Calderon, F. 1997. Lipids: Their value as molecular markers and their role in the carbon cycle of arbuscular mycorrihizae. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA..

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Arbuscular mycorrhizae are associations between fungi and the roots of vascular plants. Part of this dissertation is devoted to analyzing the fatty acid and sterols of mycorrhizal and non-mycorrhizal Sorghum with the aim of identifying universal molecular markers of mycorrhizal infection. The mycorrhizal fungi contain high amounts of unusual lipids that may be used to mark their presence in infected roots. My results show that phosopholipid fatty acid 16:1, as well as campesterol are molecules that can be used to consistently identify mycorrhizal infection. In addition, lipid profiles may provide insight as to which fungal species is present in the roots.

In a second experiment, the fatty acids and sterols of several isolates of root pathogenic fungi were surveyed to assess the taxonomic value of lipid profiles. My results show that the genera Rhizoctonia and Pythium can be reliably identified because of their characteristic lipid composition.

Another question that I address in this work is: What is the C turnover time of mycorrhizal lipids? For this purpose, I carried out a pulse-chase experiment in which I followed the incorporation and subsequent turnover of C in mycorrhizal lipids. Mycorrhizal and non-mycorrhizal plants were subjected to a pulse exposure to 14CO2followed by sequential harvesting. Infected plants assimilated more 14C than non-mycorrhizal plants, and had a higher absolute and percentage allocation of 14C to root tissue, below ground respiration, and soil. Despite the increased fixation of C by mycorrhizal plants, mycorrhizal shoots had reduced biomass. This indicates that the C drain imposed by the results in a reduced shoot growth, suggesting that the mycorrhizal fungus was acting as a parasite. The pulse-chase experiment demonstrated that the lipids of mycorrhizal roots are a dynamic pool of C with measurable turnover of 14C. The C turnover time of the mycorrhizal fatty acid 16:1 w 5 was calculated at 210 h-1. The lipids of non-mycorrhizal roots incorporated less radiolabel, underscoring the difference in the lipid C cycle between the arbuscular mycorrhizae and non-mycorrhizal roots. To my knowledge, this is the first measurement of the C turnover of a biomass component of the mycorrhizal fungus.

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