Rhizosphere was first studied by Hiltner (1904) and according to him soil within the vicinity of root system is having more bacterial population than nearby soil. Those bacteria also get profit as root secrets some metabolites which are the sources of nutrient for them and around 5-21% of plant fixed carbon is released from root exudates (Marschner 1995). Vessey (2003) reported that in plant rhizosphere several species of soil bacteria increase their population through various processes that promote plant growth are known as plant growth promoting rhizobacteria (PGPR). There are about 2-5% of PGPR in soil and they enhance the growth of the plant by plant infection (Kloepper and Schroth 1978). According to Somers et al. (2004), there are four types of PGPR as per the growth promoting traits and they are biofertilizers, phyto-stimulators, rhizo-remediators and biopesticides (Fig. 1).

Another classification of PGPR has given by Gray and Smith (2005) as per the plant tissue compartment.

Extracellular PGPRs (ePGPRs) that exist in the rhizosphere, on the rhizoplane or in the spaces between cells of the root cortex. Example of these PGPRs are Agrobacterium, Arthrobacter, Azotobacter, Bacillus, Burkholderia, Caulobacter, Chromobacterium, Enterobacter, Enterococcus, Erwinia, Flavobacterium, Klebsiella, Micrococcus, Pseudomonas, Serratia and so on.

Intracellular PGPR (iPGPR) that exist inside root cells, generally in specialized nodular structures. Ex. Azorhizobium, Allorhizobium, Bradyrhizobium, Mesorhizobium/Ensifer, Rhizobiumand so on.

Most of the rhizobacteria are gram ‘–’ve and pleomorphic in nature and may be rod or cocci shaped with various colours such as creamy, white or translucent. Most of them show positive effect on plant growth. Microbial variation observed in rhizosphere are mainly with different plant growth promoting traits (Bhattacharyya and Jha 2012).

Legumes are well known soil fertility improver since ancient times and rhizobia was identified on later period. A publication on root nodulation in legumes was first written by Leonhard Fuchsius in 1542 (Fuchsius 1542) and Malpighi (1679) also found root nodules of Phaseolus vulgaris and Vicia faba. Boussingault (1838) in an experiment on legume crops and he recorded that there was increase in soil N content. Further, he was also the first who gave the concept of biological nitrogen fixation (BNF). Moreover, Lachmann (1858) took root nodules for microscopic study and reported that it consisted of vibrio like particles. After few years, Woronin (1866) mentioned that those particles were having properties like bacteria and the root nodules obtained from legumes belonging to a specific class of bacteria. Henceforth, in 1886, German scientists Hellriegel and Wilfarth observed that root nodules were able to fix atmospheric nitrogen and that created a milestone on symbiotic relation of Rhizobium and legumes. After two years,i.e., in 1888,the Dutch microbiologist Beijerinck for the first time identified and isolated one bacterium from root nodule and he named it as Bacillus radicicola (Beijerinck 1888). But one year later, another microbiologist, Frank (1889) named it as Rhizobium leguminosarum. To utilize Rhizobium culture in agriculture commercially it was used as biofertilizer production in the name of Nitragin and it was patented by Nobbe and Hiltner (1896). In 1921, Löhis and Hansen classified them into two types as per their growth ability, as fast growers and slow growers. Concept of cross inoculation between some rhizobium species and legume plants as a host was given Baldwin and Fred (1929). At very beginning, people used to transfer soil from leguminous crop field to non-leguminous crop fieldor to seeds before planting, but later on, solid carrier-based formulations used for inoculation of Rhizobium (Fred et al. 1932) and liquid carrier-based formulation used in later period (Van Schreven et al. 1953; Singleton et al. 2002). Rhizobium is highly host specific and suitable species should be chosen for different crops (Table 2). Earlier it was found that in groundnut crop the slow growing Bradyrhizobium Zhang et al. 1999; Chen et al. 2003; Yang et al. 2005) acted more efficiently for establishment of root nodules, whereas in recent past,it was observed that fast growing rhizobium efficiently acted for effective root nodulation (Khalid et al. 2015; Akhal et al. 2008; Rosália et al. 2005). NC 92, IGR 6, IGR 40, TAL 1000 and TNAU 14 are some superior stains of Bradyrhizobium that work efficiently and recommended for groundnut (ICAR 2009). It has been observed that Rhizobium activity, population and its effect on groundnut differs according to different agroecological conditions (Table 3).

