Transcriptional regulation of host NH4+ transporters and GS/GOGAT pathway in arbuscular mycorrhizal rice roots

• OsAMT3;1 expression is highly induced in mycorrhizal rice roots.
• OsAMT1;1 and OsAMT1;3 expression is down-regulated in mycorrhizal roots.
• Mycorrhizal colonization induces specific induction of OsGOGAT2 in rice roots.
• OsGOGAT2 might be involved in NH4+ assimilation in the mycorrhizal N uptake pathway.

Arbuscular mycorrhizal (AM) fungi play a key role in the nutrition of many land plants. AM roots have two pathways for nutrient uptake, directly through the root epidermis and root hairs and via AM fungal hyphae into root cortical cells, where arbuscules or hyphal coils provide symbiotic interfaces. Recent studies demonstrated that the AM symbiosis modifies the expression of plant transporter genes and that NH4+ is the main form of N transported in the symbiosis. The aim of the present work was to get insights into the mycorrhizal N uptake pathway in Oryza sativa by analysing the expression of genes encoding ammonium transporters (AMTs), glutamine synthase (GS) and glutamate synthase (GOGAT) in roots colonized by the AM fungus Rhizophagus irregularis and grown under two N regimes. We found that the AM symbiosis down-regulated OsAMT1;1 and OsAMT1;3 expression at low-N, but not at high-N conditions, and induced, independently of the N status of the plant, a strong up-regulation of OsAMT3;1   expression. The AM-inducible NH4+ transporter OsAMT3;1 belongs to the family 2 of plant AMTs and is phylogenetically related to the AM-inducible AMTs of other plant species. Moreover, for the first time we provide evidence of the specific induction of a GOGAT   gene upon colonization with an AM fungus. These data suggest that OsAMT3;1 is likely involved in the mycorrhizal N uptake pathway in rice roots and that OsGOGAT2 plays a role in the assimilation of the NH4+ supplied via the OsAMT3;1 AM-inducible transporter.