Identification of plant nutrient transporters in arbuscular-mycorrhizal roots using a micro- array approach

Using a microarray approach, 29 putative nutrient-transporter genes were identified as up-/down-regulated transcripts in the model legume Lotus japonicus during the colonization by the arbuscular mycorrizal fungus (AMF) Gigaspora margarita. Among them, transcripts for phosphate, potassium, sulfate, ammonium, nitrate, and other N-transporters were detected, and suggested to be involved in macronutrient uptake by the plant. Based on a bioinformatic analysis, the mycorrhiza-specific phosphate transporter LjPT4 was identified, offering a new tool as a functional marker for future mycorrhiza research on the model plant L. japonicus.Since recent data on AMF physiology are demonstrating that AMF have the capacities to transfer nitrogen, and not only phosphate, from the soil to their host plants, particular attention has been given to a sequence coding for an ammonium transporter (LjAMT2;2). Our results show that the LjAMT2;2 protein is a functional ammonium transporter. Its dependency on an acidic pH and the localization of its expression to arbusculated cells suggest that LjAMT2;2 plays a role in the acquisition of ammonium from the interfacial apoplast during the inorganic N-transfer from the fungus to the host plant.Ten putative amino acid and peptide transporters were identified with the array: their analysis will help to answer the question whether an alternative to the inorganic N-transfer exists in the AM symbiosis. Also transporters for other nutrients (e.g. NO3-, K+ and SO42-) which have been identified, but have not so far been characterized, will represent interesting targets for future studies on nutrient exchange between the two symbionts

Modelling an Ammonium Transporter with SCLS

The Stochastic Calculus of Looping Sequences (SCLS) is a recently proposed modelling language for the representation and simulation of biological systems behaviour. It has been designed with the aim of combining the simplicity of notation of rewrite systems with the advantage of compositionality. It also allows a rather simple and accurate description of biological membranes and their interactions with the environment.<P>In this work we apply SCLS to model a newly discovered ammonium transporter. This transporter is believed to play a fundamental role for plant mineral acquisition, which takes place in the arbuscular mycorrhiza, the most wide-spread plant-fungus symbiosis on earth. Due to its potential application in agriculture this kind of symbiosis is one of the main focuses of the BioBITs project. <p>In our experiments the passage of NH3 / NH4+ from the fungus to the plant has been dissected in known and hypothetical mechanisms; with the model so far we have been able to simulate the behaviour of the system under different conditions. Our simulations confirmed some of the latest experimental results about the LjAMT2;2 transporter. The initial simulation results of the modelling of the symbiosis process are promising and indicate new directions for biological investigations

LjLHT1.2-a mycorrhiza-inducible plant amino acid transporter from Lotus japonicus

International audienceIn mycorrhizal associations, the fungal partner assists its plant host by providing nitrogen (N) in addition to phosphate. Arbuscular mycorrhizal (AM) fungi have access to inorganic and organic forms of N and translocate them, via arginine, from the extra-to the intraradical mycelium, where N is transferred to the plant as inorganic N compounds such as ammonium. However, several putative amino acid transporters (AATs) with an altered expression in Lotus japonicus mycorrhizal roots were recorded in a previous microarray-based investigation, which led to the question of whether a transfer of organic N, mainly in the form of amino acids, could occur in AM roots. Here, we have characterized an AAT gene (LjLHT1.2) that encodes for lysine-histidine-transporter (LHT)-type amino acid transporter. We show that it is induced in mycorrhizas, but not in nodulated roots. By using in situ hybridization and laser microdissection technology, the corresponding transcripts have been demonstrated to be located above all in arbusculated cells but also in the noncolonized cells of the root cortex. The gene expression resulted to be differentially regulated by the availability of the N sources. Furthermore, functional experiments, via heterologous expression in yeast, have demonstrated that the protein was a high-affinity amino acid transporter. Taken together, the results show that LjLHT1.2 may allow the uptake of energy-rich N compounds, such as amino acids, towards the cortical cells. We suggest that LjLHT1.2 could be involved in complex mechanisms that guarantee the re-uptake and recycle of amino acids and which are particularly efficient in mycorrhizal roots