FITNESS OF GENETICALLY-MODIFIED PSEUDOMONAS-FLUORESCENS IN COMPETITION FOR SOIL AND ROOT COLONIZATION

The cryIVB gene of Bacillus thuringiensis var morrisoni and the Tn5-de rived nptII marker gene were inserted into the chromosome of a rifampi cin-resistant derivative of the grass root isolate Pseudomonas fluores cens R2f, via homologous recombination. In addition, nptII alone was i nserted. Southern hybridization analysis revealed that single copies o f nptII or nptII-cryIVB had been inserted into the genomes of clones A r-1 and Art-3, respectively. The growth rates of both modified derivat ives in different liquid media were similar to that of the parent stra in. Further, neither of the two modified strains showed altered fitnes s during intermittent growth in different liquid media in competition with the parent strain. Separately introduced parent and modified stra ins showed a similar rate of decline in loamy sand soil microcosms. In soil inoculated with parent/modified strain mixtures, both modified s trains showed impaired survival compared to that of the parent. Strain Art-3 carrying nptII-cryIVB showed reduced fitness similar to that of strain Ar-1, which carried nptII. Starvation of modified strain and p arent prior to mixed introduction into soil resulted in enhanced survi val of the total added population as compared to that of a population of vegetative cells. However, starvation did not affect the competitiv e disadvantage of the modified strain. Studies on the colonization of the rhizosphere and rhizoplane of young wheat plants by parent/modifie d strain mixtures similarly showed a disadvantage for both modified st rains, since these progressively made up smaller portions of the intro duced P. fluorescens populations. Kanamycin added to soil containing t he modified/parent strain mixes did not affect the proportion of the m odified strain in the introduced P. fluorescens populations in soil. T here was, however, an effect on rhizosphere and rhizoplane colonizatio n. A drying/rewetting cycle applied to the soil inoculated with the mo dified/parent strain mixes revealed an ability to withstand moisture s tress of the modified strain similar to that of the parent.