ELUCIDATING SORGHUM DEFENSE RESPONSES TO THE GRAIN MOLD FUNGI Fusarium moniliforme AND Curvularia lunata

120 L.F. Peterson Bldg. Department of Plant Pathology and Microbiology Texas A&M University College Station, TX 77840

Infection of florets of Sorghum bicolor (L.) Moench by several fungi, including Fusarium moniliforme Sheld. and Curvularia lunata (Wakker) Boedijn, can lead to preharvest grain molding. Elucidating sorghum defense pathways will lead to improved understanding of quantitative resistance to faculatively pathogenic fungi. Several genes and their gene products have been shown to be involved in the defense response of S. bicolor. Phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) have previously been shown to be expressed more quickly or at higher levels during pathogen attack. PAL functions very early in the phenylpropanoid pathway to remove an alpha - amino group from L - phenylalanine to form trans - cinnamic acid, the first step in phenylpropanoid metabolism. Resistant (Sureno; Tx2911) and susceptible (SC170; Tx430) panicles of S. bicolor were inoculated at flowering with F. moniliforme, C. lunata, or water. Tx430 showed an upregulation of PAL mRNA within 5 days of flowering/infection in response to F. moniliforme. Sureno (resistant) showed only brief developmental expression of PAL but no upregulation, which indicates that this genotype does not use this pathway as a major route of defense. Actin was used as a loading control. Chalcone synthase (CHS) converts p - coumarylCoA to chalcone via the condensation of three malonylCoA molecules. CHS appears to be more or less constitutive in susceptible cultivars whereas generalized upregulation was seen in inoculated treatments. The most dramatic expression was seen in Sureno, where very low levels of CHS were expressed in controls but much greater mRNA levels were observed in inoculated panicles.