Ecology and genetic stability of Tn5 mutants of bean rhizobia in Sonoran desert soils.

Abstract

Five transposon Tn5 mutants of bean rhizobia (Rhizobium leguminosarum b.v. phaseoli) and the wild type strain were used in ecological studies to evaluate the efficacy of transposon Tn5 as a phenotypic marker in rhizobia for ecological studies in two Sonoran desert soils. All mutants possessed chromosomal insertions of the transposable element. Survival of each mutant strain was compared to that of the wild type strain under non stress, moisture stress and temperature stress conditions in Pima silty clay loam and Brazil to sandy loam. The genetic stability of Tn5 in terms of transposition of the element within the chromosome and the Tn5 coded antibiotic resistant phenotype was determined in cells recovered throughout the survival period. Under non stress conditions, the viable Tn5 mutant population decreased in size. Two mutants showed significantly (p < 0.01) lower populations than the wild type at the end of 30 days in the silty clay loam. In the sandy loam, four of the five mutant populations were significantly lower than the wild type. Tn5 was genetically stable in both soils. Under moisture stress conditions, the decline of the Tn5 mutant and wild type populations corresponded to a decline in soil moisture content. The finer textured soil afforded more protection to the cells than the coarse textured soil. There were no indications of Tn5 instability under moisture stress. In both soils under temperature stress, sizes of all populations declined rapidly and after 12 days, the mutant cells when screened using the Tn5 coded markers were significantly less in numbers than the wild type indicating a loss of Tn5 coded antibiotic resistance phenotype. There were no significant differences in numbers between wild type and mutant cells when screened using only the intrinsic markers. DNA:DNA hybridizations confirmed that the lack of Tn5 coded antibiotic resistance phenotype was probably not due to a deletion or transposition of the element. Under non stress conditions Tn5 is a useful ecological marker, but each Tn5 mutant has to be evaluated independently under specific environmental conditions to determine the efficacy of Tn5 as an ecological marker.