Common bean
Phaseolus vulgaris
(Linnaeus) is the key source of protein, carbohydrates and micronutrients for over 300 million people in the tropics. Like many legumes,
P. vulgaris
can fix ...atmospheric nitrogen in symbiosis with rhizobia, alleviating the need for the expensive and polluting N-fertilizers. The crop is known to nodulate with a wide range of rhizobia and, although Brazil is not a center of genetic origin/domestication of
P. vulgaris
, a variety of rhizobial species have been found as symbionts of the legume. Mato Grosso do Sul (MS) is one of the largest common bean producer states in Brazil, with reports of high yields and abundant natural nodulation. The objective of this study was to evaluate the diversity of 73 indigenous rhizobia isolated from common bean grown in 22 municipalities of MS. Great morphophysiological and genetic diversity was found, as indicated by the six and 35 clusters formed, considering the similarity level of 75 and 70%, respectively, for the phenotypic and rep-PCR dendrograms. Eleven representative isolates were selected for detailed genetic characterization using 16S rRNA and three protein-coding housekeeping genes,
glnII
,
gyrB
and
recA
. We identified species originated from the centers of origin/domestication of the legume,
R. etli and R. phaseoli,
species probably indigenous of Brazil,
R. leucaenae
and others of the
Rhizobium/Agrobacterium
clade, in addition to putative new species. The results highlight the great rhizobial diversity of the region.
Phaseolus lunatus
(Lima bean) is an important legume for the poor population of the Brazilian northeast region. The legume is able to take advantage of the nitrogen fixation process, but the ...diversity of indigenous microsymbionts is poorly known. In this study, 29 bacteria isolated from root nodules of
P. lunatus
inoculated under greenhouse conditions with soils from Piauí State, in the northeast semi-arid region of Brazil, were obtained and characterized. Classical morphological and biochemical essays revealed high phenotypic diversity, splitting the bacteria into four clusters. Genetic fingerprinting by BOX-PCR indicated outstanding diversity, with the 29 strains positioned in 19 different clusters with 30% of final similarity. Nine genera were confirmed in the analysis of the 16S rRNA gene, with two typical nodulating N
2
-fixing clades,
Bradyrhizobium
and
Agrobacterium/Rhizobium,
accounting for 38% and 21% of the isolates, respectively; the results were confirmed with the housekeeping
gyrB
gene, that also indicated putative new species.
Bradyrhizobium
was confirmed as the main symbiont, being present in nodules of all plants. All rhizobia except for those with higher resemblance to agrobacteria carry
nifH
genes. Six other genera were isolated as nodules endophytes,
Bacillus, Burkholderia, Enterobacter, Franconibacter, Pseudomonas
and
Williamsia
. Several of these endophytes exhibited one or more important biochemical properties, such as the synthesis of catalase, gelatinase and the ability to solubilize phosphate, that might confer ecological advantages to the rhizobia associated with them in the harsh environment of the Brazilian semi-arid, explaining the high rate of co-infection detected in the nodules.
Introduction
Smallholder farmers in Sub-Saharan Africa (SSA) are increasingly producing soybean for food, feed, cash, and soil fertility improvement. Yet, the difference between the smallholder ...farmers’ yield and either the attainable in research fields or the potential from crop models is wide. Reasons for the yield gap include low to nonapplication of appropriate fertilizers and inoculants, late planting, low plant populations, recycling seeds, etc.
Methods
Here, we reviewed the literature on the yield gap and the technologies for narrowing it and modelled yields through the right sowing dates and suitable high-yielding varieties in APSIM.
Results and Discussion
Results highlighted that between 2010 and 2020 in SSA, soybean production increased; however, it was through an expansion in the cropped area rather than a yield increase per hectare. Also, the actual smallholder farmers’ yield was 3.8, 2.2, and 2.3 times lower than the attainable yield in Malawi, Zambia, and Mozambique, respectively. Through inoculants, soybean yield increased by 23.8%. Coupling this with either 40 kg ha−1 of P or 60 kg ha−1 of K boosted the yields by 89.1% and 26.0%, respectively. Overall, application of 21–30 kg ha-1 of P to soybean in SSA could increase yields by about 48.2%. Furthermore, sowing at the right time increased soybean yield by 300%. Although these technologies enhance soybean yields, they are not fully embraced by smallholder farmers. Hence, refining and bundling them in a digital advisory tool will enhance the availability of the correct information to smallholder farmers at the right time and improve soybean yields per unit area.
