AUTHOR=Frikha-Gargouri Olfa , Ben Abdallah Dorra , Bhar Ilhem , Tounsi Slim 

TITLE=Antibiosis and bmyB Gene Presence As Prevalent Traits for the Selection of Efficient Bacillus Biocontrol Agents against Crown Gall Disease

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01363

DOI=10.3389/fpls.2017.01363

ISSN=1664-462X

ABSTRACT=<p>This study aimed to improve the screening method for the selection of <italic>Bacillus</italic> biocontrol agents against crown gall disease. The relationship between the strain biocontrol ability and their <italic>in vitro</italic> 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 <italic>in vitro</italic> antibacterial activity was shown to be not suitable in some cases. A direct plant-protection strategy was performed to screen the 32 <italic>Bacillus</italic> biocontrol agent candidates. Moreover, potential <italic>in vitro</italic> biocontrol traits were investigated including biofilm formation, motility, hemolytic activity, detection of lipopeptide biosynthetic genes (<italic>sfp, ituC</italic> and <italic>bmyB</italic>) and production of antibacterial compounds. The obtained results indicated high correlations of the efficiency of the biocontrol with the reduction of gall weight (<italic>p</italic> = 0.000) and the antibacterial activity <italic>in vitro</italic> (<italic>p</italic> = 0.000). Moreover, there was strong correlations of the efficiency of the biocontrol (<italic>p</italic> = 0.004) and the reduction in gall weight (<italic>p</italic> = 0.000) with the presence of the <italic>bmyB</italic> 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 <italic>Bacillus</italic> 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 <italic>in vitro</italic> or those harbouring the <italic>bmyB</italic> gene. Further selection has to be performed on tomato plants <italic>in vivo</italic>. Moreover, based on the results of the biocontrol assay, five potent strains exhibiting high biocontrol abilities were selected. They were identified as <italic>Bacillus subtilis</italic> or <italic>Bacillus amyloliquefaciens</italic>. 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 <italic>Bacillus</italic> 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 <italic>Agrobacterium tumefaciens</italic> strains.</p>