Introduction

Application of various chemicals as fertilizers frequently deteriorate soil eco-profile, along with imparting negative influence on the environment and human health, thereby increasing the input cost for crop production especially for the marginal farmers (Bhatt et al. 2019a, 2019b; Gangola et al. 2018; Sharma et al. 2019). These chemicals not only affect soil microbiota but also increase the residue of toxic chemicals at different trophic levels in the food chain (Pankaj et al. 2016a, b). Soil is a vital natural resource decisive for the maintenance of any ecosystem which we need to manage efficiently. Soil is confronting grave threats of declension owing to inexorable human pressure and its incompatibility with carrying capacity (Kumar et al. 2019; Bhatt et al. 2020b). There can be significantly severe effects concerning soil erosion, loss of soil fertility, and therefore, decreased plant growth or crop productivity. It is figured that some types of land degradation comprising 75% of the earth’s usable landmass influence 4 billion people in the world that constitute around 15% population of the globe which is anticipated to get worse if adequate and instant measures are not taken to check the degradation processes. The maximum degradation occurs due to water and wind erosion, which accounts for 80% of total degradation followed by salinization/alkalization and waterlogging. The hazardous effects of the herbicides, fungicides, insecticides and organic pollutants in agricultural soil depends on their half-life, degradability, adsorption, desorption, bioavailability, bioactivity, persistence, concentration and toxicity of agrochemicals along with soil factors such as texture, vegetation, tillage system and organic matter (Meena et al. 2020). The data on land degradation are required for several purposes such as designing reclamation programs, sound land-use planning, for bringing additional areas into cultivation and also to enhance the productivity levels in degraded lands (Bhatt 2019; Bhatt and Barh 2018). The indigenous microbial strains have the ability to degrade the toxic pesticides from the environment using their metabolic pathways (Bhatt et al. 2020a, b, c, d). Microbial degradation of such toxic chemical fertilizers from the environment is eco-friendly and sustainable approach to resource recovery of the contaminated agricultural fields (Bhatt et al. 2020e, f, g).

Sesbania is a plant of high importance due to the exceptional merit of adapting to a wide range of environments under stress conditions such as salinity, waterlogging and at very high altitudes with high-nitrogen fixation ability. Sesbania rostrata has been reported for its role in biological nitrogen fixation and pollution remediation in rice production (Naher et al. 2020). The extent of the diversity of symbiotic nitrogen-fixing bacteria (rhizobia) in the soil is observed to be significant for the conservation of soil health and value, as a broad variety of rhizobia possessing attributes, viz., plant growth promotion (PGP) is involved in important soil functions. Mounting curiosity has arrived with respect to the significance of rhizobia infecting different Sesbania species, viz. S. sesban, S. grandiflora, S. aculeata and S. rostrata for enhancing plant biomass, which will combat desertification of marginal lands and rehabilitate destructive lands into productive croplands for rigorous crop yield (Fig. 1). They are tremendous nitrogen fixers and adapt quickly in nitrogen poor soils, therefore, have immense usefulness in agroforestry as intercrop, cover crop, green manure, mulch and fodder. Table 1 representing the amount of nitrogen fixed by various legume in crops. Activity and species composition of rhizobia, usually influenced by a number of environmental factors, viz., soil physicochemical properties, temperature as well as vegetation. The research must continue for the legume crops having high nitrogen fixation ability with a beneficial effect on plant and soil health. Swarup (1992) reported the beneficial influence of Sesbania spp., as green manure on electrochemical properties and nutrient availability in sodic soils with additional grain yield of 1.48 t ha−1 of rice and 0.66 t ha−1 of wheat. To date, different groups of researchers isolated and characterized the rhizobial isolates with Sesbania plant. Due to fragmented information, farmers are not able to use these groups of plants and associated bacterial strains for crop improvement. This review increases our understanding of Sesbania and associated microbial strains (Fig. 1). The effect of chemical fertilizers can be reduced via using various biofertilizers to recover the agricultural resources. This review throws light on the detailed mechanism of biofertilizers and their application in reduction of chemical fertilizers.

