Rhizobia

Drought is one of the environmental factors most affecting crop production. Under drought, symbiotic nitrogen fixation is one of the physiological processes to first show stress responses in nodulated legumes. This inhibition process involves a number of factors whose interactions are not yet understood. This work aims to further understand changes occurring in nodules under drought stress from a proteomic perspective. Drought was imposed on Medicago truncatula ‘Jemalong A17’ plants grown in symbiosis with Sinorhizobium meliloti strain 2011. Changes at the protein level were analyzed using a nongel approach based on liquid chromatography coupled to tandem mass spectrometry. Due to the complexity of nodule tissue, the separation of plant and bacteroid fractions in M. truncatula root nodules was first checked with the aim of minimizing cross contamination between the fractions. Second, the protein plant fraction of M. truncatula nodules was profiled, leading to the identification of 3...

Over the past few decades, a combination of physiology, biochemistry, molecular and cell biology, and genetics has given us a basic understanding of some of the key transport processes at work in nitrogen-fixing legume nodules, especially those involved in nutrient exchange between infected plant cells and their endosymbiotic rhizobia. However, our knowledge in this area remains patchy and dispersed over numerous legume species. Recent progress in the areas of genomics and functional genomics of the two model legumes, Medicago truncatula and Lotus japonicus is rapidly filling the gap in knowledge about which plant transporter genes are expressed constitutively in nodules and other organs, and which are induced or expressed specifically in nodules. The latter class in particular is the focus of current efforts to understand specialised, nodule-specific roles of transporters. This article briefly reviews past work on the biochemistry and molecular biology of plant transporters in nodu...

A lysogenic Streptomyces sp., strain NS.A4, which was isolated from the nodule surface of black gram ( Vigna mungo (L.) Hepper), was found to inhibit rhizobia of fast-and slow-growing strains of cowpeas and soybeans. It exhibited plaques when there was a change in cultural conditions. Repeated culturing of the organism in nutrient agar and broth confirmed the infection of Streptomyces sp. strain NS.A4 by an actinophage. Addition of the culture filtrate of Streptomyces sp. strain NS.A4 to shaken broth cultures of three other Streptomyces spp. resulted in phage infection.

The interaction of legumes with rhizobia leads to formation of the symbiotic nodules on their roots, which are specialized plant organs for nitrogen fixation. Considerable progress has been made in deciphering the molecular-genetic and cellular mechanisms of symbiotic nodule development in recent years. However, some aspects of nodule development clearly merit much more attention.

The garden pea (Pisum sativum L.), like most members of Fabaceae family, is capable of forming symbioses with beneficial soil microorganisms such as nodule bacteria (rhizobia), arbuscular mycorrhizal (AM) fungi and plant growth promoting bacteria (PGPB). The autoregulation of nodulation (AON) system is known to play an important role in controlling both the number of nodules and the level of root colonization by AM via root-to-shoot signaling mediated by CLAVATA/ESR-related (CLE) peptides and their receptors. In the pea, mutations in genes Sym28 (CLV2-like) and Sym29 (CLV1-like), which encode receptors for CLE peptides, lead to the supernodulation phenotype, i.e., excessive nodule formation. The aim of the present study was to analyze the response of pea cv. ‘Frisson’ (wild type) and mutants P64 (sym28) and P88 (sym29) to complex inoculation with rhizobia, AM fungi and PGPB, with regard to biomass accumulation, yield and transcriptomic alterations. The plants were grown in quartz sa...

A total of 354 indigenous bradyrhizobia were isolated from soybean nodules collected from five major crop grown regions. Host-specific 12 phages, each active on particular strains were selected. Factors, which influence the interaction between the host and phage, were examined. Four different types of plaques were detected. Nearly 17% of isolates were found resistant to all phages. Phage sensitivity patterns revealed a total of 32 distinct phage genotype groups. Different set of phage combinations expressed variation in specificity for parasitizing against particular group of rhizobia. Distributions of isolates in each phage types differed markedly between regions. Interestingly, nine strains belonging to phage group 16 exhibited high ex planta nitrogenase activity in culture. However, no correlation could be established between high ex planta nitrogenase activity and their symbiotic effectiveness with soybean cultivars. Soybean cv. JS335 showed relatively superior performance than ...

