Cadmium-induced ER stress, autophagy and ethylene biosynthesis in leaves and roots of Arabidopsis thaliana and their dependence on stress intensity

• IRE1-mediated ER stress responses depend on the Cd stress intensity. • Severe Cd stress gradually decreases transcript levels of autophagy inhibitors. • Autophagy is progressively induced by increasing Cd stress intensity. • The stress hormone ethylene follows the ER stress responses of Cd-exposed plants. • ACC conjugation increases with Cd stress intensity and may limit ethylene emission. Cadmium (Cd) pollution in soils and uptake by plant roots is a widespread problem, illustrating the requirement to enhance our knowledge of stress responses underlying its phytotoxicity. Acute Cd exposure induces an oxidative challenge in roots through the use of glutathione (GSH) for Cd-chelating phytochelatin (PC) production. To further uncover the acute Cd stress response of varying intensities, especially related to endoplasmic reticulum (ER) stress, autophagy, and the ethylene response, wild-type (WT) Arabidopsis thaliana were exposed to 2 and 5 µM Cd for 24 h. The dependence of these responses on stress intensity was explored by comparing Cd-sensitive cad2–1 and cad1–3 mutants with disturbed biosynthesis of GSH and PCs, respectively, to the WT. Indicative of ER stress, inositol requiring 1 (IRE1)-dependent basic region/leucine zipper 60 ( bZIP60 ) splicing and target genes were induced with increasing Cd stress in roots. In leaves, this response was already initiated at a lower stress intensity, but also reached its limit more quickly upon increasing stress. On the other hand, more severe Cd stress decreased transcript levels of two autophagy-inhibiting targets of IRE1-dependent decay of mRNAs (RIDD). Lower levels of these transcripts correlated with autophagy induction, suggesting a connection between ER stress and autophagy upon increasing Cd stress. Furthermore, while higher stress intensity stimulated the ethylene response, it also steered the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) towards conjugation, forming malonyl-ACC. Lastly, this study demonstrates that the increased Cd sensitivity of the cad2–1 mutant is mostly related to lower PC production rather than depleted GSH levels.