Fig. 1
figure 1

Benefits of Sesbania plant on soil environment and development of a sustainable environment

Full size image
Table 1 Amount of nitrogen fixed by different legumes in the soil environment
Full size table
Fig. 2
figure 2

Direct and indirect mechanisms of plant growth-promoting rhizobacteria (PGPR)

Full size image

Microbial nitrogen fixation

Following photosynthesis, nitrogen fixation is treated to be another important mechanism to determine the main yield of the crops and is the center of life on earth as nitrogen (N) is essential for the organization of nucleic acids, proteins, enzymes and chlorophyll. It is mainly restrictive and supplied nutrients to nearly all the plants and the decisive of plant development (Puri et al. 2018). The environmental concern is due to the rising quantity of the active mode of nitrogen in the atmosphere, originating from the manufacture and uses of chemical fertilizers have resulted to re-focus on the biological nitrogen fixation (BNF), particularly by legumes (Bhatt et al. 2019c). BNF has been extensively practiced as an alternative of various nitrogen fertilizers in legume production as a consequence of its economic capability in terms of sustainable agroecosystem services. Annually, around 2.5 × 1011 kg NH3 is fixed from the atmosphere due to BNF (by legumes and Cyanobacteria) and about 8 × 1010 kg NH3 are manufactured by ammonia industry. Legumes have an advantageous impact on the yield of cereals and alternative crops in agricultural rotations thereby the application of nitrogenous fertilizers is reduced. BNF contributed by the Rhizobium-legume association is considered a highly efficient process and may supply up to 90% of the nitrogen requirements for host plants. Hence, the symbiotic association between rhizobia and legume for biological nitrogen fixation is the leading practice because it is environmentally safe and secure. Problems of soil infertility due to salinity, alkalinity and waterlogging could be alleviated through the use of biologically fixed nitrogen by Sesbania spp. The current approach comprises the finding of unique rhizobial species, which have an adjustment to numerous abiotic stresses, viz., elevated temperature, drought, alkalinity and salinity. Sesbania spp. is well recognized for pursuing a central role in nutrient cycling and nutrient enrichment in various cropping systems. The low productivity in legumes is frequently connected by means of diminishing soil productiveness of the farmland and decreased nitrogen fixation. The yield reduction of legume can be improved through the inoculation of adaptable and effective rhizobia. This practice is constrained mostly to leguminous plants in agricultural systems, thereby generating great curiosity among researchers to examine whether similar symbiosis can be developed in non-legumes as well, delivering maximum food yield for mankind (Mus et al. 2016). Green manuring associated with legumes is an ancient procedure providing biologically fixed N2 to succeeding crops grown in alternation (Table 2). The rotational profit of legumes and N credit for subsequent cereal crops are extensively known. Sesbania is able to fix nitrogen not just via its roots in the soil, but also in its aerial parts together with stems and branches. Rao and Gill (1993) performed large-scale field experiments on Sesbania spp. in alkaline soil throughout the summer period (1986, 1987 and 1989) to examine the nodulation, nitrogen fixation, biomass production and uptake of nutrients (N, P, K, Ca, Na, S and Mg). It was observed that natural nutrients (N, P, K, Ca, S and Mg concentrations) uptake in the shoots were towering, while the Na concentration was low, reflecting its usefulness as an integrated biofertilizer source. Hasan et al. (2015) procured 19 rhizobial isolates from Sesbania root nodules, from 5 different agro-ecological regions of Bangladesh designated as AEZ 9, AEZ 11, AEZ 12, AEZ 13 and AEZ 28. Rhizobia inoculation on varieties of Sesbania, viz., Deshi dhaincha and African dhaincha bear major and assured consequence on the nitrogen content of shoot biomass, shoot parched weight, root parched weight, nodule quantity and nodule parched weight in sterile conditions. Yan et al. (2017) isolated a Gram-negative, non-spore-forming, aerobic rods strain YIC4121T, isolated from root nodule of Sesbania cannabina grown in Dongying (Yellow River Delta), Shandong Province, PR China. Based on phylogenetic analysis of 16S rRNA gene sequences, strain YIC4121T was assigned to the genus Agrobacterium with 99.7, 99.3, 99.0, 98.8 and 98.7% sequence similarities to Agrobacterium radiobacter LMG140T, A. pusense NRCPB10T, A. arsenijevicii KFB 330T, A. nepotum 39/7T and A. larrymoorei ATCC51759T.

