Intensive research attempts are proceeding to alleviate the effects of climate change and improve soil fertility and disease resistance of plants using organic agricultural approaches, comprising microbial inoculants. This is mainly because microbial inoculants are beneficial to mankind and can be used in the field of agriculture. The most antique microbe used as inoculants is “rhizobia,” bacteria that are able to colonize the rhizosphere and establish a symbiotic association with legumes, which are used as a plant growth promoter and protector through biological nitrogen fixation, mobilization and solubilization of nutrients, production of siderophores, and discharge of phytohormones (Brockwell and Bottomley, 1995; Makoi et al., 2013; Koskey et al., 2021; Yadav et al., 2021). Particularly, research on inoculants and inoculation with rhizobia and legumes raised pronounced interest from researchers and companies in the 1970s (Santos et al., 2019). The practice of using rhizobial inoculants as bio-inoculants and/or bio-pesticides signifies a potential promise for improving plant health and soil productivity and controlling and/or suppressing plant diseases (Deaker et al., 2006; Khan et al., 2017; Wolde-meskel et al., 2018; Volpiano et al., 2019; Matse et al., 2020). Consequently, this is part of an environmentally friendly approach for sustainable nutrient management and can be supplements or alternatives to chemical fertilizers.

Rhizobial inoculants usually contain live or dormant cells and are developed as carrier-based inoculants comprising effective microbes, and their formulation with different carrier materials allows long-term storage and higher efficacy (Ahemad and Kibret, 2014; Gopalakrishnan et al., 2015; Matse et al., 2020; Yadav et al., 2021). Following the inoculation, the organisms in the inoculants colonize plant roots, stimulate root growth, make nutrients available to the plants, and protect plants from various diseases. Hence, legume inoculation with rhizobia is a way of assuring that the strain of rhizobia appropriate for the legume cultivar being grown exists in the soil at the proper period and in numbers adequate to make a rapid and effective infection and succeeding nitrogen fixation (Zahran, 1999). The inventive objective of this practice is to stimulate biological nitrogen fixation to offer nitrogenous nutrients to a particular legume and other crops. The return of this practice is an increment in plant growth, nutrient uptake, yield, seed protein, and other traits and a reduction in the use of chemical fertilizers with the subsequent decrease of ecological contamination (Wolde-meskel et al., 2018).

Different studies have confirmed that rhizobial inoculation in legumes is recognized for stimulating growth and is an alternative to the expensive inorganic nitrogen fertilizers (Ndakidemi et al., 2006; Abbasi et al., 2010). Therefore, the use of appropriate inoculants in legumes offers an opportunity for improving productivity of legumes and other crops grown in integrated cropping systems such as crop rotation, intercropping, alley cropping, and green manuring. As a result, the phenomenon of rhizobial inoculation has gotten consideration due to its increasing contribution to agricultural productivity. The overview of the benefits of rhizobial inoculation, the nitrogen-fixing process due to the application of rhizobia, and mechanisms of biological nitrogen fixation in legumes are described in Figure 1.

Commercial rhizobial inoculants can be supplied to farmers in different forms such as solid, liquid, and freeze-dried formulations (Brockwell and Bottomley, 1995). Rhizobial inoculants need to be competitive and superior to the native rhizobial populations for nodule residence and effectiveness in fixing nitrogen. Therefore, rhizobial inoculants should be chosen for specific host legumes and ecological conditions (Stephens and Rask, 2000). An additional essential trait that needs to be considered is the persistence of the inoculants in the soils over the period (Zahran, 1999). At the recommended rates of inoculation, a suitable inoculant often dominates in nodulation, nitrogen fixation, and aftermath provision of the fixed nitrogen to crops (Brockwell and Bottomley, 1995). A study by Lindström et al. (1990) revealed that rhizobial inoculants can prevail in nodules for 5–15 years after their first inoculation into the soil. This confirms that rhizobial inoculants are competitive and effective saprophytes and reside in the soil for several years even in the non-existence of their legume host.

