Abstract

Plant growth-promoting rhizobacteria (PGPR) are well-known to influence plant growth via a variety of mechanisms such as nitrogen fixation, production of volatile organic compounds and enzymes, and bioremediation contaminants from the environment. PGPR have been previously identified by other researchers using laboratory screening methods. It was hypothesized that relying on these routine laboratory tests, some PGPR species are being overlooked. These species could promote growth through genes that encode for the synthesis of specific growth stimuli or other growth-promoting traits such as vitamins, antibiotics, and secondary metabolites. To evaluate this hypothesis, PGPR (MA-7, ON-4, SP-7, and RA-9) and previously overlooked PGPR (SE-7, LE-26, SQ-7, and SQ-9) were tested both with sterilized and non-sterilized soil in pot and greenhouse experiments. The PGPR isolates significantly increased pea plant growth, albeit to different degrees based on isolate, in both types of soil. The increases were recorded in shoot and root length and fresh matter in non-sterilized soil whereas increases in root length and root fresh weight were observed in sterilized soil.  Interestingly, strains SE-7 and SQ-7 of the four overlooked PGPR isolates tested were also able to promote pea plant growth similarly to the PGPR isolates under both pot and greenhouse conditions. Morphological and biochemical characterization of the four original PGPR isolates revealed that they were rod-shaped, gram-positive, and spore-forming. Sequencing of 16S ribosomal RNA showed that these strains were mostly similar to Bacillus sp. (99% similarity). Using the EzBioCloud 16S rRNA database, it was found that one strain was likely to be Bacillus paramycoides based on 100% similarity, two strains were Bacillus wiedmannii based on 99.05 and 100% similarity, and the remaining strain was Bacillus amyloliquefaciens based on 99.64% similarity.

Key words: Plant growth-promoting rhizobacteria (PGPR), pea, soil, 16S rRNA, Bacillus.