1. Introduction
Rhizobium species have been successfully utilized worldwide as a biofertilizer leading to effective establishment of nitrogen fixing symbiotic association with leguminous crops. In the Rhizobium-legme symbiosis, the bacterial partner differentiates into non-dividing endo-cellular symbionts. The application of rhizobia as biofertilizer leads to increase nutrients composition and plant water uptake and tolerance to both biotic and abiotic stresses. Rhizobia are gram-negative rod-shaped bacteria and they are characterized and classified by different systems. Members of the genus Bacillus are large, gram-positive, aerobic and sporeformin rods. They are catalase positive and Bacillus species occur widely in nature, being found in the soil, air and water. The Bacilli are among the most investigated rhizobacterial species, after Pseudomonas mostly o their bio-control activities. These bacteria are capable of increasing plant growth through production of a number of substances such as antifungal and antibiotics metabolites. Members of the Bacillus genus are particularly popular candidates for plant growth promoting bacteria (PGPR) because thy sporulate an dare easier to subject to commercial formulation. Pseudomonas spp. is an unique microorganism for application bioremediation due to its flexibility and plasticity of its metabolic pathways. Streptomyces is the type of genus of the family Streptomycetaceae belonging to the order Actinomycetals of the class Schizomycetes. The bacteria belonging to this genus are mainly found in soil but are also occasionally separated from other sources such as manures. They are widely reported to have plant diseases suppression against wide variety of plat pathogens. They are one of the most important plant growth promoting bacteria due to unique characteristics like multiplication rate, cellulases, quorum sensing controlled gen expression, antibiotics, amino acid synthesis, lipase, chitinase, phytohormones, and β-1,3-glucanase production. They have wonderful ability to control plant pathogens stem because of antibiotic production, siderophore production, competition for nutrients, volatile compound secretion and synthesis of plant growth regulators.
Sorghum (
Sorghum bicolor (L.) Moench) is a worldwide food crop with strong drought tolerance [
1,
2,
3]. It is a C4 plant with a wide range of adaptations and resistance to different negative abiotic and biotic factors [
4,
5]. It is also the main staple food crop in many developing countries, and is a now biofuel source in some countries with 30% grain use for ethanol production [
6,
7]. Plant associated microorganisms, particularly plant growth promoting rhizobacteria, are known to play an important function in promoting plant growth and also in remediating soils from organic and metal pollutants by numerous mechanisms [
8,
9,
10,
11]. Plant growth promoting bacteria has a notable role in sustainable agriculture and disease management, and it acts as potential alternatives against chemical fertilizers and provides stress tolerance in addition to rhizosphere inhabitants. There are many literature reviews dealing with the role of PGPR in mobilization, phytoremediation and phytoextraction of heavy metals from soil [
12,
13,
14]. By different mechanisms such as solubilizing metal minerals, increasing root surface area for heavy metal uptake, acidifying the rhizosphere environment, and increasing the release of root exudates, these PGPR effectively increase the mobilization of heavy metals [
15,
16,
17]. Inoculation of plants with strains of plant growth promoting bacteria boosts plant growth and biomass production, and promoting phytoextraction, through different mechanisms including the exudation of phytohormones, siderophores, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase enzyme, as well as phosphate solubilization, nitrogen fixation, and antibiotic synthesis [
18,
19,
20]. Plant growth promoting bacteria increase drought tolerance in plant in different ways, such as the microbial enzyme ACC synthase [
21]. Microorganisms stimulate plant hormones homeostasis by generating auxin and producing 1-aminocyclopropane-1-carboxylate (ACC) deaminase to modulate the concentration of the plant stress phytohormone ethylene by decomposing its precursor ACC [
22,
23]. Microorganisms produce cytokinins, auxin indole-3-acetic acid (IAA), and gibberellins, which help to modify plant hormone balance [
24,
25,
26]. Microorganisms decrease the plant s
, antioxidant system to lower the number of reactive oxygen species (ROS) [
27,
28]. Microorganisms contribute to improving the plant-induced systemic tolerance to drought by changing the host physiology and the metabolic procedures [
29]. The production of exopolysaccharides protects against extreme conditions like desiccation, forming a protective capsule around soil aggregates and increasing permeability by improving soil aggregation and maintaining a higher water potential around the roots [
30,
31]. Rhizosphere engineering with PGPR for sustainable ecosystem and agriculture are nitrogen fixation; Phytohormone production such as auxins, gibberellins, cytokinins, ABS; Nutrient mobilization for P, K, Zn, Ca, Fe, etc.; abtiotic stress mitigation for drought, heat and salinity; control of biotic stress for 1-aminocyclopropane-1-carboxylate (ACC)-deaminase, antibiosis, siderophores, lytic enzymes, induced systemic resistance [
32].
Pseudomonas can synthesize enzymes which can limit the available iron by production of siderophores, kill pathogen, and modulate plant hormone levels. Pseudomonas also have modulated the plant growth by increasing uptake of minerals into the host plants, especially by production of cytokinin, phosphate production of IAA, modulating ethylene production in roots, and solubilizing nutrients. The principle components of rhizosphere engineering and intricate connection of these components are plant (plant breeding, cultivar selection, transgenic plants), soil (application of biochar, silicon, sewage sludge, zeolites, manure, etc.), and microbes (application of microbial inoculants) [
32]. The goals are improve rhizodeposition, microbial activity, root exudates, growth and yield; improve soil structure, soil aeration, organic matter, soil buffering capacity, soil fertility, and balanced nutrients; improve nutrient availability, photosynthesis, plant growth and yield, stress, soil fertility (abiotic and biotic) tolerance, and plant immunity [
32]. The goal of this review article is a survey about research progress on rhizosphere microbial species of sorghum and their impacts on increasing yield of sorghum plants by considering sustainable agriculture.
Different Kinds of Plant Growth Promoting Bacteria
Among PGPB, bacteria belonging to
Serratia genus are recognized to be capable to serve as phytostimulators, biocontrol, biofertilizer agents, and biopesticides with ability to suppress both fungal (e.g.,
Rhizoctonia solani and
Fusarium culmorum), and bacterial plant pathogens (e.g.,
Pectobacterium carotovorum and
Erwinia amylovora) [
33,
34,
35]. It is a Gram-negative bacterium belonging to the family
Enterobacteriaceae [
36,
37].
