HORIZONTAL GENE-TRANSFER FROM A TRANSGENIC POTATO LINE TO A BACTERIALPATHOGEN (ERWINIA-CHRYSANTHEMI) OCCURS, IF AT ALL, AT AN EXTREMELY-LOW-FREQUENCY

The frequency of possible ''horizontal'' gene transfer between a plant and a tightly associated bacterial pathogen was studied in a model sy stem consisting of transgenic Solanum tuberosum, containing a beta-lac tamase gene linked to a pBR322 origin of replication, and Erwinia chry santhemi. This experimental system offers optimal conditions for the d etection of possible horizontal gene transfer events, even when they o ccur at very low frequency, Horizontal gene transfer was not detected under conditions mimicking a ''natural'' infection. The gradual, stepw ise alteration of artificial, positive control conditions to idealized natural conditions, however, allowed the characterization of factors that affected gene transfer, and revealed a gradual decrease of the ge ne transfer frequency from 6.3 x 10(-2) under optimal control conditio ns to a calculated 2.0 x 10(-17) under idealized natural conditions, T hese data, in combination with other published studies, argue that hor izontal gene transfer is so rare as to be essentially irrelevant to an y realistic assessment of the risk involved in release experiments inv olving transgenic plants.