BdNRT2A and BdNRT3.2 Are the Major Components of the High-Affinity Nitrate Transport System in Brachypodium distachyon.

An efficient nitrate uptake system contributes to the improvement of crop nitrogen use efficiency under low nitrogen availability. The High Affinity nitrate Transport System (HATS) in plants is active in low range of external nitrate and is mediated by a two-component system (high affinity transporters NRT2 associated to a partner protein NRT3 (NAR2)). In Brachypodium, the model plant for C3 cereals, we investigated the role of BdNRT2A and BdNRT3.2 through various experimental approaches. Expression profile of BdNRT2.A and BdNRT3.2 genes in response to nitrate availability fits perfectly with the characteristics of the HATS components. ¹⁵Nitrate influx measurements decreased in bdnrt2a mutants (one NaN₃ induced mutant with a truncated NRT2A protein and two amiRNA mutants). In addition, the N limited phenotype of the mutant with a truncated NRT2A protein confirmed that BdNRT2A is a major contributor of the HATS in Brachypodium. An effective nitrate transport in the heterologous expression system Xenopus oocytes required the coexpression of BdNRT2A and BdNRT3.2 that characterizes two-component system of the HATS. Functional interaction between BdNRT2A-GFP and BdNRT3.2-RFP fusion proteins was observed at the plasma membrane in Arabidopsis protoplasts in transient expression experiments with BdNRT3.2 being necessary for the plasma membrane localization of BdNRT2A. The role of a conserved Ser residue in BdNRT2A (S461) specific to monocotyledons was evaluated in the BdNRT2A and BdNRT3.2 interaction leading to plasma membrane targeting. Assuming that S461 could be regulated by phosphorylation, a directed mutagenesis was performed to mimic a nonphosphorylated (S461A) or a constitutively phosphorylated (S461D), However, the mimicking the phosphorylation status of S461 by mutagenesis did not modify the BdNRT2A and BdNRT3.2 interaction, suggesting a more complex regulating mechanism. In conclusion, our data show that BdNRT2A and BdNRT3.2 are the main components of the nitrate HATS activity in Brachypodium (Bd21-3) and allow an optimal growth in low N conditions.