Many bacterial strains have been shown to mediate protection to biotic stress and promote growth of plants. Different bacteria can mediate protection in different ways e.g. by inhibition, competition or increasing plant resistance. Examples of bacteria that mediate protection to plants include different Pseudomonas, Serratia and Bacillus strains. Bacillus strains have one major advantage toward other biocontrol strains and that is the ability to form spores that are resilient against chemicals and mechanical damage. I have studied the effect of four closely related Bacillus strains on plants in two different projects, one concerned with oilseed rape (Brassica napus) and the other using Arabidopsis thaliana to allow mechanistic studies of the interaction. The bacterial strains are all classified as Bacillus amyloliquefaciens. These bacterial strains have been tested for phenological effects on plants and for plant protection towards pathogens like Alternaria brassicae, Botrytis cinerea, Leptosphaeria maculans, and Verticillium longisporum. Production of antifungal compounds by the strains and the effects on the different pathogens were investigated. Two potential candidates for biocontrol were identified. Both Bacillus strains were found to provide significant protection of oilseed rape against the four pathogens. The effects of Bacillus treatment on the B. napus transcriptome were studied using the cDNA-AFLP technique. Bacillus priming had strong systemic effects on leaf transcripts but small effects on roots. This far 65 differentially expressed plant genes have been identified due to Bacillus treatment, of which many seem related to metabolism. The effect of Bacillus seed treatment has also been studied on Arabidopsis. Significant protection was achieved also here using the same two strains toward Alternaria and Leptosphaeria as well as Pseudomonas syringae as pathogens. Arabidopsis signalling mutant studies showed that functional jasmonic acid (JA) and ethylene (Et) signalling as well as Npr1 were needed for Bacillus biocontrol. Expression levels of marker genes depending on these signalling pathways showed no increase upon Bacillus treatment, while an increase of the JA dependent marker occurred after Bacillus treated plants were infected by P. syringae. Altogether, Bacillus primed biocontrol seems to be based on induced systemic resistance (ISR)
