THE ROLE OF MORPHOGENETIC CELL-DEATH IN THE HISTOGENESIS OF THE MYCELIAL CORD OF AGARICUS-BISPORUS AND IN THE DEVELOPMENT OF MACROFUNGI

Hyphal growth of the white button mushroom Agaricus bisporus on spawn grains and compost is typically vegetative. Hyphae are loosely arrange d, in contrast to the organized texture in tissues, and needle-like ca lcium oxalate crystals are frequently present on the surface of vegeta tive hyphae. The mycelial cord is the first well-organized tissue of t he fruiting mycelium; it is surrounded by fluffy white hyphae that gro w vegetatively. The hyphae of the cord are held together through a sem i-fluid medium, the extracellular matrix, which aids in creating a thr ee-dimensional pseudoparenchymatous structure. The matrix material see ms to be secreted into the extracellular environment by specifically d ifferentiated cells, but the vegetatively growing hyphae of A. bisporu s initially exploit a different mechanism in the production of matrix which involves a type of cell death different from cell necrosis. This primary matrix production leads to the formation of minute cord tissu es in which oxalate crystals are no longer present. Once the hyphal ce lls of A. bisporus pass the threshold from a vegetative form into orga nized structures, they become differentiated and self-maintaining in t he production of the extracellular matrix material. Morphogenetic cell death has been observed before in A. bisporus development and here we show that it occurs in various species of macrofungi: a mucoid zone o f the pileipellis typically found in developing fruit bodies of Psiloc ybe and Panaeolus spp, contains numerous, dying or dead hyphal cells w hich show ultrastructural features comparable to those observed during the mycelial cord formation of A. bisporus. Studies performed using s pecimens of Stropharia rugoso-annulata, Coprinus domesticus, Psathyrel la candolleana, Tremella mesenterica, Otidea onotica and Peziza ostrac oderma in representative growth stages revealed supporting evidence fo r the view that morphogenetic cell death plays a key role at different stages during the development of fungal fruit bodies. This phenomenon may be related to the programmed cell death occurring in developing p lants and animals.