Groundnut (Arachis hypogaea L.) is a self-fertilized annual leguminous oilseed crop and it is widely cultivated in the arid and semi-arid regions in the world (40°N and 40°S) from equatorial region to warm temperate region. Semiarid tropics are the major growing zone of groundnut crop (Fletcher et al. 1992; Tarimo 1997). In the world, after soybean, rapeseed and mustard and sunflower, it is the fourth most common source of edible oil and third most important source of vegetable protein (Ojiewo et al. 2020). Groundnut is one of the most vital cash crops of India (Madhusudhana 2013). On an average, groundnut kernels contain 45-51% oil, 25% protein and 24.2% carbohydrates (Rajgopal et al. 2000). Groundnut is also rich in vitamins like B group and E and rich in all the 20 essential amino acids among which arginine content is maximum. The calorific value of groundnut seed is 567 Kcal/100g of seed (Arya et al. 2016). Globally major amount (53%) of groundnut kernels is used for edible oil, 32% is consumed in confectionary and remaining quantity is used for food and seed production (Rajgopal et al. 2000). After oil extraction, the byproduct, i.e., oil cake, is used as cattle feed and concentrated organic manure (Rajgopal et al. 2000). Groundnut oil is rich in unsaturated fatty acids, such as poly unsaturated fatty acid (PUFA) (linoleic acid 25-35%) and mono unsaturated fatty acid (MUFA) (oleic acid 40-50%), moreover, it is very much suitable for deep frying because of its higher iodine value (ICAR-DGR, 2015). Worldwide groundnut area, production and productivity are 296 lakh ha, 488 lakh tone and 1647 kg/ha respectively in 2019 (FAOSTAT, 2021). Globally, China is the largest producer of groundnut and India ranks second in groundnut production. Groundnut can be produced in diverse soil inclusive of light to heavy textured soil and the most suitable soil is light sandy-loam soil. Moreover, it requires warm climatic conditions with a nicely distributed rainfall ranging from 500 to 1000 mm (ICAR-DGR, 2015). In India, groundnut is grown in an area of 4.89 million hectare, with an yield of 1893 kg/ha and production of 9.25 Million tons. In India, Gujarat is the highest producer of groundnut and its contribution is 42.55% of total production in the country. In 2013-14, India imported 0.11 thousand tons of groundnut by spending rupees 0.36 crore, whereas, in the same year it exported 509.75 thousand tons of groundnut and earned 3187.66 crore revenue (Agricultural statistics at a glance 2019, GOI).

According to Singh et al. (1997) biological criteria for lesser yield in oilseeds is due to conversion of one gram glucose produced from photosynthesis into 0.83 g starch, 0.4 g protein and only 0.32 g of lipids. Therefore, it needs additional inputs for higher yield. Nutrients required for groundnut to produce 2000-2500 kg/ha of pod yield are 160-180 kg of N, 20-25 kg of P, 80-100kg of K, 60-80 kg of Ca, 15-20 kg S, 30-45 kg Mg, 3-4 kg Fe, 300-400 g Mn, 150-200 g Zn, 140-180 g B, 30-40 g Cu and 8-10 g Mo (Singh, 1999). Both primary and secondary macronutrients have distinct role in kernel filling and oil synthesis, whereas, sulphur (Tandon 1991) and calcium are most essential of oil synthesis and pod formation respectively. Biofertilizers are prepared by using microorganisms which can reduce cost of cultivation and make it environment friendly (Gupta et al. 2003). Gradually, use of biofertilizer is getting more importance as it includes microbial culture either for seed inoculation or soil application which results in higher crop yield (Mahdi 1992). As the biofertilizers consists of living microbial culture, so it has beneficial influence on plant performances both directly and indirectly (Fuentes-Ramirez and Caballero-Mellado 2005). Rhizobium forms nodule and fixes nitrogen biologically and thus, improve crop yield and soil quality (Fig. 2). (Chetti et al. 1995) found that highest growth parameters obtained from use of both Rhizobium and Phosphobacterium and that ultimately showed maximum dry matter accumulation in groundnut. Beneficial micro-organisms present in the soil facilitate to increase nutrient availability. That’s why plant growth promoting microorganisms (PGPM) has a distinct role in agriculture. Generally, PGPM includes both bacterial and fungal species. Use of plant growth promoting rhizobacteria (PGPR) is increasing steadily in all over world and showing positive influence on crop growth and productivity in most of the field crops which is a path towards sustainable agriculture (Sharma et al. 2016).

