Abstract
Verticillium wilt causes severe yield losses in a broad range of economically important crops worldwide. As many soil fumigants have a severe environmental impact, new biocontrol strategies are needed. Members of the genus Bacillus are known as plant growth-promoting bacteria (PGPB) as well as biocontrol agents of pests and diseases. In this study, we isolated 267 Bacillus strains from root-associated soil of field-grown tomato plants. We evaluated the antifungal potential of 20 phenotypically diverse strains according to their antagonistic activity against the two phytopathogenic fungi Verticillium dahliae and Verticillium longisporum. In addition, the 20 strains were sequenced and phylogenetically characterized by multi-locus sequence typing (MLST) resulting in 7 different Bacillus thuringiensis and 13 Bacillus weihenstephanensis strains. All B. thuringiensis isolates inhibited in vitro the tomato pathogen V. dahliae JR2, but had only low efficacy against the tomato-foreign pathogen V. longisporum 43. All B. weihenstephanensis isolates exhibited no fungicidal activity whereas three B. weihenstephanensis isolates showed antagonistic effects on both phytopathogens. These strains had a rhizoid colony morphology, which has not been described for B. weihenstephanensis strains previously. Genome analysis of all isolates revealed putative genes encoding fungicidal substances and resulted in identification of 304 secondary metabolite gene clusters including 101 non-ribosomal polypeptide synthetases and 203 ribosomal-synthesized and post-translationally modified peptides. All genomes encoded genes for the synthesis of the antifungal siderophore bacillibactin. In the genome of one B. thuringiensis strain, a gene cluster for zwittermicin A was detected. Isolates which either exhibited an inhibitory or an interfering effect on the growth of the phytopathogens carried one or two genes encoding putative mycolitic chitinases, which might contribute to antifungal activities. This indicates that chitinases contribute to antifungal activities. The present study identified B. thuringiensis isolates from tomato roots which exhibited in vitro antifungal activity against Verticillium species.
Highlights
Verticillium wilt occurs in a wide range of plant species including economical important crops
Our experiments demonstrate that the classification of ChiA from Bacillus thuringiensis (Bt) serovar colmeri should be reconsidered, as the deduced amino acid size of the protein and domain analysis revealed that this chitinase belongs to the ChiB group of chitinases (Group I)
We evaluated the antifungal potential of 20 phenotypically diverse Bacillus isolates toward V. dahliae and V. longisporum
Summary
Verticillium wilt occurs in a wide range of plant species including economical important crops. V. dahliae is known as a vascular pathogen with a broad host range including trees, legumes, ornamental crops, and vegetables such as tomato (Pegg and Brady, 2002; Luo et al, 2014) This fungus forms extremely outlasting melanized resting structures (microsclerotia) that are able to survive in soil for many years (Schnathorst, 1981). As many soil fumigants have a severe environmental impact, no pesticide which meets the standards of a sustainable agriculture is currently available to control the expansion of V. dahliae and V. longisporum (Frank, 2003; Depotter et al, 2016) Other control strategies such as crop rotation, the usage of resistant plant species, and soil solarization have only limited efficiency (Haas and Défago, 2005; Depotter et al, 2016). They are either ineffective, time-consuming, costly, may have a severe environmental impact, or even could affect human health (Angelopoulou et al, 2014)
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