Pigeonpea is an important grain legume. It contributes to the improvement of soil fertility through biological nitrogen (N) fixation. However, the symbiotic efficiency of pigeonpea with native soil rhizobia has not been determined adequately. This study was designed to determine the variation in the N fixation ability of pigeonpea inoculated with the native rhizobia. Forty soil samples were collected from diverse locations across South Africa and used for inoculating pigeonpea seed. Each pigeonpea genotype was inoculated separately with each soil sample and raised in a nitrogen-depleted growth medium in the greenhouse. A split-plot experimental design was used in the study. Several N fixation variables of pigeonpea were measured. There was >40.0% difference in the number of nodules between genotypes ‘Ex-PP-MD-321’ and ‘Mpuma-B-Spot’ but the nodule dry weight between the two genotypes was >80.0%. In contrast, the heaviest dry shoots (0.4513 g), weighed 52.0% heavier than those that were observed for ‘Mpuma-B-Spot’. Pigeonpea showed differential N fixation ability with the nodules, suggesting that there was potential to select for optimum host × rhizobial isolate combinations for the process and to expand the production area of the crop.

In the Northern mountainous region of Vietnam, cassava–cowpea intercropping system has been widely promoted with support from the local agricultural department. However, cowpea yield is often limited because of a low Biological Nitrogen Fixation (BNF) activity due to its low natural nodulation and lack of available effective Rhizobium products. The aim of this study was to identify the most effective native rhizobia isolate nodulating cowpea with the potential to increase BNF and yield of cowpea. A greenhouse experiment was initially conducted with five treatments: three native rhizobia isolates (CMBP037, CMBP054, and CMBP065); a control (no inoculation and no N application); and N+ (no inoculation, application of N as KNO3). Field inoculations were carried out and the treatments were as follows: a control (no inoculation); CMBP (037+054) – a mixture of strains from Mau Dong; CMBP065 strain from Cat Thinh. CMBP054 and CMBP065 had the highest nodulation in the greenhouse (46.4 and 60.7 nodules plant−1, respectively) and were rated as effective with symbiotic efficiency (SEF) of 54.56 and 55.73%, respectively. In the field, CMBP (037+054) recorded significantly higher nodulation (19.4 nodules plant−1) than the control (11.7 nodules plant−1). CMBP (037+054) also increased cowpea shoot dry weight, shoot N, and yield by 28.6, 4.9, and 10.5%, respectively, compared to the uninoculated control. This effect was slope dependent (statistically significant in moderate and steep slope, not with gentle slope). Besides, the high expansion rate of intercropping with cowpea showed the high adoption level of these agroecological practices by local farmers. This study reveals the potential of native rhizobia inoculation to enhance soil fertility and sustainable agriculture in the Northern mountainous region of Vietnam and proposes enhanced efforts to promote the availability and utilization of effective inoculants for cowpea.

This is the report of the first open field release of genetically modified microorganisms (GMMs) in Italy. It covers ten years of monitoring, and follows in-field GMM dynamics from strain release to disappearance below detection limits, as well as assessment of impact on resident microorganisms. The bacteria released belong to the nitrogen fixing legume endosymbiont Rhizobium leguminosarum bv. viciae, and were engineered with non-agronomically-proficient traits, in order to assess their behavior and fate without GMM-specific positive feedback from the plant. A DNA cassette containing mercury resistance and ß-galactosidase genes was introduced in either plasmid-borne or chromosomally integrated versions, in order to test the resulting strain stability. A synthetic promoter was used to drive the lacZ gene, conferring high catabolic activity to the GMM. Two different wild-type Rhizobium backgrounds were tested, comparing a non-indigenous vs. an indigenous, highly competitive strain. The latter had much greater persistence, since it was able to survive and establish at technically detectable levels for over four years after release. Selection factors, such as reiterated presence of the plant host, or lactose substrate supply, enhanced long-term survival to different extents. The lactose treatment showed that even a single trophic supplementation can surpass the benefits of symbiotic interaction for a period of several years. Concerning impact, the GMMs did not alter substantially the other soil community general microbiota. However, there were some significant differences in microbiota as a consequence of the Rhizobium inoculation. This effect was observed with either the WT or GMM, and was more evident in the release of the indigenous Rhizobium. Moreover, as the indigenous GMM had its parental, dominant wild-type in the same soil, it was possible to evaluate to what extent the GMM version could result in parent displacement (“self-impact”), and how much the two rhizobia would additively contribute to nodulation.