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. Improved nutrient use efficiency is therefore a highly desired trait for developing a more sustainable agriculture. Compartmenting nitrate into vacuoles is one of the option to develop N-efficient crop adapted to less fertilizers. Only few proteins involved in nitrate transport on the tonoplast have been identified. CLCa is the major transporter involved in nitrate storage in Arabidopsis. Several other nitrate transporters amongst 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 for electrophysiology analysis. We then investigated its function by ectopically overexpressing it in a clca knock-out mutant. Although the growth on nitrogen sufficient medium was complemented, nitrate homeostasis was not restored by NRT2.7 activity like for CLCa overexpression. Moreover, NRT2.7 ectopic overexpression in wild-type background increased growth under limited nitrogen supply, suggesting that NRT2.7 stimulates nitrate efflux from vacuoles. This result was confirmed by electrophysiology performed on isolated vacuoles. Possible means of the growth stimulation by NRT2.7 versus CLCa are discussed based on nitrate fluxes through plasma membrane and nitrate homeostasis.