Gerald H. Elkan

Bio: Gerald H. Elkan is an academic researcher from North Carolina State University. The author has contributed to research in topics: Rhizobium & Bradyrhizobium japonicum. The author has an hindex of 18, co-authored 42 publications receiving 1204 citations.

Transmissible Resistance to Penicillin G, Neomycin, and Chloramphenicol in Rhizobium japonicum

Abstract: The genetic basis for resistance to a number of antibiotics was examined in Rhizobium japonicum. Resistance to penicillin G, neomycin, and chloramphenicol appears to be mediated by an extrachromosomal element similar to that found in the Enterobacteriaceae. Resistance to these antibiotics was eliminated from cells by treatment with acridine orange, and resistance to all three antibiotics could be transferred en bloc to Agrobacterium tumefaciens under conditions excluding transformation or transduction as possible genetic mechanisms.

Rhizobium fredii sp. nov., a fast-growing species that effectively nodulates soybeans

Abstract: A new species, Rhizobium fredii, is proposed for fast-growing root nodule bacteria isolated from soybeans. The type strain was isolated from a root nodule of Glycine max growing in Honan Province, China, and is designated strain USDA 205 (= ATCC 35423 = PRC 205). This new species is differentiated from currently recognized Rhizobium and Bradyrhizobium species by deoxyribonucleic acid hybridization comparisons, plant specificity, generation times, antibiotic resistance, and serology. The strains of R. fredii are differentiated into two proposed chemovars, R. fredii chemovar fredii chemovar nov. and R. fredii chemovar siensis chemovar nov., by deoxyribonucleic acid hybridization tests, growth in the presence of erythromycin (20 μg/ml), final pH of yeast extract-mannitol medium, and serology.

Glucose catabolism in Rhizobium japonicum.

Abstract: Gluconate catabolism in Rhizobium japonicum ATCC 10324 was investigated by the radiorespirometric method and by assaying for key enzymes of the major energy-yielding pathways. Specifically labeled gluconate gave the following results for growing cells, with values expressed as per cent 14CO2 evolution: C-1 = 93%, C-2 = 57%, C-3 = 30%, C-4 = 70%, C-6 = 39%. The preferential release of 14CO2 from C-1 and C-4 indicate that gluconate is degraded primarily by the Entner-Doudoroff pathway but the inequalities between C-1 and C-4 and between C-3 and C-6 indicate that another pathway(s) also participates. The presence of gluconokinase and a system for converting 6-phosphogluconate to pyruvate also indicate a role for the Entner-Doudoroff pathway. The extraordinarily high yield of 14CO2 from C-1 labeled gluconate suggests that the other participating pathway is a C-1 decarboxylative pathway. The key enzyme of the pentose phosphate pathway, 6-phosphogluconate dehydrogenase, could not be demonstrated. Specifically labeled 2-ketogluconate and 2,5-diketogluconate were oxidized by gluconate grown cells and gave ratios of C-1 to C-6 of 2.73 and 2.61, respectively. These compare with a ratio of 2.39 obtained with specifically labeled gluconate. Gluconate dehydrogenase, the first enzyme in the ketogluconate pathway found in acetic acid bacteria, was found. Oxidation of specifically labeled pyruvate, acetate, succinate, and glutamate by gluconate-grown cells yielded the preferential rates of 14CO2 evolution expected from the operation of the tricarboxylic acid cycle. These data are consistent with the operation of the Entner-Doudoroff pathway and tricarboxylic acid cycle as the primary pathways of gluconate oxidation in R. japonicum. An ancillary pathway for the initial breakdown of gluconate would appear to be the ketogluconate pathway which enters the tricarboxylic acid cycle at α-ketoglutarate.

Genetic diversity among Bradyrhizobium isolates that effectively nodulate peanut (Arachis hypogaea)

Abstract: Symbiotic gene diversity and other measures of genetic diversity were examined in Bradyrhizobium isolates that form an effective symbiosis with peanut (Arachis hypogaea). Initially, restriction fra...

Bradyrhizobium (Arachis) sp. strain NC92 contains two nodD genes involved in the repression of nodA and a nolA gene required for the efficient nodulation of host plants.

Abstract: The common nodulation locus and closely linked nodulation genes of Bradyrhizobium (Arachis) sp. strain NC92 have been isolated on an 11.0-kb EcoRI restriction fragment. The nucleotide sequence of a 7.0-kb EcoRV-EcoRI subclone was determined and found to contain open reading frames (ORFs) homologous to the nodA, nodB, nodD1, nodD2, and nolA genes of Bradyrhizobium japonicum and Bradyrhizobium elkanii. Nodulation assays of nodD1, nodD2, or nolA deletion mutants on the host plants Macroptilium atropurpureum (siratro) and Vigna unguiculata (cowpea) indicate that nolA is required for efficient nodulation, as nolA mutants exhibit a 6-day nodulation delay and reduced nodule numbers. The nolA phenotype was complemented by providing the nolA ORF in trans, indicating that the phenotype is due to the lack of the nolA ORF. nodD1 mutants displayed a 2-day nodulation delay, whereas nodD2 strains were indistinguishable from the wild type. Translational nodA-lacZ, nodD1-lacZ, nodD2-lacZ, and nolA-lacZ fusions were created. Expression of the nodA-lacZ fusion was induced by the addition of peanut, cowpea, and siratro seed exudates and by the addition of the isoflavonoids genistein and daidzein. In a nodD1 or nodD2 background, basal expression of the nodA-lacZ fusion increased two- to threefold. The level of expression of the nodD2-lacZ and nolA-lacZ fusions was low in the wild type but increased in nodD1, nodD2, and nodD1 nodD2 backgrounds independently of the addition of the inducer genistein. nolA was required for increased expression of the nodD2-lacZ fusion. These data suggest that a common factor is involved in the regulation of nodD2 and nolA, and they are also consistent with a model of nod gene expression in Bradyrhizobium (Arachis) sp. strain NC92 in which negative regulation is mediated by the products of the nodD1 and nodD2 genes.

