Nitrogen-fixing Rhizobacteria Research Articles

Isolation and Identification of Nitrogen-Fixing Rhizobacteria associated with Cocoa plantation (Theobroma cacao L) as Biofertilizer Agent

Ringinkembar Village, Sumbermanjing Wetan District, Malang Regency, is one of the centers for cocoa plantations using an organic farming system. However, over time this organic farming system experienced a decrease in fruit production, possibly from soil fertility and biofertilizers that were less available in the soil. This study aims to analyze the nitrogen-fixing ability and identify rhizosphere isolates that excel in nitrogen-fixing obtained from the rhizosphere of cacao (Theobroma cacao L) plant. Bacteria were isolated from the soil surrounding cocoa plant roots and grown on Nfb (Nitrogen free Bromothymol Blue) agar media. The nitrogen-fixing bacteria were analyzed with quantitative and qualitative methods. Six potential nitrogen-fixing isolates were identified based on the 16S rDNA sequence. The total number of isolates obtained from nitrogen-fixing isolation was six isolates. The R3.FN1 isolate showed the highest ammonia index at 0.52 µg.L-1 and was identified as Stenotrophomonas maltophilia KB13 with 99.87% similarity to the 16S rDNA sequence.

Complete Genome Sequence of Bradyrhizobium sp. Strain C-145, a Nitrogen-Fixing Rhizobacterium Used as a Peanut Inoculant in Argentina.

ABSTRACTWe present the complete genome sequence of Bradyrhizobium sp. strain C-145, one of the most widely used nitrogen-fixing rhizobacteria for inoculating peanut crops in Argentina. The genome consists of 9.53 Mbp in a single circular chromosome and was determined using a hybrid long- and short-read assembly approach.

MtWRP1, a Novel Fabacean Specific Gene, Regulates Root Nodulation and Plant Growth in Medicago truncatula.

Fabaceans symbiotically interact with nitrogen-fixing rhizobacteria to form root nodules. Some fabacean specific proteins play important roles in the symbiosis. WRKY-related Protein (WRP) is a novel fabacean specific protein, whose functions have not been well characterized. In this study, MtWRP1 was functionally characterized in Medicago truncatula. It contains a WRKY domain at C-terminal and a novel transmembrane (TM) domain at N-terminal, and its WRKY domain was highly similar to the N-terminal WRKY domain of the group I WRKY proteins. The TM domain was highly homologous to the eukaryotic cytochrome b561 (Cytb561) proteins from birds. Subcellular localization revealed that MtWRP1 was targeted to the Golgi apparatus through the novel TM domain. MtWRP1 was highly expressed in roots and nodules, suggesting its possible roles in the regulation of root growth and nodulation. Both MtWRP1-overexpression transgenic M. truncatula and MtWRP1 mutants showed altered root nodulation and plant growth performance. Specifically, the formation of root nodules was significantly reduced in the absence of MtWRP1. These results demonstrated that MtWRP1 plays critical roles in root nodulation and plant growth.

Distribution of Heterotrophic and Nitrogen fixing Rhizobacteria in Irrigation Sites of Lake Alau Borno State Northeastern Nigeria

This study was conducted to know the population of rhizobacteria in both irrigation and non-irrigation sites of the dam. The dense population of these organism indirectly promote plant growth and development. Five sites (A, B, C, D and E) were used to collect soil samples randomly and transported to the laboratory for analysis. Total heterotrophic bacterial count was done using nutrient agar (NA) and nitrogen fixing bacteria was counted using Ashbey’s media (AM). The result shows that highest number of total heterotrophic bacteria in site C (46.0×106) cfu/g in irrigation site whereas higher count in non-irrigation site was (13.0×106) site D, the nitrogen fixing bacterial count in irrigation site was higher at site E with (12.0×106) and for the non-irrigation site was higher at site D with (14.0×106) The total heterotrophic bacteria isolated in the soil sample are the species of Bacillus alvei, Bacillus alvei, Bacillus cereus, Bacillus cereus, Pseudomonas putida, Klebsiella aeruginosa and Enterobacter aeruginosa. Likewise, the Nitrogen fixing bacteria isolated are the species of Rhizobium leguminosarum, Klebsiella pneumonia, Bacillus lentus, Azotobacter nigricans, Azotobacter tropicalis, Azotobacter spp, and Azotobacter tropicalis. The long history of agricultural activities in the lake area has directly influenced the diversity of microbial population in the area.

