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Ultrastructure of Conidia and the Conidium Aging Process in the Plant Pathogenic Fungus Colletotrichum graminicola

The influence of age on conidial ultrastructure was investigated in Colletotrichum graminicola. The cytoplasmic volume of young conidia-those harvested from 5-7-d-old cultures-was dominated by lipid bodies. These conidia also were characterized by numerous small vacuoles with electron-dense contents. Other structures present included mitochondria, microbodies, ribosomes, elements of tubular ER, short strands of rough ER, multivesicular bodies, simple cisternal elements, Woronin bodies, microtubules, and large amounts of what appeared to be glycogen. Older conidia harvested from 30-35-d-old cultures exhibited a significantly different ultrastructure. They contained fewer lipid bodies, lacked the large glycogen deposits, were very highly vacuolate, and contained sheetlike strands of ER and extensive aggregations of tubular ER. Structures thought to be autophagic vacuoles also were present. The conidial wall appeared similar in conidia of both ages and consisted of a thicker inner electron-transparent layer and a very thin outer layer composed of fine electron-dense fibers. The inner layer stained moderately for polysaccharides and could be labeled with wheat germ agglutinin-gold complexes indicating the presence of chitin. The outer layer stained intensely for polysaccharides but did not label with wheat germ agglutinin. The outer surface of the conidium could be labeled with α-amylase-gold complexes specific for α-1,4 glucans. The observed changes in ultrastructure may serve as indicators of physiological changes that affect the ability of the fungus to survive.