Vapors, fluorescence and bioluminescence: New approaches and insights into plant and microbial processes in the rhizosphere
Soil microbial activity controls the decomposition of organic material and ultimately the recycling of key nutrients up into higher trophic levels. The availability of water and energy (carbon) in soils are critical determinants of this activity. Yet soils are biologically and structurally complex systems that present significant technical hurdles to studying these processes. We have applied novel methodologies in three experiments to understand soil microbial activity at the intersections of water availability, root activity, and carbon availability. In the first experiment, we injected isotopically-labeled acetic acid vapor into soils to test whether water availability influenced microbial growth efficiency (MGE). MGE is the efficiency with which microbes build biomass from assimilated carbon and fundamentally binds together the immobilization and availability of carbon and nitrogen in soil. Interestingly, microbial growth efficiencies in our test cores, determined using 13C-acetic acid gaseous injections, did not vary in soil ranging from 9.7% to 20.7% soil moisture; they hovered around 47%. To study how roots were influencing processes, we applied microbial biosensors to study patterns of water and carbon availability in the rhizosphere. Microbial biosensors were tested extensively in liquid cultures that demonstrated their ability to produce reliable reports in nonsterile soils. The biosensor, Pantoea agglomerans BRT98 hosted a reporter system consisting of a proU-GFP construct that responded to variation in water potential (a function of both osmotic and matric potentials) in nonsterile soil. The proU-GFP microbiosensor reported more negative soil water potentials as a function of axial distance from tips of Zea mays L. roots, reflecting the gradient in water potential hypothesized to develop during transpiration. The second biosensor used the common soil bacterium Pseudomonas putida KT2440 as host to a plasmid we developed to report on energy availability in the rhizosphere. Plasmid pZKH2 contains a fusion between the strong constitutive promoter nptII and the genes that enocode for a light-emitting reaction, luxCDABE, from Vibrio fischeri. Biosensor KT2440/pZKH2 was able to continuously report on carbon availability around plant roots for as long as six days.