The endoplasmic reticulum in plant immunity and cell death

Abstract

The endoplasmic reticulum (ER) is a highly dynamic organelle in eukaryotic cells and a major production site of proteins destined for vacuoles, the plasma membrane, or apoplast in plants. At the ER, these secreted proteins undergo multiple processing steps, which are supervised and conducted by the ER quality control system. Notably, processing of secreted proteins can considerably elevate under stress conditions and exceed ER folding capacities. The resulting accumulation of unfolded proteins is defined as ER stress. The efficiency of cells to re-establish proper ER function is crucial for stress adaptation. Besides delivering proteins directly antagonizing and resolving stress conditions, the ER monitors synthesis of immune receptors. This indicates the significance of the ER for the establishment and function of the plant immune system. Recent studies point out the fragility of the entire system and highlight the ER as initiator of programed cell death (PCD) in plants as was reported for vertebrates. This review summarizes current knowledge on the impact of the ER on immune and PCD signaling. Understanding the integration of stress signals by the ER bears a considerable potential to optimize development and to enhance stress resistance of plants.

Keywords: endoplasmic reticulum quality control; plant immunity; programed cell death; stress; unfolded protein response.

FIGURE 1

Signaling in mammals and plants under mild (A,B) and prolonged ER stress (C,D). Models indicate overlaps and differences in ER stress signaling. Conservation in mammalian (A) and plant (B) UPR signaling in response to mild ER stress. Various components involved in mammalian ER-PCD signaling under prolonged ER stress have been identified (C), whereas plant ER-PCD signaling is almost unknown (D). Question marks (in D) indicate postulated orthologs or structural homologs of plant ER-PCD signaling. XBP1u/bZIP60u, unspliced mRNA; XBP1s/bZIP60s, spliced mRNA.