Several studies have shown that inoculation of legumes with rhizobia can improve the growth and development of plants through other mechanisms in addition to the provision of fixed nitrogen to plants (Chernin and Glick, 2012; Namvar et al., 2013; Tagore et al., 2013; Uddin et al., 2014; Singh and Singh, 2018; Wolde-meskel et al., 2018; Matse et al., 2020). Duan et al. (2009) reported that the presence of amino cyclopropane carboxylate (ACC) deaminase activity in some strains of rhizobia stimulates plant growth by reducing the levels of ethylene in the plants. Das et al. (2017) stated that legume seed inoculation with effective rhizobia leads to stimulation and accumulation of phenolic compounds, like isoflavonoid phytoalexins, and triggering of enzymes such as l-phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), peroxidase (POX), and polyphenol oxidase (PPO), which are involved in phenylpropanoid and isoflavonoid pathways, a rich source of metabolites in plants. Avis et al. (2008) reported that rhizobia help to solubilize phosphorus by producing low-molecular-weight organic acids that perform on inorganic phosphorus and can also promote improved resistance against plant pathogens.

Rhizobia consist of a diverse range of genera in the Alphaproteobacterial and Betaproteobacterial classes and are termed “Alpha-rhizobia” and “Beta-rhizobia,” respectively (Sprent et al., 2017). The Alphaproteobacteria contains about 20 families; however, there are currently 13 genera of Alphaproteobacteria comprising legume-nodulating species, viz., Aminobacter, Azorhizobium, Bradyrhizobium, Devosia, Ensifer (Sinorhizobium), Mesorhizobium, Methylobacterium, Microvirga, Neorhizobium, Ochrobactrum, Phyllobacterium, Rhizobium, and Shinella. On the other hand, the Betaproteobacteria contains about 12 families of bacteria although there are currently two nodulating genera of Betaproteobacteria, viz., Burkholderia and Cupriavidus (Howieson and Dilworth, 2016). Presently, the number of correctly described species of legume-nodulating rhizobia is increasing at a rate of more than 10 species per year; however, rhizobial strains in only five genera of rhizobia are currently used as inoculants in agriculture (Table 1).

Almost all plants nodulated by rhizobial species in the genus indicated in Table 1 are in the family Leguminosae, which includes more than 700 genera and about 20,000 species, which are in turn classified into three subfamilies (Caesalpinioideae, Mimosoideae, and Papilionoideae), although the legume taxonomy is currently under revision (Sprent et al., 2017). Similarly, the taxonomy of rhizobia nodulating different legumes is also huge, and the wide-ranging lists of valid rhizobial species are continually updating. Rhizobial taxonomy is nowadays based on divergence chromosomal genes such as ribosomal RNA, and the genetics of symbiotic host specificity allowed variation of host range among rhizobial species (Shamseldin et al., 2017; Sprent et al., 2017). These indicate that there are relationships between rhizobial taxonomic groups (genera and species). A phylogeny of the currently confirmed symbiotic genera and species of Alpha- and Beta-rhizobia and their relationships are shown in Figure 2.

In the field of sustainable agriculture, rhizobial inoculation has the competency to promote plant growth and stress resistance, recycle nutrients, improve soil fertility, and rectify soil contamination. These microorganisms have been used for improving plant productivity by directly contributing biologically fixed nitrogen, nutrient solubilization, and phytohormone production. At present, the use of these inoculants, especially in legume crops, is considered as a strategic component of the agricultural system as they improve the productivity of the crops sustainably without causing harm to the environment (Yadav et al., 2021). Reports from different authors have revealed that legume inoculation with rhizobia subsequently increased the nodulation, growth, and yields of legume crops. As stated by Herridge (2008), rhizobial inoculation is a method of insurance where a farmer, by paying a minor premium charge of inoculation, is protected against the possibility of nitrogen-deficient crops, which helps to overcome the reduction in crop yield and income. Among the growth and symbiotic parameters of legumes, the number of nodules and dry weight of root nodules can be an index for the degree of infection of the inoculated rhizobial species, resulting in nodule development, nodulation, and improvement of plant growth (Singh and Singh, 2018).

The positive results of inoculating legumes with rhizobia on the number of nodules and nodule dry weight per plant in different legumes were documented well (Ndakidemi et al., 2006; Singh and Singh, 2018; Wolde-meskel et al., 2018; Matse et al., 2020). Kumaga and Ofori (2004) reported an increment in nodulation and plant growth after inoculation of soybean varieties, both promiscuous and non-promiscuous, which can be attributed to the highly competitive capability of the rhizobial inoculant used. Likewise, Dey et al. (2004) showed that nodulation and plant growth were increased in peanuts following the inoculation of seeds with a diversity of rhizobial species. Huang and Erickson (2007) inoculated pea and lentil seeds with Rhizobium leguminosarum and stated that nodulation and shoot and root growth of both plants increased as a result of rhizobial inoculation. Similarly, the inoculation of lentil with Rhizobium leguminosarum enhanced seedling height, nodule number, and shoot biomass of the crop.