Table 2 Sesbania plants application as biofertilizer with various crops
Full size table

Rhizobial taxonomy and groups of rhizobia infecting Sesbania plants

The taxonomy of Rhizobium is frequently changing along with the discoveries of newer nodule inhabitants. The currently explained legumes symbionts generally belong to α, β and γ-proteobacteria, the three main phylogenetic subclasses. The genera Rhizobium, Mesorhizobium, Ensifer (formerly Sinorhizobium), Bradyrhizobium, Phyllobacterium, Microvirga, Azorhizobium, Ochrobactrum, Methylobacterium, Devosia, Shinella (Class of α-proteobacteria), Burkholderia, Cupriavidus (formerly Ralstonia) (Class of β-proteobacteria) and some γ-proteobacteria form the set of the bacteria known as legumes symbionts. The advent of new genera and species from nodules of different legumes, currently, rhizobia consist of 118 species belonging to 17 different genera, viz., Azorhizobium, Bradyrhizobium, Burkholderia, Cupriavidus, Devosia, Ensifer, Herbaspirillum, Mesorhizobium, Methylobacterium, Microvirga, Ochrobactrum, Phyllobacterium, Pseudomonas, Ralstonia, Rhizobium, Shinella and Sinorhizobium (Pongsilp 2017). The recent use of whole-genome sequencing-based taxonomy (genomotaxonomy) will obviously change the current concept of this important group of bacteria. Molecular study on the basis of the 16S ribosomal DNA sequence, presently described legumes symbionts belong to α, β and γ-proteobacteria phylogenetic subclasses, in which 238 species were grouped into 18 genera and 2 clades (Pankaj et al. 2014; Shamseldin et al. 2017).

The compatibility of host and microsymbiont in legume–Rhizobium symbiosis is a prerequisite for nodule formation. Sesbania nodules may be induced by a variety of rhizobia, including Azorhizobium spp., Ensifer spp., (syn. Sinorhizobium spp.), Mesorhizobium spp., and Rhizobium spp. In addition to the standard rhizobial types, strains of the genus Agrobacterium have also been frequently isolated from Sesbania nodules. Sesbania rostrata and other Sesbania species may also form a symbiosis with other rhizobia (Dreyfus et al. 1988), including the newly described species Sinorhizobium saheli and S. teranga. The latter species has been subdivided into bv. Sesbania (Sesbania-nodulating strains) and bv. Acaciae (Acacia-nodulating strains). The ability of such distantly related bacteria to establish interactions with Sesbania species raises the questions whether Sinorhizobia exhibit the unusual characteristics of Azorhizobia, i.e. stem nodulation and free-living N2 fixation, or whether they have similar or specific symbiotic properties.

Effect of rhizobial inoculation on nodulation efficiency and plant growth promotion

