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Cytology and Histology

Resistance to Fungal Penetration in Gramineae. R. T. Sherwood, Plant pathologist, U.S. Regional Pasture Research Laboratory, AR, SEA, U.S. Department of Agriculture, University Park, PA 16802; C. P. Vance, plant physiologist, AR, SEA, U.S. Department of Agriculture, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul 55108. Phytopathology 70:273-279. Accepted for publication 15 May 1979. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1980. DOI: 10.1094/Phyto-70-273.

Twelve species in 11 tribes of the Gramineae were inoculated with incompatible leaf-infecting fungi to obtain evidence on how grasses resist epidermal penetration. We inoculated reed canarygrass, timothy, big bluestem, switchgrass, smooth bromegrass, corn, oat, wheat, and barley leaves with Stemphylium botryosum and examined 781 epidermal sites where the fungus had initiated direct penetration. At 779 sites, appositions developed in the epidermal cell walls and penetration was unsuccessful. At two sites the wall failed to thicken and penetration occurred. When inoculated leaves were floated on cycloheximide solutions (12 μg/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 nine species. We also inoculated the nine species with an incompatible isolate of Curvularia lunata. Of 1,828 penetration attempts studied, 1,800 showed appositions without penetrations, 25 showed penetrations without appositions, and three 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 1,469 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, 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 three fungi. The results indicted that 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.