An extra repABC locus in the incRh2 Ti plasmid pTiBo542 exerts incompatibility toward an incRh1 plasmid

Complete sequence of the tumor-inducing plasmid pTiChry5 from the hypervirulent Agrobacterium tumefaciens strain Chry5

Agrobacterium tumefaciens strain Chry5 is hypervirulent on many plants including soybean that are poorly transformed by other A. tumefaciens strains. Therefore, it is considered as a preferred vector for genetic transformation of plants. Here we report the complete nucleotide sequence of its chrysopine-type Ti-plasmid pTiChry5. It is comprised of 197,268 bp with an overall GC content of 54.5%. Two T-DNA regions are present and 219 putative protein-coding sequences could be identified in pTiChry5. Roughly one half of the plasmid is highly similar to the agropine-type Ti plasmid pTiBo542, including the virulence genes with an identical virG gene, which is responsible for the supervirulence caused by pTiBo542. The remaining part of pTiChry5 is less related to that of pTiBo542 and embraces the trb operon of conjugation genes, genes involved in the catabolism of Amadori opines and the gene for chrysopine synthase, which replaces the gene for agropine synthase in pTiBo542. With the exception of an insertion of IS869, these Ti plasmids differ completely in the set of transposable elements present, reflecting a different evolutionary history from a common ancestor.

Effective removal of a range of Ti/Ri plasmids using a pBBR1-type vector having a repABC operon and a lux reporter system

Ti and Ri plasmids of pathogenic Agrobacterium strains are stably maintained by the function of a repABC operon and have been classified into four incompatibility groups, namely, incRh1, incRh2, incRh3, and incRh4. Removal of these plasmids from their bacterial cells is an important step in determining strain-specific virulence characteristics and to construct strains useful for transformation. Here, we developed two powerful tools to improve this process. We first established a reporter system to detect the presence and absence of Ti/Ri plasmids in cells by using an acetosyringone (AS)-inducible promoter of the Ti2 small RNA and luxAB from Vibrio harveyi. This system distinguished a Ti/Ri plasmid-free cell colony among plasmid-harboring cell colonies by causing the latter colonies to emit light in response to AS. We then constructed new "Ti/Ri eviction plasmids," each of which carries a repABC from one of four Ti/Ri plasmids that belonged to incRh1, incRh2, incRh3, and incRh4 groups in the suicidal plasmid pK18mobsacB and in a broad-host-range pBBR1 vector. Introduction of the new eviction plasmids into Agrobacterium cells harboring the corresponding Ti/Ri plasmids led to Ti/Ri plasmid-free cells in every incRh group. The Ti/Ri eviction was more effective by plasmids with the pBBR1 backbone than by those with the pK18mobsacB backbone. Furthermore, the highly stable cryptic plasmid pAtC58 in A. tumefaciens C58 was effectively evicted by the introduction of a pBBR1 vector containing the repABC of pAtC58. These results indicate that the set of pBBR1-repABC plasmids is a powerful tool for the removal of stable rhizobial plasmids.

Ecological and evolutionary dynamics of a model facultative pathogen: Agrobacterium and crown gall disease of plants

Many important pathogens maintain significant populations in highly disparate disease and non-disease environments. The consequences of this environmental heterogeneity in shaping the ecological and evolutionary dynamics of these facultative pathogens are incompletely understood. Agrobacterium tumefaciens, the causative agent for crown gall disease of plants has proven a productive model for many aspects of interactions between pathogens and their hosts and with other microbes. In this review, we highlight how this past work provides valuable context for the use of this system to examine how heterogeneity and transitions between disease and non-disease environments influence the ecology and evolution of facultative pathogens. We focus on several features common among facultative pathogens, such as the physiological remodelling required to colonize hosts from environmental reservoirs and the consequences of competition with host and non-host associated microbiota. In addition, we discuss how the life history of facultative pathogens likely often results in ecological tradeoffs associated with performance in disease and non-disease environments. These pathogens may therefore have different competitive dynamics in disease and non-disease environments and are subject to shifting selective pressures that can result in pathoadaptation or the within-host spread of avirulent phenotypes.