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Bacteriocins play a key role in Pseudomonas syringae competition in the plant environment

Hanareia Ehau-Taumaunu: Dept. of Plant Pathology and Environmental Microbiology, The Pennsylvania State University

<div>Bacteria can employ narrow-spectrum toxins called bacteriocins to inhibit the growth of competing strains closely related to the producer. Bacteriocin-based biocontrol has been proposed as an alternative to broad-spectrum chemical biocides. However, the fitness benefit provided by bacteriocins to producing strains is poorly understood in the plant environment. Here, we investigated the competition between <em>Pseudomonas syringae</em> pv. <em>syringae </em>(<em>Psy</em>) and <em>P. syringae</em> pv. <em>phaseolicola </em>(<em>Pph</em>) which are both able to infect and cause disease on <em>Phaseolus vulgaris </em>(common bean). Importantly, <em>Psy </em>encodes a bacteriocin that inhibits <em>Pph</em>. A series of <em>in vivo</em> experiments tested the direct competition of <em>Psy</em>, or derived mutants with reduced or abolished virulence and/or bacteriocin production, against <em>Pph</em>. The leaf epiphytic and endophytic populations of <em>Psy</em> and <em>Pph</em> were measured at days 0, 4, and 6 after co-inoculation. Additionally, the levels of bacteriocins present were monitored. Co-inoculations with <em>Psy</em> and <em>Pph</em> (1:1) indicate the bacteriocin producing <em>Psy</em> had a statistically significant increase in population size compared to <em>Pph</em> at days 4 (<em>P</em>= 0.0002) and 6 (<em>P</em>=0.0028). However, the <em>Psy</em> bacteriocin deficient mutant population remained similar to the <em>Pph</em> population growth. These results demonstrate a fitness benefit for the bacteriocin producing <em>Psy</em> strain against <em>Pph</em> in the plant environment, and begins to aid our understanding of how to effectively employ bacteriocins.</div>