A total of eight acid tolerant strains of bradyrhizobia isolates from indigenous cowpea plants grown in acid soil in Varanasi, Uttar Pradesh, India, were examined for their ability to survive in soil and yeast extract mannitol broth at low pH levels. All these isolates survived in acidic (pH 3.5-6.5) conditions. Survival capacity of rhizobia was higher in soil than in nutrient medium at low pH 3.5-6.5 levels. Symbiotic effectiveness of these strains under polyhouse conditions in sterilized soil of pH 4.5 recorded the highest and lowest symbiotic characters for dry matter production and nitrogen improvement per plant in CR09 and CR20 inoculated plants, respectively. All the examined isolates showed variability in their symbiotic performances. The strain found to be more tolerant to stress were more effective N2 fixers in symbiosis with cowpea cv. Paiyur1 under acid-soil conditions. Symbiotic variation among different strains showed that there is potential to improve strain performanc...

The ability of indigenous rhizobia to nodulate a legume crop effectively, or to act as barrier to the successful establishment of inoculant strains, is critical to successful establishment and growth of legumes. Effective groups serve both as a guide to inoculant preparation and as a basis for predicting the need for inoculation. This review included all aspects of rhizobial work, starting from isolation and characterization of root nodule bacteria from the field to authentication of the symbiotic effectiveness test under sterile sand condition. This review shows the process on how to evaluate the symbiotic effectiveness along with growth responses to varied conditions of pH, temperature, antibiotics as well as carbon and nitrogen sources. Goal of Rhizobium scientists is to discover new and better strains for use in legume inoculants. This pursuit entails the collection of isolates, strain characterization, assessment of symbiotic capacity and comparison to strains currently include...

Rhizobia are symbiotic bacteria that synthesize lipochitooligosaccharide Nod factors (NFs), which act as signal molecules in the nodulation of specific legume hosts. Based on the structure of their N‐acyl chain, NFs can be classified into two categories: (i) those that are acylated with fatty acids from the general lipid metabolism; and (ii) those (= αU‐NFs) that are acylated by specific α,β‐unsaturated fatty acids (containing carbonyl‐conjugated unsaturation(s)). Previous work has described how rhizobia that nodulate legumes of the Trifolieae and Vicieae tribes produce αU‐NFs. Here, we have studied the structure of NFs from two rhizobial species that nodulate important genera of the Galegeae tribe, related to Trifolieae and Vicieae. Three strains of Mesorhizobium huakuii, symbionts of Astragalus sinicus, produced as major NFs, pentameric lipochitooligosaccharides O‐sulphated and partially N‐glycolylated at the reducing end and N‐acylated, at the non‐reducing end, by a C18:4 fatty a...

The common bean production system is majorly based on the use of granular fertilizers to provide nutrients for the crop. Studies on the use of liquid fertilization at an appropriated depth and, seed inoculation with Rhizobium tropici can provide significant increases in the grain yield of the common bean. The objective of this study was to determine the growth and productivity of common bean as affected by N-P formulations, application depths and the inoculation with R. tropici. Field experiments were carried out in 2015 and 2016 cropping years using a complete block design, in a 2x2x2 factorial arrangement, with four replicates. The treatments involved the combination of N-P formulation (granular and liquid), two application depths of the N-P formulation (6 and 12 cm) and with or without rhizobia inoculant. The plant density (PD), number of pods (NP), number of grains (NG), mass of 100 grains (M100) and grain yield (GY) were determined. The granular and liquid N-P formulations prov...

