Aims Verticillium wilt is difficult to suppress, and causes severe yield losses in a broad range of crops. Five Serratia and five Paenibacillus endophytic isolates showing antagonistic properties against fungal pathogens were compared for their plant growth-promoting (PGP) potential under different plant growth conditions with the objective of evaluating the PGP of endophytic strains in different ad planta systems. Methods Preselected isolates were applied to the surfacesterilized seeds of oilseed rape and cauliflower using biopriming. The isolates' PGP effect and root colonization capacities were compared under gnotobiotic conditions. One strain from each genus was selected and tested for its PGP qualities in sterile and non-sterile soil. Results Serratia treatment resulted in different levels of PGP, while Paenibacillus strains damaged roots under gnotobiotic conditions. P. polymyxa Sb3-1 did not have a significant effect on plant growth in non-sterile soil; however it did promote plant growth in the sterile soil. S. plymuthica 3RP8 and P. polymyxa Sb3-1 were selected for further testing of their biocontrol effect under field conditions. Conclusions The choice of growth environments in the investigation of plant-bacterium interaction is crucial. Non-sterile soil is suggested as the ideal medium for use in studying the PGP effect.

The genome of Paenibacillus polymyxa Sb3-1, a strain that shows antagonistic activities against pathogenic fungi and bacteria, consists of one 5.6-Mb circular chromosome and two plasmids of 223 kb and 8 kb. The genome reveals several genes that potentially contribute to its antagonistic and plant growth promotion activity.