Induction of Polyphenol Oxidase Activity in Dormant Wild Oat (Avena fatua) Seeds and Caryopses: A Defense Response to Seed Decay Fungi

Abstract

Persistence of the soil seed bank requires both dormancy and resistance to seed decay organisms. However, there is little or no information evaluating biochemical responses of dormant weed seeds to pathogens. Wild oat caryopses were incubated with four pathogenic fungal isolates to evaluate the response of the pathogen defense enzyme, polyphenol oxidase (PPO). Caryopsis PPO activity was induced by threeFusariumspp. isolates previously obtained from whole seeds incubated in the field whereas caryopsis PPO activity was decreased by aPythiumisolate.Fusarium avenaceumisolateF.a.1 caused the greatest PPO induction and was studied in more detail. When whole wild oat seeds were incubated onF.a.1, PPO activity was induced in seeds, hulls (lemma and palea), and caryopses. Incubation of whole seeds onF.a.1 gradually induced caryopsis PPO activity over an 8-d period, whereas incubation of caryopses onF.a.1 over a 4-d period caused a greater and more rapid induction of PPO activity. Very little PPO activity could be leached from untreated caryopses, but nearly all of the induced PPO activity inF.a.1-treated caryopses was readily leached when incubated in buffer. In Western blots, both untreated andF.a.1-treated leachates contained a ∼57-kilodalton (kD) protein, putatively the mature and relatively inactive form of PPO. However, lower molecular weight antigenic proteins between ∼52 and ∼25 kD were strongly induced inF.a.1-treated caryopses, with this induction being correlated with the increase in PPO activity. We hypothesize that dormant weed seeds possess biochemical defenses against pathogens and, more specifically, that proteolysis in the presence of fungal pathogens may release an activated form of PPO from the surface of wild oat caryopses and hulls.