Induction and accumulation of polyphenol oxidase activities as implicated in development of resistance against pearl millet downy mildew disease

Polyphenol oxidase (PPO) activity was analysed in seedlings of resistant and susceptible pearl millet [Pennisetum glaucum (L.) R.Br] cultivars with or without inoculation of the downy mildew pathogen Sclerospora graminicola (Sacc.) Schroet. Seedlings of resistant varieties had greater PPO activity than susceptible seedlings, and inoculated seedlings had significantly higher PPO levels than uninoculated seedlings. Temporal accumulation of PPO showed a maximum activity at 24 h post-inoculation in resistant seedlings, whereas in susceptible seedlings it peaked at 48 h. PPO activity was positively correlated with levels of downy mildew resistance in different pearl millet cultivars under field conditions. Native PAGE staining showed four isoforms of PPO, which were difterentially induced in relation to the time of appearance and intensities in the uninoculated seedlings, whereas a fifth PPO isoform appeared after inoculation with S. graminicola. PPO activity was significantly higher in the shoot and leaves of pearl millet than in the root. Tissue printing analysis of the enzyme expression showed that the enzyme is predominantly expressed after pathogen inoculation and is localised in the epidermal and vascular regions. Temporal analysis of transcript accumulation showed that in resistant seedlings PPO mRNAs was expressed earlier and more abundantly than in susceptible seedlings. Our studies demonstrate, for the first time, that PPO is actively involved in plant defence and can be used as a marker of resistance to downy mildew infection in pearl millet.