Increasing production of grain legumes is well recognized as a vital component of sustainable intensification strategies. Legumes capability of converting and thereby fixing atmospheric nitrogen by symbiotic rhizobia offers the potential substitute of nitrogen fertilizers along with enhancing the biological yield of crops. For the establishment of legume root nodules to provide a functioning N2-fixing symbiosis requires an ample number of root-nodulating bacteria in the soil or to be provided during the course of sowing (Thilakaranthna and Raizada 2017). However, the introduction of rhizobial strain establishment, the persistence, as well as the efficacy, generally decreases with an increase in the population density, perhaps due to the possibility of negative microbial interaction or incompatibility with the other symbionts within the rhizosphere. Therefore, the application of native rhizobial strain serves better as biofertilizers, which would improve soil biodiversity conservation (de Mandal and Bhatt 2020). Bio-fertilization limits the negative effects resulting from the inorganic fertilizers on below-ground biodiversity. Since rhizobia are poorly motile in soils, so the point of delivery of rhizobia into the soil is a determinant of nodulation pattern. Inoculation with compatible and appropriate rhizobia may be necessary, where a low population of native rhizobial strains predominates and is one of the solutions, which can be utilized by the farmers for optimizing grain legume yields. Mondal et al. (2017) observed that inoculation of abiotic stress-tolerant cluster bean rhizobial isolates enhanced 80–90% in nodulation efficiency and plant growth over uninoculated control. Van Heerwaarden et al. (2018) found inoculation response of Rhizobium to soybean across sub-Saharan Africa, where the average yields were estimated to be about 1343 (with inoculation) and 1227 kg·ha−1 (without inoculation). Li et al. (2019) isolated Strain KG2 from soybean nodule which was identified as Rhizobium pusense KG2 by phylogenetic analysis. Rhizobium pusense KG2 showed the 120 mg·L−1 of minimal lethal concentration for Cd2. In 50 and 100 mg·L−1 of Cd2+ liquid, approximately 2 × 1010 cells removed 56.71% and 22.11% of Cd2+, respectively. In pot soil containing 50 and 100 mg·kg−1 of Cd2+, strain KG2 caused a 45.9% and 35.3% decrease in soybean root Cd content, respectively. The strain KG2 improved the root and shoot length, nitrogen content and biomass of soybean plants and superoxide dismutase activity.

Recent strategies to increase biological nitrogen fixation (BNF) in legumes

Legumes have been approximately contributing 20% of the nitrogen required for overall grain and oilseed making. Owing to their advantageous assets, legumes potentially fix nearly 80% of their own N requirement, besides aiding as a supplement to the succeeding crops. However, all these potential benefits can be utilized merely in the assured situation. Pure legumes addition in a cropping practice does not guarantee high BNF. For exploitation of BNF, there lie two most common approaches, viz., first, enhanced crop, soil and water management to attain the utmost ability of BNF and second, inoculation of Rhizobium or selection of host genotypes to assure an elevated amount of nitrogen fixation in the plant. The rhizobia–legume symbiosis accounts for a significant proportion of nitrogen available to the leguminous crop. The use of efficient rhizobial strains as biofertilizers to enhance legume production is a significant method in sustainable agriculture (Saharan and Nehra 2011). Among both approaches, the first one is well recognized for approximately 50 years and will continue to play its correct function. While the second one on the host plant’s choice is the latest. Therefore, there arises a requirement to enhance rhizobia for improving their symbiotic effectiveness and vast host choice. Recent advancements in the next-generation high-throughput techniques allows to explore the depth of biological nitrogen fixation. The omics-based techniques are highly efficient and informative for the enhancement of nitrogen fixation in the legumes (Afzal et al. 2020).

Plant growth-promoting rhizobacteria (PGPR) as multifunctional agents

Current agriculture faces challenges, for instance, impairment of soil productiveness, varying climatic parameters and increase in pathogen and pest attacks. Sustainability and environmental security of agricultural production rely on various eco-friendly tactics such as biofertilizers, biopesticides and crop residue management. The range of valuable microbial inoculants effects, mainly in plant growth promotion, indicates the need of further investigation for their exploitation in terms of present agriculture (Gopalakrishnan et al. 2015). PGPR promotes plant growth and development using several mechanisms such as the production of phytohormones, siderophore production, phosphate solubilisation and decrease of plant ethylene level by ACC utilization or nitrogen fixation associated with roots. Several bacterial isolates directly control plant physiology by mimicking the synthesis of phytohormones, whereas some enhance the availability of various minerals and nitrogen in the soil, thereby supplementing plant growth and development. Plants choose PGPR that is competitively well to inhabit niches without causing any pathological stress on them (Fig. 2). PGPR may use different mechanisms to augment the growth and development of the plant, as investigational proof suggests that the encouragement of plant growth is the net product of numerous tools that may be functioning at the same time. Sultana et al. (2019) isolated Rhizobium spp. that retained a symbiotic relationship with leguminous plants including Sesbania bispinosa by fixing N2 through nodule formation. Several researches suggest that Exopolysaccharides (EPSs) are required for nodule formation. Rhizobial growth parameters as well as the EPS production are affected by the presence of pesticides. Table 3 provides a list of multifunctional microbial strains recovered from different species of Sesbania.

