Abstract
Nutrient acquisition and transfer are essential steps in the arbuscular mycorrhizal (AM) symbiosis, which is formed by the majority of land plants. Mineral nutrients are taken up by AM fungi from the soil and transferred to the plant partner. Within the cortical plant root cells the fungal hyphae form tree-like structures (arbuscules) where the nutrients are released to the plant-fungal interface, i.e., to the periarbuscular space, before being taken up by the plant. In exchange, the AM fungi receive carbohydrates from the plant host. Besides the well-studied uptake of phosphorus (P), the uptake and transfer of nitrogen (N) plays a crucial role in this mutualistic interaction. In the AM fungus Rhizophagus irregularis (formerly called Glomus intraradices), two ammonium transporters (AMT) were previously described, namely GintAMT1 and GintAMT2. Here, we report the identification and characterization of a newly identified R. irregularis AMT, GintAMT3. Phylogenetic analyses revealed high sequence similarity to previously identified AM fungal AMTs and a clear separation from other fungal AMTs. Topological analysis indicated GintAMT3 to be a membrane bound pore forming protein, and GFP tagging showed it to be highly expressed in the intraradical mycelium of a fully established AM symbiosis. Expression of GintAMT3 in yeast successfully complemented the yeast AMT triple deletion mutant (MATa ura3 mep1Δ mep2Δ::LEU2 mep3Δ::KanMX2). GintAMT3 is characterized as a low affinity transport system with an apparent Km of 1.8 mM and a Vmax of 240 nmol-1 min-1 108 cells-1, which is regulated by substrate concentration and carbon supply.
Highlights
Nitrogen is an essential, often limiting, macronutrient for plants
The coding exon sequence (1,365 bp) was confirmed by expressed sequence tag (EST) alignment and cDNA sequencing, and it is interrupted by four short introns of 92 bp, 130 bp, 125 bp, and 86 bp length, typical of R. irregularis (Tisserant et al, 2012)
Location of intron 2 is even conserved in all six ammonium transporter (AMT) genes of R. irregularis and G. pyriformis, indicating its presence in a common ancestral gene before these glomeromycotan species split
Summary
Since the availability of nitrogen (N) in form of ammonium (NH4+) or nitrate (NO3−) in the environment is quite low, plants have evolved different strategies to overcome this problem. Under natural conditions 70–90% of land plant species are associated with nearly ubiquitous AM fungi, which can increase nutrient and water supply of their host. This goes along with improved plant fitness, growth, and disease resistance. It has been assumed that AMF play only a minor role in N nutrition of their host plant. Several studies testing the contribution of AM fungi to plant N supply revealed that N uptake of the host plant via mycorrhizal uptake pathway can reach 42% (Mäder et al, 2000)
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