Serratia spp. Includes multiple plant growth promoting characteristics and stimulate, and colonize the growth of multiple (non-homologous, homologous) hosts, and they lead to root hair development controlled by their IAA-production and acyl homoserine lactones (AHLs) signaling procedures [
38,
39]. Hanaka et al. [
39] reported that
Serratia plymuthica (with and without inoculation) influenced most physiological factors of
Phaseolus coccineus grown with copper and indicated some impacts on biomass, and inoculation decreased catalase and guaiacol peroxidase activities, while organic acids content was higher. It has been reported that
Serratia marcescens, strain S27 is important for the bioremediation of environments contaminated with Cr and Cd in tropical regions [
40]. Abreo et al. [
41] reported that
Serratia ureilytica which has activity as a biological control parameter and dimethyl disulfide (DMDs) produced by this bacterial strains actively involved in seedling and seed protection against
Pythium cryptoirregulare.
Serratia sp. S119 is a potential important biofertilizer for maize and peanut [
42].
Serratia proteamaculans suppressed
R. solani in vivo and
in vitro and it modulated tomato defence machinery [
43].
Klebsiella is one of the principle PGPB that improve plant growth by fixing nitrogen; producing 1-aminocyclopropane-1-carboxylate deaminase, gibberellic acid, indole-3-acetic acid (ACC), and siderophores, and induction of systemic resistance [
44,
45,
46]. Singh et al. [
47] concluded that
Klebsiella sp. SBP-8 promoted growth of the host wheat plants and protected them from salt stressors via more than one mechanism such as K
+/Na
+ ration, and the effect of 1-aminocyclopropane-1-carboxylatedeaminase (ACCD) activity in plants. Noman et al. [
48] displayed that
Klebsiella pneumoniae strain NST2 as nanofactories for green copper nanoparticles (CnNPs), biogenic CuNPs positively promoted antioxidative defense response of maize plant against salinity, and boosted the maize growth under salt stress.
Streptomyces is the important genus of actinobacteria, and a filamentous bacteria presenting the family
streptomycetaceae and other actinomycetales, and it has been extensively applied for biofortification, plant growth improvement, mineral availability, nutrient assimilation, and biologic control of both plant pathogens and insect pests of agriculturally important crops [
49,
50].
Streptomyces is a bacterial genus with biotechnological activity [
51]. Samgangi and Gopalakrishnan [
52] reported that the application of strains of
Streptomyces griseus (MMA-32 and KAI-26) revealed a significant enhancement of Fe, K, Mn, and Zn contents in the harvested chickpea grains. Puppala et al. [
53] reported that
Streptomyces sp. having the capability to produce acidic and thermostable phytase as well as plant growth promoting characteristics showing increased plant growth could have implications in soils having acidic nature and in arid region. Djuidje et al. [
54] indicated the probability of using endophytic
Streptomyces strains, particularly isolate PERM2 in the protection of cocoyams (
Xanthosoma sagittifolium L. Schott) against root rot disease and the improvement of root growth. Kaur and Manhas [
55] showed that
Streptomyces hydrogenans strain DH16 and extracellular metabolites produced by it can be utilized as soil amendment as biofertilizer and biofungicide with several benefits over the chemical fungicides and fertilizers.
Streptomyces eurocidicus has antagonistic activity against a broad spectrum of phytopathogenic fungi, and it has a positive impact on soybean plants growth and development [
56].
Streptomyces microflavus G33 application efficiently controlled tomato bacterial wilt disease, and showed strong antagonistic activity against
Ralstonia solanacearum [
57]. The most important points about plant growth promoting bacteria is shown in
Table 1.
Enterobacter, any of a group of rod-shaped bacteria of the family
Enterobacteriaceae.
Enterobacter are gram-negative bacteria that are categorized as facultative anaerobes, which means that they are capable to thrive in both aerobic and anaerobic environments [
89,
90,
91].
Enterobacter sp. SA187 increased the yield of the forage crop alfalfa when submitted to different saline irrigation in field trials. Luo et al. [
92] showed that highly Cd-resistant alkalizing strain
Enterobacter sp. LYX-2, as a novel microbial passivator, has considerable ability and reuse value to acquire the remediation of Cd-contaminated soil together with safe production of vegetables simultaneously. Luduena et al. [
93] discovered that strain
Enterobacter sp. J49 is an endophytic plant growth promoting bacteria, is a notable source of information to continuing the research of its potential industrial production as a biofertilizer of maize, peanut and other economically important crops.
Enterobacter cloacae had strong insoluble P/Cd mobilization capabilities, improved diversity of rhizospheric microbial community, and could be applied as bio-inoculants for assisting phytoremediation of Cd-polluted soil in the
Solanum nigrum L. rhizosphere [
94]. Application of
Enterobacter ludwigii CDP-14 resulted in meaningful increase in the Zn content in wheat plant under metal stress, and also promoted the various compatible solutes such as total soluble sugar, proline content, and decreased the malondialdehyde (MDA) content as compared to control [
95].
Enterobacter sp. P23 was shown to improve rice seedling growth under salt stress, and the impact was connected with a decrease in antioxidant enzymes and stress-induced ethylene [
96]. Sabir et al. [
97] concluded that combined application of biochar and
Enterobacter sp. MN17 retrieved negative impacts of Cd in
Brassica napus, mobilized Cd in soil, but reduced Cd uptake by plants. Ghosh et al. [
98] also displayed that
Enterobacter cloacae AS10 significantly promoted rice plant growth at the seedling stage through Cd immobilization, and it prevented the surge of stress ethylene and oxidative stress in rice seedlings, leading to plant growth improvement.
The entomopathogenic fungus
Beauveria bassiana is extensively applied for insect pest control and can produce three distinctive infective unit kinds under different nutritional and environmental conditions: blastospores, aerial conidia, and submerged conidia [
99,
100,
101,
102,
103]. Espinoza et al. [
104] concluded
that Beauveria bassiana could be applied to promote the yield of active chemical components in cultivated medicinal plants such as chive (
Allium schoenoprasum L.).
Beauveria bassiana had great potential to be applied in the integrated management of
Hypothenemus hampei (Ferrari) in coffee crops [
105]. Its strain GHA is the main component of an area-wide pest management study for coffee berry borer [
106].
Beauveria bassiana strains were effective to kill almost all the Colorado potato beetles, and it can be suggested for the development of a new biopesticide [
107].