Inoculation of soybean
Glycine max
(L.) Merr. with rhizobia strains is a low-cost investment which can increase yields of smallholder farmers in Mozambique. The performance of four
Bradyrhizobium
...strains was evaluated to identify the best strain to inoculate soybean grown in different agro-ecologies. Field experiments were conducted in three ecological zones in 2018 and 2019 using soybean variety Zamboane inoculated with
Bradyrhizobium diazoefficiens
strain USDA 110,
B. japonicum
strains USDA 136, USDA 442 and WB74, and a non-inoculated control in a randomized complete block design with four replications. Indigenous rhizobia populations at the sites ranged from 9.0 x 10
1
to 2.2 x 10
3
cells g
−1
soil. All four strains increased nodulation, but USDA 110 was superior at two sites with low native rhizobia population, whereas USDA 442 and WB74 were the best at the site with relatively high native rhizobia population. On an average, the strains doubled the number of nodules and increased the dry weight up to 5.8-fold. Inoculation increased shoot dry weight and N content at podding, plant biomass, and number of pods plant
−1
across sites but the effects of the strains on seeds per pod, and 100-seed weight were inconsistent. Shoot N content did not differ among inoculant strains and ranged from 15.70 g kg
−1
in the control to 38.53 g kg
−1
across inoculation. All four strains increased soybean grain yield across sites in 2018 but USDA 110 outperformed the other strains and was also the best at one of the two sites in 2019. Grain yield responses associated with USDA 110 ranged from 552 kg ha
−1
(56%) to 1,255 kg ha
−1
(76%). Positive correlations between nodule dry weight plant
−1
and seed yield, and number of pods plant
−1
and grain yield were observed. The gross margin ranged from $343.50–$606.80 ha
−1
for the control, but it increased to $688.34–$789.36 when inoculants were applied. On an average, inoculation increased gross margin by $182.57-$395.35 ha
−1
over that of non-inoculated control in 2018 but drought stress in 2019 reduced the benefit. The results demonstrate that USDA 110 was the best inoculant strain and has the potential of increasing smallholder productivity and net returns.
•Biological nitrogen fixation (BNF) is a key process for soybean production in Africa.•The selection of elite African indigenous soybean Bradyrhizobium strains is a feasible strategy.•Eighty-seven ...isolates were obtained from soybean nodules in Mozambique.•Isolates fit into the Bradyrhizobium (75%) and Agrobacterium-Rhizobium (25%) clades.•Five Bradyrhizobium isolates with outstanding symbiotic performance were obtained.
Soybean inoculation with effective rhizobial strains makes unnecessary the use of N-fertilizers in the tropics. A frequently reported problem is the failure of the inoculant strains to overcome the competition imposed by indigenous rhizobial populations. The screening of indigenous rhizobia, already adapted to local conditions, searching for highly effective strains for use as inoculants represents a promising strategy in overcoming inoculation failure. The objective of this study was to isolate and characterize indigenous rhizobia and to identify strains that hold potential to be included in inoculant formulations for soybean production, with both promiscuous and non-promiscuous soybean cultivars, in Mozambican agro-climatic conditions. A total of 105 isolates obtained from nodules of promiscuous soybean grown at 15 sites were screened for N2-fixation effectiveness in the greenhouse along with five commercial strains. Eighty-seven isolates confirmed the ability to form effective nodules on soybean and were used for genetic characterization by rep-PCR (BOX) and sequencing of the 16S rRNA gene, and also for symbiotic effectiveness. BOX-PCR fingerprinting revealed remarkable genetic diversity, with 41 clusters formed, considering a similarity level of 65%. The 16S rRNA analysis assigned the isolates to the genera Bradyrhizobium (75%) and Agrobacterium/Rhizobium (25%). Great variability in symbiotic effectiveness was detected among the indigenous rhizobia from Mozambique, with ten isolates performing better than the commercial strain B. diazoefficiens USDA 110, the best reference strain, and 51 isolates with lower performance than all reference strains. Thirteen of the best isolates from the first greenhouse trial were evaluated, along with the five commercial strains, in two promiscuous (TGx 1963-3F and TGx 1835-10E) and one non-promiscuous (BRS 284) soybean cultivars in a second greenhouse trial. In general the promiscous soybeans responded better to inoculation. The 13 isolates were also characterized for tolerance to acidity and alkalinity (pH 3.5 and 9.0, respectively), salinity (0.1, 0.3 and 0.5molL−1 of NaCl) and high temperatures (35, 40 and 45°C) in vitro. Five isolates, three (Moz 4, Moz 19 and Moz 22) belonging to the superclade B. elkanii and two (Moz 27 and Moz 61) assigned to the superclade B. japonicum, consistently showed high symbiotic effectiveness, suggesting that the inoculation with indigenous rhizobia adapted to local conditions represents a possible strategy for increasing soybean yields in Mozambique. Phylogenetic position of the five elite isolates was confirmed by the MLSA with four protein-coding housekeeping genes, dnaK, glnII, gyrB and recA.
•Soybean N demand can be fulfilled by biological nitrogen fixation (BNF).•Bradyrhizobium strains from Brazil and USA were tested in Brazil and Mozambique.•Inoculation resulted in grain yield gains of ...4–5% in Brazil and 20–29% in Mozambique.•Transference of BNF technologies is feasible, speeding up the production system.•Exotic soybean Bradyrhizobium strains can highly benefit soybean in Mozambique.
