Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2009
Publication Date: 7/29/2009
Citation: Jin, H., Pfeffer, P., Govindarajulu, M., Douds, D.D., Piotrowski, E.G., Nagahashi, G., Bucking, H., Lammers, P.J., Shachar-Hill, Y., Gachomo, E. 2009. Germinating spores of glomus intraradices can use internal and exogenous nitrogen sources for a de novo biosynthesis of amino acids. New Phytologist. 184:399-411. Interpretive Summary: Under a program of low input sustainable agriculture (LISA), new, more efficient, methods of growing food crops with lower inputs of fertilizers and pesticides are presently being evaluated. One important means for achieving this goal is through the development a plant/microbe symbiotic system known as arbuscular mycorrhizas (AM), whereby a low concentration of widely dispersed nutrients can be efficiently delivered from the soil to the growing plant. In addition the association of such micoorganisms or fungi impart disease resistance to the plant making the need for pesticide application considerably lower. Ideally, one would want to inoculate fields with such beneficial organisms (“biofertilizers”) to facilitate the LISA approach to food production, however since these organisms are presently obligate symbionts they can only be grown in association with the host plant. This limits our ability to make large quantities of pure cultures of these fungi needed for field inoculation. In this study we determined the metabolic pathways and genes that are activated in the fungus during germination and what forms of nitrogen the spores can use most efficiently at this phase of their life cycle. This information can help us optimize the germination step of the fungus in the absence of a plant host.
Technical Abstract: The uptake preferences, metabolic activity and gene expression shown by germinating AM fungal spores under various conditions with different nitrogen sources were examined. AM spores take up inorganic nitrogen (nitrate, ammonium), urea, and all amino acids except tyrosine; however, inorganic nitrogen and urea are incorporated into amino acids of AM spores more quickly. Arginine was found to be the most abundant amino acid in AM spores. In the absence of an exogenous nitrogen source soluble amino acids were produced from the breakdown of proteins. 15N enrichment in the amino acids labeled with 15NH4 in the presence of 14NO3 was considerably higher than that by 15NO3 in the presence of 14NH4, indicating that the AM spores have a preference for the uptake and utilization of NH4 over NO3. The gene expression for the enzymes, glutamate dehydrogenase (GDH), urease accessory protein (UAP) and ornithine-oxoacid transaminase (OAT) in germinating spores increased in the presence of a high concentration of purified root exudates, however, this trend was not evident in the isotopic enrichment of the synthesized amino acids.