Table 3 Multifunctional bacterial isolates associated with Sesbania plants
Full size table

Phosphate solubilization

Among all macronutrients, phosphorus (P) is essential for plant growth and advancement aiding in photosynthesis, production of energy and sugar, besides improving nitrogen fixation in legumes. Only 0.1% of the P is accessible to plants from the total P content of soils, while the rest in an insoluble form, making it unavailable for plants uptake. Rock phosphate (RP) is the sole economic basis of P. However, its accessibility is limited and skewed. Generally, it is found to be in precipitated form (mono- or orthophosphate) in the soil or is adsorbed by Al or Fe oxides via ligand exchange. Mobilization of inaccessible P to plant-available P is must to prolong crop yields (Bhatt and Maheshwari 2020). Phosphate-solubilizing microorganisms (PSM) play a very significant function in transforming unavailable form of phosphorus to the available form, by lowering the soil pH, as a result producing organic acids, besides mineralization of organic phosphorus by acid phosphatases and alkaline phosphatases, thus making it available for plants uptake (Baby et al. 2016; Bhatt and Maheshwari 2019). Bacteria-solubilizing phosphate, i.e. PSB and PGPR altogether could decline the practice of phosphorus fertilizer by 50% without causing any adverse effect on the crop yield. The most significant PSB belonging to the genera, viz., Azotobacter, Bacillus, Beijerinckia, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Microbacterium, Pseudomonas and Serratia, are reported well. The ability of P-solubilization is found even among Rhizobiacae, comprising of Rhizobium, Bradyrhizobium, Mesorhizobium and other non-specified legume-nodulating bacteria (LNB). Singh and Gera (2018) reported that out of 20 Sesbania grandiflora rhizobia, around 80% of the isolates were able to form significant phosphate solubilization zone on the media supplemented with P (Pikovskaya’s medium), where their solubilization index (SI) varied from 1.96 to 4.85.

Production of indole-3-acetic acid (IAA)

IAA is a growth phytohormone considered to be the most important representative of auxin class of plant hormone. Some common precursors for natural biosynthetic pathways are tryptophan and derivatives of indole. Microbes isolated from the rhizospheric region of different crops have the capability of IAA production, as secondary metabolites due to the rich supply of substrates. IAA functions as a regulator of several biological processes, viz., cell division, elongation and differentiation to tropic responses, seed germination, growth of root hairs, fruit development and senescence, thus performing a key role in plant growth and development. Parallel to plant, IAA also affects the survivability of bacteria along with its resistance to plant defence. As per the reported data, about 80% of rhizospheric microbes possess the capability of synthesizing and liberating auxins as secondary metabolites. Sridevi and Mallaiah (2007a, b) reported 26 IAA-producing Rhizobium strains from root nodules of Sesbania sesban (L.) Merr., from different locations of Andhra Pradesh. However, only five strains were able to produce the maximum amount of IAA in yeast extract mannitol (YEM) medium supplemented with L-tryptophan. Etesami et al. (2014) reported enhanced shoot biomass and root length after inoculation of seedlings (host plant) with IAA-producing isolates. IAA-producing bacteria enhanced the plant vegetative growth.

Siderophore production

Since iron is one of the requisite microelements for all living cells but its availability is limited, as the dominant form of iron in the soil is present as ferric iron (Fe3+), which has very low solubility. Rhizospheric bacteria fix the atmospheric nitrogen in a symbiotic association with the leguminous plants via iron-containing enzyme nitrogenase. During evolution, microorganisms have established the strategies of acquiring and assimilating iron, by secreting iron chelators called siderophores. Siderophores chelate iron and alter it to a soluble form, and they are commonly produced by aerobic, facultatively anaerobic bacteria and fungi under iron-limiting conditions. Sridevi and Mallaiah 2008 studied 26 rhizobium strains isolated from root nodules of Sesbania sesban (L.) Merr. which were studied for their ability to produce siderophores. It was found that only nine strains have the ability to produce catechol-type of siderophores in culture after 4 h of incubation and at neutral pH. Mannitol and sucrose at the concentration 2% stimulated growth and siderophore production. To date, nearly 500 siderophores are reported from selected microorganisms. In general, siderophores are classified as hydroxamates, catecholate, salicylates, carboxylates and new group polycarboxylates (Kannahi and Senbagam 2014). Joshi et al. (2018) isolated siderophore-producing bacteria from agricultural soil belonging to genus Pseudomonas and maximum siderophore production was observed at 6.5 pH and 30 °C using glucose as the carbon source. The type of siderophore was determined and was found to be a hydroxamate type. Singh et al. (2018) reported that out of 14 Sesbania sesban rhizobia, 6 of them possess the capability of siderophore production on Chrome-Azurol S agar medium after 7 days of incubation.

