Action of the antipenetrant, tricyclazole, on appressoria of Pyricularia oryzae

Abstract

The antipenetrant, tricyclazole, blocked melanin biosynthesis and protected unwounded, but not wounded, rice plants from infection by a wild-type isolate of Pyricularia oryzae. Two buff mutants genetically deficient in melanin biosynthesis infected wounded but not unwounded plants. Tricyclazole (0·1–10 μg ml−1) did not inhibit spore germination or appressorial formation by spores of P. oryzae on detached epidermal sections from Bryophyllum pinnatum, but completely blocked appressorial melanization and prevented fungal penetration of the epidermal cell walls. The walls were penetrated by 80–95% of melanized control appressoria. Penetration of 135–145 nm Formvar (polyvinyl formal) plastic membranes was accomplished by 54–77% of control appressoria but only by 0–2% of tricyclazole treated appressoria. The cutinase inhibitor, diisopropylfluorophosphate, at 50 μm or less had little or no effect on spore germination or penetration of B. pinnatum epidermal walls. Concentrations of 100 and 150 μm strongly inhibited penetration, but were also toxic to fungal growth. Melanized walls of control appressoria were impermeable to sucrose and various other solutes, but walls of tricyclazole treated appressoria were freely permeable to these solutes. Ultra-structural studies revealed an electron-dense layer in walls of control appressoria which thickened at the base to form a reinforced region surrounding the periphery of the thin underwall in contact with the cuticle. This electron-dense layer was absent in walls of treated appressoria. The data suggest that failure of tricyclazole treated appressoria to accomplish cuticular penetration is due to the lack of rigidity of the unmelanized appressorial walls.