Biological control of plant pathogens by Bacillus species

Abstract

Bacteria from the Bacillus group are microorganisms that inhabit a large number of different habitats. They are well known as producers of a wide array of antagonistic compounds of different structures, having between 5 to 8% of the total genome devoted to biosynthesis of secondary metabolites. Most important bioactive molecules from the genus Bacillus are non-ribosomally synthesized peptides and lipopeptides, polyketide compounds, bacteriocins and siderophores. Lipopeptides from Bacillus have very complex mechanisms of biosynthesis catalyzed by non-ribosomal peptide synthetases (NRPSs), large enzyme complexes with modular structure, with each module being in charge for the incorporation of a particular amino acid. In general, they have a broad spectrum of antagonistic activity against plant pathogenic bacteria, fungi and viruses. Most important molecules from this group, circular lipopeptides from surfactin, iturin and fengycin families affect the target cells on the membrane level. Bacillus strains exhibit their biocontrol capacity predominantly through inhibitory activity on the growth of plant pathogens, as well as inducing systemic resistance in plants and competing for ecological niches with plant pathogens. Our previous studies showed the presence of multiple biosynthetic operons for synthesis of non-ribosomal lipopeptides in the collection of natural isolates of Bacillus, with many strains having more than one of them. Several strains of Bacillus sp. that we have recently characterized showed very strong antibacterial and antifungal activity against phytopathogens. The PCR analysis showed the presence of biosynthetic operons for iturin, bacillomycin, fengycin and surfactin in tested strains. Measurement of the kinetics of production of antimicrobial substances showed that, in most cases, synthesis started at the beginning of exponential phase of growth, reaching the maximum of antimicrobial activity at the beginning of the stationary growth phase and stayed at this level for the whole duration of observed period. Preparations of cell-free supernatants of tested strains were active against many fungal and bacterial pathogens, in vitro and in vivo. Mass spectrometry and HPTLC bioautography analysis of purified compounds confirmed the presence of lipopeptides of mentioned families, hence confirming the biocontrol capacity of Bacillus isolates.