Root peroxisomal responses in Arabidopsis plants exposed to cadmium toxicity

Peroxisomes are single membrane bound organelles that play key roles in plant growth, development, and stress response. They are also the site of an active ROS/RNS metabolism and of the conversion of the auxin precursor indole-3-butyric acid (IBA) into active indole-3-acetic acid (IAA) [1]. In plant cells, peroxisomal functions are dependent upon different PEROXIN (PEX) receptors. Among them, PEX5 and PEX7 cytosolic receptors recognize peroxisomal matrix proteins carrying specific C- or N-terminal aminoacidic sequences, the Peroxisomal Targeting Signal type 1 (PTS1) or type 2 (PTS2), respectively, and traffic them into the peroxisomal matrix [2]. Cadmium (Cd) is a non-essential metal whose excessive levels in soils reduce crop yield and inhibit root development and plant growth [3]. To encounter Cd toxicity, plants must rely on an effective response of the root system to the stressor, a process in which peroxisomes could play a key role. To deepen the knowledge about the role of this organelle during Cd-related stress conditions, seedlings of Arabidopsis thaliana pex7-1 mutant, characterized by a low PTS2 protein import [2], and of its wild-type were grown in the presence of 30 or 60 μM CdSO4 and their roots analysed for ROS and RNS content by histochemical or epifluorescence analysis. In addition, using a fluorescent probe labelling peroxisome, the presence and distribution of the organelles in root protoplasts of both pex7-1 and wild-type were investigated. Moreover, root morphological traits and the activity of root catalase (CAT) in response to Cd-treatments were also evaluated, such as IBA and IAA contents. Our findings show that the PEX7 receptor is involved in peroxisome responses to Cd toxicity. In fact, peroxisomal presence in pex7-1 and the wild-type roots are differently affected by Cd such as CAT activity and ROS levels. Finally, pex7-1 mutation affects auxin homeostasis in both root and shoot, ultimately resulting into changes in root morphogenesis in the mutant in response to the pollutant compared to wild-type.