Is any pTi disarmed plasmid naturally maintained in Agrobacterium strains (i.e. GV2260, LBA4404) without any selection? I assume that it should derive from wild plasmid only missing of the T-DNA region, so naturally capable to persist in the Agrobacterium cells.
Does it strictly imply that cured strain are not (or less?) virulent?
Most, if not all, of Ti plasmids contain genes that increase the plasmid stability. But several Ti plasmids such as pTiC58 are frequently lost during stress conditions such as high temperature. When you concern the loss problem, simply check its presence by PCR or marker characters in a single colony. Refer Yamamoto et al (2007, 2009) in our publication list for the stability genes.
Most, if not all, of Ti plasmids contain genes that increase the plasmid stability. But several Ti plasmids such as pTiC58 are frequently lost during stress conditions such as high temperature. When you concern the loss problem, simply check its presence by PCR or marker characters in a single colony. Refer Yamamoto et al (2007, 2009) in our publication list for the stability genes.
In non-oncogenic (a.k.a. disarmed) strains of Agrobacterium tumefaciens and Agrobacterium rhizogenes the T-DNA of the virulence plasmid has been removed or mutated (to knockout oncogenicity) by either homologous recombination or transposon mutagenesis. There are conflicting anecdotal reports about the need for selective agents to maintain the modified virulence plasmid within these bacteria, and all I can say is that for the last 15 years I have not ever used the selective agent supposedly required for virulence plasmid maintenance and yet have had no difficulty in obtaining transgenic plants. As Dr. Suzuki suggests PCR can be used to confirm that the virulence plasmid isn't being lost, but probably you won't need to do that because the virulence plasmid is remarkably stable, much more so than binary vectors.
Point of clarification, at the end of your question you said: "Does it strictly imply that cured strain are not (or less?) virulent?". A cured strain is one that is lacking altogether the virulence plasmid, so a strain like this would be completely avirulent because all of the virulence machinery encoded by genes located on the virulence plasmid is gone. There aren't many cured strains (C58 is one, but maybe someone else knows of another), and if you were to use one of these you should expect to produce exactly zero transgenic plants.
In addition to previous remarks, I'd like to add that Ti plasmids are substantially big (we are talking about plasmids ranging 200 Kb). Therefore, it's much less likely for them to be cured than for helper or binary plasmids. That explains why a selective agent, in the case of Ti plasmids, acts more as an ID to the strain than as a means to keep the plasmid in the cell population.
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