Heavy metal toxicity leads to accumulation of insoluble proteins and induces endoplasmic reticulum stress–specific unfolded protein response in Arabidopsis thaliana

Abstract

Unfolded protein accumulation in the endoplasmic reticulum (ER) triggers ER stress leading to a unique transcriptomic response called unfolded protein response (UPR). While ER stress is linked to various environmental stresses its role in plant responses to heavy metal toxicity remains unclear. This study aimed to elucidate if heavy metals Fe Zn Cu and As induce ER stress in plants. For this purpose Arabidopsis thaliana seedlings were treated with Fe (200 400 µM) Zn (500 700 µM) Cu (25 50 µM) and As (250 500 µM) for 7 days which resulted in 50–70% decrease in plant growth. All treatments increased insoluble protein levels indicating unfolded protein accumulation with the highest induction observed for 50 µM Cu treatment (fivefold). Expressions of genes involved in the perception and signaling of ER stress (IRE1 bZIP28 bZIP60 bZIP17) indicate that Zn toxicity specifically induces bZIP28 but not the IRE1 branch of UPR. All metals except Fe also induced genes associated with protein folding in the ER (BIP1 BIP3 and CNX) and ER-associated protein degradation (ERAD) (HRD1). This finding indicates Zn Cu and As but not Fe cause ER stress in plants. Furthermore increased expression of ER oxidoreductase 1 (ERO1) suggests that metal toxicity also disrupts oxidative protein folding in the ER lumen. This study enhances our understanding of the intricate interplay between essential nutrients metal toxicity protein folding machinery and ER stress demonstrating that heavy metal toxicity has an ER stress component in plants alongside its established effects on energy metabolism membrane integrity and oxidative stress. © 2024 Elsevier B.V. All rights reserved.