Correlation between endoglucanase secretion and cell wall strength in oomycete hyphae: implications for growth and morphogenesis

Oomycete fungi of the family Saprolegniaceae secrete a variety of enzymes, including endoglucanases, amylases, and proteases. This study is concerned with endoglucanase function in Achlya ambisexualis, Achlya bisexualis, and Saprolegnia ferax. Under conditions of osmotic stress, produced by supplementing the growth medium with nonionic osmolytes, extracellular endoglucanase activity increased markedly while the activities of other exoenzymes remained relatively constant. This change in endoglucanase activity correlated with measured changes in the tensile strength of the apical hyphal wall, consistent with the idea that these enzymes play a role in controlling the mechanical properties of the cell wall. This type of active modulation of wall compliance may allow pressurized hyphae to control the force they exert against their surroundings during invasive growth. Endoglucanase secretion may also loosen the wall sufficiently to allow the actin cytoskeleton to drive extension growth in the absence of turgor pressure.