Overexpressing NRT2.7 induces nitrate export from the vacuole and increases growth of Arabidopsis

Nitrogen nutrition is essential for crop yield, but applying fertilizers has detrimental effects on the environment. Compartmenting nitrate into vacuoles is one of the options to develop nitrogen-efficient crops adapted to less fertilizers. Only a few proteins involved in nitrate transport in the tonoplast have been identified. CLCa is the major transporter involved in nitrate storage in Arabidopsis, but it can also facilitate nitrate remobilization from the vacuole in guard cells. Several other nitrate transporters, including NRT2.7, have been localized in this membrane. The transport mechanism of NRT2.7 has not yet been defined as this protein is present mainly in seed cells that are not easily amenable to electrophysiology analysis. Here, we investigated NRT2.7 function through its ectopic overexpression in a clca knockout mutant. Although the decrease in growth of clca on nitrogen-sufficient medium was complemented, nitrate homeostasis was not restored by NRT2.7 activity. Moreover, NRT2.7 ectopic overexpression in the wild-type background increased growth under limiting nitrogen supply, suggesting that NRT2.7 stimulates nitrate efflux from vacuoles. This hypothesis was demonstrated by electrophysiological nitrate flux measurements on isolated vacuoles. This discovery of NRT2.7 function, and more particularly the coupling of vacuolar nitrate fluxes with growth under low nitrate supply, will enable new strategies for engineering better NUE for a more sustainable agriculture.