Fungal cell wall is essential for the protection of protoplast from various type of stressful environmental conditions e.g. changes of temperature, nutrient starvation, availability of water or oxygen, high concentration of salts or the presence of xenobiotics. The shape of the fungal cell, correct morphogenesis or the osmotic balance depends on their cell walls functions. The ligninolytic mechanisms of white rot fungi have often been described in the recent literature. In contrast, only few data are available on their general stress response system and the possible role of ligninolytic enzymes in this type of defense. Fungi described in the present work, are a very interesting group of organisms, some of them can colonize only very specific environments while others are able to occupy niches that are physically, chemically and biologically different. The aim of this study was to investigate the effect of two oxidative stress factor, menadione on the cell wall structure of white rot fungus: Abortiporus biennis. Menadione doses were added to 10-days old, stationary cultivated cultures. The basal medium was prepared according to Lindeberg. 120h after prooxidant treatment the fungal material was prepared and examined using electron microscopy. Because the main component of Basidiomycetes cell walls is chitin we determined also the changes of chitinase activity in the mycelium as well as in the culture fluid. This enzyme is an important element of recycling pathway of chitin. Our earlier investigations showed that the prooxidants treatment significantly changed the ligninolytic metabolism of this fungus e.g. especially significantly increased the extracellular laccase activity. Because the mechanisms of secretion and activation of this important enzyme are still not exactly explain, interesting was to determine the connection between the cell wall structure and activities of intra-and extracellular laccase activities under oxidative stress conditions. Laccase activity was analyzed spectrophotmetrically and electrophoretically. Using the electron microscopy we detected distinct differences between the cell wall of the menadione treated fungal cells in comparison to the control cultures. The menadione addition caused the growth of the cell wall thickness and changes of their density and structure. Those differences were strictly connected with stimulation of laccase activity. Apart from the experiments described above the activity of superoxide dismutase as a typical enzymatic antioxidant was also measured.

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