Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2000
Publication Date: N/A
Citation: Interpretive Summary: Our studies focus on biological control of plant disease, and specifically the factors that influence the success of biological control under various conditions in a field environment. The biological control agent Pseudomonas fluorescens Pf-5 is a soil bacterium that suppresses plant diseases caused by certain soilborne organisms. Strain Pf-5 produces an antibiotic called pyoluteorin (Plt) that is toxic to Oomycetes, a class of organisms that include many important plant pathogens. Pf-5 does not produce pyoluteorin all of the time; instead, it produces different amounts of the antibiotic under different conditions. We think that this variable antibiotic production is one reason why Pf-5 doesn't work as well in all field conditions. Understanding the factors that influence antibiotic production by Pf-5 in culture helps us understand the factors that influence biological control by Pf-5 on plants. In this paper, we identify ya gene (Lon) in Pf-5 that controls the production of pyoluteorin. The Lon gene also influences the capacity of Pf-5 to survive exposure to environmental stresses and it is the fourth gene we've identified that influences both antibiotic production and stress response in Pf-5. This is important because it tells us that Pf-5 is not likely to be as successful in biological control if the soil environment is unfavorable to the bacterium.
Technical Abstract: Pseudomonas fluorescens Pf-5 is a soil bacterium that suppresses plant pathogenic fungi due in part to its production of the antibiotic pyoluteorin (Plt). Previous characterization of Pf-5 revealed three global regulators, including the stationary phase sigma factor RpoS and the two-component regulators GacA and GacS, that influence both antibiotic production and stress response. In this report we describe the serine protease Lon as a fourth global regulator influencing these phenotypes in Pf-5. Lon mutants over-produced Plt, transcribed pyoluteorin biosynthesis genes at enhanced levels, and were more sensitive to UV exposure than Pf-5. The Lon gene was preceded by sequences that resembled promoters recognized by the heat-shock sigma factor of Escherichia coli and Lon accumulation by Pf-5 increased after heat shock. Therefore, the heat-shock sigma factor represents the third sigma factor implicated in the regulation of antibiotic production by P. fluorescens. Lon protein levels were similar in stationary-phase and exponentially-growing cultures of Pf-5 and were not positively affected by the global regulators RpoS or GacS. The association of antibiotic production and stress response has practical implications towards the success of disease suppression in the soil environment where biological control organisms such as Pf-5 is likely to encounter environmental stresses.