Assessing host range, symbiotic effectiveness, and photosynthetic rates induced by native soybean rhizobia isolated from Mozambican and south African soils

Article purchasedHost range and cross-infectivity studies are important for identifying rhizobial strains with potential for use as inoculants. In this study, 10 native soybean rhizobia isolated from Mozambican and South African soils were evaluated for host range, symbiotic effectiveness and ability to induce high rates of photosynthesis leading to enhanced plant growth in cowpea (Vigna unguiculata L. Walp.), Bambara groundnut (Vigna subterranean L. Verdc.), Kersting’s groundnut (Macrotyloma geocarpum Harm) and soybean (Glycine max L. Merr). The test isolates had different growth rates and colony sizes. Molecular analysis based on enterobacterial repetitive intergenic consensus (ERIC)-PCR revealed high genetic diversity among the test isolates. The results further showed that isolate TUTLBC2B failed to elicit nodulation in all test plants, just as TUTNSN2A and TUTDAIAP3B were also unable to nodulate cowpea, Kersting’s bean and Bambara groundnut. Although the remaining strains formed ineffective nodules on cowpea and Kersting’s bean, they induced effective nodules on Bambara groundnut and the two soybean genotypes. Bacterial stimulation of nodule numbers, nodule dry weights and photosynthetic rates was generally greater with isolates TUTRSRH3A, TUTM19373A, TUTMCJ7B, TUTRLR3B and TUTRJN5A. As a result, these isolates elicited significantly increased accumulation of biomass in shoots and whole plants of Bambara groundnut and the two soybean genotypes. Whole-plant symbiotic nitrogen (N) of soybean and Bambara groundnut was highest for the commercial strains CB756 and WB74, as well as for TUTRLR3B, TUTMCJ7B and TUTRSRH3A, suggesting that the three native rhizobial isolates have potential for use as inoculants

Response of promiscuous-nodulating soybean (Glycine max L. Merr.) genotypes to Bradyrhizobium inoculation at three field sites in Mozambique

Published Online: 27 January, 2016Soybean cultivation in Mozambique is dominated by smallholder farmers who use little or no inputs such as bacterial inoculants, often resulting in low yields. This study assessed the ability of TGx and non-TGx soybean genotypes to nodulate with native rhizobia in Mozambican soils, and evaluated the yield and symbiotic response of TGx and non-TGx soybean to inoculation with Bradyrhizobium japonicum strain WB74 at three IITA experimental sites in Mozambique. The data revealed significant inoculation, location and genotypic effects. Both TGx and non-TGx soybean genotypes showed effective nodulation in Bradyrhizobium-inoculated and uninoculated field plots. Inoculant applicaion increased plant growth by 32 %, %N by 18 %, N content by 45 %, %Ndfa by 11 %, fixed-N by 64 %, and grain yield by12% when compared to uninoculated control. Symbiotically, the soybean genotypes performed better at Ruace than at Nampula, and poorly at Mutequelesse. Genotypes TGx1910-14F, 427/5/7, TGx1937-1F and Solitaire accumulated the most biomass at Ruace and Nampula when compared to Mutequelesse, and individually contributed over 200 kg N.ha−1 during the cropping season. However, grain yield was much higher at Nampula than at Ruace and Mutequelesse. Independent of the location, four soybean genotypes produced similar amounts of grain yield in inoculated and uninoculated plots. Whether with or without inoculation, Solitaire and TGx1908-8F each produced about 2.0 t.ha−1 of grain, while TGx1910-14F and TGx1937-1F yielded between 1.5 and 1.7 t.ha−1. These four varieties were identified as the best for use by resource-poor farmers in Mozambique, while genotype 427/5/7, which produced about 2.4 t.ha−1 of grain with inoculation, was seen as ideal for well-resourced farmers

Identification and distribution of microsymbionts associated with soybean nodulation in Mozambican soils