Beauveria bassiana strains RGM-644, RGM-557, and RGM-721 showed fewer nymphs of whiteflies (
Trialeurodes vaporariorum), RGM-577 and RGM-731 produced the highest plant height, and RGM-731 obtained the greatest plant biomass in tomato [
108]. It can alleviate Fe chlorosis symptoms in both wheat and tomato plants during early growing stages, but the intensity of the impact related to the plant species and available Fe on substrate [
109].
Beauveria bassiana (Balsamo)
Vuillemin, can exist as an endophyte in banana (
Musa spp.) and potentially provide management against the banana weevil
Cosmopolites sordidus (Germar) [
110].
Beauveria bassiana ARP14 can decrease the egg period of
Bemisia tabaci [
111]. It can promote bean plant growth, and it has important benefits as seed inoculants to suppress spider mites in bean and the technique appears to have no conflict with application of predatory mites [
112].
Beauveria bassiana displayed promising nematicidal activity, particularly against
Meloidogyne incognita in both cucumber and tomato [
113]. It can be utilized for management of two hemipteran cotton insect pests, namely,
Phenacoccus solenopsis Tinsley and
Dysdercus cingulatus Fab. [
114]. The yields of experimental maize samples showed a significant improvement after
Beauveria bassiana treatment, and it produced a notable mortality rate in grasshopper [
115].
Beauveria bassiana endophytically colonized cotton, melon, and tomato plants [
116]. Ramakuwela et al. [
117] demonstrated the ability to establish endophytic
B. bassiana in pecan and the potential to use this ability in pecan pest management. The application of
B. bassiana strain Naturalis in the presence of
Ditylenchus destructor and
D. dipsaci might lead to a detrimental interaction leading to higher nematode population densities and higher potato tuber damage [
118]. Donga et al. [
119] showed that
B. bassiana foliar spray and soil drench improved the numbers of sugarcane sett roots. In one experiment, it has shown that the large pine weevil (
Hylobius abietis) adults may be infected by
B. bassiana after only a short exposure to carriers [
120].
Frankia are Gram-positive actinobacteria, filamentous nitrogen-fixing actinobacteria that are free-living microbes in the soil and in symbiotic associations with actinorhizal plants [
121,
122].
Frankia can fix nitrogen by converting atmospheric N
2 into biologically practical ammonia and provide the host plants with a source of reduced nitrogen [
123,
124,
125]. Some
Frankia strains are resistant to abiotic stresses including heavy metal, pH, salinity and temperature and have developed complicated mechanisms to adapt to these conditions [
126,
127]. The
Frankia species can cause the development of nodules on the roots of an immeasurable variety of actinorhizal plants [
128,
129,
130]. Marappa et al. [
131] reported that
Frankia casuarinae is an effectual biofilm inhibition agent against
Pseudomonas sp. and
Candida sp.
The genus
Microbacterium belongs to the family
Microbacteriaceae within the surborder
Micrococcineae [
132,
133].
Microbacterium are nonspore-forming, Gram-positive, and rod-shaped bacteria [
134].
Microbacterium species have been recently characterized as effective plant growth-promoting bacteria under abiotic and biotic stresses [
135], and it is also found that members of this genus could be effectual against phytopathogenic bacteria, nematodes, and fungi [
136,
137,
138]. Suarez-Estrella et al. [
139] reported that
Microbacterium aerolatum,
Microbacterium profundi, and
Microbacterium foliorum showed positive results in the
in vitro germination promotion test in lettuce seeds, increasing the radicle weight at levels close to 45%, and indicating significant palliative impacts of
Botrytis cinerea and promoting the development of lettuce seedlings both at root and aerial levels.
Burkholderia species have various lifestyles establishing mutualism or pathogenic associations with animals and plants [
140]. Bacteria of the genus Burkholderia are very heterogeneous, consisting of over 30 species (Gram-negative, β-Proteobacteria) that occupy extensively differing ecological niches, and the members are usually encountered in the rhizosphere of crop plants and their connection with plants ranges from pathogenic to plant-growth-promoting and symbiotic associations [
141].
Burkholderia phytofirmans strain PsJN promoted maize plants biomass production even in heavy metal contaminated soil, and its inoculation alone had just minor impacts on alter microbiome structures [
142]. Bernabeu et al. [
143] discovered that seedling inoculation with
Burkholderia tropica led to efficient root colonization of tomato plants followed by bacterial spreading to aerial tissues.
Burkholderia sp. SSG suppressed diverse pathogens: 5 bacteria, 9 fungi and 6 oomycetes, and it provided significant control of twelve plant diseases, and it is a favorable broad-spectrum biocontrol agent for plant disease management [
144]. Inoculation of
Burkholderia sp. AQ12 promoted the rice growth, improved the cellular levels of antioxidant enzymes and boosted the expression of tillering related genes [
145]. Application of
Burkholderia phytofirmans PsJN and tree twigs biochar together improved grain quality parameters, and growth physiology of mung bean (
Vigna radiata), immobilized Pb in soil and decreased its uptake, translocation and bioaccumulation in
V. radiata tissues, reduced Pb induced oxidative stress by intensifying antioxidant activities while meaningfully decreasing activities of ROS [
146].
Burkholderis sp. CC-AI74-infected maize show improved growth, and N, P contents, increased
in planta P-utilization rate and reveal significant escalation of root phosphatase activity [
147].
Bacillus is an aerobic, Gram-positive, and endospore-forming bacteria [
148,
149]. Manasa et al. [
150] observed that
Bacillus spp. Discovered from the marine water and the rhizosphere of the medicinal plant
Coscinium fenestratum increased plant growth, iron content, and yield of sorghum. Sorghum plants inoculated with
Bacillus spp. strains showed better growth in terms of root biomass and shoot length with dark greenish leaves due to high chlorophyll content, and inoculation also increased leaf relative water content and soil moisture content under water stress conditions [
151]. Prathibha and Siddalingeshwara [
152] reported that application of
Pseudomonas fluorescens and
Bacillus subtilis were effective in improving seed quality such as vigour index, seed germination and nutritional quality such as carbohydrate and protein content.
Bacillus is one of the highly studies plant growths promoting rhizobacteria [
153,
154], and they are common in rhizosphere communities [
155]. Most important bioactive molecules from the genus
Bacillus are non-ribosomally synthesized peptides and polyketide compounds, lipopeptides, bacteriocins and siderophores [
156].
