Spore germination in the fungus Syncephalastrum racemosum

The asexual part of the life cycle of the fungus Syncephalastrum racemosum was investigated using light microscopy, transmission and scanning electron microscopy. The structure and organisation of the vegetative mycelium and spore producing hyphae was studied in detail. Freeze-fracturing, surface replica, metal shadowing and thin sectioning techniques were used to investigate the ultrastrueture of dormant and germinated spore walls subjected to various sequential chemical treatments. The spore wall had a highly complex organisation, consisting of four, major ultrastructurally definable layers. (1) A surface layer consisting of a cross-patched network of rodlets, having a 25nm periodicity. (2) A plate-like layer covered by amorphous material.(3) A layer of thick 17»5nm diameter microfibrils. (4) An innermost layer of thin 8nm diameter microfibrils. The emergent germ tube has a smooth outer surface and an inner layer of thin microfibrils. The breakpoint between spore and germ tube is clearly delimited at various levels within the spore wall. Tentative identification of the wall layers showed: (1) the rodlet layer was mainly protein; (2) the thick microfibrils were probably a β-glucan; (3) the thin microfibrils were chitin. Freeze-fracturing of dormant and germinated spores revealed differences in plasmamembrane ultrastructure. Large particles (34nm) with complimentary large depressions (33nm) were present only in fractures of the plasmamembrane of dormant spores. During germination there is an increase in the numbers of small particles (8.4nm) in the membrane. The physiology of spore germination was studied in shake-flask and batch fermenter cultures. In the presence of glucose, spore swelling preceded outgrowth of germ tubes. Glucose was essential for initiation of these events. Swelling ceased quickly when glucose was removed. Analysis of size distributions within spore populations indicated that the larger spores germinated first. A self-inhibitor of germination was extracted from culture and spore washings, and was identified as nonanoic acid.