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

Abstract

In 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 non-colonized 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.