NnSR1, a class III non-S-RNase specifically induced in Nicotiana alata under phosphate deficiency, is localized in endoplasmic reticulum compartments.

A combined strategy of phosphate (Pi) remobilization from internal and external RNA sources seems to be conserved in plants exposed to Pi starvation. Thus far, the only ribonucleases (RNases) reported to be induced in Nicotiana alata undergoing Pi deprivation are extracellular S-like RNase NE and NnSR1. NnSR1 is a class III non S-RNase of unknown subcellular location. Here, we examine the hypothesis that NnSR1 is an intracellular RNase derived from the self-incompatibility system with specific expression in self-incompatible Nicotiana alata. NnSR1 was not induced in self-compatible Nicotiana species exposed to Pi deprivation. NnSR1 conjugated with a fluorescent protein and transiently expressed in Arabidopsis protoplasts and Nicotiana leaves showed that the fusion protein co-localized with an endoplasmic reticulum (ER) marker. Subcellular fractionation by ultracentrifugation of roots exposed to Pi deprivation revealed that the native NnSR1 migrated in parallel with the BiP protein, a typical ER marker. To our knowledge, NnSR1 is the first class III RNase reported to be localized in ER compartments. The induction of NnSR1 was detected earlier than the extracellular RNase NE, suggesting that intracellular RNA may be the first source of Pi used by the cell under Pi stress.