A species-specific latitudinal distribution of soybean rhizobia has been reported; Bradyrhizobium japonicum and B. elkanii dominate in nodules in northern and southern areas, respectively. The aim of this study was to elucidate whether temperature-dependent proliferation in soil or infection is more reliable for determining the latitudinal characteristic distribution of soybean-nodulating rhizobia under local climate conditions. Three study locations, Fukagawa (temperate continental climate), Matsue and Miyazaki (humid sub-tropical climate), were selected in Japan. Each soil sample was transported to the other study locations, and soybean cv. Orihime (non-Rj) was pot-cultivated using three soils at three study locations for two successive years. Species composition of Bradyrhizobium in the nodules was analyzed based on the partial 16S rRNA and 16S–23S rRNA ITS gene sequences. Two Bradyrhizobium japonicum (Bj11 and BjS10J) clusters and one B. elkanii (BeL7) cluster were phylogenetica...

IntroductionBradyrhizobium diazoefficiens, a symbiotic nitrogen fixer for soybean, forms nodules after developing a symbiotic association with the root. For this association, bacteria need to move toward and attach to the root. These steps are mediated by the surface and phenotypic cell properties of bacteria and secreted root exudate compounds. Immense work has been carried out on nodule formation and nitrogen fixation, but little is known about the phenotype of these microorganisms under the influence of different root exudate chemical compounds (RECCs) or how this phenotype impacts the root attachment ability.MethodsTo address this knowledge gap, we studied the impact of 12 different RECCs, one commonly used carbon source, and soil-extracted solubilized organic matter (SESOM) on attachment and attachment-related properties of B. diazoefficiens USDA110. We measured motility-related properties (swimming, swarming, chemotaxis, and flagellar expression), attachment-related properties...

The type strain of the prospective 10.1601/nm.30737 sp. nov. ERR11, was isolated from a nodule of the leguminous tree native to Ethiopia. The type strain 10.1601/nm.1463 10.1601/strainfinder?urlappend=%3Fid%3DCCBAU+10071 , was isolated from the nodules of in Beijing, China. The genomes of ERR11 and 10.1601/strainfinder?urlappend=%3Fid%3DCCBAU+10071 were sequenced by DOE-JGI and deposited at the DOE-JGI genome portal as well as at the European Nucleotide Archive. The genome of ERR11 is 9,163,226 bp in length and has 102 scaffolds, containing 8548 protein-coding and 86 RNA genes. The 10.1601/strainfinder?urlappend=%3Fid%3DCCBAU+10071 genome is arranged in 108 scaffolds and consists of 8,201,522 bp long and 7776 protein-coding and 85 RNA genes. Both genomes contain symbiotic genes, which are homologous to the genes found in the complete genome sequence of 10.1601/nm.24498 10.1601/strainfinder?urlappend=%3Fid%3DUSDA+110 . The genes encoding for nodulation and nitrogen fixation in ERR11 ...

This study was undertaken to observe the influence of seasonal fluctuation on native peanut rhizobia populations in Coastal Plain soils in cultivated fields with or without a peanut history, in pasture, and in forest environments. Soil samples were collected from Ap horizons every 30 days during a 16-month period and rhizobia populations were enumerated using a most probable number (MPN) technique with the peanut (Arachis hypogaea L.) as the host for plant infectivity tests. Cultivated fields with a peanut production history had higher peanut rhizobia populations than cultivated fields without a peanut history, pastures, or forest environments. Seasonal fluctuation was evident in cultivated fields with a peanut history; peanut rhizobia populations were greatest in July whether peanuts or another crop was being grown that year and very low from September to May. Peak peanut rhizobia population reached 10,000 per gram of soil in peanut fields in July. Populations were generally less t...

IntroductionWheat (Triticum aestivum L.) is a strategic crop and one of the world's most essential cereals, providing most of the world's calories and protein needs. Drought stress is one of the main limitations for crop production such as wheat in arid and semi-arid regions. Plants can accumulate antioxidants, carbohydrates, and stress hormones that stimulate cell and molecular regeneration under stress conditions. Irrigation saves water, improves crop photosynthesis, and increases plant ability to absorb water and elements from soil. Therefore, irrigation at the right time or supplementary irrigation can help plant growth and crop yield under drought conditions. Appropriate nutrition with fertilizers increases plants' stress tolerance. Bio-fertilizers are restorative elements used in soil to improve tolerance to stresses such as drought stress. A well-known class of bio-fertilizers is plant growth promoting rhizobacteria (PGPR). These rhizosphere bacteria affect plant ...

