Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2001
Publication Date: N/A
Interpretive Summary: Mycorrhiza are symbiotic soil born fungi that colonize plant roots and supply the host plant with needed nutrients including phosphorus, nitrogen and trace metals. In return the plant provides the fungus with carbohydrate. In this relationship, crop plants produce higher yields on a lower input of fertilizer, are more resistant to pathogens and can tolerate estress more effectively. This symbiosis is therefore considered important to the future of agriculture, especially where we wish to reduce the impact of growing food on our environment. Although this symbiosis has been going on for some 400 million years and is very widespread and important to the life of land plants, relatively little is known about many of the metabolic processes involved. In addition, since our agriculture has become dependent on the use of excessive amounts of fertilizers and pesticides, we have diminished our natural soil populations of these valuable organisms. We are studying the uptake, transport, exchange and metabolism of carbon compounds by the host roots and fungus with the goal of being able to produce these organisms in large quantities, in the absence of a host plant as field inoculum. This study has revealed that the fungus requires molecules of fat to survive during critical times in its life cycle. Thus by providing this nutrient at the appropriate time we hope to extend the growth and development of mycorrhizal fungi in a newly proposed culture system.
Technical Abstract: The arbuscular mycorrhizal (AM) symbiosis is responsible for huge fluxes of photosynthetically fixed carbon from plants to the soil. Carbon is transferred from the plant to the fungus as hexose, but the main form of carbon stored by the mycobiont at all stages of its life cycle is triacylglycerol. Previous isotopic labeling experiments show that the fungus exports this lipid from the intraradical to the extraradical mycelium. Here in vivo multiphoton microscopy was used to observe the movement of lipid bodies through the fungal colony and to determine their sizes, distribution and velocities. The distribution of lipid bodies along fungal hyphae suggests that they are progressively consumed as they move toward growing tips, and we report the isolation of an AM fungal EST for a putative acyl-CoA dehydrogenase whose deduced amino acid sequence suggests that it may function in the anabolic flux of carbon from lipid to carbohydrate. Time-lapse image sequences show lipid bodies moving in both directions along hyphae and NMR analysis of labeling patterns after supplying (13)C labeled glycerol to either extraradical mycelium or colonized roots shows that there is indeed significant bi-directional translocation between intraradical and extraradical mycelium. We conclude that large amounts of lipid are translocated within AM fungi, and that while net movement is from the intraradical to the extraradical mycelium, there is also substantial recirculation throughout the fungus.