INVESTIGATING PSEUDOMONAS KILLING OF GENETICALLY IDENTICAL BACTERIA

Abstract

Pseudomonas syringae pv. tomato (Pst) is a plant pathogenic bacteria and causal agent of bacterial speck, which can have serious impacts on the yield and quality in tomato. Previous work has shown that Pst grown in planta is able to limit the growth of in vitro grown cultures of Xanthomonas hortorum pv. gardneri (Xg), Salmonella enterica, and genetically identical Pst that has not been exposed to the plant environment. Here, we investigate the conditions Pst can limit the growth of other bacteria and explore the underlying mechanism of this limitation. Assays comparing the growth of a target population mixed with Pst grown either in planta or in vitro revealed that Pst’s ability to limit other bacteria is activated in the plant environment. Growing Pst in homogenized leaf tissue revealed plant-derived factors are suficient to trigger inhibition. Co-inoculating cultures of Pst and Xg indicated that Pst more efectively suppressed target bacteria when populations resemble that of a pre-established infection. Finally, using a reporter plasmid, we found that Pst’s T6SS is expressed at higher levels in planta, indicating that it may be the mechanism responsible for Pst’s limiting ability. Together these findings indicate that during infection, Pst may utilize its T6SS to inhibit the growth of immigrating bacteria, allowing it to maintain dominance in its infection court, the apoplast.