Biosynthesis and regulation of the antifungal herbicolin A in a Pantoea agglomerans rhizobacterium active against a broad spectrum of plant-pathogenic fungi

Abstract

The plant microbiome plays a crucial role in plant growth and protection against phytopathogens through multiple mechanisms, for example, improving nutrient acquisition and disease resistance. The bacterium Pantoea agglomerans 9Rz4 was isolated from the rhizosphere of oilseed rape due to its ability to antagonise the growth of different fungal pathogens of agronomic relevance [1]. However, the mechanisms behind these antifungal properties are unknown. By screening a library of random transposon mutants, we have isolated mutants of 9Rz4 deficient in their antifungal properties. The combination of de novo genome sequencing and chemical analysis allowed us to determine that the strain 9Rz4 produces the non-ribosomal peptide antifungal herbicolin A, as well as to identify and characterise the corresponding biosynthetic cluster. The expression of the herbicolin A gene cluster is maximal in stationary phase of growth. A quorum sensing mutant of 9Rz4, defective in the biosynthesis of acyl-homoserine lactones, shows reduced production of herbicolin A and, as a consequence, a lower antifungal activity. In planta assays revealed that 9Rz4 does not negatively impact on plant growth, but also that herbicolin A does not affect rhizosphere colonisation by the 9Rz4. Assessing bacterial potential as biopesticides is fundamental for transitioning to green alternatives to agrochemicals. Pantoea agglomerans 9Rz4 is active against a broad range of fungal phytopathogens and our results lay the foundation for the possible future use of this bacterium as a biocontrol agent in agriculture.