Immunocytochemical location of the (1→3) (1→4)-β-glucan lichenin in the lichen-forming ascomycete Cetraria islandica (Icelandic moss)

Thalli of Cetraria islandica (Icelandic moss) and Evernia prunastri (oak moss) contain considerable amounts of the linear (1 -->3), (1 -->4)-beta -glucan lichenin (lichenan), which different proportions of linkage groups in either species. The immunocytochemical location of lichenan within the thallus is reported using a monoclonal antibody (barley anti-(1 -->3), (1 -->4)-beta -glucan antibody) and low-temperature scanning electron microscopy (LTSEM) techniques. The antibody labelled ultrathin sections of C. islandica, but not of E. prunastri. In C. islandica, lichenin was located in the extracellular matrix of the peripheral cortex and in a thick outer wall layer of medullary hyphae. LTSEM of fully hydrated and desiccated thalli of C. islandica showed that both the cortical extracellular matrix and the outer wall layer of medullary hyphae shrink dramatically during drought stress, indicating that these are major sites of water storage. A mycobiont-derived, hydrophilic wall surface layer prevents the thallus interior from becoming waterlogged at full hydration. The LTSEM observations and immunocytochemical data strongly suggest that lichenin is a structural compound, rather than a storage product, with important functions in thallus water relations.