Inhibition of Sterol Biosynthesis Reduces Tombusvirus Replication in Yeast and Plants

ABSTRACT The replication of plus-strand RNA viruses depends on subcellular membranes. Recent genome-wide screens have revealed that the sterol biosynthesis genes ERG25 and ERG4 affected the replication of Tomato bushy stunt virus (TBSV) in a yeast model host. To further our understanding of the role of sterols in TBSV replication, we demonstrate that the downregulation of ERG25 or the inhibition of the activity of Erg25p with an inhibitor (6-amino-2-n-pentylthiobenzothiazole; APB) leads to a 3- to 5-fold reduction in TBSV replication in yeast. In addition, the sterol biosynthesis inhibitor lovastatin reduced TBSV replication by 4-fold, confirming the importance of sterols in viral replication. We also show reduced stability for the p92pol viral replication protein as well as a decrease in the in vitro activity of the tombusvirus replicase when isolated from APB-treated yeast. Moreover, APB treatment inhibits TBSV RNA accumulation in plant protoplasts and in Nicotiana benthamiana leaves. The inhibitory effect of APB on TBSV replication can be complemented by exogenous stigmasterol, the main plant sterol, suggesting that sterols are required for TBSV replication. The silencing of SMO1 and SMO2 genes, which are orthologs of ERG25, in N. benthamiana reduced TBSV RNA accumulation but had a lesser inhibitory effect on the unrelated Tobacco mosaic virus, suggesting that various viruses show different levels of dependence on sterol biosynthesis for their replication.

Management of Late Leaf Spot of Peanut with Benomyl and Chlorothalonil: A Study in Preserving Fungicide Utility

Recent registration of sterol biosynthesis inhibitor and strobilurin fungicides for control of early (Cercospora arachidicola) and late (Cercosporidium personatum) leaf spot diseases of peanut (Arachis hypogaea) has renewed interest in the potential for loss of disease control due to fungicide resistance. The objectives of this study were to use the systemic fungicide benomyl, the protectant fungicide chlorothalonil, and late leaf spot of peanut as a model system to compare fungicide application strategies for fungicide resistance management. Field experiments were conducted at Tifton and Plains, GA, in 1995 and 1996 to determine the effects of alternate applications, mixtures, and alternating block applications of chlorothalonil and benomyl compared with full-season applications of two rates of chlorothalonil and two rates of benomyl alone on late leaf spot of peanut and on the proportion of the pathogen population resistant to benomyl following the various regimes. Tank mix combinations of half rates of the two fungicides and alternations of the full rates of the two fungicides provided better (P ≤ 0.05) control of late leaf spot than full-season applications of either rate of benomyl alone, and were comparable to full rates of chlorothalonil alone. Neither tank mixes nor alternating sprays prevented an increase in the relative frequency of benomyl-resistant isolates compared with other treatments in which benomyl was used. Both mixtures and alternate applications of chlorothalonil and benomyl were effective for management of leaf spot in fields where benomyl alone did not provide season-long leaf spot control.