The increment in grain yields of various legumes because of rhizobial inoculation which also resulted in soil fertility improvements has been revealed by different authors (Tagore et al., 2013; Wolde-meskel et al., 2018). In Brazil, inoculation improved soybean grain yields by up to 750 kg/ha (Coutinho et al., 1999). Nyoki and Ndakidemi (2013) inoculated cowpea with Bradyrhizobium japonicum in Tanzania and described that inoculation improved the number of pods per plant by 13.7%, the number of seeds per pod by 11.6%, the mean pod weight by 24.6%, and the 100-seed weight by 8.5%. Ravikumar (2012) also revealed that inoculating both Vigna mungo and Vigna radiata varieties with rhizobia resulted in greater plant height, number of nodules, number of roots, shoot growth, number of leaves, number of pods, length of pods, fresh weight, and seed weight than those of their corresponding controls.

In chickpea, significant improvement in the grain yields and protein contents in the grain and straw were reported following the inoculation of chickpea genotypes with dual microbial fertilizers of rhizobia and phosphate-solubilizing bacterial inoculants (Tagore et al., 2013, 2014). John (2015) reported about 16.15–27.50% grain yield increment in two dry bean (Phaseolus vulgaris) cultivars due to rhizobial inoculation. A significant increment in grain yields of soybean due to inoculation with two isolates (SB6B1 and legumfix) of Bradyrhizobium inoculants in Ethiopia was reported by Fituma (2015). Ronner et al. (2016) showed mean grain yield increments of 1.75, 1.42, and 1.42 t/ha by inoculation and application of P fertilizer, seed inoculation alone, and application of sole P fertilizer, respectively, in soybean. Santos et al. (2019) delineated that inoculation of Bradyrhizobium in soybean resulted in mean increases of 8.4% in grain yield compared with the naturalized population and that inoculation of common bean with Rhizobium tropici increased yield by 8.3% (Table 2). Yield increments can vary between legume species and rhizobial strains used due to specific cropping conditions such as soil composition, temperature, site, and environmental conditions.

In their study, Ali et al. (2000) revealed that the inoculation of mung bean (Vigna radiata L.) with rhizobia significantly increased the nodulation, growth, and components of yield like the number of pods bearing branches per plant, number of pods per plant, number of seeds per pod, and 1,000-seed weight. Similar reports were stated by Nyoki and Ndakidemi (2014) in cowpea upon inoculation with Bradyrhizobium japonicum. The increment in the root nodulation, growth, yield components, and yield of legumes by inoculation can be accredited to higher nodulation and further nutrient availability, which resulted in vigorous plant growth and development, and accumulation of dry matter leading to higher seed yields (Namvar et al., 2013; Uddin et al., 2014).

Furthermore, Shahid et al. (2009) revealed that soybean seed production can be increased by 70–75% when the right rhizobial strains are used to inoculate the crop. Ahiabor et al. (2014) and Rechiatu et al. (2015) showed significant increases in the grain yield of soybean after rhizobial inoculation. Similarly, Ibrahim et al. (2011) stated increased yield and components of the yield of soybean by inoculating the seeds with the rhizobial strain. This could be attributed to higher root nodulation and nitrogen fixation due to inoculation, which eventually increased pod number per plant, and thus higher grain yields (Singh and Singh, 2018). In their study, Nyoki and Ndakidemi (2014) presented that plants inoculated with rhizobia provided significantly greater seed and stover yield than the uninoculated control and suggested the reason as high nodulation which resulted in improved nitrogen fixation and accordingly higher seed and stover yield. Bambara and Ndakidemi (2010) also revealed that rhizobial inoculation significantly improved the yield components and yields such as pods number per plant, seed number per pod, seed number per plant, 100-seed weight, and seed yield as compared to the uninoculated control.

Improvements in legume yield have also been reported with the co-inoculation of different rhizobia resulting from diverse mechanisms of action. Jesus et al. (2018) have shown yield improvement in common bean following co-inoculation with Rhizobium tropici CIAT 899, Bradyrhizobium diazoefficiens USDA 110 (formerly Bradyrhizobium japonicum USDA 110), and Bradyrhizobium elkanii 29w. These authors revealed that Bradyrhizobium spp. improved the symbiosis effectiveness of Rhizobium, resulting in a higher number of nodules, N accumulation, and overall biomass production. The mechanism behind this positive co-inoculation effect was suggested by Santos et al. (2019) as the co-inoculated rhizobia produced signaling molecules such as nodulation factors (Nod-factors) and polysaccharides that stimulated root nodulation and improved the efficiency of biological nitrogen fixation.

