Ultrastructural Studies in Lichens

Abstract

SUMMARYThe cytology of the relationship between evolved lecanoralean lichen mycobionts and their trebouxioid photobionts were studied in the foliose Cetrelia olivetorum (Nyl.)Culb. & C. Culb., Platismatia glauca (L.)Culb. & C. Culb., and Parmelia tiUacea (Hoffm.)Ach., and in the fruticose Pseudevernia furfuracea (L.)Zopf (all Parmeliaceae) using SEM and partly TEM techniques. In freeze-dried SEM preparations of all four species crystalline secondary metabolites of fungal origin were detected on the surface of the algal symbiont. Extracellular lichen products crystallized within and on the outermost surface layer of the cell wall which spreads from the mycobiont over the surface of the photobiont. This outermost wall layer revealed an irregularly tessellated pattern in freeze-fracture preparations and was recognized as a very thin, electron-dense layer in ultrathin sections. According to histochemical tests the surface layer of medullary hyphae and algal cells is largely composed of proteins and polyesters of fatty acids. Published data on the in vitro effects of phenolic lichen products on the metabolism of isolated, cultured photobionts are summarized, and possible roles of these secondary metabolites at the myobiont-photobiont interface within the lichen thallus are discussed.

Structural and functional processes during water vapour uptake and desiccation in selected lichens with green algal photobionts

“…This confirms recent findings (Honegger 1993;Scheidegger 1994) that in many foliose and fruticose lichens water storage is predominantly apoplasmic and/or symplasmic. Free internal water in the medullary layer is probably exceptional because of the highly hydrophobic surfaces of medullary hyphae and the algal cell walls partially ensheathed by hydrophobic fungal layers (Honegger 1986). The resistance to CO2 diffusion was therefore not changed inside the photobiont or medullary layers but primarily in the upper and !ower cortex and the extrathalline water films.…”

Interfacial Self-Assembly of Fungal Hydrophobins of the Lichen-Forming Ascomycetes Xanthoria parietina and X. ectaneoides

“…Moreover, the tight interactions of rodlet layers with mycobiont-derived phenolic secondary metabolites, as commonly observed in Cladoniaceae and Parmeliaceae (Honegger, 1984(Honegger, , 1986, deserve investigation at the biochemical and molecular levels.…”

“…1). A distinct rodlet pattern was resolved in this mycobiont-derived wall surface layer, with strong similarity to the rodlet layer of aerial structures in nonlichenized fungi (Honegger, 1984(Honegger, , 1986reviews: Honegger, 1991reviews: Honegger, , 1997Wessels, 1993). Medullary secondary metabolites may crystallize on and within the rodlet layer and thus enhance its hydrophobicity, but, at the same time, obscure the characteristic rodlet pattern (reviews : Honegger, 1991: Honegger, , 1997.…”

“…The chemical nature of this functionally important wall surface hydrophobicity is of considerable interest. Numerous taxa of lichen-forming ascomycetes secrete large amounts of phenolic secondary metabolites which crystallize either within the extracellular matrix of the tissue-like peripheral cortex (cortical lichen compounds) or on the wall surfaces of the loosely interwoven aerial hyphae in the thalline interior (medullary compounds) and partly even on the algal wall surface (Honegger, 1986). These deposits of mycobiont-derived, crystalline secondary metabolites render the wall surfaces remarkably hydrophobic (Armaleo, 1993;Lange et al, 1997).…”

Physiological Traits of Organisms in a Changing Environment

“…Many are synthesised by the mycobiont and are often deposited as crystals on the surfaces of hyphae and phycobiont cells (Honegger 1986). Their role (if any) in photoprotection is unclear.…”