Progress 10/01/04 to 09/30/05
Outputs
4d Progress report. This report serves to document research conducted under a Trust Cooperative Agreement between ARS and NRI. Additional details of this research can be found in the report for the parent CRIS 1935-12000-007- 00D Monoxenic and Axenic Cultivation of Arbuscular Mycorrhizal (AM) fungi. Mycorrhizae enhance the N nutrition of their hosts although little is known about the mechanisms of uptake of various forms of N, and virtually nothing is known about its movement and transfer from the fungus to the host. We used 15N labeled ammonium and doubly labeled (13C and 15N) arginine, in conjunction with mass spectrometric analysis of amino acids in an in vitro culture system, to discover the operation of a novel metabolic route in which nitrogen is moved by the AM fungus from the soil to its host. We observed that doubly labeled arginine is taken up by the extraradical fungal mycelium and is translocated to the intraradical mycelium, however only 15N and no 13C
were transferred to the host. This mechanism of nitrogen movement consists of metabolic processes known to operate in fungi together with a new variant of the urea cycle in which the anabolic and catabolic parts are separated by the long-distance translocation of arginine. The assimilation of N into arginine allows it to be moved in a concentrated (four nitrogens per molecule), non-toxic form; and the catabolic arm of the urea cycle allows the transfer of N to the host plant with minimal loss of carbon by the fungus. These findings have provided us with insight into the most effective means of delivering nitrogen, under a low input farming regime, to crop plants colonized by AM fungi. Milestone 1A(iv) 302 Action Plan component III Mechanisms of Plant Interactions with other Organisms, Performance Measures in the ARS Strategic Plan, 5.2.2 and 5.4.2).
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Progress 09/15/02 to 12/31/04
Outputs
Progress Report 4d Progress report. This report serves to document research progress under a NRI competitive grant #20002-35318-12713 initiated 10/02. Additional details of this research can be found in the report for the parent CRIS 1935-12000-007- 00D Monoxenic and Axenic Cultivation of Arbuscular Mycorrhizal (AM) fungi. Mycorrhizae enhance the N nutrition of their hosts although little is known about the mechanisms of uptake of various forms of N by germinating spores of AM fungi. We used 15N labeled inorganic and organic sources of N, in conjunction with mass spectrometric analysis of amino acids to study preferences in N uptake and metabolism in spores of Glomus intraradices. Uptake of inorganic N exceeded that of organic N. Further, the fungus preferred NH4 to NO3 nitrogen. Arginine and asparagine were the two most heavily labeled amino acids isolated from fungal tissues. These findings have provided us with insight into the most effective means of
delivering nitrogen to axenically growing spores. 302 Action Plan component III Mechanisms of Plant Interactions with other Organisms, Performance Measures in the ARS Strategic Plan, 5.2.2.
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Progress 10/01/03 to 09/30/04
Outputs
4. What were the most significant accomplishments this past year? D. Progress Report: This report serves to document research conducted under a Trust Funding Cooperative Agreement between ARS and NRI. Additional details of this research can be found in the report for the parent CRIS 1935-12000-007- 00D Monoxenic and Axenic Cultivation of Arbuscular Mycorrhizal (AM). We have pursued the effects of ammonium and nitrate on the growth and development of AM fungi in the symbiotic and germination phases of the life cycle. The fungus readily takes up ammonium in the symbiotic state and quickly converts it to arginine, which is transported to the host root. Nitrate is also taken up by the hyphae, reduced and incorporated into arginine, however this process is slow relative to the utilization of ammonium. Arginine acts as a store of nitrogen and moves in both directions between the fungus and plant. It remains intact within the hyphae until the fungus becomes nitrogen
limited. Arginine is easily taken up by the roots but uptake is limited in the symbiotic hyphae and spores. Arginine is principally a "mover" of nitrogen from fungus to host while transfer of nitrogen to the host is in the form of ammonium. These findings have provided us with insight into the most effective means of delivering nitrogen, under a low input farming regime, to host crop plants colonized with AM fungi
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