GENETIC-ANALYSIS OF NONPATHOGENIC AGROBACTERIUM-TUMEFACIENS MUTANTS ARISING IN CROWN GALL TUMORS

Little is known about the effect of the host on the genetic stability of bacterial plant pathogens. Crown gall, a plant disease caused by Ag robacterium tumefaciens, may represent a useful model to study this ef fect. Indeed, our previous observations on the natural occurrence and origin of nonpathogenic agrobaeteria suggest that the host plant might induce loss of pathogenicity in populations of A. tumefaciens. Were w e report that five different A. tumefaciens strains initially isolated from apple tumors produced up to 99% nonpathogenic mutants following their reintroduction into axenic apple plants. Two of these five strai ns were also found to produce mutants on pear and/or blackberry plants . Generally, the mutants of the apple isolate D10B/87 were altered in the tumor-inducing plasmid, harboring either deletions in this plasmid or point mutations in the regulatory virulence gene virG. Most of the mutants originating from the same tumor appeared to he of clonal orig in, implying that the host plants influenced agrobacterial populations by favoring growth of nonpathogenic mutants over that of wild-type ce lls. This hypothesis was confirmed by coinoculation of apple rootstock s with strain D10B/87 and a nonpathogenic mutant.