Submitted to: New Phytologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/30/2003
Publication Date: 6/25/2004
Citation: Pfeffer, P.E., Douds, D.D., Schwartz, D.P., Govindarajulu, M., Shachar-Hill, Y. 2004. The fungus does not transfer carbon to or between roots in an arbuscular mycorrhizal symbiosis. New Phytologist. v. 163. pp.617-627. Interpretive Summary: Because of the possible consequences of global warming, it is important to understand the pathways of carbon sequestration in plants i.e., whether carbon finds its way back into the soil or whether it can be transferred via symbiotic fungal networks from plant to plant. Although there are studies in the literature that suggest carbon could be transported from plant to plant, there is no evidence that this carbon is ever moved out of the fungal structures within the plant to provide nutrition for the receiving host. In this study we have used isotopically labeled sugar to trace the pathway of carbon in carrot roots linked by a symbiotic arbuscular mycorrhizal fungal (AMF) network. Our findings have demonstrated that although carbon does move from plant root to plant root by way of these fungal networks there is no meaningful nutritional amount of carbon that crosses into the receiving plant. Consequently, the migrated carbon remains in the fungal structures unavailable for use by the network connected host. Carbon transfer via AMF networks is therefore irrelevant to the botanical components of a community, but is a means of distributing carbon within its own structures. This is a mechanism by which AMF can sequester excess atmospheric carbon in our environment.
Technical Abstract: Very large amounts of photosynthetically fixed carbon move from plants to their fungal partners in the Arbuscular Mycorrhizal (AM) symbiosis. There is also evidence for transfer of carbon in the reverse direction. However, the significance and even existence of fungus-to-plant carbon transfer has been called into question and the issue remains controversial (Robinson and Fitter, 1998). We have sought to directly assess whether, and how, carbon may move from fungus to plant in a functioning in vitro AM mycorrhiza. NMR and MS analyses reveal that permeable 13C labeled substrates (glycerol and acetate) supplied to the extraradical mycelium label the extraradical fungal amino acids and proteins, but host-specific metabolites i.e., sucrose and storage lipids contain no measurable label. Proteins in the mycorrhizal root are not labeled suggesting that only fungal protein in the extraradical mycelium is labeled in the mycorrhizal root. Thus N transfer from fungus to host does not result in net C transfer to the host as amino acid(s). Selectively labeled triacylglycerol in the extraradical mycelium is transferred to the intraradical mycelium (Bago et al., 2001), but does not result in labeling of host lipids or carbohydrate. In experiments utilizing Ri T-DNA transformed carrot roots connected by a common AM fungal network, 13C-1 glucose supplied to donor mycorrhizal roots resulted in extensive labeling in recipient roots growing under carbon-limited conditions. However, this label was confined to fungal compounds (trehalose and fungal storage lipids).