Nitrogen Limitation Adaptation (NLA) is involved in source-to-sink remobilization of nitrate by mediating the degradation of NRT1.7 in Arabidopsis.

Abstract

Recent studies on nitrate transporters (NRTs) have greatly increased our knowledge of the mechanisms regulating nitrogen (N) homeostasis in plants. However, an understanding of how these NRTs are regulated is still lacking. The nitrogen limitation adaptation (nla) mutant is hypersensitive to N limitation. In the nla mutant, 15 N-nitrate spotted on old leaves preferentially accumulated in the youngest leaves. Analysis of leaf cross-sections indicated that NLA expression was expressed in the sieve element and companion cell system. The results of bimolecular fluorescence complementation (BiFC), split-ubiquitin yeast two-hybrid and co-immunoprecipitation (CoIP) assays demonstrated that NLA interacts with NRT1.7 in the plasma membrane. The following findings suggest that NLA directs the ubiquitination of NRT1.7: the down-regulation of NRT1.7 protein abundance in 35S::NLA/35S::Myc-NRT1.7 double transgenic plants compared with 35S::Myc-NRT1.7 transgenic plants; the up-regulation of NRT1.7 protein abundance in the nla mutant compared with wild-type plants; and the direct degradation of truncated NRT1.7 recombinant protein by NLA. Furthermore, analysis of NLA and NRT1.7 protein abundance in mirna827 knock-out plants showed that N deficiency-guided translational repression of NLA depends on miRNA827. Our findings reveal that plants regulate source-to-sink remobilization of nitrate by the ubiquitin-mediated post-translational regulatory pathway.