The differences in inoculation responses have been reported in different studies attributing to differences in ecological conditions, crop variety, and the type of inoculant and method of inoculation used; as well as legume response to rhizobial inoculation is reported to be highly unpredictable but variable and site-specific. Moreover, a 5-year comprehensive project by the biological nitrogen fixation in tropical agricultural legumes (NifTAL) project which was aimed at determining the benefits of rhizobial inoculants application for legumes shows clear benefits associated with inoculation. The majority of the 228 standardized field trials conducted in more than 20 countries with 19 legume species gave a significant response to the inoculation of rhizobia when the experiments were carried out under farmers' fields and intensive agronomic management with higher agricultural inputs (Singleton et al., 1992). The amount of increment (in percentage) in grain yield of various legume crops in response to rhizobial inoculation under different agroclimatic conditions as adapted from Bhowmik and Das (2018) is presented in Table 3.

Absences of responses to inoculation of rhizobia have also been reported in some legumes and under different environments (Giller, 2001). These can be accredited to the inherent characteristics of both the host legume plant and the rhizobial species used, as well as the unlimited sensitivity of the fixation process to different environmental stresses such as soil acidity, high temperatures, soil dryness, soil salinity, and low soil fertility (Brockwell et al., 1991; Graham et al., 1994; Wolde-meskel et al., 2018). Egamberdieva and Adesemoye (2016) indicated that there is variability in the effectiveness of inoculants when used in different conditions or cropping systems and the success of rhizobial inoculants applied as a plant growth promoter and/or biocontrol agents to various conditions depends on the collection strategy and screening process. Besides, the success of rhizobial inoculation is restricted by the existence of highly competing native rhizobia which outcompetes and presents a barrier against nodule formation and nodulation by the introduced rhizobial strain (Thies et al., 1991). Resident soil rhizobia including native rhizobia and those naturalized through past inoculation may have an impact on inoculation success through their impact on competence for nodule occupancy with the inoculated strains of rhizobia (Denton et al., 2002). According to Thies et al. (1991), the response of several legumes to rhizobial inoculation depends largely on the population of native soil rhizobia, available soil nitrogen, and the crop's nutrient demand.

Bioavailability and uptake of elemental plant nutrients such as nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are very imperative for the growth and development of plants, especially where diverse cropping systems involving legumes are practiced. The uptake of these nutrients by plants depends fundamentally on the quantity, concentration, and activities in the soil rhizosphere, and the capability of the soil to restore them in the soil's solution. Nutrient uptake by the plant is essential for plant growth and function including symbiotic nitrogen fixation processes. Although total nutrient uptake by legume crops depend on the yield obtained, which can be varied with cropping season, type of variety, soil condition, and crop agronomic practices (Tairo and Ndakidemi, 2014), it has been reported that inoculation of legumes with rhizobia significantly increases the availability and uptake of phosphorus, potassium, calcium, and magnesium in different organs of the plants (Makoi et al., 2013).

A study by Ndakidemi et al. (2011) showed that rhizobial inoculation improved the availability and uptake of plant nutrients such as P, K, Mg, Ca, S, Fe, Mn, Zn, Cu, B, and Mo in different legumes. Makoi et al. (2013) reported that rhizobial inoculation significantly improved the uptake of nutrients such as P, K, Ca, and Mg in different plant parts such as leaves, shoots, roots, and pods of common beans (Phaseolus vulgaris L.). Allito et al. (2020) indicated that inoculation of faba bean with diverse rhizobial strain caused a significant improvement in nitrogen uptakes which ranged from 194.7 to 309.6 kg N ha⁻¹ as compared to the un-inoculated control as well as soil nitrogen balance is also increased following inoculation of rhizobia. These authors indicated that the increment in the nitrogen uptake and soil nitrogen balance is mainly accredited to improved nitrogen fixation.