1-Aminocyclopropane-1-carboxylate (ACC) deaminase activity

Symbiotic associations of rhizobia and legume are extremely significant with respect to sustainable agricultural practices. In regard to this, ethylene (phytohormone) plays a vital role in the process of nodule formation, where it acts as an inhibitor of the nodulation process (Lindstrom and Mousavi 2020). Among all the phytohormone, ethylene tends to be an important one, but its overproduction under stressful conditions directly affects the plant growth or death, especially for seedlings (Deng et al. 2020). Apart from this, ethylene may also be involved in various phases of symbiosis, including the initial response to bacterial nod factors, nodule development, senescence and abscission. To counter this, various bacteria possess the capability of production of an enzyme called ACC deaminase, which can decrease the levels of ethylene (Nadeem et al. 2020). The ACC deaminase works by the action of degradation of ACC (1-aminocyclopropane-1-carboxylate) to ammonia and α-ketobutyrate, followed by their metabolization both by bacteria and plant (Shahid et al. 2020). Besides, possessing a significant role in the nodulation process, ACC deaminase also modulates the persistence of nodules (Nascimento et al. 2016). Rhizobia, that expresses ACC deaminase enzyme, possess increased symbiotic potential. Plant growth-promoting bacteria (PGPB) producing ACC deaminase offers drought tolerance by regulating plant ethylene levels. Thus, PGPB that express ACC deaminase activity tends to protect plants growth from various unfavourable conditions, viz., flooding, drought, anoxia, high salt, fungal and bacterial pathogens, nematodes, metals and organic contaminants. The study of Chandra et al. (2019a, b); Chandra et al. (2020) and Saleem et al. (2018) reported inoculation of PGPR containing ACC deaminase enzyme in wheat, finger millet and Velvet bean could improve plant growth as compared to uninoculated plants under drought conditions.

Bacteriocin production

Bacteriocins are low-molecular-weight peptides or proteins which are extracellularly released by bacteria with the bactericidal or bacteriostatic mode of action, principally against a wide range of most closely related Gram-positive bacteria, but the producer cells are immune to their own bacteriocins (Baindara et al. 2018). Bacteriocins are well recognized to be produced by a number of Gram-positive and Gram-negative bacteria but Gram-negative bacteria are well studied for their bacteriocin production. The clear halo zone that resulted from the bacteriocin production by a particular strain in a grid was observed from the growth of the overlaid test strain (Kaskoniene et al. 2017). Strong bacteriocin production was observed by clear halo zones, while hazy zones interspersed with small, punctiform growth of the overlaid strain indicated weak bacteriocin production. Bacteriocin appears to play an important role in determining competitiveness for nodulation when assayed against some strains. Ansari and Rao (2014) assessed bacteriocin production in indigenous soybean rhizobia in vertosols of central India and other soils. The slow-growing soybean strain R33 strongly inhibited the growth of 19 rhizobia strains.

