Mechanisms of unfolded protein response signaling in plants

The endoplasmic reticulum (ER) is responsible for the synthesis of one third of the cellular proteome. A potentially lethal condition termed as ER stress occurs when environmental and physiological stimuli alter the protein-folding and glycosylation activities of the ER. A complex system of signaling pathways known as the unfolded protein response (UPR) is in place to alleviate stress. Unresolved ER stress due to inefficient or dysfunctional UPR leads to cell death. In plants, there are two distinct UPR signaling pathways. The first pathway involves the ER-membrane-bound transcription factor basic Leucine Zipper 28 (bZIP28), which is activated by proteolysis in the Golgi with consequent relocation of the active transcription factor domain to the nucleus. The second pathway is controlled by the Inositol Requiring Enzyme-1 (IRE1), an ER-anchored membrane kinase and ribonuclease (RNase). In conditions of ER stress, IRE1 oligomerizes, triggering autophosphorylation and activation of its RNase domain in the cytosol. This in turn splices the mRNA of the transcription factor bZIP60. The active forms of bZIP28 and bZIP60 translocate to the nucleus, resulting in the regulation of downstream genes that are involved in protein folding and ER quality control (ERQC). The UPR response could be induced by the defects of ER morphology that affect the proper ER functionality, or by environmental stresses. To follow up the investigation on these two aspects, first the ER ability to trigger UPR in the various ER defective mutants was determined in model plant, Arabidopsis. A specific mutation in the formation of ER tubules caused by the loss of function of RHD3 showing attenuated RNase activity of IRE1 regulated UPR reduction was characterized. It also identified RHD3 as a new UPR modulator in plants.UPR also get activated in response to environmental stresses. In these conditions, plants switch on distinct stress signaling pathways, which frequently coordinate with each other through shared signaling molecules. Upon pathogen-caused biotic stress, the elevated salicylic acid (SA) initiates the stress signaling in which the activation of the UPR machinery serves as a part of defense response to ensure the ER’s capability to cope with the rapid demand of antimicrobial protein production. Therefore, the involvement of SA and SA-mediated signaling molecules in the ER stress response was further explored by testing the UPR response in mutants defective in SA production and SA-responsive cofactor, respectively, under ER stress. Subsequently, a SA-independent role of nonexpressor of PR1 genes 1 (NPR1) —a master regulator in SA-mediated defense—in the UPR was identified. In response to ER stress-induced cytosolic reduction, NPR1 translocates to the nucleus, whereby it interacts with bZIP28 and bZIP60 and negatively regulates UPR activation. Ultimately, this study uncovered another novel UPR transducer involved in the ER surveillance system.UPR signaling throughout the whole organism is a complicated regulation network and intensive research were done at a cellular level. However, UPR signaling transmitted at an organismal level in a cell non-autonomous manner in metazoans and it is unknown in plants. Consequently, the existence of long-distance UPR signaling was evaluated through genetic approaches in combination with multiple molecular reporters and conventional micro-grafting. Toward the end, the systemic UPR regulation mediated by intracellular movement of bZIP60 was established which further extends current knowledge on the mechanism of UPR signal transduction.