Isolation of potential plant growth-promoting bacteria from nodules of legumes grown in arid Botswana soil

Abstract

As the world population increases, improvements in crop growth and yield will be needed to meet rising food demands, especially in countries that have not developed agricultural practices optimized for their own soils and crops. In many African countries, farmers improve agricultural productivity by applying synthetic fertilizers and pesticides to crops, but their continued use over the years has had serious environmental consequences including air and water pollution as well as loss of soil fertility. To reduce the overuse of synthetic amendments, we are developing inocula for crops that are based on indigenous soil microbes, especially those that enhance plant growth and improve agricultural productivity in a sustainable manner. We first isolated environmental DNA from soil samples collected from an agricultural region to study the composition of the soil microbiomes and then used Vigna unguiculata (cowpea), an important legume crop in Botswana and other legumes as “trap” plants using the collected soil to induce nitrogen-fixing nodule formation. We have identified drought-tolerant bacteria from Botswana soils that stimulate plant growth; many are species of Bacillus and Paenibacillus. In contrast, the cowpea nodule microbiomes from plants grown in these soils house mainly rhizobia particularly Bradyrhizobium, but also Methylobacterium spp. Hence, the nodule microbiome is much more limited in non-rhizobial diversity compared to the soil microbiome, but also contains a number of potential pathogenic bacteria.

The Rhizobial Microbiome from the Tropical Savannah Zones in Northern Côte d’Ivoire

“…A pioneering study of soil microbiomes from African savannah woodlands carried out in Mozambique, based on both culture-dependent (isolation directly from soil or from V. unguiculata used as trapping host) and culture-independent (HTAS analysis of the 16S rDNA’s V3-V4 region from soil DNA) methods, revealed the presence and dominance of the Bradyrhizobium genus with a trapping assay only [ 35 ]. Similar results were recently obtained from savannah soils in Botswana [ 36 ]. The apparent paucity of the cosmopolitan Bradyrhizobium genus in these recent HTAS analyses and the limited distribution pattern on the African continent of Burkholderia sensu lato ( s.l. )…”

Isolation and characterization of plant growth-promoting endophytic bacteria of wild legumes growing on sandy soils of Binh Thuan province, Vietnam

“…Bacterial isolation from the root nodules of cowpeas grown with this soil also detected bacteria other than rhizobia, including Bacillus safensis, B. pumilus, Burkholderia dolosa. DNA of Sphingomonas and many other bacterial genera was also detected in the soil but was not isolated in the root nodules [8].…”

“…Other taxa included Gemmatimonadetes, Acidobacteria, and Planctomycetes. In culture-dependent method, in the nodules of legumes including Vigna unguiculata, Macroptilium atropurpureum and Tephrosia virginiana, some PGPB rhizobia and Bacillus species and several opportunistic bacteria were finding out [8].…”

Bacterias promotoras de crecimiento en el fréjol caupí (Vigna unguiculata L. Walp.)

“…Las PGPR ayudan en el crecimiento de las plantas a través de diversos mecanismos que incluyen la fijación de nitrógeno, la solubilización de fosfatos, la producción de sideróforos para la adquisición de hierro (Kosty et al, 2020).…”