Ammonia excretion and intrinsic antibiotic resistance (IAR)

Ammonia production by rhizobia plays an important role in biocontrol activity, which may indirectly influence plant growth. Rhizobium spp. fixes atmospheric N2 in symbiotic association with a leguminous plant. The main procedure, converting atmospheric nitrogen (N2) to ammonia (NH3) takes place inside the nodule symbiosome by the bacteroides via the nitrogenase enzyme complex, which tends to be oxygen sensitive which irreversibly damaged by oxygen. The ammonia formed in the bacteroides, however, is assimilated by plant enzymes in the plant cytosol. Several processes exist utilizing which ammonia can be produced, viz., nitrite ammonification, degradation and decarboxylation of amino acids to create biogenic amines with ammonia, deamination and the urease mediated hydrolytic degradation of urea. This form of ammonia cannot be utilized by plants but may be available through the BNF process, developed only in prokaryotic cells. Inoculation with such bacteria enhances plant growth as a result of their ability to convert atmospheric nitrogen (N2) to ammonia (NH3) making it an available nutrient for plant growth. Ammonia production was reported by Joseph et al. (2007) in 95% of total isolates of Bacillus followed by Pseudomonas (94.2%), Rhizobium (74.2%) and Azotobacter (45.0%).

The resistance of rhizobia refers to their intrinsic resistance to antibiotics in terms of usual development. The mode of action of antibiotics against any bacteria relies on the chemical constitutes of antibiotics, bacterial cell morphology and primary cell walls. The resistance of PGPR to several antibiotics might have an ecological advantage of survival in the rhizosphere when they are introduced as inoculums. Diverse rhizobial strains show distinct degrees of susceptibility to antibiotics, that is why this property is being meant for their identification. Antibiotics resistance of rhizobial strains (mutants) is extensively used in investigating their survivability in various environments including soil and for monitoring their competitiveness as a marker for nodulation of the host plant and effectiveness of nitrogen fixation. Different Rhizobium strains of faba bean and soybean were found to be resistant to different antibiotics, viz., amoxicillin, ampicillin and cloxacillin while very few strains were found to be resistant to ampicillin and cloxacillin only (Abera et al. 2015). Dhull et al. (2018) reported that out of most of the rhizobial strains had good growth on ampicillin, chloramphenicol, nalidixic acid and streptomycin up to concentrations of 100 µg ml−1 while using gentamicin showed medium growth up to concentrations of 20 µg ml−1. None of the rhizobial strains showed growth on the YEMA medium supplemented with antibiotics, viz., kanamycin, neomycin and tetracycline even at concentration of 10 µg ml−1. The rhizobial strain GB-5c showed growth on all the antibiotics tested except kanamycin and neomycin. The potential indigenous microbial strains can be used for the large scale crop production which could reduce the harmful effects of chemical fertilizers (Fig. 3).

Fig. 3
figure 3

Harmful effects of chemical fertilizers

Full size image

Conclusions and future prospects

This study recognizes importance of Sesbania plants and their respective rhizobia as a non-polluting and more commercial way for improving soil fertility compared to other ways, such as application of chemical fertilizers, pesticides and sewage sludge. The fertility of the soil is of significant concern due to various alarming risks on human health because of the leaching of toxins from fertilizers and pesticides into the groundwater. Sesbania has multifarious attributes, making them attractive as multipurpose plants and potentially useful species in agricultural production systems. Sesbania rhizobia is superior to other N2-fixing systems with high N2-fixing potential. Sesbania plants and their rhizobia are useful because they have the capability of adapting to acidic, alkaline and waterlogging soil conditions. Various symbionts including seven novel genospecies within Agrobacterium, Ensifer, Neorhizobium and Rhizobium were identified among the S. cannabina rhizobia and all of them were designated as symbiovar sesbaniae based on their highly conserved symbiosis genes and nodulation test results. To achieve the maximum legume productivity, screening of native isolates for high-nitrogen fixation efficiencies tends to be significant. Several Sesbania plant rhizobia capable of tolerating extreme conditions of alkalinity, acidity, salinity, drought, metal toxicity and fertilizer were identified. In fact, the existence of Rhizobium-tree legume symbiosis, capable of fixing the appreciable amount of N2 under severe conditions, is fascinating. Thus, Sesbania represents the best source of ideal fertilizers in the present and subsequent crops, and therefore, commands great interest as the subject of future research. There is a need to implement the use of Sesbania plants and their respective rhizobia as biofertilizer from lab to field so that in the upcoming years, soil health is maintained and the problem of soil infertility can be diminished. Recent high-throughput tools and techniques in the future can also help to increase our understanding of the Sesbania plants and their associated microbes.