Some recent data by Rezakhani et al. (2020) indicate that the single-and dual cultures of PSB, Pseudomonas sp. FA1, and Bacillus simplex UT1 and different concentrations of silicon had variable impact on the morphological, nutritional uptake of P, Si and K, and physiological activities of Sorghum bicolor plant fertilized with soluble or insoluble P (RP; Rezakhani et al., 2020). In this sense, the RP-fertilized sorghum had better root and shoot biomass while the PSB strains and Si levels applied independently augmented all the measured bio-chemical properties. Application of Pseudomonas sp. FA1 and B. simplex UT1 and Si with soluble P or insoluble P significantly enhanced P-use efficiency of sorghum plants. As a co-culture, both B. simplex and Pseudomonas sp. considerably increased the dry matter and P uptake of sorghum plants when raised with both forms of P fertilizer. While comparing the inoculation effects of the two PSB, Pseudomonas sp. was more efficacious than B. simplex. The composite culture of PSB with Si had the largest increase in P uptake and other growth indices. The results clearly suggest that the co-application of PSB and Si along with RP fertilization may serve as a substitute for chemical P fertilizer in sustainable sorghum cultivation practices. Studies by (Jisha and Alagawadi, 1996) revealed a variable impact of dual inoculation of PSB bacteria, P. striata, or B. polymyxa with the cellulolytic fungus, Trichoderma harzianum, on the nutrient uptake and yield of Sorghum grown in a vertisol treated with cotton stalks. Composite application of B. polymyxa or P. striata with T. harzianum increased the size and weight of the earhead, the number of spikelets per ear, and the straw and grain yield, and N and P uptake significantly relative to uninoculated control and single PSB application. Grain yield was increased by 6–8% due to co-inoculation over mono P-solubilizers inoculation and by 28–30% over T. harzianum alone.

Among phytonutrients, N and P are the two major nutrients that affect many important cellular processes like root elongation, proliferation and changes of root architecture, seed development, and maturity of plants growing both under conventional and stressful conditions (Liu et al., 2020; Patel and Panchal, 2020). The combined inoculation of PSM and N₂ fixers benefit plants better than either group of organisms used alone while substantially reducing the application of chemical fertilizers under field conditions (Gheliya et al., 2018; Li et al., 2020). Considering this (Afifi et al., 2014), in a field experiment conducted at Ismailia Agricultural Research Station, Agriculture Research Centre (ARC), Egypt, evaluated the impact of an associative N₂ fixer, Azospirillum brasilense, PSB B. megaterium, and potassium solubilizing bacterium (KSB) B. circulans, applied either alone or in combined forms and treated individually with either humic or fulvic acids, on the availability of N, P, and K and their overall impact on morphology, physiology, and the yield attributes of S. bicolor (cv. Dorado) in the presence of 50% of the recommended dose of mineral NPK. Half of the recommended dose (175kg/fed) of ammonium sulfate (20.5%) was added during sowing and 30 and 60days after sowing with A. brasilense only. Half of the recommended dose of P (100kg/fed) as super phosphate (15.5% P₂O₅) was applied during soil preparation containing B. megaterium only, while half of the recommended dose (25kg/fed) of potassium sulfate (48% K₂O) was applied once only at 60days before the earing stage inoculated with B. circulans only. The application of bacterial cultures either alone or as a mixture was more effective and maximally enhanced the growth and the yield of sorghum plants. Among all formulations, the mixture of bacterial cultures and humic acid demonstrated the highest populations of bacterial group in the sorghum rhizosphere. Biological features such as plant height, plant dry weight, and number of branches and biochemical attributes like nitrogenase, dehydrogenase, and phosphatase activities were significantly increased in the presence of co-culture of A. brasilense, B. megaterium, and B. circulans plus humic acid after 45 and 75days of sorghum growth. Also, the NPK, crude protein, and total carbohydrates (%) were at maximum in sorghum grains. The A. brasilense, B. megaterium, and B. circulans when used together exhibited the highest grains yield (3.55 ton/fed) and 1,000 grains weight over control (full NPK). The findings clearly revealed that each bacterial stain could reduce the half doses of NPK through P and K solubilization and N-fixation processes. In a similar recent study, the impact of the asymbiotic nitrogen fixer (A. chroococcum) and P solubilizer (B. megaterium) on growth, yield parameters, and nutrient uptake of Sorghum bicolor grown under greenhouse conditions varied considerably (Vijayalakshmi et al., 2020). The formulations consisting of A. chroococcum and B. megaterium magnified the microbial populations in soil and through synergistic interaction increased the bioavailability of N and P and some other nutrients in the soil during sorghum harvest. The dual culture of N₂ fixer and P solubilizer applied with a 100% recommended dose of fertilizer (RDF) had a maximum beneficial impact on growth (plant height, length, and dry weight of root and shoot), nutrient uptake (NPK), and grain yield of sorghum compared to the uninoculated plants at harvest. The maximum number of grains (190/plant) and grain weight (5.23g/plant) were observed with 100% RDF and combined formulations of Azotobacter with PSB, which was followed by the 75% RDF with Azotobacter and PSB. A profound increase in the yield of sorghum was attributed as being largely due to the supply of N and P by the N₂ fixer and PSB to sorghum crops. This increase was well supported by enhancement in N (highest N 1.9%) and P (highest P uptake 0.41%) uptake by sorghum plants due to inoculation of Azotobacter with PSB along with 100% RDF at harvest compared to uninoculated and unfertilized control plants. In a similar greenhouse experiment (Alagawadi and Gaur, 1988, 1992), observed a significant increase in the yield of sorghum due to combined inoculation of PSB, P. striata or B. polymyxa and N₂ fixing A. brasilense which indicated a positive interaction between the dissimilar groups of bacteria. The yields were, however, markedly further enhanced by the application of 40kgN as urea and 60kg P₂O₅ (rock phosphate) per ha along with bacterial cultures over sole application of fertilizer or only bacterial inoculation. Due to this, it was suggested that 40kgN could be saved and the whole quantity of super phosphate could be replaced by RP plus inoculation of A. brasilense and P. striata or B. polymyxa. The population counts of Azospirillum and phosphate-solubilizing bacteria in the rhizosphere of sorghum were higher in the respective inoculation treatments than in uninoculated treatments.

