The molecular mechanism of stipe cell wall extension for mushroom stipe elongation growth

Stipe elongation growth is one of the remarkable characteristics of the growth and development of basidiomycete fruiting bodies. Stipe elongation is resulting from the lateral extension of stipe cells. The stipe cell is enclosed within a thin cell wall which must be loosened to expand the wall surface area for accommodation of the enlarged protoplast as the stipe cell elongates. In fungal cell walls, chitin molecules associate with each other by interchain hydrogen bonds to form chitin microfibrils which are cross-linked covalently to matrix polysaccharides. Early, some scientists proposed that stipe elongation was the result of enzymatic degradation of wall polysaccharides, whereas other researchers suggested that stipe elongation resulted from nonhydrolytic disruption of the hydrogen bonds by turgor pressure between wall polysaccharides. Recently, an extensometer was used to determine stipe wall extension for elucidation of the molecular mechanism of stipe elongation. In Coprinopsis cinerea, the native stipe cell wall is induced to extend by acidic buffers and the acid-induced native wall extension activity is located in the growing apical stipe region. A series of current experiments indicate that chitinases play a key role in the stipe wall extension, and β-glucanases mainly function in the wall remodeling for regulation of stipe wall expansibility to cooperate with chitinase to induce stipe wall extension. In addition, fungal expansin-like proteins can bind to chitin to enhance chitin hydrolysis, and their expression pattern is consistent with the stipe elongation growth, which is suggested to play an auxiliary role in the stipe wall extension.