Morphogenesis of Trichoderma: autoradiography of intact colonies labeled by ( 3 H)-acetylglucosamine as a marker of new cell wall biosynthesis

Regulation of chitin synthesis during growth of fungal hyphae: the possible participation of membrane stress

Apical hyphal extension involves very localized apical deposition of newly synthesized wall skeletal material, notably chitin. A branch forms where a new localized site of chitin deposition occurs in the lateral wall. Key enzymes involved are the chitin synthases. Their activity must be under tight regulation to achieve the orderly deposition of chitin. There is evidence that inactive chitin synthase is distributed throughout the hyphal plasma membrane and activated at the apex and at an incipient branch site. At these sites, the wall is plastic. We have investigated the hypothesis that physical stressing of the membrane, a consequence of the cell's turgor pressure acting at these weaker points, may locally activate the chitin synthase. Results show that cells that have been subjected to hypoosmotic stress have raised native chitin synthase activities. It is suggested that stressing the membrane may cause a conformational change in chitin synthase molecules in the membrane or changes in the interactions between chitin synthase and associated polypeptides, leading to activation. This process may act along with other regulatory mechanisms discussed here, such as post-translational modification and availability of allosteric effectors, to restrict the enzymic activity to sites where chitin synthesis is required. Key words: chitin synthase, zymogen, turgor pressure, membrane stress, Candida albicans, hyphal growth.

Studies on growth inhibition by lectins of Penicillia and Aspergilli

It has previously been shown in our laboratory that wheat germ agglutinin (WGA) binds to Trichoderma viride and inhibits growth of this fungus. Here we report on the effect of WGA, soybean agglutinin (SBA) and peanut agglutinin (PNA) on Penicillia and Aspergilli. Binding of the lectins to the fungi was examined with the aid of their fluorescein isothiocyanate (FITC) conjugated derivatives. FITC-WGA bound to young hyphal walls of all species, in particular to the hyphal tips and septa, in agreement with the chitinous composition of the cell walls of the two genera. Hyphae of all species examined were labelled, though in different patterns, by FITC-SBA and FITC-PNA, suggesting the presence of galactose residues on their surfaces. Young conidiophores, metulae (of the Penicillia), vesicles (of the Aspergilli), sterigmata and young spores, were also labelled. The three lectins inhibited incorporation of [3H]acetate, N-acetyl-D-[3h]glucosamine and D-[14C]galactose into young hyphae of Aspergillus ochraceus, indicating interference with fungal growth. Inhibition of spore germination by the three lectins was also observed. Preincubation of the lectins with their specific saccharide inhibitors prevented binding and the inhibitory effects. We conclude that lectins are useful tools for the study of fungal cell surfaces, and may also serve as an important aid in fungal classification. The present findings also support the suggestion that one role of lectins in plants is protection against fungal pathogens.

Hyphal walls of isolated lichen fungi: autoradiographic localization of precursor incorporation and binding of fluorescein-conjugated lectins

The hyphal walls of three mycobionts, isolated from the lichens Xanthoria parietina, Tornabenia intricata and Sarcogyne sp. were investigated by two techniques: microautoradiography of fungal colonies exposed to radioactive carbohydrate precursors: and binding, in vivo, of fluorescein conjugated lectins to hyphal walls of such colonies. N-[3H] acetylglucosamine was readily incorporated into tips, young hyphal walls and septa of the three mycobionts and the free-living fungus Trichoderma viride, but not into Phytophthora citrophthora, indicating that chitin is a major component of the mycobionts' hyphal walls. All three mycobionts, but neither of the free-living fungi, incorporated [3H] mannose and [3H] mannitol into their hyphal walls. Fluorescein-conjugated wheat germ agglutinin was bound to the hyphal walls of the three mycobionts and T. viride, but not to the walls of P. citrophthora; the binding pattern was similar to the grain pattern obtained in autoradiographs after short N-[3H]acetylglucosamine labelling. As wheat germ agglutinin binds specifically to chitin oligomers, the lectin binding tests further confirmed that chitin is a mycobiont hyphal wall component. Binding characteristics of several fluorescein-conjugated lectins to the three mycobionts indicated that this technique can yield useful information concerning the chemical composition of hyphal wall surfaces.