM have been reduced over the growth stages of B. brizantha (Lopes et al., 2018a). In Cicer arietinum L., the soil inoculation of Bacillus resulted in better nodulation and growth than when inoculated on seeds (Bhattacharjya and Chandra, 2013).

Plant growth promoting rhizobacteria inoculated by soil incorporation, improved Ranunculus asiaticus growth, increasing the efficiency of nutrient and water absorption by roots (Prisa, 2020). Burkholderia phytofirmans was more efficient in improving Lolium multiflorum biomass production, when inoculated in soil. When inoculated in seeds, root or leaves, the indigenous microbiota made it difficult for inoculated bacteria to colonize successfully and promote plant growth (Afzal et al., 2013).

In Lycopersicon esculentum, growth and productivity were increased by soil inoculation with Pseudomonas putida delivered in microcapsules. According to Hernández-Montiel et al. (2017), soil inoculation with microcapsules offered greater protection and viability, since the release was gradual, improving adhesion, stability, and colonization of roots by PGPM.

Soil pH is key for the solubility of different metal ions, nutrient availability and soil physical properties (Dutta and Bora, 2019; Msimbira and Smith, 2020). High or low soil pH is a worldwide problem to agricultural productivity (Dutta and Bora, 2019; Salwan et al., 2019; Zerrouk et al., 2019). Salinity of agriculture soils is also a serious constraint to plant growth. This stress condition causes nutrient deficiency, ion toxicity, osmotic and oxidative stress, reducing yield of agricultural crops (Dutta and Bora, 2019; Salwan et al., 2019).

Under salinity conditions or in alkaline soils, the high pH, affects the bioavailability of nutrients, causing osmotic stress, nutrient deficiency and increase in the reactive oxygen species production (Dutta and Bora, 2019; Salwan et al., 2019). In acidic soils, the low pH and high aluminum ions concentration cause toxicity and formation of phosphoric acid complexes, which makes phosphorus unavailable to plants (Dutta and Bora, 2019; Zerrouk et al., 2019).

The pH range 5.5–6.5 is optimal for plant growth and increasing production of root exudates to microbes. Bacteria are favored by neutral pH, but the fungi are favored by acidic pH conditions (Msimbira and Smith, 2020). Therefore, pH interferes in plant metabolism, which can also disrupt biological activities, inhibiting the microorganisms that inhabit the rhizosphere (Salwan et al., 2019). Saline stress inhibits seed germination, causes stomata closure affects seedling growth, the onset of flowering and fruiting set (Enebe and Babalola, 2018; Salwan et al., 2019). Aluminum toxicity reduces cell division, root growth, nutrient absorption, and phenolic metabolite production and increases reactive oxygen species production (Zerrouk et al., 2019; Msimbira and Smith, 2020).

Plant beneficial microorganism inoculation is a suitable strategy to improve plant tolerance to pH extremes, as reported to Bacillus and Trichoderma in Glycine max (Bakhshandeh et al., 2020), and Rhizobium and Paenibacillus increase pH tolerance in Triticum (El-Sayed and Hagab, 2020). Pseudomonas promote growth and increases tolerance to high salinity conditions in Capsicum annuum (Samaddar et al., 2019) and in Zea mays under salt and aluminum toxicity (Zerrouk et al., 2019) (Table 2).

Soil nutritional condition can also affect the PGPM efficiency. In soils with high nutrient profile there can be absence of root colonization; this is due to microorganism displacement to regions more abundant in nutrients (Egamberdiyeva, 2007; Bhat et al., 2019). Mathematical modeling indicates that PGPM inoculation is more efficient in nutrient poor soils, or stressed soils, because the development of the resident microflora is inhibited (Strigul and Kravchenko, 2006). Inoculation with Pseudomonas, Bacillus, and Mycobacterium are often more effective in promoting plant growth in nutrient-deficient soils (Egamberdiyeva, 2007; Mathimaran et al., 2020).

Heavy metal contamination in soils is toxic to most organisms and can also inhibit the effectiveness of inoculants. Heavy metals reduce soil fertility, affect the rhizosphere microbial community, plant photosynthetic efficiency, causes nutrient imbalance, and reduce yields (Mimmo et al., 2018). Beneficial microorganisms, such as Pseudomonas aeruginosa, Alcaligenes feacalis, and Bacillus subtilis are an effective remediation strategy in contaminated soils, increasing plant tolerance to heavy metals, as reported in Brassica juncea (Ndeddy Aka and Babalola, 2016).

Soil affects inoculation efficiency and influences the rhizosphere microflora (Egamberdiyeva, 2007; Souza et al., 2015). After inoculation, cell numbers will undergo a rapid decline, especially on unsterilized soils. In autoclaved soils, because there is no competition with other microorganisms, inoculants remain in high cell densities for many weeks. In nonsterile soils, because there is great competition with the resident soil microbiome and predation by protozoa and nematodes, inoculants populations will decline rapidly, until the population reach an equilibrium (Martínez-Viveros et al., 2010; Varma et al., 2019). For this reason, research work screening PGPM must be carried out in nonsterile soils, since the possibility of testing the competition efficiency of the inoculated microorganisms against those already native to the soil will be greater.

