ALTERNATIVE OXIDASE1a modulates the oxidative challenge during moderate Cd exposure in Arabidopsis thaliana leaves

This study aims to unravel the functional significance of alternative oxidase1a (AOX1a) induction in Arabidopsis thaliana leaves exposed to cadmium (Cd) by comparing wild-type (WT) plants and aox1a knockout mutants. In the absence of AOX1a, differences in stress-responsive transcript and glutathione levels suggest an increased oxidative challenge during moderate (5 mu M) and prolonged (72 h) Cd exposure. Nevertheless, aox1a knockout leaves showed lower hydrogen peroxide (H2O2) accumulation as compared to the WT due to both acute (24 h) and prolonged (72 h) exposure to 5 mu M Cd, but not to 10 mu M Cd. Taken together, we propose a working model where AOX1a acts early in the response to Cd and activates or maintains a mitochondrial signalling pathway impacting on cellular antioxidative defence at the post-transcriptional level. This fine-tuning pathway is suggested to function during moderate (5 mu M) Cd exposure while being overwhelmed during more severe (10 mu M) Cd stress. Within this framework, ethylene is required - either directly or indirectly via NADPH oxidase isoform C - to fully induce AOX1 expression. In addition, reciprocal crosstalk between these components was demonstrated in leaves of A. thaliana plants exposed to Cd.