The AM-fungi during interaction receive sugars, amino acids, vitamins, and other organic substances from the host plant. In return, it absorbs minerals, especially P from the soil, and enhances the growth and yield of the host plant (Johansson et al., 2004). Agronomically, when the two dissimilar groups of beneficial organisms are applied in crop cultivation, the yield of many crops increases substantially (Saxena et al., 2013; Nacoon et al., 2020). Like other PGPR, AM- fungi and PSB interact synergistically wherein PSB solubilize sparingly available P compounds into orthophosphate that AM-fungi absorbs and transport to the host plants (Ordoñez et al., 2016). The reports on the inoculation effects of single or multiple PGPR on many cereal crops are numerous (Wahid et al., 2016; Naseri et al., 2018), but there is a scarcity of information on the synergistic effect of the PSM and AM-fungi on sorghum plants. Below, the role of composite application of PSM and AM fungi in the growth and yield promotion is highlighted.

Principally, AM-fungi provide nutrients to their host plants by producing hyphae that grow out from plant roots, effectively increasing the soil volume from which immobile nutrients can be acquired. So, mycorrhizal agricultural crops perform better and can be more productive compared with non-mycorrhizal plants. In addition, AM-fungi increase and improve the amounts of secondary compounds in plants (Mohamed et al., 2014). The synergy between AM-fungi and PSB with NPK fertilizer demonstrated variable growth and yield impact on greenhouse-grown sweet sorghum under saline conditions (Ramadhani and Widawati, 2020). The dual inoculation of AM-fungi and PSB in the presence of varying doses of NPK increased the plant height, the number of leaves, plant fresh weight, plant dry weight, and P of sweet sorghum plant. The AM-fungi and PSB applied with 25% NPK produced the highest increase in all the biological features of sorghum plants, suggesting that the colonization between AM-fungi and PSB with NPK fertilizer could be a suitable and economical option for enhancing sorghum production even under saline conditions. The results of a study conducted by (Ziaeyan et al., 2016) showed that the plants inoculated with a combination of PSB and AM-fungi expressed synergistic effect to increase the efficiency of P fertilizer and growing conditions and yield of sorghum. The combined application of 25mg P₂O₅ per kg of soil along with the PSB and AM-fungi maximally enhanced the biological attributes and therefore, could be recommended to save the P application in sorghum cultivation systems. Nitroxin, a bio-fertilizer prepared from a mixture of asymbiotic N₂ fixers Azospirillum and Azotobacter and AM-fungus (Glomus mosseae), mitigated the deleterious effects of stress by increasing the amounts of photosynthetic pigments, soluble proteins and osmotic regulation and decreasing electrolyte leakage in sorghum and, hence, increased the yield significantly. Mechanistically, the interacting bacterial genera had N₂ fixation, P solubilization, and iron releasing abilities, as well as the capability to secret plant hormones such as auxin, cytokinin, and gibberellin and a growth modulating enzyme, ACC-deaminase, which together enhanced the yield substantially (Olanrewaju et al., 2017).