Inoculation of legumes with rhizobia can also noticeably enhance the availability and uptake of phosphorus in legumes. Tairo and Ndakidemi (2014) revealed that cowpea [Vigna unguiculata (L.) Walp] inoculation with Bradyrhizobium japonicum significantly enhanced uptake of the phosphorus which might be accredited to indirect effects of inoculation of Bradyrhizobium japonicum on plant growth and activities of the rhizosphere. On the other hand, increased root capacities to absorb nutrients (Ziadi et al., 2007) and mobilization of phosphorus caused by improved extracellular phosphatase activity (Agren et al., 2012) are the supreme reasons responsible for enhanced availability and uptake of phosphorus. Regar et al. (2017) indicated that rhizobial inoculation improved the development of roots and the availability of more nutrients in soybean due to boosted growth and development of the plant. Higher phosphorus uptake due to rhizobial inoculation is accredited to the capacity of introduced rhizobia to solubilize precipitated phosphorus components thereby improving the availability and uptake in plants (Fatima et al., 2007). The nitrogen and phosphorus uptakes by different parts of faba bean inoculated with six different strains of rhizobia and non-inoculated plants provided with and without N fertilizer as +N and –N controls, respectively, are shown in Table 4.

Rhizobial inoculation can also improve the contents of nutrients in plants. According to Matse et al. (2020), the separate inoculation of two Rhizobium species and their co-inoculation significantly improved the N, P, and K contents in roots and shoots of white clover as compared to the non-inoculated control. Sahai and Chandra (2011) revealed higher nitrogen and phosphorus uptake and content in both shoot and grain when legume is inoculated with Mesorhizobium ciceri in contrast to uninoculated control. Further, Kaur et al. (2015) showed that chickpea inoculation with Mesorhizobium sp. resulted in 61.1 and 11.4% greater grain N and P content, respectively, due to the improved nitrogen fixation as well as improvement in root growth and root behavior which favorably increased nutrient acquisition.

Rhizobia were reported to parasitize, distort and inhibit the hyphae and reproductive structures of fungi, and antagonize fungal pathogens by the secretion of hydrolytic enzymes (Volpiano et al., 2019). Antoun et al. (1978) evaluated the antagonistic ability of 49 Sinorhizobium meliloti strains against Fusarium oxysporum and showed that the strains suppressed the disease notwithstanding their symbiotic effectiveness and the inhibition of the fungal growth varied from 5 to 50%. Kelemu et al. (1995) reported that 15 Bradyrhizobium sp. strains evaluated in dual cultures exhibited the capacity to suppress the mycelial growth of Rhizoctonia solani. Chao (1990) evaluated six rhizobial strains for their disease antagonistic ability against ten fungi isolates and revealed that all the tested Rhizobium strains reduced the growth of fungi.

According to Ehteshamul-Haque and Ghaffar (1993), Rhizobium leguminosarum, Sinorhizobium meliloti, and Bradyrhizobium japonicum are testified to significantly hinder the growth of pathogenic fungi such as Macrophomina phaseolina (Tassi) Gold, Rhizoctonia solani Kuhn, and Fusarium species in both legume and non-leguminous plants. Moreover, Tamiru and Muleta (2018) indicated that Faba bean inoculation with rhizobial isolates suppressed the radial growth of Fusarium solani under in vitro conditions and the average disease reduction for combinations of rhizobial isolates was 45.1% as compared to 29.2% for individual strains and the highest disease severity (73.3%) suppression was detected with inoculation of a combination of Rhizobium isolates before the appearance of the pathogen (Table 5). As a result, the mixture of rhizobial strains as biocontrol agents helps to defend against a broader range of pathogens, acclimatize to the ecological fluctuations, and improve the genetic diversity of biocontrol systems. In addition, it also allows the perseverance of the biocontrol agents for an extended period in the rhizosphere, exploitation of a broader array of biocontrol mechanisms which improve the efficiency and reliability of biocontrol and offer a mixture of several mechanisms of biocontrol.

