Establishment of introduced antagonistic bacteria in the rhizosphere of transgenic potatoes and their effect on the bacterial community

In a field release experiment, rifampicin resistant mutants of two antagoni stic plant-associated bacteria were used for seed tuber inoculation of tran sgenic T4 lysozyme expressing potatoes, transgenic control potatoes and non -transgenic parental potatoes. The T4 lysozyme tolerant Pseudomonas putida QC14-3-8 was originally isolated from the tuber surface (geocaulosphere) of T4 lysozyme producing plants and showed in vitro antibacterial activity to the bacterial pathogen Erwinia carotovora ssp. atroseptica. The T4 lysozym e sensitive Serratia grimesii L16-3-3 was originally isolated From the rhiz osphere of parental potatoes and showed in vitro antagonism toward the plan t pathogenic fungus Verticillium dahliae. The establishment of the inoculat ed bacteria in the rhizosphere and geocaulosphere of the different plant li nes was monitored over one growing season to assess the effect of T4 lysozy me produced by transgenic potato plants on the survival of both inoculants. Both introduced isolates were able to colonize the rhizo- and geocaulosphe re of transgenic plants and non-transgenic parental plants, and established in the rhizosphere at levels of ca. log(10) 5 colony forming units g(-1) f resh weight of root. During flowering of plants, significantly more colony counts of the T4 lysozyme tolerant P. putida were recovered from transgenic T4 lysozyme plants than from the transgenic control and the parental line. At this time, the highest level of T4 lysozyme (% of total soluble protein ) was detected. Effects of the inoculants on the indigenous microbial commu nity were monitored by analysis of PCR-amplified fragments of the 16S rRNA genes of the whole bacterial community after separation by denaturing gradi ent gel electrophoresis (DGGE). At any sampling time, the DGGE pattern of r hizosphere and geocaulosphere communities did not show differences between the inoculated and non-inoculated potatoes. Neither of the introduced strai ns became a dominant member of the bacterial community. This work was the f irst approach to assess the establishment of plant growth promoting rhizoba cteria and potential biocontrol agents on transgenic plants. (C) 2000 Feder ation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.