Potential use of rhizobial bacteria as promoters of plant growth for increased yield in landraces of African cereal crops

Rhizobia form root nodules that fix nitrogen (N2) in symbiotic legumes. Extending the ability of these bacteria to fix N2 in non-legumes such as cereals would be a useful technology for increased crop yields among resource-poor farmers. Although some inoculation attempts have resulted in nodule formation in cereal plants, there was no evidence of N2 fixation. However, because rhizobia naturally produce molecules (auxins, cytokinins, abscicic acids, lumichrome, rhiboflavin, lipo-chito-oligosaccharides and vitamins) that promote plant growth, their colonization and infection of cereal roots would be expected to increase plant development, and grain yield. We have used light, scanning, and transmission electron microscopy to show that roots of sorghum and millet landraces from Africa were easily infected by rhizobial isolates from five unrelated legume genera. With sorghum, in particular, plant growth and phosphorus (P) uptake were significantly increased by rhizobial inoculation, suggesting that field selection of suitable rhizobia/cereal combinations could increase yields and produce fodder for livestock production

Assessing the suitability of antibiotic resistance markers and the indirect ELISA technique for studying the competitive ability of selected Cyclopia Vent. rhizobia under glasshouse and field conditions in South Africa

<p>Abstract</p> <p>Background</p> <p>Symbiotic N2 fixation in legumes is constrained by many factors, including the paucity of suitable soil rhizobia To maximise growth of legume species therefore often requires the application of effective rhizobia as inoculants. But where native strains out-compete introduced rhizobia for nodule formation, it is important that the competitiveness of selected strains is tested in the field and glasshouse prior to their recommendation as commercial inoculants. However the methodology for strain identification inside nodules has often proved difficult and thus limited this field of research. In this study, the suitability of the antibiotic resistance technique (both intrinsic low-resistance fingerprinting and high-resistance marking) and the serological indirect ELISA method were assessed for their ability to detect selected Cyclopia rhizobia under glasshouse and field conditions. The four rhizobial strains that were used, namely PPRICI3, UCT40a, UCT44b and UCT61a, were isolated from wild Cyclopia species growing in the Western Cape fynbos of South Africa.</p> <p>Results</p> <p>The test strains formed two distinct groups with regard to their intrinsic resistance to the antibiotics streptomycin sulphate and spectinomycin dihydrochloride pentahydrate, making it impossible to use intrinsic antibiotic resistance to distinguish strains from within the same intrinsic resistance group. The use of strains marked with double antibiotic resistance was also investigated. A number of these strains lost their antibiotic marker tags after one plant passage; and some also lost their competitive ability. The indirect ELISA technique provided a more satisfactory method of identifying selected Cyclopia strains under both field and glasshouse conditions. The primary antibodies raised against strains UCT40a, UCT61a and UCT44b gave absorbance readings that were unambiguously negative (0.30 OD405), while those of strain PPRICI3 were ambiguous (0.50 OD405) with many false positive readings (1.0 A405). The indirect ELISA method showed a high level of analytical sensitivity in glasshouse experiments and there were no cross-reactions between the four test strains. The method was also suitable for detecting three of the four test strains in competition studies under field conditions, and can also be used to identify some strains under field conditions.</p> <p>Conclusion</p> <p>The antibiotic marker method was found unsuitable for identifying Cyclopia rhizobia in competition experiments in both glasshouse and field conditions. However, the indirect ELISA technique was found suitable for identifying these strains in glasshouse studies. The method was also appropriate for identifying strains UCT40a, UCT44b and UCT61a, but not strain PPRICI3, in field competition studies.</p

Potential use of rhizobial bacteria as promoters of plant growth for increased yield in landraces of African cereal crops

Rhizobia form root nodules that fix nitrogen (N2) in symbiotic legumes. Extending the ability of these bacteria to fix N2 in non-legumes such as cereals would be a useful technology for increased crop yields among resource-poor farmers. Although some inoculation attempts have resulted in nodule formation in cereal plants, there was no evidence of N2 fixation. However, because rhizobia naturally produce molecules (auxins, cytokinins, abscicic acids, lumichrome, rhiboflavin, lipo-chito-oligosaccharides and vitamins) that promote plant growth, their colonization and infection of cereal roots would be expected to increase plant development, and grain yield. We have used light, scanning, and transmission electron microscopy to show that roots of sorghum and millet landraces from Africa were easily infected by rhizobial isolates from five unrelated legume genera. With sorghum, in particular, plant growth and phosphorus (P) uptake were significantly increased by rhizobial inoculation, suggesting that field selection of suitable rhizobia/cereal combinations could increase yields and produce fodder for livestock production. Key Words: Rhizobia, N2 fixation, plant growth, sorghum, millet African Journal of Biotechnology Vol.3(1) 2004: 1-

