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