The effect of different nitrogen sources on the symbiotic interaction between Sorghum bicolor and Glomus intraradices : Expression of plant and fungal genes involved in nitrogen assimilation

Fungal and plant gene expression in the Tulasnella calospora–Serapias vomeracea symbiosis provides clues about nitrogen pathways in orchid mycorrhizas

“…As no information is currently available on the N pathways inside orchid mycorrhizas, we investigated in T. calospora the expression of some genes identified in other mycorrhizal fungi. In particular, the urea cycle is a pathway reported for both AM (Tian et al ., ; Koegel et al ., ) and ECM fungi (Morel et al ., ; Wright et al ., ). Argininosuccinate lyase is involved in arginine biosynthesis and is a key enzyme of the anabolic arm of the urea cycle; in the T. calospora free‐living mycelium, the corresponding gene was found to be slightly induced by glutamine (FC = 2.5; P < 0.05), as compared with ammonium (Table ).…”

“…Although solely based on transcriptional evidence, it seems unlikely that T. calospora uses this N pathway to transfer ammonium to the orchid protocorm because the gene coding for urease, the main enzyme involved in arginine breakdown and ammonium release, is up-regulated in the intraradical AM fungal mycelium (Koegel et al, 2015) but strongly down-regulated in the symbiotic T. calospora. Moreover, argininosuccinate lyase, a marker gene of arginine biosynthesis up-regulated in the extraradical AM fungal mycelium (Koegel et al, 2015), was instead up-regulated in symbiosis in T. calospora.…”

“…Although solely based on transcriptional evidence, it seems unlikely that T. calospora uses this N pathway to transfer ammonium to the orchid protocorm because the gene coding for urease, the main enzyme involved in arginine breakdown and ammonium release, is up-regulated in the intraradical AM fungal mycelium (Koegel et al, 2015) but strongly down-regulated in the symbiotic T. calospora. Moreover, argininosuccinate lyase, a marker gene of arginine biosynthesis up-regulated in the extraradical AM fungal mycelium (Koegel et al, 2015), was instead up-regulated in symbiosis in T. calospora. It should, however, be noted that, also as a consequence of the obligate symbiotic nature of AM fungi, gene expression and enzymatic activities in AM fungi were assessed in two different but connected compartments, that is, the extraradical and intraradical AM fungal mycelium (Gomez et al, 2009;Tian et al, 2010;Koegel et al, 2015), whereas gene expression in T. calospora was measured separately in free-living mycelium and symbiotic conditions.…”

“…Nitrogen occurs in the soil in inorganic forms, such as ammonium (NH 4 + ) and nitrate (NO 3 À ), or in organic N compounds. Fungal uptake and transfer of soil-derived N to host plants has been extensively investigated in arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi (see references in Chalot et al, 2006;Martin et al, 2007;M€ uller et al, 2007;Koegel et al, 2015;B€ ucking & Kafle, 2015). Nitrate transporters and genes responsible for nitrate utilization have been identified mainly in AM fungi (Kaldorf et al, 1998;Tian et al, 2010;Koegel et al, 2015), and nitrophilous ECM fungi (Plassard et al, 2000;Jargeat et al, 2003;Montanini et al, 2006).…”

“…Fungal uptake and transfer of soil-derived N to host plants has been extensively investigated in arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi (see references in Chalot et al, 2006;Martin et al, 2007;M€ uller et al, 2007;Koegel et al, 2015;B€ ucking & Kafle, 2015). Nitrate transporters and genes responsible for nitrate utilization have been identified mainly in AM fungi (Kaldorf et al, 1998;Tian et al, 2010;Koegel et al, 2015), and nitrophilous ECM fungi (Plassard et al, 2000;Jargeat et al, 2003;Montanini et al, 2006). Genes coding for high-and low-affinity ammonium transporters have been identified and characterized in both AM (L opez-Pedrosa et al, 2006;P erez-Tienda et al, 2011;Calabrese et al, 2016) and ECM fungi (Javelle et al, 2001;Montanini et al, 2002;Willmann et al, 2007).…”