Studies have also revealed that white rot disease (Sclerotinia sclerotiorum) of Brassica campestris can be controlled by inoculation with Mesorhizobium loti in which the growth of disease was suppressed by 75% after extended incubation (Chandra et al., 2007) (Figure 3A), sheath blight of rice can be controlled by inoculation with Rhizobium leguminosarum bv. phaseoli strain RRE6 and bv. Trifolii strain ANU843 (Mishra et al., 2006), and Pythium root rot of sugar beet can be controlled by inoculation with Rhizobium leguminosarum viciae (Bardin et al., 2004). Dubey et al. (2012) assessed Bradyrhizobium sp. isolates of black gram for antifungal activities against Macrophomina phaseolina and revealed that two rhizobial isolates (VR2 and VR1) were capable of reducing the Macrophomina phaseolina mycelial growth by 71.5 and 50.5% in dual cultures and by 37.6 and 49.2% in cell-free cultures, respectively (Figure 3B). Gopalakrishnan et al. (2015) also revealed that the inoculation of Bradyrhizobium japonicum, Rhizobium leguminosarum, and Rhizobium meliloti can be a biocontrol of plant pathogenic fungi that infect okra and sunflower such as Macrophomina phaseolina, Rhizoctonia solani, and Fusarium solani. Kumar et al. (2011) isolated five rhizobial strains (TR1–TR5) from fenugreek root nodules and reported that three isolates (TR1, TR2, and TR4) suppressed the growth of Fusarium oxysporum, resulting in loss of structural integrity of the mycelium, hyphal perforation, lysis of hyphae, fragmentation, and degradation. According to Hemissi et al. (2011), the reduction of fungal growth in vitro by rhizobia and subsequent formation of zones of suppression were probably a result of the metabolites released into the culture medium by rhizobia. Microscopic investigations also revealed that rhizobial inoculation leads to abnormal intercalary swelling, unfolding, tip deformation, lysis of hyphae, and degeneration of cytoplasm of fungi such as Rhizoctonia solani, Fusarium oxysporum, Sclerotinia sclerotiorum, and Macrophomina phaseolina due to interaction with rhizobia (Deshwal et al., 2003).

Like rhizobia, plant-pathogenic bacteria establish companionable interactions with plants to obtain nutrients from the host plants upon the colonization, and both rhizobia and plant-pathogenic bacteria implemented similar strategies to colonize, invade, and form a chronic infection in the host plants (Volpiano et al., 2019). Different studies on the effects of rhizobia on diseases caused by plant-pathogenic bacteria demonstrated that rhizobia have biocontrol properties against plant-pathogenic bacteria. Osdaghi et al. (2011) indicated that inoculation of common bacterial blight (CBB)-susceptible cultivar and tolerant lines of common bean with Rhizobium leguminosarum bv. phaseoli significantly reduced the disease severity of CBB under both greenhouse and field experiments. Díaz-Valle et al. (2019) delineated that the inoculation of common bean with Rhizobium etli inhibited halo blight severity as compared to plants not inoculated with the symbiont and indicated that the size of foliar pathogen lesion was 75% lesser in the Rhizobium etli-treated plants than in the uninoculated plants. Osdaghi et al. (2011) assessed the inoculation of the Rhizobium leguminosarum bv. phaseoli strain on CBB of common bean caused by Xanthomonas axonopodis pv. phaseoli under greenhouse and field conditions and delineated that Rhizobium leguminosarum bv. phaseoli significantly reduced disease severity. Rhizobial strains such as Rhizobium leguminosarum bv. trifolii, Rhizobium leguminosarum bv. viciae, Rhizobium meliloti, Rhizobium trifolii, Sinorhizobium meliloti, and Bradyrhizobium japonicum have been reported to produce antibiotics and cell-wall-degrading enzymes that can hinder the phytopathogenic bacteria (Gopalakrishnan et al., 2015). Díaz-Valle et al. (2019) also showed that rhizobial inoculation facilitated greater and more rapid activation of defense-related genes following infection with the pathogenic bacteria and suggested that inoculation is imperative for a cellular mechanism of ISR in plants.

To date, the investigations regarding rhizobia and viruses have been mainly concerned with the effect of viral diseases on the nodulation process, nitrogen fixation, and subsequent nitrogen availability to the plants and nutrient content in plants. However, studies have revealed that rhizobial inoculation has contributed to the biocontrol of plant viral diseases. Elbadry et al. (2006) showed significant inhibition in bean yellow mosaic potyvirus (BYMV) disease in broad bean in which the incidence was reduced from 91.33% (infected control) to 43% and 27.7% when inoculated with rhizobia and Pseudomonas FB11 strains, respectively, showing that rhizobia are key biocontrol agents against plant viral diseases. Singh and Srivastava (1983) suggested that the increase in availability and nutrition of nitrogen following inoculation of Rhizobium phaseoli strain Dangeard affected the replication and symptomatic appearance of common bean mosaic virus in mung beans (Vigna radiata). Further, Elbadry et al. (2006) proved the presence of ISR against BYMV in broad bean inoculated with Rhizobium leguminosarum bv. viciae.

