Friend or foe? Exploring the fine line between Pseudomonas brassicacearum and phytopathogens

Pseudomonas brassicacearum is one of over fifty species of bacteria classified into the P. fluorescens group. Generally considered a harmless commensal, these bacteria are studied for their plant-growth promotion (PGP) and biocontrol characteristics. Intriguingly, P. brassicacearum is closely related to P. corrugata, which is classified as an opportunistic phytopathogen. Twenty-one P. brassicacearum genomes have been sequenced to date. In the current review, genomes of P. brassicacearum and strains from the P. corrugata clade were mined for regions associated with PGP, biocontrol and pathogenicity. We discovered that 'beneficial' bacteria and those classified as plant pathogens have many genes in common; thus, only a fine line separates beneficial/harmless commensals from those capable of causing disease in plants. The genotype and physiological state of the plant, the presence of biotic/abiotic stressors, and the ability of bacteria to manipulate the plant immune system collectively contribute to how the bacterial-plant interaction plays out. Because production of extracellular metabolites is energetically costly, these compounds are expected to impart a fitness advantage to the producer. P. brassicacearum is able to reduce the threat of nematode predation through release of metabolites involved in biocontrol. Moreover this bacterium has the unique ability to form biofilms on the head of Caenorhabditis elegans, as a second mechanism of predator avoidance. Rhizobacteria, plants, fungi, and microfaunal predators have occupied a shared niche for millions of years and, in many ways, they function as a single organism. Accordingly, it is essential that we appreciate the dynamic interplay among these members of the community.