Survival of Tn5 mutant bean rhizobia in desert soils: Phenotypic expression of Tn5 under moisture stress

The survival and phenotypic expression of a transposon Tn5 mutant of bean rhizobia (Rhizobium leguminosarum bv. phaseoli) and the corresponding wildtype under increasing moisture stress conditions was studied in two Sonoran Desert soil types. Survival studies in sterile and nonsterile soil showed that both biotic and abiotic factors influenced the survival of the mutant and the wild type. In the silty clay loam, the Tn5 mutant population averaged ca 10⁵cfu g^-1 soil even at a moisture content of 2% (corresponding to a soil water potential of <l.5MPa). There was no significant difference (P <0.01) between the surviving populations of the wild type and the Tn5 mutant. In the sandy loam, the surviving Tn5 mutant population averaged around 10⁵cfu g^-1 soil at a moisture content of 1% (< 1.5 MPa) and again there was no significant difference between the populations of the Tn5 mutant and its wild type. During the survival of Tn5 mutant cells under increasing moisture stress conditions, there was no indication of Tn5 instability based on the phenotypic expression (kanamycin and ncomycin resistance) coded by the transposon. These studies suggest that Tn5 is a stable useful ecological marker even under moisture stress.