Mushroom Spore Germination

This research project was initially undertaken to expand our understanding of the biochemical events that occur during mushroom spore germination. The study actually began several years ago when I worked with five undergraduate students and conducted ultrastructural examinations of ungerminated basidiospores of several species of mushrooms, including the store mushroom, Agaricus bisporus. In all species we observed a juxtaposition of lipids, mitochondria, and glyoxysomes. Since this association is a prerequisite for the glyoxylate cycle, we hypothesized the probable occurrence of this cycle in these spores. The relevance of the glyoxylate cycle to fungi lies in the fact that it plays a major role in the conversion of stored lipids, especially fatty acids, to carbohydrates. Germinating fungal spores require a constant supply of sugar for new cell wall synthesis.

The occurrence of the cycle can be detected by running assays for either of its two marker enzymes, malate synthase or isocitrate lyase. To perform the assays, we must first homogenize the ungerminated spores. To demonstrate the occurrence of the cycle, we assayed for malate synthase using a UV/Vis spectrophotometer.

Since little information was available concerning the glyoxylate cycle in mushroom spores, the purpose of Candi and Aaron's project was to determine the ubiquity of the glyoxylate cycle in ungerminated mushroom spores. To date they have assayed 90 species, representing 51 genera, and all were found to use the glyoxylate cycle. From this large sample, we can tentatively conclude that the cycle is ubiquitous in mushroom spores. Further analysis of their results has revealed additional interesting information. For example, the specific activity of malate synthase varies from group to group. (Specific activity equals the units of enzyme activity per milligram protein.) The light-spored, gilled mushroom (i.e., mushrooms with white, pink, or light brown spores) have a higher specific activity for malate synthase than do the dark-spored, gilled mushrooms (i.e., mushrooms with chocolate brown, purple brown, or black spores). Although our research was initially undertaken to expand our understanding concerning the biochemical events that occur during spore germination, the results have raised some potentially intriguing taxonomic questions.

We plan to expand our future research in two directions. First, the information concerning the glyoxylate cycle reported above was obtained from ungerminated spores. We have postulated that if the glyoxylate cycle occurs in ungerminated, dormant spores, then it must play a role in germination. Since this extrapolation may or may not be true, we plan to determine if the cycle actually does occur during germination. Second, we hope to determine if the difference in the specific activity of malate synthase between the light- and dark-spored gilled mushrooms is taxonomic or due to the presence of contaminating phenolic compounds released from the cell walls of the dark spores during spore homogenization.