Functional characterization of NRT1/PTR FAMILY transporters: looking for a needle in a haystack

NRT1/PTR FAMILY (NPF) transporters play crucial roles in plant physiology and development due to their involvement in nitrogen nutrition and their ability to transport multiple signaling molecules and metabolites. Whereas most eukaryotic and prokaryotic NPF orthologs are peptide transporters, most flowering plant NPF transport other substrates such as nitrate as well as a wide range of structurally unrelated molecules like amino acids, potassium, chloride, glucosinolates, alkaloids, nicotianamine, sugars, or glycerate. Some NPF have also been reported to transport hormones, such as auxin, abscisic acid, gibberellins, or jasmonate. Strikingly, several NPF were shown to transport more than one of these molecules. This multispecificity places NPF in a central position in the integration of plant signals. In addition, different plant NPF were reported to have a signaling activity that is independent of their transport activity, indicating a direct role in molecule sensing. Identification of NPF substrates is not trivial and requires specific heterologous expression systems such as Xenopus oocytes or yeast to be functionally characterized. The aim of this review is to highlight the techniques used for NPF characterization, provide a comprehensive overview of their different substrates, and to speculate on the role of NPF during plant evolution.