Influence of endogenous plasmids on phenotypes of Pantoea vagans strain C9-1 associated with epiphytic fitness

Authors: KLEIN, JEANNIE - University Of Florida; LOPER, JOYCE - Oregon State University; Stockwell, Virginia

Submitted to: Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2017
Publication Date: 8/4/2017
Citation: Klein, J.M., Loper, J.E., Stockwell, V.O. 2017. Influence of endogenous plasmids on phenotypes of Pantoea vagans strain C9-1 associated with epiphytic fitness. Journal of Plant Pathology. 99:81-89. https://doi.org/10.4454/jpp.v99i0.3914.
DOI: https://doi.org/10.4454/jpp.v99i0.3914

Interpretive Summary: Many members of Pantoea spp. have been shown to be effective bacterial biological control agents of serious plant diseases. The strain called C9-1 is one such biological control agent that is effective for control of fire blight of pear and apple. Previously, we sequenced the genome of C9-1 and discovered that the bacterium has one chromosome and also carries three large plasmids. Like the chromosome, plasmids have genes that may be important to the activities of the bacterium, but sometimes plasmids can be lost during growth of the bacterium. The interesting aspect of plasmids in C9-1 is that they carry 10% of the total DNA in the biocontrol agent and carry genes that could be important to biological control. For example, one plasmid carries genes for production of an antibiotic and another plasmid carries genes predicted to be important for the bacterium to survive on plant tissues. We removed two plasmids (individually and together) from C9-1 and tested the effect of plasmid loss to the capacity of the bacterium to grow and also tolerate stress. We found that loss of one of the plasmids had minor effects on growth and stress tolerance. The other plasmid significantly reduced the ability of the bacterium to spread and throughly colonize surfaces, which could affect its ability to protect plants from pathogens or the capacity of C9-1 to survive under adverse environmental conditions. Understanding the impact of genes, including those carried on plasmids, on survival and the activity of biocontrol agents is important. For example, the information on the impact of genes on fitness can be used to screen new candidate organisms for their potential to control plant disease. The plasmid data could be used to develop simple assays to ensure that the plasmids are maintained in the formulated product of Pantoea spp. biocontrol agents.

Technical Abstract: Pantoea vagans strain C9-1 is an effective biological control agent for fire blight of pear and apple. C9-1 carries three circular plasmids: pPag1 (168 kb), pPag2 (166 kb), and pPag3 (530 kb). Of these, pPag3, a member of the large Pantoea plasmid family, was proposed to contribute to epiphytic fitness. We cured C9-1 of pPag2 and pPag3, individually and together, and compared phenotypes of the plasmid-cured derivatives to the wild-type C9-1 in laboratory assays. pPag3 carries genes for a yellow zeaxanthin pigment, and pigmentation is a known mechanism of UV protection in some bacteria. We found that UV-C tolerance did not differ between yellow-pigmented C9-1 and white derivatives lacking pPag3. Swarming motility, but not swimming, was influenced by both pPag2 and pPag3. C9-1 exhibited wide dendritic swarms that coalesced, whereas C9-1 lacking pPag3 formed thin dendritic swarms. Swarms of C9-1 lacking pPag2 exhibited less radial extension. In MBMA medium containing glucose, exopolysaccharide production and biofilm formation was greater by C9-1 lacking both pPag2 and pPag3 compared to the wild-type and C9-1 derivatives lacking only one plasmid. pPag2 and pPag3, both individually and together, influenced expression of phenotypes of C9-1 associated with epiphytic fitness and survival on plant surfaces.