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

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.     

Isolation of a field strain of Brucella abortus from RB51-vaccinated- and brucellosis-seronegative bovine yearlings that calved normally

A study was carried out in Pichucalco, Chiapas (Mexico) to determine whether recently calved cows or those that aborted shed Brucella. Serological diagnosis of brucellosis was made in all animals (209). Six of the cows that calved normally and two that aborted underwent a bacteriological study of milk and vaginal exudate. Brucella abortus was isolated from vaginal exudate samples in two 3- to 4-year-old seronegative first-birth cows that had calved normally. This was confirmed through bacteriological identification and PCR as a field strain and smooth phenotypes. We conclude that seronegative cows vaccinated with RB51 which calved normally and shed B. abortus in the vaginal exudate after calving could be a serious problem because these cows are overlooked in routine diagnoses and are a source of Brucella infection.