Lux bacterial biosensors for in vivo spatiotemporal mapping of root secretion.

Plants engineer the rhizosphere to their advantage by secreting various nutrients and secondary metabolites. Coupling transcriptomic and metabolomic analysis of the Pisum sativum rhizosphere, a suite of bioreporters has been developed in Rhizobium leguminosarum bv. viciae 3841, and these detect metabolites secreted by roots in space and time. Fourteen bacterial lux-fusion bioreporters, specific for sugars, polyols, amino acids, organic acids or flavonoids, have been validated in vitro and in vivo. Using different bacterial mutants (nodC, nifH), the process of colonization and symbiosis has been analyzed, revealing compounds important in the different steps of the rhizobial-legume association. Dicarboxylates and sucrose are the main carbon sources within the nodules; in ineffective (nifH) nodules, particularly low levels of sucrose were observed suggesting that plant sanctions affect carbon supply to nodules. In contrast, high myo-inositol levels were observed prior to nodule formation and also in nifH senescent nodules. Amino-acid biosensors showed different patterns: a GABA biosensor was active only inside nodules, whereas the phenylalanine bioreporter showed a high signal also in the rhizosphere. The bioreporters were further validated in vetch, producing similar results. In addition, vetch exhibited a local increase of nod-gene inducing flavonoids at sites where nodules subsequently developed. These bioreporters will be particularly helpful to understand the dynamics of root exudation and the role of different molecules secreted into the rhizosphere.