Colonization by bacteria of the rhizosphere (the zone surrounding plant roots) is crucial to plant productivity and plants secrete a variety of nutrients and secondary metabolites to engineer the rhizosphere to their advantage. In spite of its importance, rhizosphere colonization is poorly understood but recent advances in genome sequencing and analysis makes it possible to address this complex topic in exciting new ways.
We have recently developed maps of bacterial transcription which reveal the presence of specific plant metabolites released by roots (1). To capitalize on this information we have developed bacterial lux fusions to the promoters of many of these genes, creating metabolite-specific biosensors that enable quantitative detection of metabolites secreted by roots using a sensitive charged-coupled device (CCD) camera.
Bio-reporters based on Lux technology are particularly powerful because they enable non-invasive monitoring of plants, and we are able to map the release of metabolites to precise regions of roots (e.g. presence of phenylalanine on pea roots shown). Plants grown in square Petri dishes were inoculated with bacteria containing a specific biosensor to enable the precise temporal and spatial mapping of metabolites released by plants (2). It has also allowed us to begin screening plant mutants for both decreased and increased metabolite secretion.
(1) Ramachandran, V., East, A.K., Karunakaran, R., Downie, J.A. & Poole, P.S. (2011) Adaptation of Rhizobium leguminosarum to pea, alfalfa and sugar beet rhizospheres investigated by comparative transcriptomics. Genome Biology 12:R106.
(2) Pini, F., East, A.K., Appia-Ayme, C., Tomek, J., Karunakaran, R., Mendoza-Suarez, M., Edwards, A., Terpolilli, J., Roworth, J., Downie, J.A. & Poole, P.S. (2017) Lux bacterial biosensors for in vivo spatiotemporal mapping of root secretion. Plant Physiology 174: 1289-1306.