Symbiotic functioning and bradyrhizobial biodiversity of cowpea (Vigna unguiculata L. Walp.) in Africa

<p>Abstract</p> <p>Background</p> <p>Cowpea is the most important food grain legume in Sub-Saharan Africa. However, no study has so far assessed rhizobial biodiversity and/or nodule functioning in relation to strain IGS types at the continent level. In this study, 9 cowpea genotypes were planted in field experiments in Botswana, South Africa and Ghana with the aim of i) trapping indigenous cowpea root-nodule bacteria (cowpea "rhizobia") in the 3 countries for isolation, molecular characterisation using PCR-RFLP analysis, and sequencing of the 16S - 23S rDNA IGS gene, ii) quantifying N-fixed in the cowpea genotypes using the ¹⁵N natural abundance technique, and iii) relating the levels of nodule functioning (i.e. N-fixed) to the IGS types found inside nodules.</p> <p>Results</p> <p>Field measurements of N₂fixation revealed significant differences in plant growth, δ¹⁵N values, %Ndfa and amounts of N-fixed between and among the 9 cowpea genotypes in Ghana and South Africa. Following DNA analysis of 270 nodules from the 9 genotypes, 18 strain IGS types were found. Relating nodule function to the 18 IGS types revealed significant differences in IGS type N₂-fixing efficiencies. Sequencing the 16S - 23S rDNA gene also revealed 4 clusters, with cluster 2 forming a distinct group that may be a new <it>Bradyrhizobium </it>species. Taken together, our data indicated greater biodiversity of cowpea bradyrhizobia in South Africa relative to Botswana and Ghana.</p> <p>Conclusions</p> <p>We have shown that cowpea is strongly dependant on N₂fixation for its N nutrition in both South Africa and Ghana. Strain IGS type symbiotic efficiency was assessed for the first time in this study, and a positive correlation was discernible where there was sole nodule occupancy. The differences in IGS type diversity and symbiotic efficiency probably accounts for the genotype × environment interaction that makes it difficult to select superior genotypes for use across Africa. The root-nodule bacteria nodulating cowpea in this study all belonged to the genus <it>Bradyrhizobium</it>. Some strains from Southern Africa were phylogenetically very distinct, suggesting a new <it>Bradyrhizobium </it>species.</p

Symbiotic N2 Fixation and Grain Yield of Endangered Kersting's Groundnut Landraces in Response to Soil and Plant Associated Bradyrhizobium Inoculation to Promote Ecological Resource-Use Efficiency

Kersting's groundnut (Macrotyloma geocarpum Harms) is a neglected, endangered food and medicinal legume in Africa. Efforts to harness the benefits of the legume-rhizobia symbiosis have focused on few major legumes to the neglect of underutilized ones such as Kersting's groundnut. This study assessed plant growth, N-fixed and grain yield of five Kersting's groundnut landraces in response to inoculation with Bradyrhizobium strain CB756 at two locations in the Northern Region of Ghana. The transferability of cowpea-derived Simple Sequence Repeat (SSR) markers to Kersting's groundnut was also assessed. The symbiotic results revealed significant variation in nodulation, shoot biomass, δ15N, percent N derived from fixation, amount of N-fixed and soil N uptake. The cross-taxa SSR primers revealed monomorphic bands with sizes within the expected range in all the Kersting's groundnut landraces. The results of the aligned nucleotide sequences revealed marked genetic variability among the landraces. Kersting's groundnut was found to be a low N2-fixer, with 28–45% of its N derived from fixation at Nyankpala and 15–29% at Savelugu. Nitrogen contribution was 28–50 kg N-fixed·ha−1 at Nyankpala, and 12–32 kg N-fixed·ha−1 at Savelugu. Uninoculated plants of the Kersting's groundnut landraces Puffeun, Dowie, Sigiri and Boli, respectively, contributed 22, 16, 13, and 15 kg N-fixed·ha−1 from symbiosis at Savelugu as opposed to 89, 82, 69, and 89 kg N·ha−1 from soil. Landrace Puffeun was highly compatible with the introduced strain CB756 if based on δ15N and %Ndfa values, while Dowie, Funsi and Boli showed greater compatibility with native rhizobia in Ghanaian soils. The unimproved Kersting's groundnut in association with soil microsymbionts could produce grain yield of 1,137–1,556 kg ha−1 at Nyankpala, and 921–1,192 kg ha−1 at Savelugu. These findings suggest the need for further work to improve the efficiency of the Kersting's groundnut-rhizobia symbiosis for increased grain yield and resource-use efficiency in cropping systems

