Rhizosphere and endorhiza of oilseed rape (Brassica napus L.) plant harbor bacteria with multifaceted beneficial effects

• PGP traits were different between endophytic and rhizosphere isolates.
• Both endophytic isolates and rhizosphere isolates showed good antifungal activity.
• IAA production of isolates was positively correlated with antifungal activity.
• IAA production may be a useful trait to screen antagonistic bacteria.

In this study, bacteria were isolated from the rhizosphere and inside the roots of oilseed rape (Brassica napus L.) plant grown in the field in Iran. A total of 150 isolates were obtained from the rhizosphere (90 isolates) and endorhiza (60 isolates) of B. napus plants grown in rotation with rice. All of the isolates were characterized for production of plant growth promoting (PGP) traits and antifungal activity against some rice plant pathogen fungi. The results showed the endophytic and rhizosphere isolates had different PGP traits in terms of both the number and the production amount of these traits. Only one rhizosphere isolate and two endophytic isolates showed highly inhibitory effects against the mycelial growth of all fungal rice pathogens tested in this study. In addition, thirty-seven isolates inhibited only the mycelial growth of some the fungal rice pathogens. A higher percentage of the endophytic isolates were positive for production of IAA, siderophore, and ACC deaminase as compared to rhizosphere isolates respectively. The best bacterial isolates (one endophytic isolate and one rhizosphere isolate), based on multiple PGP traits and inhibitory effects against the mycelial growth of all fungal rice pathogens, were identified. Based on biochemical tests and by comparison of 16S rDNA sequences, the endophytic isolate CEN3 and the rhizosphere isolate CEN5 were closely related to Bacillus subtilis and Bacillus cereus respectively. These bacteria exhibited a broad-spectrum antifungal activity towards all rice phytopathogenic fungi. We also studied the PGP effect of these isolates as single and co-inoculation on rice seedlings. All treatments resulted in significant increases (p < 0.05) in root length, root fresh weight, and shoot dry weight, as compared to control. When these isolates were applied as co-inoculation on rice seedlings, the highest increase in rice plant growth indices and population size of these isolates after 20 days was observed as compared to treatments receiving these isolates alone. In conclusion, the B. subtilis CEN3 and B. cereus CEN5 strains analyzed here exert multiple PGP and antagonistic mechanisms, show positive root colonization capability, and represent an excellent option to be used as either potent bio-promoting or bio-control agents in crops such as rice.

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