Environmental information perceived by chloroplasts can be translated into retrograde signals that alter the expression of nuclear genes. Singlet oxygen ( 1 O 2 ) 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 1 O 2 -triggered retrograde signaling by promoting the degradation of the EXECUTER1 (EX1) protein, a putative 1 O 2 sensor. Here, we show that a 1 O 2 -mediated oxidative post-translational modification of EX1 is essential for initiating 1 O 2 -derived signaling. Specifically, the Trp643 residue in DUF3506 domain of EX1 is prone to oxidation by 1 O 2 . Both the substitution of Trp643 with 1 O 2 -insensitive amino acids and the deletion of the DUF3506 domain abolish the EX1-mediated 1 O 2 signaling. We thus provide mechanistic insight into how EX1 senses 1 O 2 via Trp643 located in the DUF3506 domain.

Chloroplast-to-nucleus retrograde signaling is essential for the coupled expression of photosynthesis-associated nuclear genes (PhANGs) and plastid genes (PhAPGs) to ensure the functional status of chloroplasts (Cp) in plants. Although various signaling components involved in the process have been identified in Arabidopsis (Arabidopsis thaliana), the biological relevance of such coordination remains an enigma. Here, we show that the uncoupled expression of PhANGs and PhAPGs contributes to the cell death in the lesion simulating disease1 (lsd1) mutant of Arabidopsis. A daylength-dependent increase of salicylic acid (SA) appears to rapidly up-regulate a gene encoding SIGMA FACTOR BINDING PROTEIN1 (SIB1), a transcriptional coregulator, in lsd1 before the onset of cell death. The dual targeting of SIB1 to the nucleus and the Cps leads to a simultaneous up-regulation of PhANGs and down-regulation of PhAPGs. Consequently, this disrupts the stoichiometry of photosynthetic proteins, especially in PSII, resulting in the generation of the highly reactive species singlet oxygen ( 1 O 2 ) in Cps. Accordingly, inactivation of the nuclear-encoded Cp protein EXECUTER1, a putative 1 O 2 sensor, significantly attenuates the lsd1-conferred cell death. Together, these results provide a pathway from the SA-to the 1 O 2 -signaling pathway, which are intertwined via the uncoupled expression of PhANGs and PhAPGs, contributing to the lesionmimicking cell death in lsd1.

Chloroplasts overproduce reactive oxygen species (ROS) under unfavorable environmental conditions, and these ROS are implicated in both signaling and oxidative damage. There is mounting evidence for their roles in translating environmental fluctuations into distinct physiological responses, but their targets, signaling cascades, and mutualism and antagonism with other stress signaling cascades and within ROS signaling remain poorly understood. Great efforts made in recent years have shed new light on chloroplast ROS-directed plant stress responses, from ROS perception to plant responses, in conditional mutants of Arabidopsis thaliana or under various stress conditions. Some articles have also reported the mechanisms underlying the complexity of ROS signaling pathways, with an emphasis on spatiotemporal regulation. ROS and oxidative modification of affected target proteins appear to induce retrograde signaling pathways to maintain chloroplast protein quality control and signaling at a whole-cell level using stress hormones. This review focuses on these seemingly interconnected chloroplast-to-nucleus retrograde signaling pathways initiated by ROS and ROS-modified target molecules. We also discuss future directions in chloroplast stress research to pave the way for discovering new signaling molecules and identifying intersectional signaling components that interact in multiple chloroplast signaling pathways.