Use of bacterial bioluminescence for monitoring the behavior of rhizobacteria introduced to plant rhizosphere

Bioluminescence (Lux) genes from Vibrio harveyi were introduced into an artificial transposon vector pJFF350 as a constitutively expressing lux-genes set. The resultant recombinant plasmid pSOL2 was then transposed into the chromosome of fluorescent pseudomonad strains, in which the lux genes were stably integrated. Root colonization by the lux-marked strains was visually monitored by autophotography, and it was found that the bioluminescence intensities were highly correlated with the populations of the strains colonizing roots. A strain of Pseudomonas putida thus lux-marked was inoculated to spinach seedlings in nursery pots, and the root colonization was assayed. In sterile soil, the strain extensively colonized the entire part of the developing roots and the population densities at any sites were high. In non-sterile soil, only the inoculated site of the roots was weakly colonized. These results suggest that the presence of the indigenous microflora is an environmental constraint on root colonization of the introduced bacteria.