Perspectives for sustainable agriculture from the microbiome in plant rhizosphere

The ever-growing human population globally has resulted in the quest for solutions to the problem of hunger by providing food security. The importance of plant-root-associated microorganisms cannot be overlooked, plants rely on them. These root colonizers dominate the rhizosphere due to the abundance of available nutrients, relying on their host plant for nutrients and other essential requirements. The relationships between microbial communities and plants are controlled by the type of plant and microorganism involved. Advances in modern molecular techniques have led to the evolution of omic technology using nucleic acid molecules to study plant-microorganism associations capable of stimulating plant growth, improve yield, and induce disease suppression. This review elucidates the activities of microbial communities, especially nitrogen-fixing rhizobacteria associated with plant roots, nitrogen fixation as a mechanism of promoting plant growth, their importance, and the challenges employing bioinoculants. Prospecting plant growth promoters using omic technology will advance sustainable agriculture globally.

Effects of Selected Functional Bacteria on Maize Growth and Nutrient Use Efficiency.

Plant growth-promoting rhizobacteria (PGPR), which include isolates from genera Paraburkholderia, Burkholderia and Serratia, have received attention due to their numerous plant growth-promoting mechanisms such as their ability to solubilize insoluble phosphates and nitrogen-fixation. However, there is a dearth of information on the potential plant growth-promoting effects of these three groups of bacteria on non-legumes such as maize. This study determined the influences of the aforementioned strains on soil properties, maize growth, nutrient uptake and nutrient use efficiency. A pot trial using maize as a test crop was done using a randomized complete block design with 7 treatments each replicated 7 times. The treatments used in this study were: Control (no fertilizer), chemical fertilizer (CF), organic-chemical fertilizers combination without inoculum (OCF) and with inocula consisting of single strains [cellulolytic bacteria (TC), organic fertilizer and chemical fertilizer with N-fixing bacteria (TN), organic fertilizer and chemical fertilizer with P-solubilizing bacteria (TP)) and three-strain inocula (TCNP), respectively. The variables measured included plant growth and nutrient content, soil nutrient content and functional rhizospheric bacterial populations. Paraburkholderia nodosa NB1 and Burkholderia cepacia PB3 showed comparable effects on maize biomass and also improved N and P use efficiencies when compared to full chemical fertilization. Nitrogen-fixing rhizobacteria had a positive effect on above-ground biomass of maize. Paraburkholderia nodosa NB1 improved soil total C and organic matter contents, besides being the only bacterial treatment that improved K use efficiency compared to OCF. The results suggest that P. nodosa NB1 and B. cepacia PB3 have potential usage in bio-fertilizers. In contrast, treatments with Serratia nematodiphila C46d and consortium strains showed poorer maize nutrient uptake and use efficiency than the other single strain treatments. Bacterial treatments generally showed comparable or higher overall N and P use efficiencies than full chemical fertilization. These findings suggest that at least half the amounts of N and P fertilizers could be reduced through the use of combined fertilization together with beneficial bacteria.

Diversity of nitrogen-fixing rhizobacteria associated with sugarcane: a comprehensive study of plant-microbe interactions for growth enhancement in Saccharum spp.

