Galactosides in the rhizosphere: Utilization by Sinorhizobium meliloti anddevelopment of a biosensor

Identifying the types and distributions of organic substrates that support microbial activities around plant roots is essential for a full understandi ng of plant-microbe interactions and rhizosphere ecology. We have construct ed a strain of the soil bacterium Sinorhizobium meliloti containing a gfp g ene fused to the melA promoter which is induced on exposure to galactose an d galactosides. We used the fusion strain as a biosensor to determine that galactosides are released from the seeds of several different legume specie s during germination and are also released from roots of alfalfa seedlings growing on artificial medium. Galactoside presence in seed wash and sterile root washes was confirmed by HPLC. Experiments examining microbial growth on alpha -galactosides in seed wash suggested that Lu-galactoside utilizati on could play an important role in supporting growth of S. meliloti near ge rminating seeds of alfalfa. When inoculated into microcosms containing legu mes or grasses, the biosensor allowed us to visualize the localized presenc e of galactosides on and around roots in unsterilized soil, as well as the grazing of fluorescent bacteria by protozoa. Galactosides were present in p atches around zones of lateral root initiation and around roots hairs, but not around root tips. Such biosensors can reveal intriguing aspects of the environment and the physiology of the free-living soil S. meliloti before a nd during the establishment of nodulation, and they provide a nondestructiv e, spatially explicit method for examining rhizosphere soil chemical compos ition.