Symbiosis receptor kinase (SYMRK) is indispensable for activation of root nodule symbiosis (RNS) at both epidermal and cortical levels and is functionally conserved in legumes. Previously we reported SYMRK to be phosphorylated on 'gatekeeper' Tyr both in vitro as well as in planta (Samaddar et al., 2013). Since gatekeeper phosphorylation was not necessary for activity, the significance remained elusive. Herein we show that substituting gatekeeper with non-phosphorylatable residues like Phe or Ala significantly affected autophosphorylation on selected targets on activation segment/αEF and β3 αC loop of SYMRK. In addition, the same gatekeeper mutants failed to restore proper symbiotic features in a symrk null mutant where rhizobial invasion of the epidermis and nodule organogenesis was unaffected but rhizobia remain restricted to the epidermis in infection threads migrating parallel to the longitudinal axis of the root resulting in extensive infection patches at the nodule ape...

This study was devoted to exploring the natural nodulation and nitrogen fixation of wild legumes grown in different Egyptian habitats. These habitats are representative to four phytogeographical regions. Sites that inhabited by Melilotus indicus, Medicago polymorpha, Trifolium resupinatum, Trigonella hamosa and Vicia sativa in each region were selected for study. High nodulation, nitrogen fixation and plant biomass were recorded in plants grown at Nile region and Oases compared with those at Mediterranean region and Sinai. The inhibition in nodulation and potential of nitrogen fixation in legumes at MR and S were attributed to drought and low soil fertility. Differences in species, regions or their interaction have significant effect on nodulation, legheamoglobin, nitroginase activity and biomass of nodules, shoots and roots; the magnitude of effect due to different species was the greatest. Five rhizobial isolates ( Sinorhizobium fredii , Rhizobium mesosinicum, Rhizobium daejeonens...

Adsorption of Rhizobium meliloti to alfalfa roots before their infection and nodule formation shows the specificity of the symbiotic association (G. Caetano-Anollés and G. Favelukes, Appl. Environ. Microbiol. 52:377-382, 1986). The time course of specific adsorption of R. meliloti (10(3) to 10(4) cells per ml) to roots shows an initial lag period of 3 h, suggesting that either or both symbionts must become conditioned for the adsorption process. Preincubation of R. meliloti L5-30 for 3 h with dialyzed alfalfa root exudate (RE) markedly increased early adsorption of rhizobia to alfalfa roots. The activity in RE was linked to a nondialyzable, thermolabile, trypsin-sensitive factor(s), very different from the root-exuded flavonoid compounds also involved in early Rhizobium-legume interactions. The lack of activity in the RE from plants grown in 5 mM NO3- suggested its negative regulation by the nitrogen nutritional status of the plant. Preincubation of R. meliloti with heterologous clo...

Symbiosis Receptor Kinase (SYMRK), a member of the Nod factor signaling pathway, is indispensible for both nodule organogenesis and intracellular colonization of symbionts in rhizobia-legume symbiosis. Here, we show that the intracellular kinase domain of a SYMRK (SYMRK-kd) but not its inactive or full-length version leads to hyperactivation of the nodule organogenic program in Medicago truncatula TR25 (symrk knockout mutant) in the absence of rhizobia. Spontaneous nodulation in TR25/SYMRK-kd was 6-fold higher than rhizobia-induced nodulation in TR25/SYMRK roots. The merged clusters of spontaneous nodules indicated that TR25 roots in the presence of SYMRK-kd have overcome the control over both nodule numbers and their spatial position. In the presence of rhizobia, SYMRK-kd could rescue the epidermal infection processes in TR25, but colonization of symbionts in the nodule interior was significantly compromised. In summary, ligand-independent deregulated activation of SYMRK hyperactiv...