**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** BACTERIA ARE EVERYWHERE AND ARE TYPICALLY WITHIN COMMUNITIES WITH OTHER MICROBES. AS SUCH, BACTERIA NEED TO SENSE OTHERS AND DIFFERENTIATE KIN FROM NON-KIN CELLS TO MAKE DECISIONS ON WHETHER TO COMPETE OR COOPERATE FOR NUTRIENTS. UNDERSTANDING THE RULES THAT GOVERN COMMUNITY ASSEMBLAGE AND MAINTENANCE IS IMPORTANT FOR INFORMING ON MANAGING DISEASES AND IN APPLYING BACTERIA TO IMPROVE THE HEALTH OF PLANTS. THE TYPE VI SECRETION SYSTEM (T6SS) IS AN APPARATUS USED BY MANY GRAM-NEGATIVE BACTERIA TO LAUNCH EFFECTOR PROTEINS INTO OTHER BACTERIA. THESE EFFECTORS ARE OFTEN TOXIC AND CAN PREVENT BACTERIA FROM GROWING OR BE LETHAL. HOWEVER, NOT ALL BACTERIA ARE SUSCEPTIBLE, AS MANY CAN CARRY IMMUNITY GENES OR HAVE TRAITS THAT MAKE THEM UNAFFECTED BY ATTACKER CELLS. HENCE, THE T6SS HAS A DUAL ROLE IN SUPPRESSING COMPETING BACTERIA AND DIFFERENTIATING RELATED CELLS, POTENTIALLY AFFECTING HOW MICROBIAL COMMUNITIES ARE ASSEMBLED AND MAINTAINED. THIS PROPOSAL HAS THREE OVERARCHING GOALS. THE FIRST IS TO UNDERSTAND THE MOLECULAR MECHANISMS BY WHICH EFFECTORS ARE LOADED ONTO THE T6SS. CURRENT THINKING IS THAT SO-CALLED SPECIALIZED EFFECTORS ARE LOADED DIRECTLY BECAUSE THEY ARE PHYSICALLY LINKED TO A COMPONENT OF THE T6SS. CURRENT THINKING ALSO SUGGESTS THAT EFFECTORS ARE ASSOCIATED TO A SPECIFIC PROTEIN VARIANT OF THE T6SS. OUR DATA ARE NOT CONSISTENT WITH MODELS AND POTENTIALLY SHOW THAT LOADING IS MORE FLEXIBLE THAN CURRENTLY UNDERSTOOD AND REVEALING THE PROCESS WILL PROVIDE A MORE COMPLETE UNDERSTANDING OF THE COMPLEXITIES OF T6SS BIOLOGY. SECOND, THIS PROPOSAL SEEKS TO IDENTIFY TRAITS WHICH ARE NOT ASSOCIATED WITH EFFECTOR IMMUNITY THAT INFLUENCE T6SS-BASED INTERACTIONS. THIS IS IMPORTANT FOR BEING ABLE TO UNDERSTAND, AND EVENTUALLY PREDICT, WHICH BACTERIA ARE CAPABLE OF CO-EXISTING IN MICROBIAL COMMUNITIES. LAST, THIS PROPOSAL WILL EXPERIMENTALLY TEST HOW T6SS-MEDIATED INTERACTIONS INFLUENCE THE ABILITY OF BACTERIA TO COLONIZE PLANTS AND CAUSE DISEASE. ADDRESSING THE LAST GOALWILL YIELD KNOWLEDGE THAT INFORMS ON DISEASE MANAGEMENT STRATEGIES.