The association of rhizobia with plant nematodes in the rhizosphere and the beneficial effect of rhizobia on nitrogen fixation and plant nutrition have led to studies on the potential effect of nematode parasitism on nodulation and symbiotic nitrogen fixation (Taha, 1993) and the inhibition effect of rhizobial inoculation on plant nematodes (Khan et al., 2017). Consequently, rhizobial strains can act as biocontrol agents of diseases caused by parasitic nematodes through direct and/or indirect mechanisms in both legumes and non-legumes (Volpiano et al., 2019), and the galling, egg mass production, and soil population of the nematode can be suppressed by the treatment of rhizobia (Khan et al., 2017). Noreen et al. (2016) reported the reduction of root-knot nematode in chickpea following inoculation with rhizobia and suggested rhizobial inoculation as a biocontrol of root-knot nematode. Volpiano et al. (2019) revealed that inoculation of Rhizobium can decrease about 96% of galls in roots of eggplant (Solanum melongena) infected with Meloidogyne incognita. Siddiqui et al. (2007) suggested Rhizobium inoculation as the most efficient biocontrol of nematodes as inoculation of Rhizobium endorsed the decrease from 72 (infested control) to 40 galls per root systems and 14,960 (infected control) to 7,520 nematodes per kilogram of soil, apart from causing a larger increase in plant growth in the absence of Meloidogyne javanica.

Besides, Ashoub and Amara (2010) reported the ability of the Rhizobium isolate of broad bean (Vicia faba) to attain 100% mortality of Meloidogyne incognita juvenile in vitro at 72 h. Studies have revealed that the cyst nematode of potato can be controlled by inoculation with Rhizobium etli strain G12 (Reitz et al., 2000). Khan et al. (2017) reported 12% to 18% mortality in the nematode juveniles due to the culture and culture filtrate of the rhizobial strains and suggested that the inhibition in the hatching and survival of nematode larvae and mortality in the nematode juveniles were apparently due to the toxic metabolites synthesized by the bacteria. According to Sidhu (2018), the most susceptible stages of plant-parasitic nematodes to manage with biocontrol are the eggs and second-stage juveniles as these life stages survive outside of the plants, especially in water films in soil particles, providing biocontrol agents the opportunity to interact, infect, and parasitize the nematodes. Consequently, the life process and cycle of the nematodes can be interrupted, resulting in decreased population density and successful control when the eggs and second-stage juveniles of the plant-parasitic nematodes are controlled.

Rhizobia are reported to show active responses against nematodes by possessing different modes of actions such as impeding plant–nematode recognition, hindering nutrient uptake, activating systemic resistance against them, and undergoing direct inhibition by producing various enzymes, toxins, and metabolites (Khanna et al., 2019). Hallmann et al. (2001) reported a significant reduction in the number of potato root galls formed by Meloidogyne incognita following inoculation with Rhizobium etli G12 and G12 (pGT-trp) in which the number of galls was reduced by 34 and 39%, respectively, compared with plants treated with only Meloidogyne incognita. Mahdy et al. (2001) also indicated nematode control in different vegetables (tomato, cucumber, and pepper) and field crops (soybean and cotton) by Rhizobium etli G12 with a decrease in galling ranging from 17% for cotton to 50% for tomato and a significant decrease in the number of egg masses ranging from 37% for soybean to 70% for pepper.

Nematodes enter roots via the zone of elongation and differentiation, which is also a zone preferably colonized by biocontrol agents, and alterations in the physiology of plant following nematode infection might also create favorable conditions for rhizobial inoculant well (Hallmann et al., 1997). Following the inoculation, rhizobia can actively penetrate plant tissues using hydrolytic enzymes like cellulase and pectinase (Hallmann et al., 1997), or they can penetrate within galled tissue via the wounds caused by juveniles and move from the galled tissue to the stem base, thereby leaking, intercepting, and reducing the nutrients available to the nematode, hence interfering with nematode development and decreasing egg production (Hallmann et al., 2001). Another aspect of rhizobia–nematode interaction is that root nodulation may limit the pathogenesis of root-knot nematode, resulting in a significant decline in the galling and reproduction of nematodes, particularly the Meloidogyne spp. (Taha, 1993). According to Khan et al. (2017), a toxin named “rhizobitoxine” is produced by Bradyrhizobium japonicum, which may negatively affect the nematode pathogenesis, and bacteriocin was also produced by rhizobial strains, which may be involved in the reduction of nematode in plants. Reitz et al. (2000) also confirmed that lipopolysaccharides secreted by Rhizobium etli strain G12 activated ISR to infection in potato roots against the potato cyst nematode Globodera pallida.

