Oxidative Post-translational Modification of EXECUTER1 is Required for Singlet Oxygen Sensing in Plastids

Environmental information perceived by chloroplasts can be translated into retrograde signals that alter the expression of nuclear genes. Singlet oxygen (O) generated by photosystem II (PSII) can cause photo-oxidative damage of PSII but has also been implicated in retrograde signaling. We previously reported that a nuclear-encoded chloroplast FtsH2 metalloprotease coordinates O-triggered retrograde signaling by promoting the degradation of the EXECUTER1 (EX1) protein, a putative O sensor. Here, we show that a O-mediated oxidative post-translational modification of EX1 is essential for initiating O-derived signaling. Specifically, the Trp643 residue in DUF3506 domain of EX1 is prone to oxidation by O. Both the substitution of Trp643 with O-insensitive amino acids and the deletion of the DUF3506 domain abolish the EX1-mediated O signaling. We thus provide mechanistic insight into how EX1 senses O via Trp643 located in the DUF3506 domain.