Wheat leaf resistance to Pyrenophora tritici-repentis induced by silicon activation of phenylpropanoid metabolism

Abstract

Tan spot caused by Pyrenophora tritici-repentis is a disease present in all wheat-producing countries and silicon (Si) treatment of wheat plants has been shown to increase plant resistance to tan spot. In this study, the effect of phenylpropanoid metabolism on resistance to tan spot was evaluated and some phenolic compounds that accumulated in response to P. tritici-repentis attack were identified. Furthermore, the effect of Si on phenylalanine ammonia-lyase (PAL) activity and phenolic compound accumulation were determined in situ. Antifungal activity of differentially accumulated phenolic compounds was also evaluated in in vitro tests. Results showed that the increase in concentration of phenolic compounds was greatest at the onset of infection, and that some compounds showed fungitoxic effects including fungal tip swelling, granulation of germ tube and hyphae, and hyphal hyperbranching. Silicon-induced reduction in both lesion size and tan spot disease progression were associated with activation of phenylpropanoid metabolism. PAL activity and accumulation of antifungal phenolic compounds were greater in pathogen-inoculated plants supplied with Si. In these plants, fluorescence indicative of accumulation of phenolic compounds occurred early in epidermal cells and its intensity increased during the evaluation period, showing higher numbers of fluorescent cells around infected cells. Thus, the combined responses of cell fluorescence at sites of infection, increased PAL activity and accumulation of phenols indicate that Si strengthened wheat defence responses to infection by P. tritici-repentis, reducing the severity of tan spot.