CORRELATIVE ASSOCIATION BETWEEN RESIDENT PLASMIDS AND THE HOST CHROMOSOME IN A DIVERSE AGROBACTERIUM SOIL POPULATION

Soil samples collected from a fallow field which had not been cultivat ed for 5 years harbored a population of Agrobacterium spp. estimated a t 3 X 10(7) CFU/g. Characterization of 72 strains selected from four d ifferent isolation media showed the presence of biovar 1 (56%) and bv. 2 (44%) strains. Pathogenicity assays on five different test plants r evealed a high proportion (33%) of tumorigenic strains in the resident population. All tumorigenic strains belonged to bv. 1. Differentiatio n of the strains by restriction fragment length polymorphism analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cellular proteins, and utilization. patterns of 95 carbon substrates (Biolog G N microplate) revealed a diversified bv. 1 population, composed of fiv e distinct chromosomal backgrounds (chr A, C, D, E, and F), and a homo geneous bv. 2 population (chr B). chr A, B, C, and D were detected at similar levels throughout the study site. According to opine metabolis m, pathogenicity, and agrocin sensitivity, chr A strains carried a nop aline Ti plasmid (pTi), whereas chr C strains had an octopine pTi. In addition, four of six nontumorigenic bv. 1 strains (two chr D, one chr E, and one chr F) had distinct and unusual opine catabolism patterns. chr B (bv. 2) strains were nonpathogenic and catabolized nopaline. Al though agrocin sensitivity is a pTi-borne trait, 14 chr B strains were sensitive to agrocin 84, apparently harboring a defective nopaline pT i similar to pAtK84b. The other two chr B strains were agrocin resista nt. The present analysis of chromosomal and plasmid phenotypes suggest s that in this Agrobacterium soil population, there is a preferential association between the resident plasmids and their bacterial host.