Mining and characterization of antimicrobials from plant growth-promoting rhizobacteria isolated from perennial ryegrass

Plant growth-promoting rhizobacteria (PGPR) form a good alternative to chemical fertilizers and pesticides. Moreover, PGPR can produce antimicrobials that have the potential to be antibiotics, which are in great demand because of the rapidly emerging antibiotic resistance. This thesis describes the isolation and screening of seven PGPR strains for use on perennial ryegrass, a crucial pasture plant distributed worldwide. Genome mining of newly isolated PGPR strains reveals abundant biosynthetic gene clusters (BGCs) of secondary metabolites, some of which remain unknown. One of the most interesting strains, Brevibacillus laterosporus MG64, displays outstanding biocontrol activity but 80% of its BGCs have not been assigned to known products. Further characterization of its secondary metabolites led us to the discovery of three families of compounds: relacidines, bogorols, and succilins. Relacidines are a class of cationic circular lipopeptides that selectively combat Gram-negative bacteria by affecting the oxidative phosphorylation process. Bogorols are a class of cationic linear lipopeptides that are active against both Gram-positive bacteria and Gram-negative bacteria. Succilins are modified from bogorols by succinylation at Orn3. The lipoinitiation of bogorols and succilins is proven to be mediated by an adenylation domain, while the valinol at the C-terminus is formed by a two-step reduction. A comparative transcriptomics study reveals that the expression of sporulation genes and secondary metabolite encoding BGCs are regulated during the interaction with pathogens. This thesis provides a comprehensive understanding of the PGPR strain B. laterosporus MG64 and its antimicrobials and extends our knowledge on the biosynthetic machinery of lipopeptides.