Biothiols, ethylene and autophagy - an exploration of their significance and interplay during the acute cadmium stress response in Arabidopsis thaliana

Abstract

Soil pollution with the metal cadmium (Cd) constitutes a widespread problem that, once Cd is taken up by plants and enters the food chain, poses a direct risk to human health. Although plants can contribute to the valorisation of polluted soils (e.g. through phytoremediation), such practices are often limited by the phytotoxicity of the corresponding pollutant. Accordingly, Cd exposure hampers plant growth and development, which is largely attributable to an indirect increase in the levels of reactive oxygen species (ROS). Apart from eliciting cellular damage, ROS are also important signalling mediators during plant stress responses. As such, the transient depletion of the antioxidant glutathione (GSH) in the roots of Cd-exposed plants, which is related to the use of GSH to form Cdchelating phytochelatins (PC), could be seen as an oxidative signal. Hence, research suggested that early root GSH depletion upon Cd stress activates an oxidative signalling pathway that stimulates the production of 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of the phytohormone ethylene. Consequent ethylene signalling has, in turn, been linked to the Cd-induced oxidative challenge via its stimulatory effect on both pro-oxidative gene expression and GSH levels in Arabidopsis thaliana leaves. Apart from a connection between the oxidative challenge and ethylene, both mediators may also be related to the endoplasmic reticulum (ER) stress response. ER stress occurs when protein homeostasis in this organelle is disrupted and can activate an unfolded protein response (ER-UPR), which can be mediated by inositol requiring 1 (IRE1)-dependent splicing of basic region/leucine zipper 60 (bZIP60) mRNA and subsequent induction of ER stress-responsive genes. In addition, IRE1 is also known for its decay of mRNAs (RIDD), which can induce autophagy by degrading mRNAs coding for autophagy inhibitors. While Cd exposure has often been linked to an oxidative challenge and changes in phytohormone levels, its effects on ER stress and autophagy are less investigated and leave the role of autophagy in Cdinduced stress responses largely unclear.