BackgroundNitrogen is an essential element for sugarcane growth and development and is generally applied in the form of urea often much more than at recommended rates, causing serious soil degradation, particularly soil acidification, as well as groundwater and air pollution. In spite of the importance of nitrogen for plant growth, fewer reports are available to understand the application and biological role of N2 fixing bacteria to improve N2 nutrition in the sugarcane plant.ResultsIn this study, a total of 350 different bacterial strains were isolated from rhizospheric soil samples of the sugarcane plants. Out of these, 22 isolates were selected based on plant growth promotion traits, biocontrol, and nitrogenase activity. The presence and activity of the nifH gene and the ability of nitrogen-fixation proved that all 22 selected strains have the ability to fix nitrogen. These strains were used to perform 16S rRNA and rpoB genes for their identification. The resulted amplicons were sequenced and phylogenetic analysis was constructed. Among the screened strains for nitrogen fixation, CY5 (Bacillus megaterium) and CA1 (Bacillus mycoides) were the most prominent. These two strains were examined for functional diversity using Biolog phenotyping, which confirmed the consumption of diverse carbon and nitrogen sources and tolerance to low pH and osmotic stress. The inoculated bacterial strains colonized the sugarcane rhizosphere successfully and were mostly located in root and leaf. The expression of the nifH gene in both sugarcane varieties (GT11 and GXB9) inoculated with CY5 and CA1 was confirmed. The gene expression studies showed enhanced expression of genes of various enzymes such as catalase, phenylalanine-ammonia-lyase, superoxide dismutase, chitinase and glucanase in bacterial-inoculated sugarcane plants.ConclusionThe results showed that a substantial number of Bacillus isolates have N-fixation and biocontrol property against two sugarcane pathogens Sporisorium scitamineum and Ceratocystis paradoxa. The increased activity of genes controlling free radical metabolism may at least in part accounts for the increased tolerance to pathogens. Nitrogen-fixation was confirmed in sugarcane inoculated with B. megaterium and B. mycoides strains using N-balance and 15N2 isotope dilution in different plant parts of sugarcane. This is the first report of Bacillus mycoides as a nitrogen-fixing rhizobacterium in sugarcane.

Long-Term Exposure of Agricultural Soil to Veterinary Antibiotics Changes the Population Structure of Symbiotic Nitrogen-Fixing Rhizobacteria Occupying Nodules of Soybeans (Glycine max).

Antibiotics are entrained in agricultural soil through the application of manures from medicated animals. In the present study, a series of small field plots was established in 1999 that receive annual spring applications of a mixture of tylosin, sulfamethazine, and chlortetracycline at concentrations ranging from 0.1 to 10 mg · kg-1 soil. These antibiotics are commonly used in commercial swine production. The field plots were cropped continuously for soybeans, and in 2012, after 14 annual antibiotic applications, the nodules from soybean roots were sampled and the occupying bradyrhizobia were characterized. Nodules and isolates were serotyped, and isolates were distinguished using 16S rRNA gene and 16S to 23S rRNA gene intergenic spacer region sequencing, multilocus sequence typing, and RSα fingerprinting. Treatment with the antibiotic mixture skewed the population of bradyrhizobia dominating the nodule occupancy, with a significantly larger proportion of Bradyrhizobium liaoningense organisms even at the lowest dose of 0.1 mg · kg-1 soil. Likewise, all doses of antibiotics altered the distribution of RSα fingerprint types. Bradyrhizobia were phenotypically evaluated for their sensitivity to the antibiotics, and there was no association between in situ treatment and a decreased sensitivity to the drugs. Overall, long-term exposure to the antibiotic mixture altered the composition of bradyrhizobial populations occupying nitrogen-fixing nodules, apparently through an indirect effect not associated with the sensitivity to the drugs. Further work evaluating agronomic impacts is warranted.IMPORTANCE Antibiotics are entrained in agricultural soil through the application of animal or human waste or by irrigation with reused wastewater. Soybeans obtain nitrogen through symbiotic nitrogen fixation. Here, we evaluated the impact of 14 annual exposures to antibiotics commonly used in swine production on the distribution of bradyrhizobia occupying nitrogen-fixing nodules on soybean roots in a long-term field experiment. By means of various sequencing and genomic fingerprinting techniques, the repeated exposure to a mixture of tylosin, sulfamethazine, and chlortetracycline each at a nominal soil concentration of 0.1 mg · kg-1 soil was found to modify the diversity and identity of bradyrhizobia occupying the nodules. Nodule occupancy was not associated with the level of sensitivity to the antibiotics, indicating that the observed effects were not due to the direct toxicity of the antibiotics on bradyrhizobia. Altogether, these results indicate the potential for long-term impacts of antibiotics on this agronomically important symbiosis.

Correction.

