Resistance to Fungal Penetration in Gramineae

Abstract

Twelve species in 11 tribes of the Gramineae were inoculated with incompatible leaf-infecting fungi. Reed canarygrass [Phalaris arundinacea], timothy, big bluestem, switchgrass, smooth bromegrass, corn [Zea mays], oat [Avena sativa] and barley leaves were inoculated with Stemphylium botryosum and 781 epidermal sites where the fungus had initiated direct penetration were examined. At 779 sites, appositions developed in the epidermal cell walls and penetration was unsuccessful. At 2 sites the wall failed to thicken and penetration occurred. When inoculated leaves were floated on cycloheximide solutions (12 .mu.g/ml) to inhibit leaf responses, S. botryosum penetrated 767 of 771 sites examined and no wall thickening was observed. Thus, cycloheximide inhibited appositions and permitted epidermal penetration in all 9 spp. The 9 spp. were with an incompatible isolate of Curvularia lunata. Of 1828 penetration attempts studied, 1800 showed appositions without penetrations, 25 showed penetrations without appositions and 3 showed penetrations through thin appositions. The number of penetration attempts per 100 conidia of C. lunata varied among the grass species, ranging from 153 on oats to 314 on reed canarygrass. Cycloheximide treatment resulted in 1469 penetrations without appositions and 52 appositions without penetrations. In all species, cycloheximide inhibited the formation of appositions and permitted penetration by C. lunata. Big bluestem, rice [Oryza sativa], bermudagrass and Japanese lawngrass were inoculated with Helminthosporium maydis race T, because S. botryosum and C. lunata initiated few penetrations on those species. The sites of attempted penetration by H. maydis showed appositions without penetration. Cycloheximide treatment had the effect noted on other grasses and did not change the relatively low frequency of attempts. The morphology and color of appositions differed widely among grass species, but were uniform within a grass species inoculated with any of the 3 fungi. Grasses have both constitutive and inducible resistance mechanisms associated with the epidermis. The constitutive mechanism restricts frequency of penetration attempts and is not highly sensitive to cycloheximide. The inducible resistance mechanism is correlated with appositional cell wall formation and is sensitive to cycloheximide. The evidence that the process of appositional wall formation acts as a general mechanism of resistance to fungal penetration in the Gramineae is discussed.