ABSTRACT Occurrence of phase variation phenomenon in fluorescent pseudomonads resulted from the Sss and XerD recombinase activities that are encoded by sss and xerD genes, respectively. In this research, the effects of sss gene in enabling colonizing bacterial strains of fluorescent pseudomonads were investigated. Only 25 bacterial strains of fluorescent pseudomonads subjected to examined, 7 bacterial strains Um141, Um11, Um138, Um70, Um115, UmCHN5 and Pseudomonas fluorescens F113 were able to adopt themselves to root under various growing conditions and colonized the totality of wheat root parts. Finally, these strains reached to the end part of roots both in complete sterilized condition and in presence of Gaeumannomyces graminis var. tritici, the causal agent of take-all disease. The laboratory cultivation of these strains on King B and SA media, resulted in the appearance of colonies with various morphology. The polymeras chain reaction(PCR) experiment to monitore sss gene in 25 bacterial strains revealed that this gene was present in seven bacterial strains representing considerable root colonization.

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