Incompatibility properties of tartrate utilization plasmids derived from Agrobacterium vitis strains

The Ecology of Agrobacterium vitis and Management of Crown Gall Disease in Vineyards

Agrobacterium vitis is the primary causal agent of grapevine crown gall worldwide. Symptoms of grapevine crown gall disease include tumor formation on the aerial plant parts, whereas both tumorigenic and nontumorigenic strains of A. vitis cause root necrosis. Genetic and genomic analyses indicated that A. vitis is distinguishable from the members of the Agrobacterium genus and its transfer to the genus Allorhizobium was suggested. A. vitis is genetically diverse, with respect to both chromosomal and plasmid DNA. Its pathogenicity is mainly determined by a large conjugal tumor-inducing (Ti) plasmid characterized by a mosaic structure with conserved and variable regions. Traditionally, A. vitis Ti plasmids and host strains were differentiated into octopine/cucumopine, nopaline, and vitopine groups, based on opine markers. However, tumorigenic and nontumorigenic strains of A. vitis may carry other ecologically important plasmids, such as tartrate- and opine-catabolic plasmids. A. vitis colonizes vines endophytically. It is also able to survive epiphytically on grapevine plants and is detected in soil exclusively in association with grapevine plants. Because A. vitis persists systemically in symptomless grapevine plants, it can be efficiently disseminated to distant geographical areas via international trade of propagation material. The use of healthy planting material in areas with no history of the crown gall represents the crucial measure of disease management. Moreover, biological control and production of resistant grape varieties are encouraging as future control measures.

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

Ti/Ri plasmids in pathogenic Agrobacterium species are repABC replicons that are stably maintained by the function of repABC genes. Two Ti plasmids, pTiBo542 and pTiS4, belonging to incRh2 and incRh4 incompatibility groups, respectively, were reported to carry two repABC loci. In the present study, to reveal the roles of the two repABC loci in the two plasmids, we constructed mini-replicons carrying any one or both of the repABC loci (referred to as repABC1 and repABC2 here) and examined their replication and incompatibility properties. The introduction of mini-replicons into A. tumefaciens C58C1 strains suggested that repABC1 functions as replicator genes but repABC2 does not in both the Ti plasmids. Because the components of repABC2 of pTiBo542 have highly similar amino acid and nucleotide sequences to those of the incRh1-type repABC replicon, we introduced repABC2-containing replicons into cells harboring an incRh1 plasmid in order to check their incompatibility traits. As a result, the repABC2-containing replicon expelled the resident incRh1 plasmid, indicating that the extra repABC locus is dispensable for replication and could work as an incompatibility determinant against incRh1 group plasmids. We suggest that the locus contributes to plasmid retention by eliminating the burden of co-existing competitive plasmids in host cells through its incompatibility.

The repABC Plasmids with Quorum-Regulated Transfer Systems in Members of the Rhizobiales Divide into Two Structurally and Separately Evolving Groups

The large repABC plasmids of the order Rhizobiales with Class I quorum-regulated conjugative transfer systems often define the nature of the bacterium that harbors them. These otherwise diverse plasmids contain a core of highly conserved genes for replication and conjugation raising the question of their evolutionary relationships. In an analysis of 18 such plasmids these elements fall into two organizational classes, Group I and Group II, based on the sites at which cargo DNA is located. Cladograms constructed from proteins of the transfer and quorum-sensing components indicated that those of the Group I plasmids, while coevolving, have diverged from those coevolving proteins of the Group II plasmids. Moreover, within these groups the phylogenies of the proteins usually occupy similar, if not identical, tree topologies. Remarkably, such relationships were not seen among proteins of the replication system; although RepA and RepB coevolve, RepC does not. Nor do the replication proteins coevolve with the proteins of the transfer and quorum-sensing systems. Functional analysis was mostly consistent with phylogenies. TraR activated promoters from plasmids within its group, but not between groups and dimerized with TraR proteins from within but not between groups. However, oriT sequences, which are highly conserved, were processed by the transfer system of plasmids regardless of group. We conclude that these plasmids diverged into two classes based on the locations at which cargo DNA is inserted, that the quorum-sensing and transfer functions are coevolving within but not between the two groups, and that this divergent evolution extends to function.