Bacillus mojavensis strain PRC101 is a close relative of
Bacillus subtilis which have been reported to reduce mycotoxin accumulation and disease incidence of
Fusarium verticillioides in maize [
157].
Bacillus sp. CHEP5 acts synergistically with
Bradyrhizobium sp. SEMIA6144 increasing the symbiotic interaction with peanut even in
Sclerotium rolfsii challenged plants [
158]. Kim et al. [
159] reported that
Bacillus velezensis M75 was separated from cotton waste and its application as biocontrol agent against fungal plant pathogens in agricultural fields.
Bacillus amyloliquefaciens subsp.
plantarum strain 32a significantly colonized and persisted in the rhizoplane of tomato plants, promoted the growth of tomato plants, and decreased the severity of crown gall disease in tomato plants [
160]. Rocha et al. [
161] observed that among the
Bacillus strains, there are potential biocontrol agents against multiple races of
Fusarium wild.
Bacillus sp. strain PnD is a potential plant growth promoting bacterium, and the seed biopriming procedure can effectively mitigate inhibitory impacts of salinity stress in rice plants [
162]. It has been reported that
Bacillus megaterium BM18-2 could be used as a potential agent for Cd bioremediation, increasing growth and Cd absorption of hybrid
Pennisetum in Cd contaminated soil [
163].
Bacillus amyloliquefaciens B9601-Y2 treatment promoted maize growth parameters and yield, and revealed potential to promote maize grown and suppress corn leaf blight (
Bipolaris maydis) [
164].
Bacillus amyloliquefaciens PDR1 from root of karst adaptive plant increased
Arabidopsis thaliana roots to absorb ions, promoted rhizosphere acidification capacity, and improved
Arabidopsis thaliana to resist alkaline stress [
165].
Bacillus velezensis OEE1 effectively colonize and increase growth of
in vitro olive plants, and its antifungal components inhibit growth of
Verticillium dahliae [
166]. Gowtham et al. [
167] reported that
Bacillus subtilis Rhizo SF 48 enhanced the plant growth parameters in tomato plants subjected to drought stress, and down-regulation of
Le25 and
SIERF84 confirm positive impact of Rhizo SF 48 in tomato plants subjected to drought stress.
Bacillus sp. KUJM2 offered considerable potential as a PTE remediating parameter, plant growth promoter of a plant (
Lens culinaris) and regulator of potentially toxic elements (PTEs; Cd, As, Ni, Cu) translocation curtailing human health and environmental risks [
168].
Bacillus pumilus improves tomato growth and N uptake under N fertilization, but it does not effect tomato growth or N uptake under no N fertilization [
169].
B. Pumilus-induced N
2 fixation is connected with bacterial
nifH gene [
169].
Bacillus pumilus strain JPVS11 is vital for rice growth promotion, improves photosynthetic pigments, antioxidant activity, and proline, improved microbial count, soil enzyme activity, and soil properties in salt stress [
170].
Bacillus licheniformis induced resistance against gray mold in pepper plants, and both and
Bacillus pumilus and
Bacillus licheniformis inhibited the growth of
Fusarium solani and
Botrytis cinerea [
171].
Arthrobacter is a genus obligate aerobes bacteria distinguished by rod-coccus growth cycle, and it is usually are found in aerial surface of plants, soils, and wastewater sediments [
172,
173].
Arthrobacter genus belongs to the family
Micrococcaceae within the surborder
Micrococcineae [
174,
175]. Vanissa et al. [
176] reported that
Arthrobacter sp. strains can assist the growth of maize in salinity-affected and P deficient soils in a genotype-dependent fashion, and the bacterial strains was shown as able to produce the phytohormones auxin, gibberellins, abscisic acid, and cytokinins. Jiang et al. [
177] indicated that
Arthrobacter sp. DNS10 application increased nitrogen utilization by increasing the expression of genes involved in nitrogen metabolism in soybean leaves.
Pseudomonas are oxidase positive, Gram-negative, and non-spore forming bacteria that are known for inhabiting a diversity or environments [
178,
179]. The genus
Pseudomonas contains different species considered to be plant growth promoting rhizobacteria, and it is one of the most and largest diverse in the bacterial kingdom [
180]. Among the
Pseudomonas species, the
P. syringae complex including many
P. syringae pathovars,
P. cichorri,
P. viridiflava, and other species, has been associated with diseases on a broad spectrum of plant hosts [
181,
182,
183,
184].
Pseudomonas putida strain SAESo11 inoculation keeps the redox state of tomato plants at exposure to drought stress, and resist to drought stress [
185]. Moin et al. [
186] reported that
Pseudomonas EFP-121 was significantly effectual in improving fresh shoot weight and plant height of sunflower.
Pseudomonas S4, S1 serve as PGPR and improved biomass accumulation in spinach, tomato, and lettuce; S2 and S4 root inoculation boosted chlorophyll content in tomato and lettuce leaves [
187].
Pseudomonas lurida strain E0026 showed high metal and wide pH tolerance, and able to resist broad range of antiobiotics and adapt drought stressed conditions, and copper phytoremediation by
Helianthus annuus [
188]. The inoculation with the
Pseudomonas putida KT2440 mutant strains is an important technique to increase the growth of corn and soybean plants, so they might be applied for promoting regional crops growth at field scale utilizing natural saline soils [
189]. Sahebani and Gholamrezaee [
190] showed the capability of
Pseudomonas fluorescens CHA0 to control
Meloidogyne javanica and induction of systemic resistance of cucumber and tomato plants.
Rhizobia play a vital function to establish a symbiotic connection between leguminous plants and environmental productivity [
191,
192]. Rhizobia inhabit root nodules and they can convert the atmospheric nitrogenous molecule to ammonia and consequently make it available to the plant [
193]. Rhizobial-mediated nitrogen fixation is a significant agriculture, and its inoculum has been applied frequently as biofertilizers [
194,
195]. It seems that rhizobia and legumes co-evolved to change important elicitors/microbe-associated molecular patterns (MAMPs) and their corresponding receptors to prevent the development of defense reactions, as the plant immunity mechanisms depend on the recognition of microbe-associated molecular patterns [
196].
Rhizobium triggers enzyme-mediated induced resistance reactions [
197].
