APX2, the ascorbate peroxidase-related protein, regulates the levels of plastocyanin in Chlamydomonas

The function of ascorbate peroxidase-related (APX-R) proteins, present in all green photosynthetic eukaryotes, remains unclear. This study focuses on APX-R from Chlamydomonas reinhardtii, namely ascorbate peroxidase 2 (APX2). We showed that apx2 mutants exhibited a faster oxidation of the PSI primary electron donor, P700, upon sudden light increase and a slower re-reduction rate compared to the wild type, pointing to a limitation of plastocyanin. Spectroscopic, proteomic, and immunoblot analyses confirmed that the phenotype was a result of lower levels of plastocyanin in the apx2 mutants. The redox state of P700 did not differ between wild type and apx2 mutants when the loss of function in plastocyanin was nutritionally complemented by growing apx2 mutants under copper deficiency. In this case, cytochrome c6 functionally replaces plastocyanin, confirming that lower levels of plastocyanin was the primary defect caused by absence of APX2. Overall, the results presented here shed light on an unexpected regulation of plastocyanin level under copper replete conditions, induced by ascorbate peroxidase 2 in Chlamydomonas