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.