The Plant JournalVolume 94, Issue 2 p. 405-405 CorrectionFree Access Correction This article corrects the following: Pattern recognition receptors and signaling in plant–microbe interactions Yusuke Saijo, Eliza Po-iian Loo, Shigetaka Yasuda, Volume 93Issue 4The Plant Journal pages: 592-613 First Published online: February 2, 2018 First published: 02 April 2018 https://doi.org/10.1111/tpj.13875AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat In the article “Pattern recognition receptors and signaling in plant–microbe interactions” by Yusuke Saijo et al. (2018), the authors recognized some incorrect sentences that they would like to amend. In particular: in page 593:: right column, the sentence “… receptor-like cytoplasmic kinases (RLCKs) such as BIK1 in intercellular signaling …” should read “… receptor-like cytoplasmic kinases (RLCKs) such as BIK1 in intra cellular signaling …”. in page 597: left column, the sentence “… mediating chitin-triggered immunity (Miya et al., 2007; Wan et al., 2008; Iizasa et al., 2010; Petutschnig et al., 2010) …” Should read “… mediating chitin-triggered immunity (Miya et al., 2007; Wan et al., 2008; Iizasa et al., 2010; Petutschnig et al., 2010; Liu et al., 2012b). in page 597: left column, the sentence “… upregulation of Eix1, whose EIX binding in turn attenuates Eix2 signaling by functioning with BAK1 …” should read “… which functions with BAK1 after binding EIX to attenuate Eix2 signaling …”. in page 598: right column, the sentence “… Indeed, the malectin-like domain Catharanthus roseus RLK1-like (MLD-CrRLK1L) receptor …” should read “… In A.thaliana, the malectin-like domain Catharanthus roseus RLK1-like (MLD-CrRLK1L) receptor …” . in page 600: left column, the sentence “… In A. thaliana, dephosphorylation at two residues and phosphorylation at Ser 899 of AHA, promoting and inhibiting AHA activity respectively, occur during PTI …” should read “ … In A. thaliana, phosphorylation at Ser 899 of AHA, as well as dephosphorylation at several other residues, is induced during PTI to repress H+ pumping …” in page 602: right column, the sentence “… PRR homo-oligomerization or larger complex formation involving two or more PRRs may occur at such cell-surface scaffolds …” should read “… Such cell-surface scaffolds may provide a platform for PRR homo-oligomerization or larger complex formation involving two or more PRRs …”. in page 603: left column, the sentence “… pathogenic microbes, which include PTI, i.e. elicitor-inactive flg22 alleles in nitrogen-fixing rhizobacteria (Gomez-Gomez et al., 1999) and flg22-induced suppression of their colonization (Lopez-Gomez et al., 2012), and ETS/ETI, i.e. NLR-dependent rhizobacterial resistance …” should read “… pathogenic microbes. PTI constrains mutualistic microbes as evident with elicitor-inactive flg22 alleles in nitrogen-fixing rhizobacteria (Gomez-Gomez et al., 1999) or flg22-induced suppression of their colonization (Lopez-Gomez et al., 2012). Likewise, ETS/ETI also influence them, as seen with NLR-dependent rhizobacterial resistance …”. in page 604: left column, the sentence “… It is important to determine how MAMP presentation is regulated, possibly depending on microbial growth …” should read “…It is important to determine how MAMP presentation is altered depending on microbial growth …”; right column, the sentence “… This work is supported in part by JSPS KAKENHI (16KT0031 and 16H01469) and the Mitsubishi Foundation ….” should read “… The work in the Saijo Lab is supported in part by JSPS KAKENHI (16KT0031 and 16H01469), JST PRESTO (JPMJPR13B6) and the Mitsubishi Foundation…”. Reference Saijo, Y., Loo, E.P. and Yasuda, S. (2018) Pattern recognition receptors and signaling in plant–microbe interactions. Plant J., 93, 592– 613. Volume94, Issue2April 2018Pages 405-405 ReferencesRelatedInformation

Chromium(VI) Toxicity in Legume Plants: Modulation Effects of Rhizobial Symbiosis.

Most legume species have the ability to establish a symbiotic relationship with soil nitrogen-fixing rhizobacteria that promote plant growth and productivity. There is an increasing evidence of reactive oxygen species (ROS) important role in formation of legume-rhizobium symbiosis and nodule functioning. Environmental pollutants such as chromium compounds can cause damage to rhizobia, legumes, and their symbiosis. In plants, toxic effects of chromium(VI) compounds are associated with the increased production of ROS and oxidative stress development as well as with inhibition of pigment synthesis and modification of virtually all cellular components. These metabolic changes result in inhibition of seed germination and seedling development as well as reduction of plant biomass and crop yield. However, if plants establish symbiosis with rhizobia, heavy metals are accumulated preferentially in nodules decreasing the toxicity of metals to the host plant. This review summarizes data on toxic effects of chromium on legume plants and legume-rhizobium symbiosis. In addition, we discussed the role of oxidative stress in both chromium toxicity and formation of rhizobial symbiosis and use of nodule bacteria for minimizing toxic effects of chromium on plants.