Article purchasedIndigenous soybean rhizobial strains were isolated from root nodules sampled from farmers’ fields in Mozambique to determine their identity, distribution and symbiotic relationships. Plant infection assays revealed variable nodulation and symbiotic effectiveness among the 43 bacterial isolates tested. Strains from Ruace generally promoted greater whole-plant growth than the others. 16S rRNA-RFLP analysis of genomic DNA extracted from the rhizobial isolates produced different banding patterns, a clear indication of high bacterial diversity. However, the multilocus sequence analysis (MLSA) data showed alignment of the isolates with B. elkanii species. The 16S rRNA sequences of representative soybean isolates selected from each 16S rRNA-RFLP cluster showed their relatedness to B. elkanii, as well as to other Bradyrhizobium species. But a concatenated phylogeny of two housekeeping genes (glnII and gyrB) identified the soybean nodulating isolates as Bradyrhizobium, with very close relatedness to B. elkanii. The nifH and nodC sequences also showed that the majority of the test soybean isolates were closely related to B. elkanii, albeit the inconsistency with some isolates. Taken together, these findings suggest that the B. elkanii group are the preferred dominant microsymbiont of soybean grown in Mozambican soils. Furthermore, the distribution of soybean rhizobia in the agricultural soils of Mozambique was found to be markedly influenced by soil pH, followed by the concentrations of plant-available P and Mn. This study suggested that the identified isolates TUTMJM5, TUTMIITA5A and TUTLBC2B can be used as inoculants for increased soybean production in Mozambique

Table_1_Effect of rhizobia inoculation and seaweed extract (Ecklonia maxima) application on the growth, symbiotic performance and nutritional content of cowpea (Vigna unguiculata (L.) Walp.).docx

Research efforts to develop alternatives to chemical-based fertilizers for sustainable crop production has led to renewed interest in beneficial soil microbes such as rhizobia and plant growth promoting biostimulants such as the seaweed (Ecklonia maxima). This study assessed the interactive effect of the co-application of seaweed extract with two Bradyrhizobium strains (Inoculant 1 and Inoculant 2) on the growth, symbiotic performance and nutritional composition of three cowpea (Vigna unguiculata (L.) Walp.) genotypes (IT97K-390-2, Songotra and TVU13998) grown under glasshouse conditions. The response of cowpea to the treatments was genotype dependent, such that the combined application of inoculant 2 plus seaweed extract increased nodule dry matter in genotype Songotra, and together with sole inoculant 2 increased the parameter in genotype IT97K-390-2, just as the inoculation plus seaweed extract treatments increased the parameter in genotype TVU13998 when compared to their respective counterparts receiving other treatments. Sole inoculation or inoculation plus seaweed extract treatments increased shoot dry matter in all varieties (2.0 to 7.2 g.plant-1) relative to the control plants receiving sole nitrate (0.5 to 1.2 g.plant-1), sole seaweed extract (0.3 g.plant-1), nitrate plus seaweed extract (1.2 to 1.6 g.plant-1) or the absolute control (0.2 g.plant-1). Due to N2 fixation in the inoculated plants, their leaf δ15N (-2.66‰ to -1.20‰) were markedly lower (p≤0.001) than values recorded by the control plants (+3.30‰ to +510‰) which had no nodules; consequently, leaf N accumulation was greater in the inoculation-based treatments (41.2 to 258.2 mg.plant-1) relative to the uninoculated controls (1.7 to 24.7 mg.plant-1). In most instances, the sole inoculation and inoculation plus seaweed extract treatments increased leaf photosynthetic rates (except for genotype TVU13998 treated with inoculant 1 + seaweed extract), water use efficiency (δ13C) (except in genotype TVU13998) and the concentrations of macro and micronutrients in leaves (except for K in Songotra treated with inoculant 1 or inoculant 1 + seaweed extract as well as Mn in TVU13998 treated with inoculant 1 among others) of the cowpeas relative to the controls. We highlight the potential benefits of the synergistic interactions between rhizobia and seaweed extract for enhancing plant growth and nutrient accumulation in cowpea leaves.</p