Antibiosis and Gene Presence As Prevalent Traits for the Selection of Efficient Biocontrol Agents against Crown Gall Disease.

This study aimed to improve the screening method for the selection of biocontrol agents against crown gall disease. The relationship between the strain biocontrol ability and their studied traits was investigated to identify the most important factors to be considered for the selection of effective biocontrol agents. In fact, previous selection procedure relying only on antibacterial activity was shown to be not suitable in some cases. A direct plant-protection strategy was performed to screen the 32 biocontrol agent candidates. Moreover, potential biocontrol traits were investigated including biofilm formation, motility, hemolytic activity, detection of lipopeptide biosynthetic genes ( and ) and production of antibacterial compounds. The obtained results indicated high correlations of the efficiency of the biocontrol with the reduction of gall weight ( = 0.000) and the antibacterial activity ( = 0.000). Moreover, there was strong correlations of the efficiency of the biocontrol ( = 0.004) and the reduction in gall weight ( = 0.000) with the presence of the gene. This gene directs the synthesis of the lipopeptide bacillomycin belonging to the iturinic family of lipopeptides. These results were also confirmed by the two-way hierarchical cluster analysis and the correspondence analysis showing the relatedness of these four variables. According to the obtained results a new screening procedure of biocontrol agents against crown gall disease could be advanced consisting on two step selection procedure. The first consists on selecting strains with high antibacterial activity or those harbouring the gene. Further selection has to be performed on tomato plants . Moreover, based on the results of the biocontrol assay, five potent strains exhibiting high biocontrol abilities were selected. They were identified as or . These strains were found to produce either surfactin or surfactin and iturin lipopeptides. In conclusion, our study presented a new and effective method to evaluate the biocontrol ability of antagonistic strains against crown gall disease that could increase the efficiency of screening method of biocontrol agents. Besides, the selected strains could be used as novel biocontrol agents against pathogenic strains.