Rhizobium infection of common bean increased the production of N-based nutritive compounds, while inhibited the production of C-based organic compounds associated with plant resistance, and probably increasing host plant tolerance to the pathogen [
197].
Rhizobium plays the main function in mutualistic interactions with plants, and
Rhizobium sp., isolated from
Chlorella vulgaris, enhances green algal growth [
198].
Rhizobium decreased Cu-induced growth inhibition and promoted nitrogen content, reduced oxidative stress through boosting antioxidant ability, and regulated antioxidant, phytochelatin, gene expressions, and metallothioneins [
199].
Rhizobium alamii strains promoted plant growth and tolerance to water stress, and they influenced on the plant growth and root-associated microbiota changed according to the strain [
200]. The most important points about the effects different species of
Rhizobium several crops are shown in
Table 2.
Mechanism of Plant Growth Promoting Bacteria
The application of plant growth-promoting bacteria (PGPB) is considered as one of the the techniques to improve phytoremediation procedure and plant growth under various abiotic stress condition [
213]. The PGPB can remain under abiotic stress conditions by different mechanisms including biofilm formation, synthesis of heat shock proteins (Hsps), osmoprotective compounds, etc. Plant growth-promoting microorganisms can be applied to increase soil characteristics, improve plant growth, alleviate abiotic stresses and promote phytoremediation effectiveness is considered as an effective and environmentally friendly process [
214,
215]. The PGPB can improve plant growth and phytoremediation process by providing plant growth promoting (PGP) metabolites such as indole-3-acetic acid (IAA), siderophores, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, and as well as solubilizing phosphate (P) [
216,
217,
218,
219]. Certain PGBP are able to immobilizing Cr by reducing Cr
6+ to Cr
3+ can also decrease Cr toxicity in plants. Some PGPB may also lead to abiotic stress tolerance to plants by stimulating gene expression and antioxidant enzyme activity such as ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), etc. [
220,
221,
222]. Plant growth-promoting microbes support plants to tolerate abiotic stress (drought, Cd, temperature, etc.) by producing indole-3-acetic acid (IAA) and siderophores, solubilizing phosphate, reducing stress ethylene content by synthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase, and producing antioxidant enzymes like catalse (CAT), superoxide dismutase (SOD) and ascorbic peroxidase (APX) to scavenge ROS and promote nutrient uptake [
223,
224]. Orozco-Mosqueda et al. [
225] discovered that the enzyme activity of ACC deaminase results I the production of ammonia and α-ketobutyrate, which, by reducing ACC levels, hinders excessive increases in the synthesis of ethylene under different stress environments, and it one of the most effective mechanisms to lead to plant tolerance to salt stress.
Alijani et al. [
226] reported that
Serratia rubidaea Mar61-01 succeeded in controlling
Botrytis cinerea, the causal factor of strawberry gray mold, because of reduce mycelial growth, conidial germination and fruit decay development under in
in vivo and
in vitro experiments, but also that prodigiosin influenced the activity of some fruit enzymes related to the defense mechanism and resistance to gray mold infection.
Serratia marcescens strains not only increased plant growth, but also induced systemic defense responses in the host plant, and the induction of entomopathogenic bacterium
Serratia marcescens with the ability to produce IAA during the early establishment of rice seedlings can be beneficial [
227]. Many studies have shown the potential of
Serratia marcescens that induces plant growth by modulating phosphate solubilization and phytohormone producing [
228,
229]. Li et al. [
230] reported that
Klebsiella sp. strain San01 extracted from sweet potato increased the expression of the
IbLEA gene in sweet potato under drought and salinity, and the results lead to the encouraging biotechnological approach via San01 interaction to promote stress-tolerant sweet potato (
Ipomoea batatas L.) plants. Abbasi et al. [
231] reported that
streptomyces decrease drought stress in tomato plants and adjust the expression of transcription parameters of ERF1 and WRKY70 genes, and
streptomyces strains inhibit stress signals, at biochemical and molecular levels to hinder the plant
,s understanding of stress while, through boosting relative water content (RWC), compensated for the decline in the yield. The plant growth promotion by s
treptomyces is discovered which has association with the production of growth regulators including two kinds of pathways for the biosynthesis of IAA which have role in tryptophan-dependent, indole acetic acid (IAA), and tryptophan-independent have been proposed [
232].
Enterobacter species sustain productivity and plant growth include nitrogen fixation, solubilization of inorganic phosphate, production of plant hormones, reduction of ethylene levels, improvement of nutrient uptake and production of antimicrobial components [
233,
234].
Enterobacter cloacae MG00145 (OS03) isolated from surface disinfected stem of
Ocimum sanctum, which has multigrowth-promotion characteristics (IAA production, siderophore, phosphate solubilisation, and ammonia production), and promotes the growth of
Oryza sativa,
Vigna mungo,
Arachis hypogaea, and
Brassica rapa Var. Toria. Panneerselvam et al. [
235] demonstrated that
Entreobacter species were significantly increased the below and above ground response in rice plants. Inoculation of
Enterobacter cloacae MSR1 and
Pisum sativum significantly improved the growth parameters like dry weight of
Medicago sativa [
236].
According to the 16S rRNA gene sequence analysis SCAUK0330 was discovered as
Burkholderia cepacia. SCAUK0330 grew at 10-40oC and pH 4.0-10.0, tolerated up to 5% NaCL, and indicated antagonism against some pathogenic fungi [
237]. The expression of a single SCAUK0330 gene gave
E. coli a pH reduce ability to solubilize phosphate, and the nucleotide and the deduced amino acid sequences of this phosphate solubilization connected with gene indicated the high degree of sequence identity with
B. cepacia E37gabY [
237]. Jiang et al. [
238] concluded that
Burkholderia sp. J62 on the basis of the 16S rDNA gene sequence analysis, show various multiple heavy metal and antibiotic resistance characteristics, as the isolate produced siderophore, indole acetic acid and 1-aminocyclopropane-1-carboxylate deaminase, and the isolate also solubilized inorganic phosphate.
Burkholderia vietnamiensis C12 from
Ficus tikoua Bur. showed an important biocontrol factor for rice sheath blight, and lead to the defense enzyme in rice against rice sheath blight by improving the activities of defense enzymes SOD, POD, PAL, CAT, and PPO [
239]. It was identified as
Burkholderia sp. based on 16S rDNA sequence analysis presented nitrogenase activity, phosphate solubilizing ability and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity; suppressed the growth of
Sclerotinia sclerotiorum,
Verticillium dahliae, and
Gibberella zeae, and produced small quantities of indole acetic acid (IAA) [
240].