Badawi et al. (2011) reported that Bradyrhizobium treated peanut crop resulted dry matter accumulation higher pod and straw yield than control. Basu et al. (2006) observed thatgrowth parameters of groundnut like plant height, number of branches per plant, leaf area index, number of nodules per plant were significantly higher with Rhizobium inoculation than the untreated treatments. Similarly,pod yield and haulm yield were also more with biofertilizer inoculation. Gunri et al. (2014) conducted an experiment on Red and Lataritic Zone of West Bengal and reported that groundnut seed inoculation with Rhizobium I (NRCG9) and Rhizobium II (IGR6) resulted significantly higher pod yield, haulm yield and number of pods per plant over control. Didagbé et al. (2014) studied on different strains of rhizobium in groundnut and noted that compared to control plant height was increased by 23% and 25% in case of WSM 4412 and STM5945 strain. They also reported that there were 38% and 33% increase in leaf number per plant by the application of STM 5894 and WSM 4412 strains of Rhizobium respectively. Laxminarayana and Patsram (2005) observed that groundnut seed inoculation with only Rhizobium without any organic manure or fertilizer application resulted higher pod yield than untreated control, but both were statistically at par with each other. But the same treatment resulted in significantly higher haulm yieldthan control. In another study, groundnut seed inoculated with NC-92 strain gave significantly higher nodules / plant, nodule dry weight/ plant than that of strain TAL-1371 and strain TAL-1000 (Ashraf et al. 2006). However, pods/plant, pod yield and haulm yield from the strain NC-92 also resulted in significantly higher values other two strains of Rhizobium. Gomoung et al. (2017) reported that different Rhizobium strains collected from different crops improved seed yield of groundnut. A study clearly indicated that seed inoculation with Rhizobium in groundnut significantly increased the number of nodules, shoot and root dry matter yields than un-inoculated treatment (Yusif et al. (2016). Kumar and Suganya (2017) concluded that among five strains of Rhizobium (namely, RS1, RS2, RS3, RS4 and RS5) inoculated in groundnut seed, RS3 resulted in higher shoot length and root length, number of nodules and pod yield. Influence of Rhizobium inoculation on growth and yield of groundnut was also noted by Sajid et al. (2011) and they found that there was significantly higher growth parameters like plant height, number of shoots, number of leaves, number of nodules, number of pods and pod yield than un-inoculated treatment.

The interaction between Bradyrhizobium (strain LMG9283) and Arbuscular mycorrhizal fungus (Glomus intraradices) showed synergistic effect on yield of groundnut Sene et al. (2010). Combined application of Rhizobium and phosphorus solubilizing microorganism (PSM) enhanced germination percentage and root and shoot length of seedlings and 100 kernel weight than single inoculation with either Rhizobium or PSM (Singh et al. 2013).

In other leguminous crops also the combination of chemical fertilizer and different biofertilizers enhanced growth and productivity. Kumawat et al. (2017) reported that combined application of 75% recommended dose of fertilizer (RDF), Rhizobium and phosphate solubilizing bacteria (PSB) produced significantly higher seed and stover yields of fenugreek than 75% RDF and Rhizobium inoculation as seed treatment. Another prominent result indicated that yields of Vicia faba cv. Alborea co-inoculated with Rhizobium leguminosarum bv. viceae Z25c + Azotobacter vinelandii Dv42, R. leguminosarum bv. viceae Z25c + Azotobacter vinelandii ATCC 12837, R. leguminosarum bv. viceae Z25c + Azotobacter chroococcum H23 and R. leguminosarum bv. viceae Z25c + Azospirillum brasilense Sp7 were respectively more by 140.6%, 147.2%, 151.5% and 136.4%than inoculation of R. leguminosarum bv. viceae Z25 alone (Rodelas et al. 1999). Effect of co-inoculation with Rhizobium and Bacillus cereus in pigeon pea in pot culture resulted in 382% increase in nodule number, 196% increase in nodule dry weight and 116% increase in N fixation and 54% increase in grain yield (Tilak et al. 2006). In the same crop, they also tested co-inoculation of Rhizobium and Pseudomonas fluorescens and observed that 388% increase in nodule number, 267% increase in nodule dry weight and 134% increase in N fixation and 66% increase in grain yield.