The soybean-Bradyrhizobium symbiosis can be very effective in fixing nitrogen and supply nearly all plant's demand on this nutrient, obviating the need for N-fertilizers. Brazil has been investing in research and use of inoculants for soybean for decades and with the expansion of the crop in African countries, the feasibility of transference of biological nitrogen fixation (BNF) technologies between the continents should be investigated. We evaluated the performance of five strains (four Brazilian and one North American) in the 2013/2014 and 2014/2015 crop seasons in Brazil (four sites) and Mozambique (five sites). The experimental areas were located in relatively similar agro-climatic regions and had soybean nodulating rhizobial population ranging from ≪10 to 2×105 cells g−1 soil. The treatments were: (1) NI, non-inoculated control with no N-fertilizer; (2) NI+N, non-inoculated control with 200kg of N ha−1; and inoculated with (3) Bradyrhizobium japonicum SEMIA 5079; (4) B. diazoefficiens SEMIA 5080; (5) B. elkanii SEMIA 587; (6) B. elkanii SEMIA 5019; (7) B. diazoefficiens USDA 110; (8) SEMIA 5079+5080 (only tested in Brazil). The best inoculation treatments across locations and crop seasons in Brazil were SEMIA 5079+5080, SEMIA 5079 and USDA 110, with average grain yield gains of 4–5% in relation to the non-inoculated treatment. SEMIA 5079, SEMIA 5080, SEMIA 5019 and USDA 110 were the best strains in Mozambique, with average 20–29% grain yield gains over the non-inoculated treatment. Moreover, the four best performing strains in Mozambique resulted in similar or better yields than the non-inoculated+N treatment, confirming the BNF as an alternative to N-fertilizers. The results also confirm the feasibility to transfer soybean inoculation technologies between countries, speeding up the establishment of sustainable cropping systems.
The effects of sole inoculation of soybean (
Glycine max
L. Merrill) with
Bradyrhizobium
and co-inoculation with
Bradyrhizobium
and
Azospirillum
on nodulation, plant growth and yields were ...investigated in the 2013/2014 and 2014/2015 cropping seasons under field conditions in Mozambique. The treatments included (1) Control (non-inoculated control, with symbiosis depending on indigenous rhizobia), (2) Urea (non-inoculated, receiving 200 kg ha
−1
of N), (3) Sole inoculation with
B. diazoefficiens
strain USDA 110, and (4) Co-inoculation with
B. diazoefficiens
strain USDA 110 and
A. brasilense
strains Ab-V5 and Ab-V6, evaluated in a randomized complete block design with five replications. Nodule number and dry weight, shoot dry weight, biological and grain yields, grain dry weight, and harvest index were evaluated. In general, both sole inoculation and co-inoculation enhanced nodulation in relation to control. Sole inoculation increased grain yield by 22% (356 kg ha
−1
), the same enhancement magnitude attained under mineral N treatment, suggesting that
Bradyrhizobium
inoculation provides ecological and economic sustainability to the soybean crop in Mozambique or other countries with similar agro-climatic conditions. Co-inoculation did not increase grain yields in relation to neither the control nor sole inoculation, indicating that further research with adapted and high yielding soybean varieties along with effective rhizobial strains is required in Mozambique to attune the beneficial
Azospirillum
–plant cultivar–rhizobia interactions that have been reported in other countries for several legumes, including soybean.
strain CNPSo 3391 was isolated from a soybean nodule in Mozambique. Its genome size was estimated at 4,926,588 bp. This isolate carries several coding sequences for stress tolerance, but no ...identifiable nodulation or virulence genes. Possible ecological roles of bacteria isolated from legume nodules and closely related to
are discussed.
Soybean inoculation with elite strains of Bradyrhizobium to improve nodulation, N2 fixation, and grain yield is well established worldwide. However, when grown in soils where N is deficient, soybean ...undergoes an initial phase of N starvation that may last up to 20 days after seedling germination due to the lack of synchronism between the phase when seed N reserves are exhausted and the moment when plants begin to benefit from the nitrogen fixed by the bacteria. Practices that promote early nodulation may play a key role in reducing the N starvation period. Azospirillum is a plant growth promoting rhizobacteria (PGPR) that can stimulate root hair formation and root growth, creating more sites for early root infection and nodule formation by N sub( 2)-fixing Bradyrhizobium spp. In this study, the effects of co-inoculating soybeans with Bradyrhizobium spp. and Azospirillum brasilense on nodulation precocity and N sub( 2) fixation were evaluated under greenhouse and field conditions. Nodule number and dry weight, as well as plant and root dry weight and N accumulated in shoots at 15, 18, 21, 24 and 30 days after emergence (DAE) were evaluated in response to inoculation with Bradyrhizobium spp. alone or when co-inoculated with Azospirillum sp. In the greenhouse, co-inoculated plants nodulated precociously as indicated by a significant increase (p < 0.05) in nodule biomass observed at (include) 21 DAE. More pronounced effects of co-inoculation were observed in the field as early as 18 DAE, suggesting that the presence of Azospirillum helps plants to overcome environmental stresses.
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