N availability modulates the role of NPF3.1, a gibberellin transporter, in GA-mediated phenotypes in Arabidopsis.

AtNPF3.1 gene expression is promoted by limiting nitrogen nutrition. Atnpf3.1 mutants are affected in hypocotyl elongation and seed germination under conditions of low-nitrate availability. The NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER (NPF) family encodes nitrate or peptides transporters, some of which are also able to transport hormones. AtNPF3.1 has been described as a nitrate/nitrite/gibberellin transporter. Until now only its gibberellins (GAs) transport capacity have been proven in planta. We further analyzed its substrate specificity towards different GA species using a yeast heterologous system which revealed that (1) NPF3.1 transported not only bioactive GAs but also their precursors and metabolites and (2) the GAs' import activity of NPF3.1 was not affected by the presence of exogenous nitrate. Gene expression analysis along with germination assays and hypocotyl length measurements of loss of function mutants was used to understand the in planta role of NPF3.1. GUS staining revealed that this gene is expressed mainly in the endodermis of roots and hypocotyls, in shoots, stamens, and dry seeds. Germination assays in the presence of paclobutrazol, a GA biosynthesis inhibitor, revealed that the germination rate of npf3.1 mutants was lower compared to wild type when GA was added at the same time. Likewise, hypocotyl length measurements showed that the npf3.1 mutants were less sensitive to exogenous GA addition in the presence of paclobutrazol, compared to wild type. Moreover, this phenotype was observed only when plants were grown on low-nitrate supply. In addition, NPF3.1 gene expression was upregulated by low exogenous nitrate concentrations and the npf3.1 mutants exhibited a not yet described GA-related phenotype under these conditions. All together, these results indicated that NPF3.1 is indeed involved in GAs transport in planta under low-nitrate conditions.