SORGHUM IS AN ATTRACTIVE BIOENERGY CROP WITH HIGH YIELD POTENTIALS AND SIGNIFICANT TOLERANCE TO DROUGHT AND HEAT. HOWEVER, SORGHUM IS PRONE TO STALK ROTS, WHICH CAN SIGNIFICANTLY LIMIT SORGHUM BIOMASS PRODUCTION THROUGH YIELD REDUCTIONS AND LODGING. STALK ROT-CAUSING FUNGI NORMALLY GROW ENDOPHYTICALLY WITHIN SORGHUM PLANTS. WHEN SORGHUM PLANTS EXPERIENCE WATER STRESS, CHANGES IN HOST METABOLISM OFTEN TRIGGERS A DEVELOPMENTAL SWITCH CAUSING THE FUNGI TO BECOME PATHOGENIC. THE UNDERLYING PLANT MOLECULAR CIRCUITS THAT CAN LIMIT OR EXACERBATE FUNGAL TRANSITION FROM ENDOPHYTIC TO PATHOGENIC GROWTH ARE NOT KNOWN AND ARE THE FOCUS OF THIS PROPOSAL. SEVERAL PUBLICLY AVAILABLE LINES HAVE PREVIOUSLY DEMONSTRATED RESISTANCE OR TOLERANCE TO SORGHUM STALK PATHOGENS, INCLUDING THOSE WITH POSTFLOWERING DROUGHT TOLERANCE (NONSENSCENCE), WHICH APPEARS TO SUPPRESS PATHOGENIC GROWTH. IN ADDITION, WE HAVE DEVELOPED SEVERAL NEAR-ISOGENIC SORGHUM BROWN-MIDRIB (BMR) 6 AND 12 LINES WITH REDUCED LIGNIN CONTENT, WHICH WERE PREVIOUSLY DEMONSTRATED TO HAVE INCREASED RESISTANCE OR TOLERANCE TO SORGHUM STALK PATHOGENS. LIGNIN, A COMPONENT OF PLANT CELL WALLS, HAS BEEN A FOCUS FOR DEVELOPMENT OF BIOENERGY SORGHUMS BECAUSE IT INHIBITS CELLULOSIC CONVERSION TECHNOLOGIES, BUT ITS PRESENCE ALSO INCREASES TOTAL ENERGY CONTENT OF BIOMASS, WHICH IS IMPORTANT FOR THERMAL CONVERSION TECHNOLOGIES. TO INCREASE ENERGY CONTENT, WE HAVE ENGINEERED SORGHUM PLANTS OVEREXPRESSING A MYB TRANSCRIPTION FACTOR THAT INDUCES LIGNIN SYNTHESIS, AND A GENE ENCODING CAFFEOYL-COA O-METHYLTRANSFERASE (CCOAOMT), A MONOLIGNOL PATHWAY ENZYME. BOTH THE TRANSGENIC AND BMR PLANTS ACCUMULATE PHENOLIC INTERMEDIATES FROM LIGNIN SYNTHESIS THAT ARE TOXIC TO STALK PATHOGENS IN VITRO. WE RECENTLY HAVE DEVELOPED THE ABILITY TO APPLY WATER-STRESS IN A CONTROLLED ENVIRONMENT, WHICH RELIABLY INDUCES THE DEVELOPMENTAL SWITCH FROM ENDOPHYTIC TO PATHOGENIC GROWTH OF SORGHUM STALK ROT FUNGI. USING THIS UNIQUE COLLECTION OF PLANT LINES DESCRIBEDABOVE, IN COMBINATION WITH GENOMICS AND METABOLOMICS TOOLS, WE WILL DIRECTLY EVALUATE THE HYPOTHESIS THAT IDENTIFYING MOLECULAR COMPONENTS OF HOST RESISTANCE WILL LEAD TO INCREASED STALK ROT TOLERANCE IN SORGHUM, USING TWO NOTORIOUSLY VIRULENT FUNGI THAT CAUSE SORGHUM STALK ROTS, FUSARIUM THAPSINUM AND MACROPHOMINA PHASEOLINA. THE SPECIFIC OBJECTIVES OF THIS PROPOSAL ARE: 1) TO DETERMINE PATHOGENIC AND ENDOPHYTIC GROWTH OF STALK PATHOGENS IN SORGHUM LINES UNDER WATER DEFICIT CONDITIONS; 2) TO IDENTIFY HOST METABOLITES AND METABOLIC PATHWAYS INVOLVED IN RESISTANCE OR TOLERANCE TO FUNGAL STALK ROT PATHOGENS UNDER WATER DEFICIT CONDITIONS IN LIGNIN MODIFIED, NONSENESCENT AND STALK ROT RESISTANT/TOLERANT LINES; 3) TO IDENTIFY HOST GENES FROM THE PHENYLPROPANOID AND DEFENSE-RELATED PATHWAYS WITH ALTERED EXPRESSION LEVELS DURING PATHOGENIC OR ENDOPHYTIC GROWTH OF FUNGAL STALK PATHOGENS UNDER WATER DEFICIT CONDITIONS; AND 4) TO IDENTIFY GENES, GENE NETWORKS, AND METABOLIC PATHWAYS WHOSE EXPRE,SSION IS ALTERED IN STALK ROT TOLERANT SORGHUM LINES UNDER WATER SUFFICIENT VERSUS DEFICIT CONDITIONS. OUR AIM IS TO DISCOVER HOST MOLECULAR PATHWAYS THAT ENHANCE ENDOPHYTIC GROWTH OF STALK FUNGI AND INHIBIT THE DEVELOPMENTAL SWITCH TO PATHOGENIC GROWTH THAT FREQUENTLY OCCURS UNDER PERIODS OF PROLONGED ABIOTIC STRESS. FROM THIS VALUABLE RESEARCH, WE WILL IDENTIFY BIOMOLECULAR MARKERS FOR RESISTANCE THAT WILL SIGNIFICANTLY ENHANCE EFFORTS TO DEVELOP SUPERIOR BIOENERGY SORGHUM WITH RESISTANCE TO INCREASING DISEASE AND ENVIRONMENTAL STRESSES.