The Ultrastructure of Soft Rot Fungi. II. Cavity-Forming Hyphae in Wood Cell Walls

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Abstract
The hyphae of the soft rot fungi, Humicola alopallonella, Monodictys putredinis, and Phialophora hoffmannii formed cavities in the S2 layer of birch cell walls. TEM sections of mature T-branch and proboscis hyphae in cavities showed cell wall-bound hyphal compartments and an organelle content similar to that of hyphae in the wood cell lumina. Tapered cavities were produced by proboscis hyphae of P. hoffmannii, while H. alopallonella and M. putredinis formed cylindrical cavities. Amorphous deposits were consistently observed in the space between hyphal and cavity surfaces. Constrictions between cavities indicated inactive regions of wood cell wall dissolution around fine hyphae and were not necessarily associated with septa. Cavity-forming soft rot hyphae may regulate the release of exocellulases and endocellulases in response to nutrient exhaustion following apical growth of fine T-branch and proboscis hyphae.