Better to light a candle than to curse the darkness: Lux expressing biosensors in the rhizosphere

Microbial biosensors designed with the luxCDABEG reporter genes (that code for light- emitting proteins) fused to a promoter of interest offer the opportunity for continuous measurements of environmental conditions from the soil.  We use the common soil bacterium Pseudomonas putida KT2440 as host to two different plasmids we developed to report on water potential and microbial growth in the rhizosphere. The first plasmid pZKH2 contains a fusion between the strong constituitive promoter npt and the lux genes that reports on growth of the bacteria.  The second construct, pPHCA1, utilizes a proU-luxCDABEG fusion to track changes in water potential around the bacterium.  After testing biosensor response in liquid media, we used both biosensors in microcosms filled with non-sterile soil in which Zea mays L. was growing.  Using a very sensitive CCD-camera to view light production through the glass sides of the microcosm, we detected temporal and spatial patterns in carbon availability. Biosensors demonstrated pulses of bacteria growth near the root tips likely associated with the release of carbon. The biosensors were able to continuously report from around roots for as long as six days at a time with the ability to follow bacterial activity in real time around growing roots.