ROLE OF BIOLOGICAL AGENT AZOTOBACTER-TRICHODERMA ON GROWTH AND YIELD OF CHILI (Capsicum annuum L.) IN A POT EXPERIMENT

Soil microbes have suggested to be used as agricultural input. Nitrogen fixing rhizobacteria Azotobacter is responsible to maintain nitrogen nutrition and plant growth whereas Trichoderma enable to reduce soil born plant diseases through antagonistic activity. The objective of this pot experiment was to determine the dosage and application time of carrier- based Azotobacter-Trichoderma inoculant which increase nitrogen availability in soil, as well as growth and yield of chili (Capsicum annuum L.). Chili transplants were grown in low fertility soil mixed with cow manure. Experiment was set up in split plot design which tested four inoculant dosage and three application time. The result showed that effect of biological agent on plant height at three and six weeks after transplanting was not significant. Inoculation of 7,5 g/pot carrier-based Azoto-Tricho at planting time followed by soil dressing with Azotobacter liquid inoculant at 10 day after planting significantly increased NO3 ­- in soil. Carrier-based Azoto-Tricho inoculant irrespective of dosage and application time increased plant yield. The highest yield, 290 g plant-1, was showed by plant treated by 7,5 g po­1t Azoto-Tricho at planting time followed by Azotobacter liquid inoculation. This pot experiment showed that carrier-based Azotobacter-Trichoderma inoculant has potential to be used as biological agent in chili production.

Strigolactones in Plant Interactions with Beneficial and Detrimental Organisms: The Yin and Yang.

Strigolactones (SLs) are plant hormones that have important roles as modulators of plant development. They were originally described as ex planta signaling molecules in the rhizosphere that induce the germination of parasitic plants, a role that was later linked to encouraging the beneficial symbiosis with arbuscular mycorrhizal (AM) fungi. Recently, the focus has shifted to examining the role of SLs in plant-microbe interactions, and has revealed roles for SLs in the association of legumes with nitrogen-fixing rhizobacteria and in interactions with disease-causing pathogens. Here, we examine the role of SLs in plant interactions with beneficial and detrimental organisms, and propose possible future biotechnological applications.

Co-inoculation with Enterobacter and Rhizobacteria on Yield and Nutrient Uptake by Wheat (Triticum aestivum L.) in the Alluvial Soil Under Indo-Gangetic Plain of India

The aim of this work was to evaluate the effects of co-inoculation with phosphate-solubilizing and nitrogen-fixing rhizobacteria on growth promotion, yield, and nutrient uptake by wheat. Out of twenty-five bacteria isolated from the rhizosphere soils of cereal, vegetable, and agro-forestry plants in eastern Uttar Pradesh, three superior most plant growth-promoting (PGP) isolates were characterized as Serratia marcescens, Microbacterium arborescens, and Enterobacter sp. based on their biochemical and 16S rDNA gene sequencing data and selected them for evaluating their PGP effects on growth and yield of wheat. Among them, Enterobacter sp. and M. arborescens fixed significantly higher amounts (9.32 ± 0.57 and 8.89 ± 0.58 mg Ng−1 carbon oxidized, respectively) of atmospheric nitrogen and produced higher amounts (27.06 ± 1.70 and 26.82 ± 1.63 TP 100 µg mL−1, respectively) of IAA in vitro compared to S. marcescens (8.32 ± 0.39 mg Ng−1 carbon oxidized and 21.29 ± 0.99 TP 100 µg mL−1). Although both M. arborescens and S. marcescens solubilized remarkable amounts of phosphate from tricalcium phosphate likely through production of organic acids, however, Enterobacter sp. was inactive. The effects of these three rhizobacteria were evaluated on wheat in alluvial soils of the Indo-Gangetic Plain by inoculation of plants with bacterial isolates either alone or in combinations in both pot and field conditions for two successive years. Rhizobacterial inoculation either alone or in consortium of varying combinations significantly (P ≤ 0.05) increased growth and yield of wheat compared to mock inoculated controls. A consortium of two or three rhizobacterial isolates also significantly increased plant height, straw yield, grain yield, and test weight of wheat in both pot and field trials compared to single application of any of these isolates. Among the rhizobacterial treatment, co-inoculation of three rhizobacteria (Enterobacter, M. arborescens and S. marcescens) performed best in promotion of growth, yield, and nutrient (N, P, Cu, Zn, Mn, and Fe) uptake by wheat. Taken together, our results suggest that co-inoculation of Enterobacter with S. marcescens and M. arborescens could be used for preparation of an effective formulation of PGP consortium for eco-friendly and sustainable production of wheat.

