The inhibition of melanin biosynthetic reactions in pyricularia oryzae by compounds that prevent rice blast disease
Wheeler, M.H.; Greenblatt, G.A.
Experimental Mycology 12(2): 151-160
ISSN/ISBN: 0147-5975 DOI: 10.1016/0147-5975(88)90004-7
Nine compounds, known to prevent melanin biosynthesis in appressoria of Pyricularia oryzae and penetration of rice plants via appressoria, inhibited the enzymatic reduction of 1,3,6,8-tetrahydroxynaphthalene and 1,3,8-trihydroxynaphthalene to scytalone and vermelone, respectively; they did not inhibit enzymatic reactions that dehydrate scytalone and vermelone to 1,3,8-trihydroxynaphthalene and 1,8-dihydroxynaphthalene, respectively. The compounds had the same order of effect in inhibiting reductase reactions as previously reported for preventing penetration of leaf surfaces and inhibiting melanin biosynthesis. Three of the four strongest reductase inhibitors (tricyclazole, fthalide, and pyroquilon) are used commercially to prevent rice blast disease. The fourth, chlobenthiazone, is also an extremely effective agent against the disease. Extracts from buff mutants of P. oryzae that lack the ability to penetrate leaf surfaces and cause rice blast disease had minimal reductase activity, but carried out normal dehydratase reactions. The buff extracts behaved similarly to those of the wild type treated with strong melanin inhibitors, e.g., tricyclazole. The current in vitro results with cell-free extracts and HPLC techniques complement earlier in vivo results with inhibition of melanin biosynthesis and penetration via appressoria and suggest that the effect of melanin inhibitors on reductase activity can be used to predict their efficacy in preventing rice blast disease.