Efficiency of a BNF microorganisms is known in a nutrient deficient soil and it was found that root nodulation is less in soils rich in readily available nitrogen (Pitumpe et al. 2020; Zahran 1999). Soil treated with 100% RDN through inorganic fertilizers showed comparatively a smaller number of root nodule than soil treated with 100% RDN through FYM and this indicated poor rhizobial activity in rich available nutrient (Baishya et al. 2014). Basically, phosphorus is a primary macronutrient, which helps in root development and nodule formation in legumes and plant growth Israel, (1987). Swarnalakshmi et al. (2020) mentioned that the bacteria which play major role efficiently in phosphorus solubilization are Bacillus, Pseudomonas and Rhizobium. As compared to fast multiplying rhizobia, slow multiplying rhizobia shows higher phosphorus deficiency tolerance (Beck and Munns 1985). There is also involvement of potassium (K) in nitrogen fixation. Sufficiency in potassium supply to plants increases carbohydrate accumulation due to higher photosynthesis and that allows better development of root nodules that ultimately results in higher nitrogen accumulation (Premaratne and Oertli, 1994).

Thomas and Hungria (1988) also suggested that potassium helps in transportation of nitrogenous compound from the N-fixing site (nodule) to other parts of the plants which promotes Rhizobium for further N reduction. Not only macronutrients, but also there are influence of soil micronutrients on activities of Rhizobium. Adissie et al. (2020) did not find any significant grain yield enhancement in faba bean due to combined application of Rhizobium strain EAL 1035 and zinc (2 kg/ ha) compared to the separate application of both. But combined application of Rhizobium strain EAL 1035 and boron (1.5 kg/ ha) resulted in higher grain yield by 13.9% and 65.9% over second highest recorded treatment (Rhizobium strain EAL 1018 alone) and control treatment, respectively.

Khaitov et al. (2016) revealed that there was increase in soil fertility status such as carbon, nitrogen and phosphorus in their experiment on chickpea. According to them, there was increase in total nitrogen (0.022 to 0.033 g kg^-1), carbon (0.054 to 0.084 033 g kg^-1) and phosphorus (0.007 to 0.015033 g kg^-1) content in Rhizobium inoculated chickpea field soil than control. Application of biofertilizer (Rhizobium and PSM) increased fertility status of post-harvest soil of summer groundnut such as N, P₂O₅, K₂O and S by 14.79%, 8.34%, 2.96%, 30.29% respectively than untreated control (Vala et al. 2018). Effect of Rhizobium inoculation in groundnut improved residual soil fertility status higher than control in terms of enrichment of N, P and K status of the post-harvest soil (Laxminarayana and Patsram 2005). Yusif et al. (2016) also observed similar result such as Rhizobium inoculated treatment in groundnut showed higher organic carbon, available N, P, K, Ca and Mg than Rhizobium un-inoculated treatment in post-harvest soil. Further, legumes are well known for symbiotic association with micro-organisms. Other than symbiotic Rhizobium, other free-living soil bacteria like Arthrobacter simplex, Bacillus laevolacticus, B. amyloliquefaciens, Pseudomonas denitrificans, and P. rathonis, and all aerobic or facultative anaerobic heterotrophic soil species, have been found to propagate more in presence of Rhizobium to promote plant growth (Maitra and Ray 2019; Qiao et al. 2012; Spehn et al. 2000).

From the beginning of crop cultivation importance of legumes with respect to soil health and sustainability is very well known. Use of chemical fertilizers in the post Green revolution era undoubtedly enhanced agricultural productivity, but it happened at a cost of agricultural sustainability. After realizing the adverse impacts of chemical inputs in agriculture, focus has been shifted to integrated management of nutrients and pests to reduce use of chemicals. Further, organic agriculture has created a huge potential for enhancing profitability in agriculture through export of organic produces. Both the approaches have created an enormous opportunity of using microbial inputs in ecofriendly agriculture. Earlier researches evidenced that inclusion of PGPR and as specially Rhizobium in groundnut improved crop yield, nutrient uptake and facilitated maintenance of soil fertility along with multiple environmental benefits. The review article concludes that targeting a higher productivity of groundnut and improving soil quality, Rhizobium inoculation can be preferred. Further, beneficial roles of Rhizobium and other PGPRs may lead towards agricultural sustainability.