The endoplasmic reticulum (ER) takes a central role in the synthesis and processing of so called secreted proteins, which are essential for plant growth and especially important for stress integration. The ER is therefore considered as stress sensory organelle and pivotal for stress adaptation. Stress and certain growth stages induce ER stress in plant cells due to the transient inability of the ER to cope with a suddenly increased demand for secreted proteins. Cells take the accumulation of unfolded proteins in the ER as indicator for ER stress. To reinstall proper ER function, ER stress sensors at the ER membrane activate the adaptive unfolded protein response (UPR) upon mild ER stress. However, plants and animals apparently use some of the same sensors to activate an ER stress-induced programmed cell death (ER-PCD) upon failed UPR activation or severe ER stress (Fig. 2). In eukaryotes, ER stress sensors show a certain degree of functional conservation, as do the underlying UPR processes (Fig. 2). However, ER stress signalling is a black box mainly due to our inability to create conditions to separately analyse the constituents initiating and executing the adaptive UPR and rather harmful ER-PCD. Adapting ER function obviously bears a high potential to improve stress adaptation of plants in unfavourable environments. The conservation of ER stress signalling in eukaryotes indicate a true feasibility to translate findings made in model plant organisms (and probably distantly related eukaryotes) to crops in order to enhance crop productivity under stress conditions.
Figure 2 Overview of ER stress signalling in mammals and plants. While mild ER stress activates the unfolded protein response (UPR) (A, B), prolonged ER stress initiates programmed cell death (PCD) in plants and animals (C, D) (Eichmann and Schäfer, 2012, Frontiers in Plant Science).
Qiang, X., Zechmann, B,. Reitz, M.U., Kogel, K.H., Schäfer, P (2012) The mutualistic fungus Piriformospora indica colonizes Arabidopsis roots by inducing an endoplasmic reticulum stress-triggered caspase-dependent cell death. Plant Cell, 24, 794 - 809
Eichmann, R., Schäfer, P. (2012) 'The endoplasmic reticulum in plant immunity and cell death', Frontiers in Plant Science, 3, 1 - 5