The interactions between PSM, N₂ fixers, and AM-fungi in terms of facilitating plant growth, nutrient uptake, and yield have been reported in numerous studies in the literature. The information regarding the inoculative effect of PSM, N₂ fixers AM fungi on sorghum is scanty. Widada et al. (2007) demonstrated that sorghum plants inoculated with PSB (Pseudomonas sp.), N₂ fixer (Azospirillum lipoferum), and AM-fungi (Glomus manihotis and Entrophospora colombiana) and grown in acid and P deficient soils improved the plant dry weight and nutrients (such as N, P, Fe, and Zn) uptake, which were enhanced further by dual inoculation of selected microbiome compared to the single inoculation. Dual inoculation of AM-fungi with PSB and AM-fungi and N₂ fixer increased plant dry weight by 112 and 64 times compared to the uninoculated plant, respectively. The selected rhizobacteria also improved plant colonization by AM-fungi. These results suggest that the tripartite interaction between PSB, N₂ fixer, and AM-fungi can be an excellent strategy for optimizing sorghum production since no antagonisms among interacting organisms were observed.

Under iron-starved conditions, numerous PSMs, including bacteria, fungi, and actinomycetes, produce a wide range of siderophores (Maldonado et al., 2020; Aallam et al., 2021) with relatively low molecular weight (below ≈2kDa) and ferric ion-specific chelating agents at neutral to alkaline pH to solubilize, capture, and transport inorganic iron to cell (Sandy and Butler, 2009). They are, therefore, exploited in the management of plant diseases (Verma et al., 2011; Jyothi et al., 2020). The siderophore positive rhizobacteria protect the plants from damages by preventing the iron acquisition (Estrella and Chet, 1998) and, hence, limiting the proliferation and root colonization by phytopathogens (O’sullivan and O’Gara, 1992).

Charcoal rot of sorghum caused by Macrophomina phaseolina, a soil-and seed-borne disease of sorghum, is endemic to tropical and temperate regions of the world (Kumar et al., 2016). Charcoal root causes considerable yield losses due to a lack of genetic-resistant sorghum varieties. Up to 64% yield losses are reported in India by this disease during the post-rainy season (Das et al., 2008). In order to control the attack of this phytopathogen (Gopalakrishnan et al., 2011), isolated siderophore-positive PSB, Pseudomonas plecoglossicida, B. altitudinis, E. ludwigii, Acinetobacter tandoii, and P. monteilii from the rhizospheres of a system of rice intensification (SRI) fields and tested their biocontrol activity against M. phaseolina. Interestingly, all PSB showed strong antagonistic activity in dual culture assay, blotter paper assay, and in vivo greenhouse and inhibited the growth of M. phaseolina. No charcoal rot infection was observed in P. plecoglossicida-treated sorghum roots indicating the complete inhibition of pathogen whereas the roots treated with other isolates had 49–76% less charcoal rot infection compared to the control. All PSB increased root and shoot dry mass by 15–20% and 15–23% over control under greenhouse experiment. Also, all PSB adapted well to the sorghum rhizosphere as indicated by the reduction in charcoal rot disease. Similar antifungal activities (inhibition of growth, biomass, microsclerotia production, spore germination) of the secondary metabolites (antifungal volatile and siderophores) and the cell-free culture filtrates (CFCFs) of the selected fluorescent pseudomonad strains (SRB129, SRB288, and Pseudomonas chlororaphis SRB127) against the mycelial growth of M. phaseolina that varied from 30.5 to 76.5% in dual culture assay is reported (Das et al., 2008) The CFCF of the fluorescent pseudomonads @ 20% (v/v) significantly decreased the formation and germination of microsclerotia of M. phaseolina. Siderophore-positive bacterium P. chlororaphis (SRB127) controlled the charcoal rot of sorghum most efficiently under field conditions. As seed treatment, the bacterium reduced the charcoal rot incidence by >40% and crop-lodging by >20% and increased the sorghum grain mass. Under glasshouse conditions, the bacterium survived in the sorghum rhizosphere without any significant reduction in its population. Similarly, Streptomyces isolated from medicinal plant rhizosphere inhibited the fungal activity of phytopathogenic fungi A. brassicicola, C. gloeosporioides, Fusarium oxysporum, P. digitatum, and S. rolfsii through siderophores (Khamna et al., 2009). de Los Santos-Villalobos et al. (2012) reported the inhibition of Colletotrichum species (causing anthracnose in sorghum) due to the release of hydroxamate siderophore by B. cepacia in a Petri-dish bioassay test. Interestingly, even the lowest concentration (0.64μgml⁻¹) of siderophore inhibited the pathogen by 91%. Other phytopathogenic fungi capable of inflicting damage to sorghum for example, grain mold diseases of sorghum (Aspergillus and Penicillium) has also been controlled by siderophore rich culture and free supernatant of Alcaligenes sp., and P. aeruginosa (Sayyed and Patel, 2011). The siderophore based biological control measures, therefore, be adopted in sorghum cultivation practices due to reasons, as they are (i) inexpensive and non-destructive (safer) for the environment; (ii) self-replicating and do not require repeat application; and (iii) there is no emergence of resistance among target organisms (Sayyed et al., 2004).

