Dual regulation of root hydraulic conductivity and plasma membrane aquaporins by plant nitrate accumulation and high-affinity nitrate transporter NRT2.1

The water status and mineral nutrition of plants critically determine their growth and development. Nitrate (NO3-), the primary nitrogen source of higher plants, is known to impact the water transport capacity of roots (root hydraulic conductivity, Lp(r)). To explore the effects and mode of action of NO3- on Lp(r), we used an extended set of NO3- transport (nrt1.1, nrt1.2, nrt1.5 and nrt2.1), signaling (nrt1.1 and nrt2.1) and metabolism (nia) mutants in Arabidopsis, grown under various NO3- conditions. First, a strong positive relationship between Lp(r) and NO3- accumulation, in shoots rather than in roots, was revealed. Secondly, a specific 30% reduction of Lp(r) in nrt2.1 plants unraveled a major role for the high-affinity NO3- transporter NRT2.1 in increasing Lp(r). These results indicate that NO3- signaling rather than nitrogen assimilation products governs Lp(r) in Arabidopsis. Quantitative real-time reverse transcription-PCR and enzyme-linked immunosorbent assays (ELISAs) were used to investigate the effects of NO3- availability on plasma membrane aquaporin (plasma membrane intrinsic protein; PIP) expression. Whereas PIP regulation mostly occurs at the post-translational level in wild-type plants, a regulation of PIPs at both the transcriptional and translational levels was uncovered in nrt2.1 plants. In conclusion, this work reveals that control of Arabidopsis Lp(r) and PIP functions by NO3- involves novel shoot to root signaling and NRT2.1-dependent functions.