The Cryptic Plasmid of Agrobacterium tumefaciens Cointegrates with the Ti Plasmid and Cooperates for Opine Degradation

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Abstract
We crossed the Agrobacterium tumefaciens chrysanthemum strain ANT4, which harbors four plasmids, with the plasmid-free recipient C58.00RS. Transconjugants degrading the Amadori-opines chrysopine and deoxy-fructo-syl-oxo-proline (dfop) harbored the Ti plasmid of ANT4, termed pAtANT4b. Upon transfer to the recipient strain C58.00RS, pAtANT4b (pTiANT4) and pANT4a (the largest of the four plasmids of ANT4) could cointegrate. The cointegration of the two plasmids occurs at various places of the pTiANT4, a feature that may affect several functions of the Ti plasmid (e.g., opine degradation). Transcon-jugants utilizing the opine deoxy-fructosyl-glutamine (dfg) always harbored the large pAtANT4a. Other Agrobacterium strains, including nonpathogenic strains such as C58C1, naturally degraded dfg. Remarkably, strain C58C1 carries a large cryptic plasmid termed pAtC58 that also encodes dfg degradation. A screening of physiological traits additionally revealed that this plasmid allows utilization of octopine as sole nitrogen source after mutation. All these results demonstrate that the larger plasmid of A. tumefaciens is a catabolic plasmid and that both the “cryptic” plasmid and Ti plasmid cooperate for opine degradation.