Technical Abstract: Incubation of dormant wild oat (Avena fatua L., isoline M73) caryopses for 1 to 3 days with Fusarium avenaceum seed-decay isolate F.a.1 induced activity of the plant defense enzyme polyphenol oxidase (PPO). Both extracts and leachates obtained from F.a.1-treated caryopses had decreased abundance of an ~57 kD antigenic PPO and increased abundance of antigenic PPOs ranging from ~52 to 14 kD, compared to extracts and leachates from untreated caryopsis. Leachates from caryopsis incubated 2 days with F.a.1 also had 5.1- and 7.5-fold more total PPO activity gfwt-1, and specific activity, respectively. Fractionation of leachate proteins by ion-exchange chromatography associated the majority of PPO activity with an ~36 kD protein from untreated caryopses, and ~36, 25, and 24 kD proteins from F.a.1-treated caryopses. Predicted peptide sequences obtained from HPLC-MS/MS analyses indicated the ~57 and 36 kD wild oat proteins had strong similarity to wheat PPO. However, the 25 and 24 kD proteins were most similar to a chitinase and oxalate oxidase, respectively. Our results suggest that F.a.1 induces activation of latent PPO near the surface of wild oat caryopsis through a mechanism involving cleavage of a C-terminal PPO peptide, and may be part of a defense response that mobilizes pathogen-defense proteins to establish a protective barrier around the caryopses.