In comparison to the other biocontrol agents, rhizobial inoculants play an essential role in agricultural production through their capacity of symbiotic nitrogen fixation, solubilization of phosphorus and biocontrol, and suppression of plant diseases (Compant et al., 2005; Hemissi et al., 2011; Gopalakrishnan et al., 2015; Khan et al., 2017; Volpiano et al., 2019). Numerous mechanisms have been identified to be employed by rhizobia for biocontrol and suppression of plant diseases. According to Martínez-Viveros et al. (2010), enormous mechanisms are involved in rhizobia as a biocontrol, which involves direct antagonism via the production of antibiotics, siderophores, HCN, and hydrolytic enzymes (chitinases, proteases, lipases, etc.) and indirect mechanisms in which the biocontrol agent acts as a probiotic by contending with the pathogen for the infection sites and nutrients, activating acquired systemic resistance and ISR responses in plants, and modifying hormonal levels in plant tissues. Compant et al. (2005) indicated that aggressive colonization, self-protective retention of the rhizosphere niches, and biocontrol characteristics of rhizobia are facilitated by the production of allelochemicals, including antibiotics, iron-chelating siderophores, lytic enzymes, biocidal volatiles, and detoxification enzymes. Gopalakrishnan et al. (2015) reported that rhizobial inoculants famish the plant pathogens by generating high-affinity siderophores and, thus, limit the growth and development of the pathogens. Besides, Deshwal et al. (2003) stated that the biocontrol mechanisms of rhizobia may involve antibiotics, HCN, and siderophores. Rhizobia also appear to influence the plant defense mechanism by enhancing the production of phytoalexins by plants. According to Tariq et al. (2020), the overall mechanisms of biocontrol depend essentially on antibiosis, competition for nutrients, mycoparasitism, production of hydrolytic enzymes, induction of systemic resistance in host plants, and rhizosphere competence. Numerous mechanisms of biocontrol against plant pathogenic microorganisms are indicated in Figure 4.

Furthermore, Gopalakrishnan et al. (2015) stated the mechanisms of biocontrol employed by rhizobia like competition for plant nutrients, antibiotic production, enzyme production to degrade cell walls, siderophore production, and the synthesis of metabolites such as HCN, phenazines, pyrrolnitrin, viscoinamide, and tensin. According to Ahemad and Kibret (2014), the principal modes of biocontrol properties in the usage of rhizobial inoculants are competition for nutrients, niche exclusion, ISR, and production of antifungal metabolites. In general, seed biopriming and plant inoculation with rhizobia can offer systemic resistance against a broad spectrum of plant pathogens. Das et al. (2017) also stated the mechanisms involved in biocontrol by rhizobia like mycoparasitism, production of antibiotics, antifungal secondary metabolites such as HCN, siderophore production and subsequent competition for iron between pathogens and rhizobia, competition for nutrients, induction of plant defense mechanisms, and plant growth promotion, which decreases vulnerability to pathogenic attack. Diseases of bacterial, fungal, and viral origin and damages caused by nematodes can be decreased by the application of plant growth-promoting bacteria such as rhizobia (Compant et al., 2005). In general, rhizobia-mediated plant growth promotion and biocontrol of diseases occur by the change in the entire microbial community of the rhizosphere niche and through the production of different substances, as indicated in Table 6.

The use of rhizobial strains also helps to produce plant resistance against diseases-causing pathogens. Interaction of rhizobial inoculants with the plant roots can result in plant resistance against some pathogenic bacteria, fungi, viruses, nematodes, etc. by a phenomenon known as ISR (Ahemad and Kibret, 2014; Gopalakrishnan et al., 2015). Gopalakrishnan et al. (2015) reported that rhizobial strains trigger the resistance of plants against pathogens by producing signals such as the jasmonate and/or ethylene pathway, lipopolysaccharides, flagella, homoserine lactones, cyclic lipopeptides, acetoin, and butanediol, leading to the induction of host plant's defense response against pathogens. Rhizosphere competence is also one of the mechanisms utilized by rhizobia as biocontrol agents comprising effective root colonization mixed with the capacity to survive and multiply along growing plant roots over a substantial period, in the existence of the native microflora (Compant et al., 2005). Given the significance of rhizosphere competence as an essential and effective biocontrol of plant diseases, understanding root–microbe communication, which can be affected by genetic and ecological determinants in spatial and time-based contexts, will also considerably contribute to the improvement in the effectiveness of rhizobia as a biocontrol agent.