Papilla response of barley epidermal cells caused by Erysiphe graminis: rate and method of deposition determined by microcinematography and transmission electron microscopy

Abstract

Papillae were deposited in barley epidermal cells directly beneath appressoria of Erysiphe graminis f. sp. hordei and appeared as hemispherical, internal wall appositions. The papilla response began shortly after the formation of a rapidly moving cytoplasmic aggregate beneath the appressorium. As documented in coleoptile tissue by time-lapse light microcinematography, the papillae grew rapidly for 20–30 min after becoming visible, their radii increasing by 0.1 μm/min. For small papillae, deposition continued for about 30 min; for larger papillae, deposition continued for 120–180 min. Results with transmission electron microscopy on leaf epidermal cells suggested that papilla deposition by host cytoplasmic aggregates can be divided into four sequential stages: (i) the deposition of osmiophilic (lipidic) materials, (ii) the deposition and partial compaction of nonosmiophilic, amorphous material (probably insoluble polysaccharides), (iii) compaction of nonosmiophilic, amorphous material, and (iv) the incorporation of osmiophilic material into the host wall and into the compacted nonosmiophilic, amorphous material. At maturity, the papillae are hardened, electron-opaque wall appositions that may be effective in preventing fungal penetration and development. Failure of papillae to prevent fungal penetration and development may be related to the inability of the epidermal cells to complete the entire sequence of events in papilla deposition before attempted fungal penetration.