BLZ8 activates a plastidial peroxiredoxin and a ferredoxin to protect Chlamydomonas reinhardtii against oxidative stress.

Reactive oxygen species (ROS) cause damage to various cellular processes in almost all organisms, in particular photosynthetic organisms that depend on the electron transfer chain for CO 2 fixation. However, the detoxifying process against ROS damage has not been studied intensively in microalgae. Here, we characterized a ROS detoxifying role of a bZIP transcription factor, BLZ8, in Chlamydomonas reinhardtii. To identify downstream targets of BLZ8, we carried out comparative genome-wide transcriptomic profiling in BLZ8 OX and its parental CC-4533 under oxidative stress conditions. Luciferase reporter activity assays and RT-qPCR were performed to test whether BLZ8 regulates downstream genes. We performed an in silico functional gene network analysis and an in vivo immunoprecipitation assay to identify the interaction between downstream targets of BLZ8. Comparative transcriptomic analysis and RT-qPCR revealed that overexpression of BLZ8 increased the expression levels of plastid peroxiredoxin1 (PRX1) and ferredoxin-5 (FDX5) under oxidative stress conditions. BLZ8 alone could activate the transcriptional activity of FDX5, and required bZIP2 to activate the transcriptional activity of PRX1. Functional gene network analysis using FDX5 and PRX1 orthologs in Arabidopsis thaliana suggested that these two genes were functionally associated. Indeed, our immunoprecipitation assay revealed the physical interaction between PRX1 and FDX5. Furthermore, the complemented strain, fdx5 (FDX5), recovered the growth retardation of the fdx5 mutant under oxidative stress conditions, indicating that FDX5 contributes to oxidative stress tolerance. These results suggest that BLZ8 activates PRX1 and FDX5 expression, resulting in the detoxification of ROS to confer oxidative stress tolerance in microalgae.