Transfer of virulence in vivo and in vitro in Agrobacterium

Abstract

TRANSFER of virulence from a pathogenic to a non-pathogenic strain of Agrobacterium has been reported to occur in vivo^1,2 and is due to the transfer of a plasmid which codes for virulence^3,4. In the experimental conditions described, virulence transfer can be detected only after several weeks. The reason for this delay is not clear. The most likely explanations are (1) that infrequent transfer occurs long before it can be detected, detection being dependent on a subsequent increase in the ratio of converted to unconverted recipients or (2) that conditions suitable for virulence transfer occur only after donor and recipient have been in contact for several weeks. The main difficulty in distinguishing between these two alternatives is technical. When pathogenicity testing is used to detect virulence transfer, low frequency transfer is impossible to detect unless thousands of inoculations are made at each assessment. It is known, however, that the virulence plasmid codes for characters other than virulence^5–7. One of these is utilisation of octopine and, as Kerr⁸ has pointed out, this could be used instead of pathogenicity testing to detect plasmid transfer. This communication describes both in vivo and in vitro transfer of virulence using octopine utilisation to detect plasmid transfer.