** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** NATURAL PLANT IMMUNITY TO PATHOGENS IS THE MOST ECONOMICAL PLANT PROTECTION AND IN SOME CASES IS THE ONLY FEASIBLE OPTION TO PROTECT CROPS FROM DISEASES. IN THE ENCOUNTER BETWEEN PLANTS AND THEIR PATHOGENS, THE INTERACTION BETWEEN PLANT IMMUNE RECEPTORS AND THE PATHOGEN-DERIVED MOLECULES THEY DETECT IS A KEY FACTOR IN DETERMINING WHICH SIDE WILL GAIN THE UPPER HAND. SOME IMMUNE RECEPTORS RECOGNIZE FEATURES OF THE PATHOGEN THAT ARE ESSENTIAL FOR ITS LIFESTYLE AND ARE HIGHLY CONSERVED, MAKING IT DIFFICULT FOR THE PATHOGEN TO EVADE DETECTION WITHOUT COMPROMISING VIABILITY. TO PREVENT DETECTION OF SUCH CONSERVED MICROBIAL FEATURES OR IN OTHER WAYS MANIPULATE THE HOST CELL, PATHOGENS INTRODUCE SUITES OF PROTEINS CALLED EFFECTORS INTO THE HOST CELL. THESE EFFECTOR SUITES ARE NOTORIOUSLY FLUID AND VARIABLE. NEVERTHELESS, THE HIGHLY SPECIFIC RECOGNITION OF INDIVIDUAL EFFECTORS BY CORRESPONDING PLANT IMMUNE RECEPTORS INSIDE THE CELL COLLECTIVELY PROVIDES A BROAD SPECTRUM OF RESISTANCE TO A PLANT SPECIES. IN MANY CASES, THIS EFFECTOR-TRIGGERED IMMUNITY RELIES ON INDIRECT DETECTION OF THE ACTION OF AN EFFECTOR INSIDE THE PLANT CELL, THUS BETRAYING THE PRESENCE OF A PATHOGEN. TO ASSESS THE VALUE OF A GIVEN IMMUNE RECEPTOR FOR CROP BREEDING PURPOSES, WE THEREFORE NEED TO UNDERSTAND WHAT AN EFFECTOR IS TRYING TO ACCOMPLISH FOR THE PATHOGEN ONCE IT IS INSIDE A PLANT CELL. THE BACTERIAL EFFECTOR AVRRPS4 IS PROCESSED AFTER DELIVERY INTO PLANT CELLS AND IN SOME PLANTS BOTH PARTS ARE REQUIRED TO TRIGGER IMMUNITY. LITTLE IS KNOWN ABOUT THE ACTION OF THE FRONT PART OF AVRRPS4. WE WILL USE THE RECOGNITION CAPABILITY OF LETTUCE THAT WE RECENTLY DESCRIBED TO DETERMINE WHAT THE FRONT HALF OF AVRRPS4 DOES INSIDE PLANT CELLS. THE RESULTS OF THIS PROJECT WILL FILL LONG-STANDING GAPS IN OUR KNOWLEDGE OF HOW MEMBERS OF THE AVRRPS4 BACTERIAL EFFECTOR FAMILY MANIPULATE THE HOST. THIS AND EXPANDING SUCH PLANT IMMUNITY STUDIES TO LETTUCE WILL ALLOW NOVEL INSIGHTS INTO THE ACTIONS OF PATHOGEN EFFECTORS AND A PLANT'S RESPONSE TO THEM, PROVIDING AVENUES FOR CROP IMPROVEMENT.