Genetic diversity of rhizobia and plant growth promoting rhizobacteria of soil under the influence of Piliostigma reticulatum (DC.) Hochst and their impact on shrub growth

Piliostigma reticulatum shrub is a native legume found in fallow areas in dry and semi-dry savanna soil and is used in intercropping systems. The aim was to understand the functioning of the rhizosphere, particularly the involvement of symbiotic and free living-N fixing bacteria. Soil extracts collected from P. reticulatum roots were sampled in two contrasting areas and endophytic bacterial communities were isolated using three trap host species (F. albida, A. bivenosa and V. seyal). Potential endophytic bacteria (PEB) were characterized by RFLP, nifH PCR and by 16S rRNA gene sequencing. The subsequent behavior of P. reticulatum was monitored in vitro by measuring leaf weight, biomass and chlorophyll content, after inoculation with PEB. This approach enabled isolation of 59 bacteria belonging to different genotypes. The most abundant genera were Cohnella (27.65%) among which 11 isolates clustered together and could represent a new species closely related to C. plantaginis. The other dominant genera were Paenibacillus (21.27%), Bradyrhizobium (14.89%) and Ensifer (8.5%). The nitrogen fixing gene (nifH) was detected in 21 strains and in particular, detected in a single isolate (PZS_S04) close to Cohnella xylanilytica. The strains PZS_S05 (Ensifer) and PZG_A18 (Cohnella) significantly increased certain parameters including shoot dry weight, shrub height at 90 days and photosynthetic activity (SPAD), compared to  non-inoculated controls.The result obtained showed that soil under the influence of P. reticulatum roots harbored a specific diversity of endophytic bacteria including two free living-N fixing bacteria with the potential to improve the growth of P. reticulatum in natural conditions. Key words: Piliostigma reticulatum, microbiology, phylogeny, Potential endophytic bacteria (PEB), nitrogen-fixing bacteria.