Transport and survival of GFP-tagged root-colonizing microbes: Implications for rhizodegradation

Many recalcitrant organic contaminants pose unique and vast environmental challenges, potentially addressed through phytoremediation. Using molecular engineering to enhance enzymatic capabilities of root-colonizing microbes, additional contaminants can be targeted and contaminant fate can be altered to promote rhizosphere degradation of contaminants, which is desired among phytoremediation mechanisms. In this paper, rhizosphere bacteria were tagged with the green fluorescent protein genes (gfp) in order to monitor colonization, survival, and transport within the root zone and to evaluate the effectiveness of visual identification using GFP. Transport of the gfp-tagged root colonizers was observed to a one meter depth against a hydraulic gradient in less than 180 days revealing that plant's roots clearly enhanced movement of the recombinant strains through the rhizosphere although proliferation of the recombinant bacteria was not substantial. Over a 49-day plant growth period survival and colonization by the recombinant bacteria was monitored in soil and on roots, revealing a decreasing trend. Overall, this study showed that enhanced rhizosphere degradation is mechanistically promising, but that the specific plant-microbe pairing studied herein was not ideal. Using GFP for visual identification is not 100% efficient but provides a quick and simple marker to detect tagged microorganisms. Selection of root colonizing organisms to be engineered in enhanced rhizoremediation is a critical step in advancing the technology.