Germinating spores of Glomus intraradices can use internal and exogenous nitrogen sources for de novo biosynthesis of amino acids

Summary Here, nitrogen (N) uptake and metabolism, and related gene expression, were analyzed in germinating spores of Glomus intraradices to examine the mechanisms and the regulation of N handling during presymbiotic growth. The uptake and incorporation of organic and inorganic N sources into free amino acids were analyzed using stable and radioactive isotope labeling followed by high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS) and liquid scintillation counting and the fungal gene expression was measured by quantitative polymerase chain reaction (Q-PCR). Quiescent spores store Asp, Ala and Arg and can use these internal N resources during germination. Although not required for presymbiotic growth, exogenous N can also be utilized for the de novo biosynthesis of amino acids. Ammonium and urea are more rapidly assimilated than nitrate and amino acids. Root exudates do not stimulate the uptake and utilization of exogenous ammonium, but the expression of genes encoding a putative glutamate dehydrogenase (GDH), a urease accessory protein (UAP) and an ornithine aminotransferase (OAT) were stimulated by root exudates. The transcript levels of an ammonium transporter (AMT) and a glutamine synthetase (GS) were not affected. Germinating spores can make effective use of different N sources and the ability to synthesize amino acids does not limit presymbiotic growth of arbuscular mycorrhizal (AM) spores.