Root colonization and growth enhancement in wheat and tomato by rhizobacteria isolated from the rhizoplane of grasses

Rhizobacteria isolated from the rhizoplane of grasses growing at the Nylsvlei Nature Reserve in South Africa were investigated for growth promotion and root colonization in wheat (Triticum aestivum L.) and tomato (Lycopersicon esculentum Mill.) under greenhouse and microplot field conditions. The identities of the isolates were determined by means of 16S rRNA gene sequencing as Bacillus simplex (KBS1F-3), Bacillus megaterium (NAS7-L), Bacillus cereus (KFP9-F) and Paenibacillus alvei (NAS6G-6). The three Bacillus strains were isolated from the perennial grass Themeda triandra while the Paenibacillus strain was isolated from another perennial grass Sporobolus fimbriatus. Enhanced plant shoot and root weight in wheat was achieved by single inoculation with three of the isolates whereas no significant increase was observed in root length. Combined inoculation of Paenibacillus alvei (NAS6G-6) and Bacillus cereus (KFP9-F) on wheat resulted in significant increase in these parameters. Single inoculations of Bacillus simplex (KBS1F-3) and Bacillus cereus (KFP9-F) resulted in significant increase in root and shoots fresh weight, root dry weight and total root length in tomatoes. Indoleacetic acid production, phosphate solubilization and siderophore secretion were studied as possible mechanisms by which the bacterial isolates enhanced plant growth. Root colonization was studied by means of spontaneous rifampicin resistant strains of the wild type isolates. Except for B. megaterium (NAS7-L), the rest of the isolates colonized the roots efficiently resulting in concentrations of 10⁶–10⁸ cfu g⁻¹ root. The root colonization of Bacillus simplex (KBS1F-3) and Paenibacillus alvei (NAS6G-6) was visualized by confocal scanning laser microscope (CSLM) after successful transformation of the isolates with the pNF8 plasmid carrying the gene for the green fluorescent protein (gfp).