Rainfall is the major water source for growing crops in many parts of the world (Enebe and Babalola, 2018; Danish et al., 2020). Water availability, lack (drought) or excess (flood), can result in abiotic stress, limiting crop production (Enebe and Babalola, 2018; Ipek et al., 2019; Danish et al., 2020). PGPM can improve plant tolerance to drought stress (Fleming et al., 2019; Asghari et al., 2020), as Azotobacter chroococcum and Azospirillum brasilense in Mentha pulegium L. (Asghari et al., 2020), and Pseudomonas sp. and Azotobacter sp. in Cymbopogon citratus (Mirzaei et al., 2020) and Zea mays (Danish et al., 2020). Klebsiella variicola and Azospirillum sp. can improve flooding stress tolerance by adaptations, such as the formation of adventitious roots resulting from endogenous hormonal regulation, as reported in Glycine max (Kim et al., 2017) and Zea mays (Czarnes et al., 2020).

However, water stress can influence the plant-microbe interactions. Drought increases soil temperature, which can inhibit multiplication of beneficial microorganisms. Flooding reduces O₂ availability in soil, restricting microorganisms that are not capable of anerobic respiration (Enebe and Babalola, 2018; Hartman and Tringe, 2019; Ipek et al., 2019). In addition, drought and flooding are stress that affect plant metabolism and photoassimilates production, interfering with production and composition of root exudates (Enebe and Babalola, 2018; Fleming et al., 2019; Hartman and Tringe, 2019; Danish et al., 2020). This will affect plant-microorganism interaction, which may inhibit the microorganism potential to promote plant growth. For this reason, it is important to know where the target plant species is usually adapted. Considering climate change, it is important to study how the excess and the lack of water would affect the interaction between plants and PGPM.

Light intensity influences plant metabolism, growth and production (Venturi and Keel, 2016). Light can interfere in plant-microorganisms interaction by modifying the amount and the chemical composition of root exudates (Venturi and Keel, 2016; Lopes et al., 2018b). Microbial inoculation demands carbohydrate allocation in exchange of nutrients delivered to plants. Beneficial microorganism inoculation can increase plant growth under limited light conditions, by increasing shade tolerance, as Kaistobacter sp. and Pseudomonas sp. in Ophiopogon japonicus and Lolium perenne (Fu et al., 2020).

However, under light-limited conditions (i.e., shade) the microbial root symbionts can create additive costs, inhibiting plant growth, as Glomus sp. Paraglomus sp. Rhizophagus sp. and Rhizobium in Phaseolus lunatus (Ballhorn et al., 2016). Aguilar-Chama and Guevara (2016) describe that a mycorrhizal inoculation had a positive effect on stem mass, root mass, and leaf nitrogen content in Datura stramonium, but only when light was not a limiting factor. This is because, under limited light conditions, photosynthesis is reduced, consequently, carbohydrate production is also reduced, turning mutualistic microbes into parasites (Ballhorn et al., 2016).

As previously explained, during screening for PGPM, light conditions commonly experienced by the targeted plant species must be taken into account. This is because light intensity can interfere with the PGPM efficiency. For example, Brachiaria brizantha is a tropical forage grass grown either under full sun, or under moderate shade, as when cultivated in silvipastoral systems. Therefore, Lopes et al. (2018b) report that they tested the efficiency of rhizobacteria in promoting growth of this forage grass under both full sun and moderate shade. According to Lopes et al. (2018b), the PGPM efficiency varied with the type of microorganism and light intensity. When the bacteria were inoculated individually, plants under full sun showed the highest growth with Pseudomonas fluorescens, while under moderate shade Burkholderia pyrrocinia was more efficient in promoting plant growth (Lopes et al., 2018b).

High and low temperatures potentially caused by climatic change may become a major threat to global agriculture, reducing crop production, with drastic economic results (Ipek et al., 2019; Mukhtar et al., 2020). Beneficial microorganism inoculation is efficient in enhancing plat growth and mitigating adverse stresses caused by extreme temperature, as Pseudomonas putida in Triticum sp. (Ali et al., 2011) and Bacillus cereus in Solanum lycopersicum (Mukhtar et al., 2020) under high temperature. Fernandez et al. (2012) related that under low temperature Burkholderia sp. increased tolerance to low temperature by modification of carbohydrate metabolism and increased plant yield in Vitis vinifera.

Extreme temperatures are recognized stress in agriculture, reducing seed germination, seedling growth, yield and altering plant metabolism (Ipek et al., 2019; Mukhtar et al., 2020). Temperature impacts morphological, biochemical and physiological attributes of plants, interfering with plant-PGPM interaction by changing root exudation composition (Ali et al., 2011; Meena et al., 2015; Ipek et al., 2019). Therefore, for PGPM to be able to withstand environmental transformations that crop plants are exposed, it is necessary to isolate these microorganisms from different rhizosphere environments, under diverse environmental conditions, such as prevalent high and low temperatures (Etesami, 2020). This is because rhizobacteria that persist under change temperatures have the ability of improving plant growth and productivity on these adverse environmental conditions (Meena et al., 2015).