Cycloheximide Treatments Implicate Papilla Formation in Resistance of Reed Canarygrass to Fungi

Abstract

Reed canarygrass [Phalaris arundinacea] leaf-disks were floated on water and inoculated with spore suspensions of Helminthosporium, catenarium, H. avenae, Stemphylium botryosum, Leptosphaerulina trifolii, Botrytis cinerea and an Ascochyta sp. Only H. catenarium was pathogenic on reed canarygrass. With the noninfecting species, a papilla (host cell wall thickening) formed in the epidermal wall beneath each fungal appressorium from which penetration was attempted, and no penetration occurred. With H. catenarium most appressoria induced papilla formation and did not initiate penetration, abut about 1% of appressoria initiated direct penetration through a thin-walled, poorly-formed papillar swelling. When leaf-disks were floated on aqueous solutions of cycloheximide (10-25 .mu.g/ml), protein synthesis was inhibited papilla formation was prevented, and more than 80% of appressoria of all fungi gave rise to direct penetrations. Primary infection hyphae grew well and invaded adjacent epidermal cells. Transfer of leaf-disks from cycloheximide solutions to water prior to appressorium formation, restored protein synthesis, restored papilla formation, and prevented penetration. No inducible antifungal compounds could be detected by the drop-diffusate technique. Papilla formation appears to be a defense mechanism against fungal penetration that requires response-dependent protein synthesis.