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Endomembrane system of aspen root cells plays a key role in defense against a common fungal root endophyte, Cryptosporiopsis radicicola

     Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9

Ichiko Tsuneda

     Medicine/Dentistry Electron Microscopy Unit, University of Alberta, Edmonton, Alberta Canada T6G 2N8

Randolph S. Currah

     Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9

The host-endophyte interaction between roots of aspen (Populus tremuloides) and Cryptosporiopsis radicicola was examined primarily by transmission electron microscopy. Hyphae growing on the exterior of the inoculated roots had a thick, electron-dense, adhesive sheath. At hyphal contact and penetration, host epidermal cells exhibited a series of defense responses (viz. formation of papillae and partition walls, general wall thickening and walling-off of internal hyphae). In papilla formation, loop-shaped, rough endoplasmic reticula (rER) gave rise to globose secretory vesicles that accumulated around and then fused to the developing papilla. Unlike papillae, general wall thickening was associated with the Golgi apparatus (GA) that produced cell wall materials; 1–3 layers of Golgi cisternae were in contact with or in the immediate proximity (mostly within 0–0.5 µ m) of and lying parallel to the host cell wall, where they budded out numerous subglobose vesicles that fused directly to the host cell wall and made it thicker. Partition wall formation and walling-off of internal hyphae also were common; the former was associated with an extended single cisterna, which was indistinguishable from rER or individual cisternae of GA, and in the latter phenomenon internal hyphae were encased by electron-dense material containing numerous ribosomes and membranous elements that were derived apparently from proliferated rER. These pronounced defense responses protected the stele and contributed to making C. radicicola endophytic rather than pathogenic.

Key words: cell wall apposition, ER, Golgi apparatus, plant defense response, transmission electron microscopy