SYNRGE (SYMBIOTIC NODULATION IN A REDUCED GRAVITY ENVIRONMENT) WILL INVESTIGATE MICROGRAVITY EFFECTS ASSOCIATED WITH MICROBE-HOST INTERACTIONS AND CELL-CELL COMMUNICATION USING A PLANT-BACTERIA MODEL SYSTEM. SEEDS FROM A SINGLE CULTIVAR OF THE MODEL LEGUME SPECIES MEDICAGO TRUNCATULA (BARREL MEDIC) WILL BE GERMINATED ON-ORBIT IN THE PRESENCE OF GENETICALLY MARKED STRAINS OF NITROGEN-FIXING BACTERIA OF THE SPECIES SINORHIZOBIUM MELILOTI CAPABLE OF FORMING A MUTUALISTIC ENDOSYMBIOSIS WITH LEGUMINOUS PLANTS. CELLULAR, MOLECULAR, AND MORPHOLOGICAL ANALYSES OF PLANT TISSUE SPECIMENS AND BACTERIA FIXED AT SERIAL TIME INTERVALS ON-ORBIT WILL BE COMPARED TO GROUND CONTROLS TO ELUCIDATE THE EFFECT OF MICROGRAVITY ON PLANT AND BACTERIAL PHYSIOLOGICAL RESPONSES, MECHANISMS OF CELL-CELL COMMUNICATION AND RESOURCE EXCHANGE (I.E., CARBON AND NITROGEN), AND THE EXTENT OF SYMBIOTIC NODULE FORMATION. SYMBIOTIC NITROGEN-FIXING BACTERIA AND LEGUMINOUS PLANTS HAVE EVOLVED COMPLEX SIGNAL EXCHANGE MECHANISMS THAT ENABLE SPECIFIC BACTERIAL SPECIES TO INDUCE SPECIFIC HOST PLANT SPECIES TO FORM INVASION STRUCTURES THROUGH WHICH THE BACTERIUM ENTERS THE PLANT ROOT. ONCE INSIDE, THE BACTERIA ARE ENCLOSED WITHIN A MICRO-AEROBIC CELL COMPARTMENT BY THE HOST PLANT AND DIFFERENTIATE INTO SPECIALIZED CELLS THAT EXPRESS AN OXYGEN-SENSITIVE ENZYME TO CATALYZE THE CONVERSION OF NITROGEN TO AMMONIA FOR USE BY THE HOST PLANT. ALTHOUGH THE RHIZOBIA-LEGUME MICROBE-HOST RELATIONSHIP IS MUTUALLY BENEFICIAL, RHIZOBIAL INFECTIONS OF PLANTS CAN BE DELETERIOUS. RHIZOBIA ARE SIMILAR TO BRUCELLA SPP. THAT INFECT ANIMALS (INCLUDING HUMANS) IN THAT THEY BOTH FORM CHRONIC INFECTIONS OF EUKARYOTIC CELLS WITHIN A HOST-DERIVED MEMBRANE COMPARTMENT AND REQUIRE HOST-DERIVED FACTORS FOR SURVIVAL WITHIN THE HOST. BASED ON RECENT REPORTS OF INCREASED VIRULENCE OF SALMONELLA TYPHIMURIUM, PSEUDOMONAS AERUGINOSA, AND STREPTOCOCCUS PNEUMONIAE IN REDUCED GRAVITY, DIFFERENCES IN THE INFECTIOUS AND/OR PATHOGENIC RESPONSES OF RHIZOBIAL BACTERIA OR THE SYSTEMIC ACQUIRED RESISTANCE OF THE HOST PLANT ARE ALSO LIKELY.