Plant growth-promoting rhizobacteria (PGPR): emergence in agriculture

Abstract: Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to agricultural improvement along with their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this review.

An introduction to agroforestry

Abstract: Preface. I: Introduction. 1. The History of Agroforestry. 2. Definition and Concepts of Agroforestry. II: Agroforestry Systems and Practices. 3. Classification of Agroforestry Systems. 4. Distribution of Agroforestry Systems in the Tropics. 5. Shifting Cultivation and Improved Fallows. 6. Taungya. 7. Homegardens. 8. Plantation Crop Combinations. 9. Alley Cropping. 10. Other Agroforestry Systems and Practices. III: Agroforestry Species. 11. General Principles of Plant Productivity. 12. Agroforestry Species: the Multipurpose Trees. 13. Component Interactions. IV: Soil Productivity and Protection. 14. Tropical Soils. 15. Effects of Trees on Soils. 16. Nutrient Cycling and Soil Organic Matter. 17. Nitrogen Fixation. 18. Soil Conservation. V: Design and Evaluation of Agroforestry Systems. 19. The Diagnosis and Design (D&D) Methodology. 20. Field Experiments in Agroforestry. 21. On-Farm Research. 22. Economic Considerations. 23. Sociocultural Considerations. 24. Evaluation of Agroforestry Systems. 25. Agroforestry in the Temperate Zone. Glossary. SI Units and Conversion Factors. List of Acronyms and Abbreviations. Subject Index.

Superoxide Radical: An Endogenous Toxicant

Abstract: The Two Faces of Oxygen Molecular oxygen is both benign and malign On the one hand it provides enormous advantages and on the other it imposes a universal toxicity This toxicity is largely due to the intermediates of oxygen reduction, ie 0;, H202, and OH·, and any organism that avails itself of the benefits of oxygen does so at the cost of maintaining an elaborate system of defenses against these intermediates We will here concern ourselves with the superoxide dismutases which, by catalytically scavenging 0;, provide a defense against it and against any reactive radical species which can be derived from it

Fatty Acids, Antibiotic Resistance, and Deoxyribonucleic Acid Homology Groups of Bradyrhizobium japonicum

Abstract: The fatty acid compositions and multiple antibiotic resistance patterns of 32 strains of Bradyrhizobium japonicum correlated with two major deoxyribonucleic acid homology groups. In group I, the fatty acid composition was 1.3% 16:1 cis9 acid, 3.6% 16:1C acid, 8.8% 16:0 acid, 1.2% 19:0 cyclopropane acid, and 81.2% 18:1 acid. Group II contained 0.5% 16:1C acid, 11.1% 16:0 acid, 0.8% 17:0 cyclopropane acid, 24.7% 19:0 cyclopropane acid, and 62.3% 18:1 acid. Group I strains were susceptible to rifampin (500 μg/ml), tetracycline (100 μg/ml), streptomycin (100 μ/ml), chloramphenicol (500 μg/ml), erythromycin (250 μg/ml), carbenicillin (500 μg/ml), and nalidixic acid (50 μg/ml), whereas group II strains were resistant to these antibiotics. Both groups were resistant to trimethoprim (50 μg/ml) and vancomycin (100 μg/ml).

Intracellular and extracellular PGPR: commonalities and distinctions in the plant–bacterium signaling processes

Abstract: Plant growth promoting bacteria (PGPR) associations range in degree of bacterial proximity to the root and intimacy of association. In general, these can be separated into extracellular PGPR (ePGPR), existing in the rhizosphere, on the rhizoplane or in the spaces between cells of the root cortex, and intracellular PGPR (iPGPR), which exist inside root cells, generally in specialized nodular structures. The latter includes rhizobia and Frankia species, both of which fix nitrogen in symbiosis with higher plants. There has been considerable development in understanding signaling mechanisms of rhizobia (iPGPR) during the establishment of the rhizobia–legume symbiosis, and this may serve as a model of knowledge regarding cross-talk and plant growth promoting mechanisms. We provide a detailed review of this process, including plant-to-bacteria signal molecules, followed by bacterial perception and consequent production of bacteria-to-plant signals. A history of PGPR discovery is also provided, indicating progress in understanding each of the PGPR groups. Recent advances in understanding plant growth responses to microbial signals are reviewed, along with the research areas that require attention. Based on new understandings of signaling mechanisms in the iPGPR (rhizobia) and recent findings with ePGPR we are able to speculate regarding general patterns of signaling in the ePGPR.