Fruiting Body Formation in Basidiomycetes

Establishment of the dikaryotic mycelium and formation of fruiting bodies are highly complex developmental programmes that are activated by a combination of environmental cues. A wide variety of proteins are expected to regulate and coordinate these programmes or to fulfil enzymatic conversions or structural roles. With the identification of the first genes involved in mushroom development, we are only at the beginning of understanding fruiting body formation. The process of identification of genes will be accelerated by whole genome expression studies and increased availability of molecular tools to assign functions to genes. Establishment of the dikaryon and emergence of fruiting bodies in basidiomycetes are regulated by the mating-type genes. These genes encode DNA-binding proteins and pheromones and their receptors. Regulation of fruiting by the mating-type genes is mediated by downstream transcription factors. Several genes encoding such regulatory proteins have now been identified. Regulatory circuits ultimately activate genes encoding structural proteins or enzymes that are involved in fruiting body formation. The role of hydrophobins is well established. They enable hyphae to escape the aqueous environment to allow fruiting body development. Moreover, they coat aerial structures and line air channels in mushrooms. The hydrophobic coating irreversibly directs growth of hyphae into the air, allows dispersal of spores and ensures gas exchange in fruiting bodies under humid conditions. Apart from hydrophobins, phenolics polymerised by the action of laccases may contribute to surface hydrophobicity of fruiting bodies. These enzymes have also been proposed to cross-link cell walls of hyphae in the fruiting bodies but this still has to be established. Experimental evidence indicates that cytochrome P450 enzymes, lectins, haemolysins and expansins also function in mushroom development. Lectins may be involved in aggregation of hyphae, haemolysins in signalling particularly to induce apoptosis of selected hyphae in the fruiting body, while expansins may be involved in cell wall modification and extension.