The Chlamydomonas alternative oxidase 1 is regulated by cadmium stress: New insights into control of expression

• Chlamydomonas AOX1 transcript and protein abundance are increased under Cd-treatment.
• Exposure of cells to Cd results in the enhanced AOX capacity.
• Cd- and H2O2-induced AOX1 expression is significantly lower under light conditions.
• AOX1 is controlled by darkness independently of Cd- or H2O2-treatments.

Although mitochondrial alternative oxidase (AOX) has been proposed to play essential roles in stress tolerance of plants, the effects of cadmium (Cd), an important environmental pollutant, on AOX are largely unknown. The green alga Chlamydomonas reinhardtii reacts to Cd stress conditions with the induced expression of many genes. In this work, we demonstrated a sharp increase in AOX1 transcript and protein abundance under Cd stress. Furthermore, C. reinhardtii cells displayed a large increase in alternative respiration in response to Cd treatment. Cd-induced AOX1 production and alternative pathway capacity were lower in illuminated cells than in dark-adapted cells. Moreover, in dark-adapted unstressed cells, AOX1 was significantly expressed. The results clearly indicate that the dark-mediated signals function in an additive manner with Cd-mediated signals, and thus likely act in parallel pathways to regulate the expression of AOX1.