Deciphering the role of Acyl-CoA oxidase in response to Cd toxicity in Arabidopsis plants

The role of acyl-CoA oxidase 1 (ACX1), a key enzyme of fatty-acid β-oxidation that generates H2O2 in peroxisomes, in plant responses to Cd toxicity, was investigated using the Arabidopsis mutant acx1-2 lacking functional ACX1. Although silencing ACX1 did not significantly alter overall growth inhibition under Cd stress, it markedly affected ion homeostasis. The acx1-2 mutant accumulated substantially higher Cd levels in both roots and shoots. Transcriptomic analysis of genes encoding for metal transporters revealed only modest expression differences, suggesting that ion homeostasis is mainly regulated through post-transcriptional mechanisms. Consistently, ion flux measurements confirmed enhanced Cd²⁺ influx and altered Ca²⁺ fluxes in acx1-2, indicating regulation involving jasmonic acid (JA) and H2O2 signalling. Analysis of oxidative stress markers and antioxidant responses showed Cd-dependent increases in H2O2 and NO accumulation in wild-type leaves after 5 days, accompanied by increased catalase (CAT) and glutathione reductase (GR) activity. These responses were not significant in acx1-2. The mutant also displayed intrinsically reduced PSII efficiency and electron transport under control conditions, indicating impaired energy partitioning associated with the loss of ACX1. Cd exposure further impaired photosynthetic performance in both genotypes, with stronger effects in acx1-2. Hormone analyses revealed a specific reduction in JA levels in acx1-2 which appears to be a key integrative factor shaping Cd stress responses. ACX1 also slightly influenced differential stomatal aperture and density in abaxial and adaxial leaf surfaces. Thus, ACX1 acts as a fine-tuner integrating peroxisomal metabolism, redox signalling, jasmonate homeostasis, and ion transport during Cd stress.