Burkholderia ambifaria T16 showed extraordinary plant growth-promoting characteristics, such as phosphate-solubilization, siderophore production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity,
in vitro antagonism against
Fusarium spp. and enhancement of grain yield when inoculated to barley grown under greenhouse conditions [
241].
Bacterial species from genus
Bacillus can produce IAA, solubilize phosphate and can fix nitrogen and develop biocontrol attributes like HCN, antibiotics, hydrolytic enzymes, and siderophore [
242]. Both
Bacillus paranthracis NT1 and
Bacillus megaterium NCT-2 showed cadmium resistance;
Bacillus paranthracis NT1 sequestered 96.75% of the adsorbed Cd
2+ on the cell wall,
Bacillus megaterium NCT-2 accumulated 43.11% of the adsorbed Cd
2+ intracellularly, and the procedure modulated by the genes
cadA,
khtT,
trkA,
zitB,
czcD, and
bshA, respectively [
243]. It has been reported that improved effectiveness of
Bacillus safensis primed plants in alleviating heat stress is mainly connected with preserving functional and structural integrity of essential cellular organelles like mitochondria and chloroplast through elimination of excess reactive oxygen species (ROS) [
244]. Two
Bacillus species,
B. altitudinis FD48 and
B. methylotrophicus RABA6 indicated efficient groups representing abscisic acid (ABA) in rice plants, and as co-inoculant priming triggered ROS enzyme activity
viz., super oxide dismutase, ascorbate peroxidase, peroxidase, and catalase [
245]. Tahir et al. [
246] reported that consortium of
Bacillus strains may modulate sodium (Na
+)/potassium (K
+) efflux and antioxidant activities of potato under salt stress; under NaCl stress, the species were positive for P-solubilization, IAA and ACC deaminase, and consortium inoculation to potato improve RWC while decreases antioxidant enzyme activity and MDA contents. Myo et al. [
247] concluded that
Bacillus velezensis NKG-2 secreted the fungal cell wall degrading enzymes and volatile organic compounds (VOCs), and it meaningfully decreased the disease severity of
Fusarium oxysporum wilt disease on tomato plants both under
in vivo and
in vitro conditions. The expression of plant hormone, DNA replication and Fe-Zn transporter related proteins in wheat roots was improved in the presence of
Bacillus megaterium strain N3 compared with uninoculated roots under Cd stress [
248].
Bacillus subtilis exhibits both a indirect and direct biocontrol mechanism to suppress disease attributed to pathogen, and the indirect process includes the stimulation of plant growth and the induction of needed systemic resistance, and the direct process consists of the synthesis of several secondary hormones, metabolites, cell-wall-degrading enzymes, and antioxidants that help the plant in its defense against pathogen attack [
249,
250].
Bacillus produced various types of antimicrobial peptide substances such as subtilin, bacilysin, oligomycin A, iturin, zwittermicin A, and kanosamine and, which may be accountable for the inhibition of pathogen [
251,
252,
253]. It also have a basic function in plant growth promotion by production of IAA and GA3, production of cytokinin, synthesize different kinds of lipopeptides and zinc solubilization [
254,
255,
256]. Sampathkumar et al. [
257] also reported that
Bacillus amyloliquefaciens (ETL2) secreted number of antibacterial secondary metabolites and endophytic
Bacillus spp. decreased cotton bacterial blight disease in cotton plants. Chi et al. [
258] illustrated that both
Bacillus megaterium NCT2 and
Bacillus paranthracis NT1 could modulate phytohormones secretion, decrease oxidative stress and promote
Solanum nigrum growth when exposed to Cd, and according to the results of qPCR the two strains impacted the detoxification of Cd in
Solanum nigrum by influencing the expression of antioxidant enzyme genes and gene PDR2, and differential expression of heavy metal transport genes HMA and IRT1 may leaf to the difference Cd accumulation in
Solanum nigrum. Dhali et al. [
259] reported that horse gram (
Macrotyloma uniflorum) inoculated with
Bacillus sp. (AS03) has notably lesser chromosomal aberration in the roots, and the photosynthetic performance of Cr treated plants co-inoculated with AS03 was meaningfully increased compared with non-inoculated Cr treated plants on the basis of photosynthetic yield.
Azospirillum brasilense Sp245, a diazotrophic plant growth-promoting bacterium impacts host plants through distinctive mechanisms, gene interactions and confers biotic and abiotic stress tolerance mediated by signaling molecules [
260].
Pseudomonas spp. belong to
Pseudomonadaceae family (10 groups on the basis of rRNA-DNA hybridization) categorized into 6-subgroups of rRNA gene homology and RFLP [
261,
262].
Pseudomonas species produce antagonistic procedure such as induced growth resistance (ISR) and components like cell wall degradation enzymes, and antibiotics to keep a mutualistic connection with the associated plant, and they can synthesize auxins having characteristics similar to phytohormones like indole-3-acetic acid (IAA), which behave as signaling molecules for modulating plant growth [
261].
Pseudomonas spp. significantly decreased the stress ethylene emission and promoted a significant increase in plant growth in comparison with single inoculation and non-inoculated control, as catalase activity which was significantly boosted in co-inoculatred red pepper plants prove its ability to effectively neutralize the hydrogen peroxide ions formed because of oxidative stress in plants under salinity stress, and significant decline in malondialdehyde (MDA) content can be associated to the increased salt tolerance in co-inoculated red pepper plants [
263].
Pseudomonas fragi stimulated tomato immunity to control
Meloidogyne incognita by jasmonic acid and salicylic acid dynamic defence, as it increased the expression of
PR1,
PR2,
PR3,
PR5, and
PDF1.2 [
264]. Anand et al. [
265] reported methyltransferase positive
Pseudomonas oleovorans as most efficient strain for reduced grain As content due to presence of
arsM gene, and improve plant nutrient status of rice.
