The dar genes of Pseudomonas chlororaphis PCL1606 are crucial for biocontrol activity via production of the antifungal compound 2-hexyl, 5-propyl resorcinol.

To determine the genetic basis by which 2-hexyl, 5-propyl resorcinol (HPR) is produced by the biocontrol rhizobacterium Pseudomonas chlororaphis (formerly known as P. fluorescens) PCL1606, the presence and role of dar genes were investigated. To accomplish this aim, the pCGNOV-1 plasmid was isolated from a PCL1606 genomic library and was shown to hybridize to various dar probes by Southern blot. An analysis of the pCGNOV-1 genomic DNA revealed the presence of five open reading frames that were homologous to dar genes and had an organization that resembled the arrangement of previously described P. chlororaphis strains. Phylogenetic studies resulted in the clustering of PCL1606 with the P. chlororaphis subgroup, which supported the renaming of this strain from P fluorescens to P chlororaphis PCL1606. The construction of insertional mutants for each homologous dar gene in P chlororaphis PCL1606 along with their corresponding complemented derivative strains restored HPR production and confirmed the key role of the darA and darB genes in HPR production and in the antagonistic phenotype. Finally, biocontrol assays were performed on avocado Rosellinia and tomato Fusarium test systems using the HPR-defective and -complemented derivative strains generated here and demonstrated the crucial role of the biosynthetic dar genes in the biocontrol phenotype of P. chlororaphis PCL1606. This biocontrol phenotype is dependent on the dar genes via their production of the HPR antibiotic. Some of the dar genes not directly involved in the biosynthesis of HPR, such as darS or darR, might contribute to regulatory features of HPR production.