Production of 1-aminocyclopropane-1-carboxylate (ACC) deaminase by PSM is considered yet another most powerful management biological weapon for plants growing under abiotic stress (Glick, 2014). After infection by phytopathogens, ethylene, a stress hormone is produced in all higher plants, which causes senescence, chlorosis, abscission, and wilting in plants, worsening the detrimental impact of pathogens (Dubois et al., 2018; Sharma et al., 2019; Etesami and Glick, 2020). The damaging impact of stress hormone can be annulled by the ACC deaminase, a multimeric enzyme, secreted by PSM bacteria (Zhao et al., 2021), fungi (Aban et al., 2017), and actinomycetes (Chukwuneme et al., 2020), which cleaves ACC (precursor of ethylene) to produce α-ketobutyrate and ammonia and thereby decreases the ethylene levels in host plants (Gupta and Pandey, 2019; Orozco-Mosqueda et al., 2021). The decrease in the levels of ACC and ethylene prevents the ethylene-mediated plant growth inhibition. PSM endowed with this capability can benefit the sorghum plants by reducing the stress and increasing plant growth (Santana et al., 2020). As an example, (Mareque et al., 2015) reported that the majority of the PSB, such as Pantoea, Enterobacter, Serratia, Pseudomonas, Acinetobacter, and Stenotrophomonas (Gammaproteobacteria phylum), Achromobacter, Herbaspirillum, and Ralstonia (Betaproteobacteria), Rhizobium (Alphaproteobacteria), Chryseobacterium (Bacterioidetes), Bacillus, Staphylococcus, Brevibacillus, and Paenibacillus (Firmicutes), and Kocuria (Actinobacteria), isolated from roots and stems of sweet sorghum plants exhibited ACC deaminase activity. Of these, ACC deaminase positive Rhizobium sp. UYSB13 and PSB bacterium Pantoea sp. UYSB45 showed a significant difference from the negative control in the stem height and dry weight (roots and shoots). Additionally, the ACC deaminase positive Rhizobium sp. UYSB12 and Enterobacter sp. UYSB34 significantly enhanced the root and the shoot dry weights of sweet sorghum.

