Organelle-originated ROS and endoplasmic reticulum responses

Accumulation of unfolded proteins caused by inefficient chaperone activity in the endoplasmic reticulum (ER) is termed ‘ER stress’, and it is perceived by a complex gene network. Induction of these genes triggers a response termed the ‘unfolded protein response’ (UPR). If a cell cannot overcome the accumulation of unfolded proteins, the ER-associated degradation (ERAD) system is induced to degrade those proteins. In addition to other factors, reactive oxygen species (ROS) are also produced during oxidative protein-folding in the ER. It has been shown in animal systems that there is a tight association between mitochondrial ROS and ER stress. However, in plants there are no reports concerning how induced ROS production in mitochondria and chloroplasts affects ER stress and if there is a possible role of organelle-originated ROS as a messenger molecule in the unfolded protein response.

Hierarchical clustering and heat map of ER stress-related genes in leaves treated with H<sub>2</sub>O<sub>2</sub>, Rot, MV, DCMU and 3-AT (average linkage and Euclidean distance as similarity measure). — Hierarchical clustering and heat map of ER stress-related genes in leaves treated with H₂O₂, Rot, MV, DCMU and 3-AT (average linkage and Euclidean distance as similarity measure). qRT-PCR data for each gene were log₂n transformed, so that controls have a value of 0, increased expressions have positive and decreased expressions have negative values. Genes were grouped according to their expression patterns. Figure by Ozgur et al.

Ozgur et al. induce the formation of reactive oxygen species (ROS) in chloroplasts, mitochondria and peroxisomes of Arabidopsis thaliana and find that relatively low concentrations of ROS are more effective for induction of the ER stress response, and that mitochondrial and chloroplastic ROS production have different induction mechanisms for the UPR and ER stress responses. The results suggest that ROS may act as a secondary messenger during ER stress.

This article appears in the special issue ROS and NO Reactions in Plants.