Syntheses of O-antigen polysaccharide fragments of nitrogen-fixing rhizobacteria of the genus Azospirillum

Fragments of O-antigen polysaccharides of nitrogen-fixing rhizobacteria of the genus Azospirillum isolated from natural sources may greatly vary in the monosaccharide composition, molecular weight, as well as in the presence of side chains and branching sites along the main chain. These variations, along with the differences in purity and homogeneity of the samples, hamper investigations of the role of individual components of O-antigen polysaccharides of the rhizobacteria Azospirillum in interactions with plants, including important cereal crops. Therefore, synthetic oligosaccharides, which correspond to O-antigen polysaccharide fragments of rhizobacteria of the genus Azospirillum and have a strictly defined structure and high purity, are highly demanded. The review summarizes the data on the synthesis of these oligosaccharide fragments as promising tools for investigation of the mechanism of formation of a symbiotic association with plants.

Role of ACC-deaminase and/or nitrogen fixing rhizobacteria in growth promotion of wheat (Triticum aestivum L.) under cadmium pollution

Excessive cadmium concentrations in agricultural soils result in minimizing the soil fertility and health which leads to decrease in crop production. Plant growth promoting rhizobacteria (PGPR) are beneficial bacteria, which can protect the plants against many abiotic stresses. Current study aimed to identify important rhizobacterial strains by using enrichment technique and examine their inoculation effects on the growth and physiological parameters of wheat, under cadmium pollution. Cadmium was added to 3 kg soil in each pot (with 6 seeds/pot) using cadmium chloride at the rate of 0, 50, 100, 150, and 200 mg kg−1 with three replications in completely randomized design. Rhizobacterial isolates performed considerably better under all cadmium levels, i.e. 50–200 mg kg−1 soil, compared to control. Nevertheless, rhizobacterial isolates containing both abilities, i.e. deaminase and nitrogen fixing, e.g. SAN1 had the highest effect and caused a significant (P < 0.05) increase in the shoot (25.1-fold) and root length (30.2-fold), seedling fresh (17.1-fold) and dry weights (31.1-fold), chlorophyll a (13.1-fold), chlorophyll b (8.2-fold), carotenoids (5.1-fold), protein (50.1-fold), proline (18.8-fold), glutathione S-transferase (26.2-fold), peroxidase (26.8-fold) and catalase (30.5-fold), while lowest cadmium uptake in the shoot (10.1-fold) and root (8.7-fold), respectively, at the highest cadmium level, i.e. 200 mg kg−1 soil compared to control. Results revealed that PGPR significantly decreased the deleterious effects of cadmium pollution by chelating and influencing its bioavailability and increased the wheat growth. The PGPR with both deaminase and nitrogen fixing activities are more resilient against cadmium pollution than PGPR having either deaminase or nitrogen fixing activity alone. The enrichment technique is an efficient approach to select promising PGPR.

Structure and serology of O-antigens of nitrogen-fixing rhizobacteria of the genus Azospirillum

The review presents a discussion of the published data of 2002 to 2014 dealing with structural studies of O-specific polysaccharides, representatives of the Azospirillum diazotrophic rhizobacteria, summarizing the results of a series of authors´ studies. Some general structures incorporated in O-specific polysaccharides responsible for classification of bacterial cultures into serogroups on the basis of immunochemical studies were characterized. The structures of O-specific polysaccharides of seven azospirillum strains identical to those of previously studied strains are presented in this review for the first time.

Rhizobial root hair infection requires auxin signaling.

Legumes can enter into a mutualistic relationship with nitrogen-fixing rhizobacteria. A recent study by A. Breakspear et al. sheds new light on the mechanisms involved in rhizobial infection of their host root hair during symbiosis establishment and reveals a new role for auxin signaling in this process.

Robust biological nitrogen fixation in a model grass-bacterial association.