PGBP inoculation tends to modify the expression of the genes associated to phytohormones such as Jasmonic acid and ABA which assists plants to tolerate salt-stressed environments [
266]. There are reports that indicate that PGPB can promote expression of some principle genes associated to phosphate transport and increase PO
42- in plants under salinity stress [
267]. It has been showed that the inoculation of
Arthrobacter protophormiae in wheat plants reduced concentration of ethylene and ABA and regulated the expression of regulatory
TaDREB2 and
TaCTR1 transcription parameters in ethylene signaling pathway [
268]. Inoculation of salt-tolerant PGPB with protective and suitable carrier materials will not just increase crop yield but also significantly reduce electrical conductivity (EC) and regulate Na
+ content in plants and rhizosphere [
266].
Pseudomonas simiae up-regulated genes include Peroxidase (POX), catalase (CAT), vegetative storage protein (VSP), and nitrate reductase (NR), and down-regulated genes include high affinity K + transporter (HKT1), lipoxygenase (LOX), polyphenol oxidase (PP!), and pyrroline-5-carboxylate synthase (P5CS) of soybean which are are responsible for antioxidative enzymes, nitrite reduction, vegetative storage protein, and ion transporters related genes [
269].
Bacillus amyloliquefaciens upregulated genes include
AT1G17420,
AT1G72520,
AT2G26010,
AT5G44420,
ACS2,
ACS8 and
ACS11 which have roles in ACC synthase, and
Brassinostreoids regulator genes of
Arabidopsis thaliana [
270].
Burkholderia phytofirmans may modify genes of
Arabidopsis K +
Transporter 1 (
AKT1),
High-affinity K + Transporter 1 (
HKT1),
Sodium Hydrogen Exchanger 2 (
NHX2), and
Arabidopsis Salt Overly Sensitive 1 (
SOS1),
RD29A, and
RD29B,
Ascorbate Peroxidase 2 (
APOX2),
Glyoxalase 17, and down-regulated the expression of
Lipoxygenase 2(
LOX2) which have roles in ion homeostasis-related genes, desiccation, ROS scavenging, detoxification, and jasmonic acid biosynthesis-related genes of
A. thaliana [
271].
Bacillus firmus induced expression of genes
APX,
CAT,
POD,
Fe-SODGmVSP,
GmPHD2,
GmbZIP62,
GmWRKY54,
GmOLPb, and
CHS which have roles as antioxidant enzymes and salt tolerance related genes of soybean [
272].
Bacillus subtilis up-regulated
PtHKT1;
5 and
PtSOS1 down-regulated
PtHKT2; 1 gene expression and ion transporter genes of
Puccinellia tenuiflora [
273].
Pseudomonas sp. enhanced expression of genes
TaP5CS,
TaP5CR, and
TaOAT which are associated with proline synthesis of wheat [
274].
Bacillus oryzicola enhanced expression of
RD29A,
RD22,
KIN1,
RD29B,
RD20,
RD22, and
ERD1 which may lead to salt stress-responsive genes of
A. thaliana [
275].
Enterobacter sp. has roles in upregulation of ROS pathway genes
CAT,
APX,
GR, and
DHAR and antioxidative enzymes related genes of Okra [
276].
Pseudomonas knackmussii modulated
NHX1,
HKT1,
SOS2 and
SOS3,
PR1 as well as
SAG13 which are related to salt stress, defense-related gene and senescence associated gene of
A. thaliana [
277].
Azotobacter chroococcum modulated the expression of
NRT2.1 and
LATE EMBRYOGENSIS ABUNDANT (
LEA) of tomato [
278].
Arthrobacter woluwensis up-regulated genes include
GmLAX1,
GmAKT2,
GmST1, and
GmSALT3, and down-regulated genes include
GmNHX1 and
GmCLC1 [
279,
280].
Plant Growth-Promoting Bacteria and Sorghum
The physiological responses and growth of sorghum plants, nitrogen metabolites, and nitrogen fixation efficiency increased after inoculation. PGBP decreased free ammonia, increased soluble protein and nitrate in sorghum plants [
281]. El-Meihy et al. [
282] reported that utilization of heavy metal-tolerant plant growth promoting bacterial strains (HMT-PGPB) namely
Alcaligenes faecalis MG966440.1,
Alcaligenes faecalis MG257493.1 and
Bacillus cereus MG257494.1 to lessen the heavy metal
,s toxic impacts on sorghum plant, in addition to their along with increase in plant growth. Beneficial bacteria are notably enhanced in the rhizosphere of sorghum, and they have positive interaction with sorghum nitrogen uptake under low nitrogen environment [
283,
284]. Sahib et al. [
285] indicated that
Pseudomonas sp.,
Pseudomonas poae,
Microbacterium sp.,
Serratia marcescens,
Pantoea agglomerans and
Bacillus pumilus may improve total productivity, and soil-plant interactions under inadequate resources. El-Meihy et al. [
282] concluded that the usage of these HTM-PGPB strains as biofertilizer of sorghum support plant to avoid the toxic impacts of heavy metals and enhance growth parameters. Bruno et al. [
286] found that the chromium reducing-thermotolerant inoculation with plant growth promoting bacteria (CRT-PGPB) strains selected from Cr contaminated plots and distinguished as
Myroides odoratimimus TCR22,
Providencia rettgeri TCR21 and
Bacillus cereus TCR17 through molecular characterization, indicated the possibility of reduction of Cr
6+ to Cr
3+, produce siderophores, solubilize phosphate and indole-3-acetic acid. Inoculation of sorghum with CRT-PGPB boosted antioxidant status such as catalase, superoxide dismutase and ascorbate peroxidase, plant growth, and declined malondialdehyde contents and decreased proline in plants under Cr stress; moreover, escalation of atmospheric temperature (IAT), and Cr+IAT stress proved that PGPB was appropriate to decrease stress induced oxidative damage [
286]. Under IAT stress, inoculation of CRT-PGPB reduced the accumulation of Cr in plants in comparison with uninoculated control propose that CRT-PGPB may have the capability to increase phytostabilization procedure in Cr contaminated soils [
286]. Dhawi et al. [
287] indicated that among treatment group, PGPB group induced to maximum number of upregulated proteins, and PGPB in combination with arbuscular mycorrhizal (AM) fungi or alone decreased the adverse influences of radical scavenging system (ROS), and boosted levels of particular proteins as a result changing the metabolism towards carbohydrates
, synthesis in sorghum biomass and absorption of nutrients. Plant growth promoting actinobacterial strains selected from metal-polluted soils and identified as
Nocardiopsis sp. strain RA07 and
Streptomyces sp. strain RA04 showed diversified plant growth promoting parameters like production of indole-3-acetic acid, siderophores, 1-aminocyclopropane-1-carboxylate deaminase, and phosphate solubilization under Cd stress (CdS), high-temperature stress (HTS), and drought stress (DS) conditions [
288]. Silambarasan et al. [
288] also reported that the actinobacterial inoculants decreased malondialdehyde concentration and increased antioxidant enzymes in plants cultivated under different abiotic stress environments, revealing that actinobacterial inoculants alleviated oxidative damage. Both strains RA07 and RA04 increased the translocation of Cd from root to shoot, and accumulation of Cd in plant tissues, which proves they could be effectual bio-inoculants to escalate phytoremediation of Cd polluted soil even in high-temperature stress and drought stress conditions [
288]. Dhwai et al. [
289] reported that plant growth promoting bacteria enhanced metabolic activities which resulted in increase in element uptake and sorghum root biomass whether accompanied with mycorrhiza or used solely, as treated plants two biochemical pathways, fatty acid biosynthesis and galactose metabolism were regulated in all treatment groups. Three heavy metals tolerant-plant growth promoting bacteria (HMT-PGBP) (
Alcaligenes faecalis MG966440.1,
Alcaligenes faecalis MG257493.1 and
Bacillus cereus MG257494.1) increased the microbial activities such as improving dehydrogenase activity (DHA), decrease of heavy metal bioaccumulation in plants and roots, and stimulate heavy metals bioaccumulation parameter in rhizosphere and sorghum plants, as well as improved the microbial activities and decreased the accumulation of heavy metals in soil sorghum plants [
290].
