Characteristics of Bacteria from Oilseed Rape in Relation to their Biocontrol Activity against Verticillium dahliae

The potential of bacteria that are adapted to the oilseed rape root environment for use in the biological control of Verticillium dahliae, Kleb was investigated in both controlled and non-sterile growth conditions. Bacterial strains dominated by the red-pigmented members of enterobacteriaceae were isolated from thoroughly washed and air-dried root segments of symptomless young rape plants. Other associated strains found either belonged to Alcaligenes sp., Stenotrophomonas spp. and Pseudomonas spp. (Pseudomonas acidovorans and Pseudomonas putida) or were unidenti®ed according to fatty acid methyl ester pro®le analysis. A total of 19 strains isolated in this study together with two previ- ously studied strains, Serratia proteamaculans and Pseudomonas chlororaphis, were characterized on the basis of their interactions with V. dahliae and a number of functional characteristics. In line with earlier obser- vations with root-colonizing fungi also from oilseed rape, all bacterial strains suppressed the pathogen not only directly and but also indirectly in in vitro assays. Mechanisms of suppression were apparently multifold among the strains, but production of hydrogen cyanide does not seem to be involved in indirect inhibition. The majority of the strains possessed the ability to produce cellulases, proteases and phosphatases and some even produced chitinases and induced hypersensitive respons- es, indicating their potential for nutrient acquisition as well as colonization capacity and active recognition by the plant cells. Investigations in non-sterile ®eld soil revealed that some strains protected rape plants from V. dahliae partly by delaying symptom development. None of the strains, however, was strongly deleterious to rape growth either in the presence or absence of the pathogen. Light microscopic observations of roots and results based on agar printing techniques revealed the potential of the studied strains to colonize or interfere with the pathogen colonization. This study provides some insight into the evolved relationship of bacterial residents with their host in terms of their potential importance in its ®tness.