The antibiotic-mediated inhibition of plant pathogens by rhizosphere-inhabiting biocontrol microorganisms is well (Babalola, 2010; Raaijmakers and Mazzola, 2012) are the most common and have been used in disease management through antibiotics, for example, the suppression of take all disease in wheat by 2,4-diacetylphloroglucinol (Weller et al., 2007). A variety of antibiotics have been identified: amphisin, 2,4-diacetylphloroglucinol (DAPG), oomycin A, phenazine, pyoluteorin, pyrrolnitrin, tensin, tropolone, and cyclic lipopeptides produced by pseudomonads (Raaijmakers and Mazzola, 2012; Saraf et al., 2014), and oligomycin A, kanosamine, zwittermicin A, and xanthobaccin produced by Bacillus, Streptomyces, and (Milner et al., 1996; Hashidoko et al., 1999; Mavrodi et al., 2012) diffusible products like 2, 4-diacetylphloroglucinol, phenazines, pyoluteorin, pyrrolnitrin, etc., or volatile compounds such as dimethyl disulfide, cyanogenic compounds, etc. (Olanrewaju and Babalola, 2019; Pandey and Gupta, 2020). One problem in depending too much on PSM-based antibiotics as biocontrol agents is, however, that with the increased use of PSM, phytopathogens may also develop resistance to microbe-mediated antibiotics in a manner similar to those exhibited for conventional antibiotics. This can though, be obviated by incorporating an HCN-positive PSM (Mahdi et al., 2020) with antibiotics producing PSM (Hamdali et al., 2008) so that the HCN-positive strain kills the pathogens while avoiding the emergence of antibiotic resistance among phytopathogens. The combination of HCN and antibiotics positive PSM looks a promising strategy that will synergistically control the phytopathogen attack.

A number of hydrolytic enzymes, such as, cellulases, chitinase, β-1,3-glucanase, protease, lipase, and peroxidase, are released by the PSM (Liu et al., 2016; Nandimath et al., 2017). The cell wall of plant pathogenic fungi, for instance F. oxysporum, is mainly composed of β-1,3-glucan layers that are highly susceptible to lysis by β-1,3-glucanase. Following degradation, there occurs the leakage of inner cellular contents to the exterior environment and, finally, due to the osmotic imbalance, the pathogenic fungi collapses (Singh et al., 1999). For instance, some PSFs, such as Acrocalymma sp., otryobambusa fusicoccum, and Phoma sp., produce lytic enzymes such as chitinases and glucanases (Silveira et al., 2021) and catalase and cellulases (Amin et al., 2020), and many PS actinomycetes such as S. fulvissimus, Streptoverticillium olivoverticillatum, S. nogalater, S. longisporoflavus, and S. cellulosae produce cellulase, chitinase, pectinase, lipase, and amylase (Nandimath et al., 2017), which lyse the cell walls of pathogenic fungi attacking sorghum. Similarly, number of PSB belonging to the genera Chrysobacterium, Bacillus, Pseudomonas, Mycobacterium, Staphylococcus, Curtobacterium, Enterobacter, Agrobacterium, Ochrobactrum, Serratia, Stenotrophomonas, and Acinetobacter produced lytic enzymes, such as proteases, celluloses, lipases, esterases, and amylases, which exhibited activity against Fusarium, Aspergillus, and Colletotrichum (Herrera-Quiterio et al., 2020). In a similar study, phosphate solubilizing B. subtilis BN1 isolated from the chirpine (Pinus roxburghii) rhizosphere exhibited strong antagonistic activity resulting in vacuolation, hyphal squeezing, swelling, abnormal branching, and lysis of M. phaseolina, F. oxysporum, and R. solani. The inhibition of fungal growth by B. subtilis BN1 was attributed due to the secretion of lytic enzymes, chitinase, and β-1,3-glucanase, which degrades the hyphae and digest the fungal cell wall (Singh et al., 2008). The CFCF of BN1 also showed a strong but concentration-dependent antifungal activity and completely inhibited the fungal growth at 60% CFCF concentration. Pseudomonas putida in yet another experiment demonstrated antifungal activity against A. alternata, F. oxysporum, and R. solani through chitinase, β-1,3 glucanase, salicylic acid, siderophore, and HCN (Selvakumar et al., 2009). The PS bacterium Serratia marcescens inhibited the growth of Sclerotium rolfsii (Ordentlich et al., 1988) while Paenibacillus sp. strain 300 and actinomycetes Streptomyces sp. strain 385 suppressed F. oxysporum fsp. Cucumerinum. Extracellular chitinase and laminarinase synthesized by Pseudomonas stutzeri degraded the mycelia of F. solani (Lim et al., 1991). These and other related studies that are not included in this review suggests that, in the absence of high level of genetic resistance in high-yielding sorghum varieties, the PSM as bio-antagonists could safely be used to effectively manage the biotic stresses of sorghum and hence to reduce losses in yield and quality of sorghum cultivated in different regions of the world.