Singlet Oxygen Signaling

Singlet oxygen is a major type of reactive oxygen species that is generated in excess light by energy transfer from excited chlorophylls. We discovered that Chlamydomonas exhibits an acclimation response to singlet oxygen stress that enables cells to survive singlet oxygen levels that would otherwise be lethal. This response involves retrograde signaling from the chloroplast to the nucleus to regulate expression of nuclear genes, and we have taken a genetic approach to investigate this signaling pathway.

During oxygenic photosynthesis, singlet oxygen is generated mainly in the photosystem II reaction center, where it can cause photo-oxidative damage and inhibit photosynthesis. Like all aerobic organisms, algae and plants have evolved sensing, signaling, and protective mechanisms to cope with various reactive oxygen species, including singlet oxygen. To dissect the signal transduction pathway involved in sensing and responding to singlet oxygen in Chlamydomonas, we have isolated and studied mutants that are constitutively more resistant to singlet oxygen, as well as mutants that are unable to acclimate to singlet oxygen. Analysis of the sor1 mutant led to the identification of a putative bZIP transcription factor that regulates the expression of various genes involved in the detoxification of reactive electrophile species. The sak1 mutant identified a cytosolic phosphoprotein that is necessary for much of the gene expression response during acclimation to singlet oxygen; this regulator represents a long-sought missing link in retrograde signaling.