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United States Department of Agriculture

Agricultural Research Service

Research Project: Improved Strategies for Management of Soilborne Diseases of Horticultural Crops

Location: Horticultural Crops Research

Title: pA506: A conjugative plasmid of the plant epiphyte Pseudomonas fluorescens A506

Authors
item Stockwell, Virginia -
item Davis, Edward
item Carey, Alyssa
item Shaffer, Brenda
item Mavrodi, Dmitri -
item Hassan, Karl -
item Hockett, Kevin
item Thomashow, Linda
item Paulsen, Ian -
item Loper, Joyce

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 21, 2013
Publication Date: September 1, 2013
Citation: Stockwell, V.O., Davis, E.W., Carey, A.B., Shaffer, B.T., Mavrodi, D.V., Hassan, K.A., Hockett, K.L., Thomashow, L.S., Paulsen, I.T., Loper, J.E. 2013. pA506: A conjugative plasmid of the plant epiphyte Pseudomonas fluorescens A506. Applied and Environmental Microbiology. 79:5272-5282.

Interpretive Summary: Pseudomonas fluorescens A506 is a biological control bacterium that is used commercially in the United States for management of fire blight disease of pear and apple. Here, we demonstrate that A506 has a conjugative plasmid, called pA506, that can transfer to other strains of Pseudomonas spp. and to members of the Enterobactericeae and is stably maintained in those bacterial hosts. We obtained the nucleotide sequence of the plasmid. By comparing the sequence of pA506 to other plasmids, we show that the plasmid is different from other plasmids that have been described before. It has some unique features but also has typical genes involved in plasmid conjugation and stable maintenance in bacteria. We derived a plasmid-cured derivative of A506 and compared it to the plasmid-containing wildtype strain for UV-resistance, twitching motility, and epiphytic fitness on pear blossoms in an experimental orchard. pA506 conferred UV resistance but did not detectably influence epiphytic fitness of A506 on pear or apple blossoms in the field. In contrast to many other Pseudomonas plasmids, pA506 does not appear to confer resistance to antibiotics or other toxic elements. Based on the conjugative nature of pA506 and the large proportion of its genes that are shared with plasmids of from diverse groups of environmental bacteria, the plasmid is likely to serve as a vehicle for genetic exchange between A506 and other bacteria on plant surfaces.

Technical Abstract: Pseudomonas fluorescens A506 is an plant-epiphytic bacterium that is used commercially in the United States for the biological control of fire blight disease of pear and apple. Here, we demonstrate that A506 has a 57 kB conjugative plasmid that can transfer to other strains of Pseudomonas spp. and to members of the Enterobactericeae and is stably maintained in those bacterial hosts. We localized the origin of vegetative replication (oriV) of pA506 to a 440-nt intergenic sequence that was sufficient for plasmid replication in A506. Through comparative analysis of the pA506 sequence, we show that the plasmid has a mosaic structure with many distinct modules, each composed of genes having shared functions and similar phylogenies. Genes encoding the type IV secretion system for conjugative transfer are similar to those of pPT23A family plasmids present in many pathovars of P. syringae and, more broadly, in the MOBP6 family plasmids, which are distributed throughout the Proteobacteria. In contrast, regions of pA506 involved in plasmid replication and partitioning are atypical for plasmids in those families; consequently, pA506 has a structure and gene composition that is unique among plasmids described to date. We derived a plasmid-cured derivative of A506 and compared it to the plasmid-containing wildtype strain for UV-resistance, twitching motility, and epiphytic fitness. pA506 conferred UV resistance, presumably due to the plasmid-borne rulAB genes, but did not detectably influence epiphytic fitness of A506 on pear or apple blossoms in the field. In contrast to many other Pseudomonas plasmids, pA506 does not appear to confer resistance to antibiotics or other toxic elements. Based on the conjugative nature of pA506 and the large proportion of its genes that are shared with plasmids of from diverse groups of environmental bacteria, the plasmid is likely to serve as a vehicle for genetic exchange between A506 and its coinhabitants on plant surfaces.

Last Modified: 8/27/2014
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