Nitrate transport across the tonoplast of Cucumis sativus L. root cells

Tonoplast-localized nitrate uptake transporters involved in vacuolar nitrate efflux and reallocation in Arabidopsis

A great proportion of nitrate taken up by plants is stored in vacuoles. Vacuolar nitrate accumulation and release is of great importance to nitrate reallocation and efficient utilization. However, how plants mediate nitrate efflux from vacuoles to cytoplasm is largely unknown. The current study identified NPF5.11, NPF5.12 and NPF5.16 as vacuolar nitrate efflux transporters in Arabidopsis. Histochemical analysis showed that NPF5.11, NPF5.12 and NPF5.16 were expressed preferentially in root pericycle cells and xylem parenchyma cells, and further analysis showed that these proteins were tonoplast-localized. Functional characterization using cRNA-injected Xenopus laevis oocytes showed that NPF5.11, NPF5.12 and NPF5.16 were low-affinity, pH-dependent nitrate uptake transporters. In npf5.11 npf5.12 npf5.16 triple mutant lines, more root-fed ¹⁵NO₃⁻ was translocated to shoots compared to the wild type control. In the NPF5.12 overexpression lines, proportionally less nitrate was maintained in roots. These data together suggested that NPF5.11, NPF5.12 and NPF5.16 might function to uptake nitrate from vacuoles into cytosol, thus serving as important players to modulate nitrate allocation between roots and shoots.

NO₃⁻/H⁺ antiport in the tonoplast of cucumber root cells is stimulated by nitrate supply: evidence for a reversible nitrate-induced phosphorylation of vacuolar NO₃⁻/H⁺ antiport

Studies in the last few years have shed light on the process of nitrate accumulation within plant cells, achieving molecular identification and partial characterization of the genes and proteins involved in this process. However, contrary to the plasma membrane-localized nitrate transport activities, the kinetics of active nitrate influx into the vacuole and its adaptation to external nitrate availability remain poorly understood. In this work, we have investigated the activity and regulation of the tonoplast-localized H⁺/NO₃⁻ antiport in cucumber roots in response to N starvation and NO₃⁻ induction. The time course of nitrate availability strongly influenced H⁺/NO₃⁻ antiport activity at the tonoplast of root cells. However, under N starvation active nitrate accumulation within the vacuole still occurred. Hence, either a constitutive H⁺-coupled transport system specific for nitrate operates at the tonoplast, or nitrate uses another transport protein of broader specificity to different anions to enter the vacuole via a proton-dependent process. H⁺/NO₃⁻ antiport in cucumber was significantly stimulated in NO₃⁻-induced plants that were supplied with nitrate for 24 hours following 6-day-long N starvation. The cytosolic fraction isolated from the roots of NO₃⁻-induced plants significantly stimulated H⁺/NO₃⁻ antiport in tonoplast membranes isolated from cucumbers growing on nitrate. The stimulatory effect of the cytosolic fraction was completely abolished by EGTA and the protein kinase inhibitor staurosporine and slightly enhanced by the phosphatase inhibitors okadaic acid and cantharidin. Hence, we conclude that stimulation of H⁺/NO₃⁻ antiport at the tonoplast of cucumber roots in response to nitrate provision may occur through the phosphorylation of a membrane antiporter involving Ca-dependent, staurosporine-sensitive protein kinase.