Determination of bean nodule occupancy by Rhizobium tropici using the double gfp and gusA genetic markers constitutively expressed from a new broad-host-range vector

A new broad-host-range vector expressing constitutively the reporter genes gfp and gusA was used to evaluate nodule occupancy of Phaseolus vulgaris nodules by Rhizobium tropici. The results showed that the pHRGFPGUS plasmid was stably maintained in R. tropici over 45 generations and can therefore be used in nodule competitiveness assays. A new method for determining the nodule occupancy using the green fluorescent protein as a marker is described and is shown to be quick, inexpensive and reliable.