Natural products with antifungal activity from Pseudomonas biocontrol bacteria.

It is now well established that some bacteria and fungi are aggressive colonizers of soil and the roots of plants, and are able to protect plants from infection by soil-borne fungal pathogens. In most cases that have been well studied, the biological mechanisms underlying this phenomenon, known as biocontrol, include the production of antifungal compounds, involving both metabolites and enzymes. Bacteria of the genus Pseudomonas comprise a large group of the active biocontrol strains as a result of their general ability to produce a diverse array of potent antifungal metabolites. These include simple metabolites such as 2,4-diacetylphloroglucinol, phenazine-1-carboxylic acid and pyrrolnitrin [3-chloro-4-(2'- nitro-3'-chlorophenyl)-pyrrole], as well as the complex macrocyclic lactone, 2,3-de-epoxy-2,3-didehydro-rhizoxin. Study of the biochemistry and mechanism of formation of these metabolites has proved useful in several ways. Pyrrolnitrin is active against Rhizoctonia spp, Fusarium spp, and other plant pathogenic fungi, and it has been used as a lead structure in the development of a new phenylpyrrole agricultural fungicide. In addition, pyrrolnitrin has been used for years as a model for the study of the mechanisms involved in the chlorination of organic molecules. We have cloned a four-gene cluster from a P fluorescens biocontrol strain that encodes the enzymes required for the production of pyrrolnitrin. Using these genes and strains mutated in the individual genes, we have elucidated the biochemical pathway by which pyrrolnitrin is synthesized. Studies of the genes involved in pyrrolnitrin biosynthesis have demonstrated that a new class of halogenase enzyme is involved in the chlorination reactions in pyrrolnitrin biosynthesis. The P fluorescens mutants that do not produce pyrrolnitrin have been used to demonstrate clearly the important role of pyrrolnitrin in the overall biocontrol activity shown by the strain. In addition, we have modified the pyrrolnitrin genes within the P fluorescens strain, which has resulted in significant increases in the production of this metabolite. The strains which overproduce pyrrolnitrin are also significantly more active than the wild-type strain in biocontrol. These studies suggest that the biocontrol activity of bacteria can be dramatically increased to rival the activity of commercial chemical fungicides through directed genetic modification. Bacteria remain important sources of natural products with diverse activities. In order to capture the full potential of these compounds, it will be necessary in the future to take a multidisciplinary approach to their study and development.