A New branch of endoplasmic reticulum stress signaling and the osmotic signal converge on Plant-specific Asparagine-rich proteins to promote cell death

Abstract

NRPs (N-rich proteins) were identified as targets of a novel adaptive pathway that integrates endoplasmic reticulum (ER) and osmotic stress signals based on coordinate regulation and synergistic up-regulation by tunicamycin and polyethylene gly- col treatments. This integrated pathway diverges from the molecular chaperone-inducing branch of the unfolded protein response (UPR) in several ways. While UPR-specific targets were inversely regulated by ER and osmotic stresses, NRPs required both signals for full activation. Furthermore, BiP (binding protein) overexpression in soybean prevented activa- tion of the UPR by ER stress inducers, but did not affect activa- tion of NRPs. We also found that this integrated pathway trans- duces a PCD signal generated by ER and osmotic stresses that result in the appearance of markers associated with leaf senes- cence. Overexpression of NRPs in soybean protoplasts induced caspase-3-like activity and promoted extensive DNA fragmen- tation. Furthermore, transient expression of NRPs in planta caused leaf yellowing, chlorophyll loss, malondialdehyde pro- duction, ethylene evolution, and induction of the senescence marker gene CP1. This phenotype was alleviated by the cytoki- nin zeatin, a potent senescence inhibitor. Collectively, these results indicate that ER stress induces leaf senescence through activation of plant-specific NRPs via a novel branch of the ER stress response.