Pseudomonas fluorescens Tn5-B20 mutant RA92 responds to carbon limitation in soil

Abstract

Tn5-B20 (lacZ as reporter gene) transcriptional fusion mutants of Pseudomonas fluorescens R2f Rpr were screened for their response to carbon limitation. Reporter gene expression was specifically induced by this stress factor in one mutant, designated RA92, and to a lower extent by phosphorus and nitrogen starvation. Expression was not detected during exponential growth. Reporter gene activity in RA92 cells increased shortly after exposure to carbon limitation and reached a plateau level within 48 h. Survival of RA92 in mineral medium without carbon source, as determined by CFU enumerations, was impaired compared to that of the parent strain, R2f Rpr. Survival and induction of reporter gene activity in exponentially grown as well as in carbon-starved RA92 cells were measured after introduction into Ede loamy sand soil. Both the exponentially grown and carbon-starved RA92 cells responded to carbon limitation in Ede loamy sand soil by induction of reporter gene expression. RA92 cells introduced into soil developed enhanced resistance to different stress factors in a fashion similar to that of the parent strain R2f Rpr. RA92 CFU numbers dropped progressively in Ede loamy sand soil portions at a rate greater than that of its parent strain. The addition of glucose to Ede loamy sand inoculated with RA92 cells gave no reduction in the fitness of RA92. Also in soil microcosms planted with wheat, the fitness of RA92 was enhanced in the layer 0–3 mm from the roots as compared to that in the deeper soil layer (3–8 mm from the roots).