Li et al. [
291] showed that the endophytic bacterial strain K3-2 may be applied for improving sorghum biomass production and Cu phytostabilization in the Cu mining wasteland soils. Shinde and Borkar [
292] showed that
Serratia marcescens strain L2FmA4 and
Serratia marcescens strain L1SC8 promoted 1-aminocyclopropane-1-carboxylic acid deaminase and indole-3-acetic acid (IAA) production, increased germination, functional leaves, promoted height and final yield of sorghum in semi-arid and arid regions.
Streptomyces pactum (Act 12) promoted potential toxic trace elements (PTEs) absorption in root and shoot of sorghum and it increased soil and plant enzymatic activities in polluted soil [
293]. Sorghum root, shoot dry, weight and chlorophyll significantly improved after Act 12, also β-glucosidase, alkaline phosphatase and urease activities were significantly increased by Act12, however lipid peroxidation (MDA) and antioxidant enzymatic activities were decreased after the consumption of Act12 [
293]. The AgNPs were synthesized by extracellular extract of
Streptomyces griseoplanus SAI-25, and it has shown antifungal activity against charcoal rot pathogen
Macrophomina phaseolina of sorghum [
294]. Gopalakrishnan et al. [
295] reported that five strains of
Streptomyces (CAI-121, CAI-24, KAI-32, CAI-127, and KAI-90) significantly increased all PGP parameters such as plant height, stem weight, leaf area, leaf weight, root surface area, root length, root volume, and root dry weight over the control. Alekhya et al. [
296] concluded that two streptomyces strains,
CAI-8 and
BCA-546 have the potential to control charcoal rot disease in sorghum. Gopalakrishnan et al. [
297] indicated that
Streptomyces albus strain CAI-21 and its active metabolite organophosphate have the ability to control charcoal rot in sorghum.
Enterobacter sp. K3-2 was identified from the roots of sorghum which exhibited Cu resistance and produced 1-aminocyclopropane-1-carboxylate (ACC) deaminase, indole-3-acetic acid (IAA), arginine decarboxylase and siderophores [
297]. Chiarini et al. [
298] reported that
Enterobacter sp. strain BB23T4d was capable of colonize the root system of sorghum, but it could not stimulate plant growth such as
Pseudomonas fluorescens strain A23T3c and
Burkholderia cepacia strain PHP7. Singh and Jha [
299] concluded that under salinity stress,
Enterobacter sp. SBP-6 can be applied for promoting the sorghum plants growth.
Beauveria bassiana strain GHA could possibly control
Melanaphis sacchari infestations in sorghum if utilized under suitable environmental conditions [
300].
Beauveria bassiana can become an endophyte in sorghum and lead to protecting from stem borer, and it has been suggested that stem tunneling by shoot borer was significantly greater compared to
Beauveria bassiana treated sorghum plants [
301]. The endophytic colonization of sorghum by
Beauveria bassiana indicates that this isolate is well adjusted to a wide range of conditions consisting of endophytic in plants and pathogenic to insects, and it can recognize as an endophyte in sorghum without negatively impacting plant growth [
302]. Ramos et al. [
303], and Liu et al. [
304] reported that
Beauveria bassiana resulted in appropriate endophytic colonization in crop tissues and regulated stem height, fresh root, fresh stem weight, and root length, and it has been suggested due to its high application potential in eco-agricultural fields including biofertilizers and biopesticides. Raya-Diaz et al. [
305] showed that
Beauveria bassiana boosted the Fe nutrition of the sorghum plants, but their impacts associated to the inoculation method. Schlemper et al. [
306] reported that
Burkholderia tropica strain IAC/BECa 135 is an appropriate PGPB strain for application as inocula for sustainable sorghum cultivation because it can notably improved the plant biomass of sorghum plants. A significant increase in the yield level of sorghum because of combined inoculation of phosphate solubilizing bacteria and
Azospirillum brasilense over single inoculation showing a positive interaction between the 2 groups of bacteria [
307]. The interactions between
Azospirillum brasilense and
endomycorrhizae may increase shoot-to-root rations, plant dry weight, and the nitrogen content of infected plants [
308].
Pseudomonas sp. P17 strain as a potential PGPR for plant growth and nutrient uptake in sorghum, as inoculation lead to better growth, higher nutrient uptake and seedlings treated, as well as appropriate root volume, maximum dry mass, shoot length, chlorophyll, leaf area, phosphorus, nitrogen, carbohydrates and other nutrients [
309,
310,
311,
312]. Saad and Abo-Koura [
313] reported that inoculation of sorghum plants with
Bacillus cereus could improved vegetative growth, RWC% but reduced antioxidant enzymes under drought stress. Wu et al. [
314] found that
Bacillus amyloliquefaciens can be applied to manipulate the microbial communities of the sorghum rhizosphere soil.