Enhancing the Efficiency of Soybean Inoculant for Nodulation under Multi-Environmental Stress Conditions

Characterization of rhizobia for beneficial traits that promote nodulation in legumes under abiotically stressed conditions

The growing interest in using rhizobia as inoculants in sustainable agricultural systems has prompted the screening of rhizobia species for beneficial traits that enhance nodulation and nitrogen fixation under abiotic stressed conditions. This study reports phenotypic and phylogenetic characterization of rhizobia strains previously isolated from the root nodules of several indigenous and exotic legumes growing in South Africa and other countries. The Rhizobia strains were screened for their ability to tolerate various abiotic stresses (temperature 16, 28, and 36 °C; acidity/alkalinity pH 5, 7, and 9; heavy metals 50, 100, and 150 mM AlCl3.6H2O; and salinity 50, 100, and 150 mM NaCl). Phylogenetic characterization of the isolates was determined using multilocus sequence analysis of the 16S rRNA, recA, acdS, exoR, nodA, and nodC genes. The analysis indicated that the isolates are phylogenetically related to Sinorhizobium, Bradyrhizobium, Rhizobium, Mesorhizobium, and Aminobacter genera and exhibited significant variations in their tolerance to abiotic stresses. Amid the increasing threats of the global stresses, these current results provide baseline information in the selection of rhizobia for use as inoculants under extreme temperatures, acidity/alkalinity, and salinity stress conditions in South Africa.

Bioprospecting and Challenges of Plant Microbiome Research for Sustainable Agriculture, a Review on Soybean Endophytic Bacteria

This review evaluates oilseed crop soybean endophytic bacteria, their prospects, and challenges for sustainable agriculture. Soybean is one of the most important oilseed crops with about 20-25% protein content and 20% edible oil production. The ability of soybean root-associated microbes to restore soil nutrients enhances crop yield. Naturally, the soybean root endosphere harbors root nodule bacteria, and endophytic bacteria, which help increase the nitrogen pool and reclamation of another nutrient loss in the soil for plant nutrition. Endophytic bacteria can sustain plant growth and health by exhibiting antibiosis against phytopathogens, production of enzymes, phytohormone biosynthesis, organic acids, and secondary metabolite secretions. Considerable effort in the agricultural industry is focused on multifunctional concepts and bioprospecting on the use of bioinput from endophytic microbes to ensure a stable ecosystem. Bioprospecting in the case of this review is a systemic overview of the biorational approach to harness beneficial plant-associated microbes to ensure food security in the future. Progress in this endeavor is limited by available techniques. The use of molecular techniques in unraveling the functions of soybean endophytic bacteria can explore their use in integrated organic farming. Our review brings to light the endophytic microbial dynamics of soybeans and current status of plant microbiome research for sustainable agriculture.

Genetic and phenotypic diversity of microsymbionts nodulating promiscuous soybeans from different agro-climatic conditions

Background

Global food supply is highly dependent on field crop production that is currently severely threatened by changing climate, poor soil quality, abiotic, and biotic stresses. For instance, one of the major challenges to sustainable crop production in most developing countries is limited nitrogen in the soil. Symbiotic nitrogen fixation of legumes such as soybean (Glycine max (L.) Merril) with rhizobia plays a crucial role in supplying nitrogen sufficient to maintain good crop productivity. Characterization of indigenous bradyrhizobia is a prerequisite in the selection and development of effective bioinoculants. In view of this, bradyrhizobia were isolated from soybean nodules in four agro-climatic zones of eastern Kenya (Embu Upper Midland Zone, Embu Lower Midland Zone, Tharaka Upper Midland Zone, and Tharaka Lower Midland Zone) using two soybean varieties (SB8 and SB126). The isolates were characterized using biochemical, morphological, and genotypic approaches. DNA fingerprinting was carried out using 16S rRNA gene and restricted by enzymes HaeIII, Msp1, and EcoRI. 

Results

Thirty-eight (38) bradyrhizobia isolates obtained from the trapping experiments were placed into nine groups based on their morphological and biochemical characteristics. Most (77%) of the isolates had characteristics of fast-grower bradyrhizobia while 23% were slow-growers. Restriction digest revealed significant (p < 0.015) variation within populations and not among the agro-climatic zones based on analysis of molecular variance. Principal coordinate analysis demonstrated sympatric speciation of indigenous bradyrhizobia isolates. Embu Upper Midland Zone bradyrhizobia isolates had the highest polymorphic loci (80%) and highest genetic diversity estimates (H’ = 0.419) compared to other agro-climatic zones.

Conclusion

The high diversity of bradyrhizobia isolates depicts a valuable genetic resource for selecting more effective and competitive strains to improve promiscuous soybean production at a low cost through biological nitrogen fixation.