Genetic diversity of plant growth promoting rhizobacteria and their effects on the growth of maize plants under greenhouse conditions

Engineering of plant rhizosphere with beneficial plant growth promoting (PGP) bacteria offers a great promise for sustainable crop productivity. In this context, 49 rhizospheric/endophytic bacterial isolates were purified using N-free medium, screened in vitro for PGP characteristics and evaluated for their beneficial effects on the early growth of maize (Zea mays L.). The biodiversity of isolated bacteria was analyzed by amplified ribosomal DNA restriction analysis (ARDRA) using four restriction enzymes. Out of the 49 isolates, 7 produced high levels (32.1-82.8 mu g mL(-1)) of indole-3-acetic acid (IAA); 11 had potential phosphate solubilizing abilities (101-163 mu g mL(-1)), while significant acetylene reduction activities (100-1800 nmole C2H4 mg(-1) protein h(-1)) were observed in 12 isolates. Five ribogroups (A-E) were identified using ARDRA. The 16S rRNA-sequence analysis of bacterial representatives from different ribogroups revealed that, 89% of isolates belonged to phylum Proteobacteria, while 11% of them were assigned into phylum Bacteroidetes. Phylum Proteobacteria included Achromobacter, Agrobacterium, Bordetella, Cupriavidus, Ochrobactrum, Pseudoxanthomonas and Stenotrophomonas genera. While, phylum Bacteroidetes was represented by Chryseobacterium and Flavobacterium genera. Under greenhouse conditions, all the selected isolates significantly increased shoot and root fresh and dry biomass of maize plants, compared to the non-inoculated control. Inoculation with Chryseobacterium sp. NGB-29 and Flavobacterium sp. NGB-31 showed the highest beneficial effects on plant growth parameters that were tentatively associated with the high efficiency of these isolates to fix nitrogen and produce high amounts of IAA. The study indicates the potential use of these isolates for production of maize biofertilizers under field conditions.