Nitrogen-fixing rhizobacteria can promote plant growth; however, it is controversial whether biological nitrogen fixation (BNF) from associative interaction contributes to growth promotion. The roots of Setaria viridis, a model C4 grass, were effectively colonized by bacterial inoculants resulting in a significant enhancement of growth. Nitrogen-13 tracer studies provided direct evidence for tracer uptake by the host plant and incorporation into protein. Indeed, plants showed robust growth under nitrogen-limiting conditions when inoculated with an ammonium-excreting strain of Azospirillum brasilense. (11)C-labeling experiments showed that patterns in central carbon metabolism and resource allocation exhibited by nitrogen-starved plants were largely reversed by bacterial inoculation, such that they resembled plants grown under nitrogen-sufficient conditions. Adoption of S. viridis as a model should promote research into the mechanisms of associative nitrogen fixation with the ultimate goal of greater adoption of BNF for sustainable crop production.

ACC-deaminase and/or nitrogen fixing rhizobacteria and growth of wheat (Triticum Aestivum L.)

Plant growth promoting rhizobacteria (PGPR) are beneficial bacteria, which can enhance the growth of the plants, when applied to crops. A pot experiment was conducted to examine the effect of six PGPR isolates on the growth of wheat. Inoculation with rhizobacterial isolates increased the all measured physical, chemical and enzymatic growth parameters compared to control (CK). However, the WAN1 isolate had the highest effect, and significantly (P < 0.05) increased the root length (3.51-fold), shoot length (3.22-fold), seedling fresh (3.41-fold) and dry (3.91-fold) weight, chlorophyll a (3.90-fold), chlorophyll b (3.51-fold), carotenoid contents (7.23-fold), plant macronutrient uptake i.e. N (7.20-fold, 6.71-fold), P (7.41-fold, 5.01-fold), K (5.51-fold, 3.91-fold), Ca (6.40-fold, 5.21-fold) and Mg (5.82-fold, 7.11-fold) in shoot and root, plant micronutrient uptake i.e. Zn (6.40-fold, 9.11-fold), Cu (7.31-fold, 7.02-fold), Fe (6.41-fold, 7.52-fold) and Mn (4.57-fold, 5.21-fold) in shoot and root and plant antioxidant enzymes i.e. glutathione S-transferase (7.51-fold), peroxidase (5.21-fold) and catalase (5.01-fold) respectively. Our results revealed that inoculation of agricultural crops with PGPR is a very useful approach to increase the plant growth. The ACC (1-aminocyclopropane-1-carboxylate) enrichment technique is an efficient approach to select promising PGPR. The PGPR containing dual abilities i.e. both ACC-deaminase and nitrogen fixing ability are more effective than PGPR possessing either ACC-deaminase or nitrogen fixing activity alone for growth promotion of crops.

Overexpression of citrate operon in Herbaspirillum seropedicae Z67 enhances organic acid secretion, mineral phosphate solubilization and growth promotion of Oryza sativa

Herbaspirillum seropedicae Z67, nitrogen fixing endophyte, significantly promotes the growth of cereals. Organic acid secreting nitrogen fixing rhizobacteria have better plant growth promotion abilities due to mineral phosphate solubilization. Plasmids pAB7, pJNK3 and pJNK4 containing Escherichia coli cs (gltA), NADH insensitive cs (gltA1), and citrate operon consisting of gltA1 gene along with Salmonella typhimurium Na+ dependent citrate transporter (citC) gene under constitutive lac promoter were constructed in broad host range plasmid pUCPM18-Kmr. The plasmid transformants of H. seropedicae Z67 were obtained by electroporation. Hs (pAB7) and Hs (pJNK3) had increased CS activity but citric acid secretion was not significant. Hs (pJNK3) secreted 45 mM acetic acid while Hs (pJNK4) secreted 2.7 mM citric and 51 mM acetic acids. Hs (pJNK3) and Hs (pJNK4) released 80 μM and 110 μM amount of P from rock phosphate, respectively, in buffered medium under both aerobic and micro aerobic conditions. These transformants showed better plant growth promoting factors. Upon inoculation to rice plants (Gujarat – 17), increase of Fresh weight, Dry weight N, P and K content was observed. Thus the study demonstrates that artificial citrate operon in H. seropedicae Z67 enhances phosphate solubilization and plant growth promotion abilities.