Plant-associated symbiotic Burkholderia species lack hallmark strategies required in mammalian pathogenesis

Burkholderia is a diverse and dynamic genus, containing pathogenic species as well as species that form complex interactions with plants. Pathogenic strains, such as B. pseudomallei and B. mallei, can cause serious disease in mammals, while other Burkholderia strains are opportunistic pathogens, infecting humans or animals with a compromised immune system. Although some of the opportunistic Burkholderia pathogens are known to promote plant growth and even fix nitrogen, the risk of infection to infants, the elderly, and people who are immunocompromised has not only resulted in a restriction on their use, but has also limited the application of non-pathogenic, symbiotic species, several of which nodulate legume roots or have positive effects on plant growth. However, recent phylogenetic analyses have demonstrated that Burkholderia species separate into distinct lineages, suggesting the possibility for safe use of certain symbiotic species in agricultural contexts. A number of environmental strains that promote plant growth or degrade xenobiotics are also included in the symbiotic lineage. Many of these species have the potential to enhance agriculture in areas where fertilizers are not readily available and may serve in the future as inocula for crops growing in soils impacted by climate change. Here we address the pathogenic potential of several of the symbiotic Burkholderia strains using bioinformatics and functional tests. A series of infection experiments using Caenorhabditis elegans and HeLa cells, as well as genomic characterization of pathogenic loci, show that the risk of opportunistic infection by symbiotic strains such as B. tuberum is extremely low

A Functional Relationship Between Leghaemoglobin and Nitrogenase Based on Novel Measurements of the Two Proteins in Legume Root Nodules

A combination of physiological and structural measurements made on nodulated cowpea and soybean plants cultured with roots in different pO2 permitted the expression of data in various ways. Values of leghemoglobin concentration and nitrogenase activity from the two legumes were expressed conventionally either on a per plant or per gram nodule fresh weight basis, and where microscopy was done, on the basis of nitrogenase-containing, N2-fixing units (i.e. per bacteroid, per infected cell, or per gram infected tissue). In both legumes, acetylene reduction, N fixed and ureide content expressed on the basis of whole plants or per nitrogenase-containing units were very significantly correlated with values of leghaemoglobin concentrations expressed in a similar manner. The use of mathematical correlations in this study involving leghaemoglobin concentrations and various indices of N2 fixation indicated a strong functional relationship between the two proteins in symbiotic legumes. These findings confirm previous suggestions that leghaemoglobin and the nitrogenase complex are two proteins closely associated with N2-fixing efficiency in legume root nodules

Symbiotic Functioning and Photosynthetic Rates Induced by Rhizobia Associated with Jack Bean (<i>Canavalia ensiformis</i> L.) Nodulation in Eswatini

Improving the efficiency of the legume–rhizobia symbiosis in African soils for increased grain yield would require the use of highly effective strains capable of nodulating a wide range of legume plants. This study assessed the photosynthetic functioning, N2 fixation, relative symbiotic effectiveness (%RSE) and C assimilation of 22 jack bean (Canavalia ensiformis L.) microsymbionts in Eswatini soils as a first step to identifying superior isolates for inoculant production. The results showed variable nodule number, nodule dry matter, shoot biomass and photosynthetic rates among the strains tested under glasshouse conditions. Both symbiotic parameters and C accumulation differed among the test isolates at the shoot, root and whole-plant levels. Although 7 of the 22 jack bean isolates showed much greater relative symbiotic efficiency than the commercial Bradyrhizobium strain XS21, only one isolate (TUTCEeS2) was statistically superior to the inoculant strain, which indicates its potential for use in inoculant formulation after field testing. Furthermore, the isolates that recorded high %RSE elicited greater amounts of fixed N