A Progressive Report on Chloroplast ROS Sensors EXECUTER1 and EXECUTER2

On December 28, 2021, Chanhong Kim's research group from CAS Center for Excellence in Molecular Plant Sciences (CEMPS) published a research paper entitled "EXECUTER2 modulates the EXECUTER1 signalosome through its singlet oxygen-dependent oxidation" in Molecular Plant. This research has unveiled the long-standing question of how EXECUTER2 (EX2) protein modulates the singlet oxygen (1O2)-triggered EX1-mediated chloroplast-to-nucleus retrograde signaling.
Under photooxidative stress conditions, Photosystem II (PSII) produces a harmful oxygen molecule, namely 1O2, implicated in photodamage and signaling in plants. β-carotene pigment and EXECUTER1 (EX1) protein mediate 1O2-triggered retrograde signaling pathways in the grana core and grana margin, respectively. Previously we demonstrated that EX1 protein senses 1O2 via EX1-Trp643 oxidation in the grana margins, and the subsequent proteolysis of EX1 by the membrane-bound FtsH protease involved in PSII quality control is indispensable for initiating the retrograde signaling (Dogra et al., 2019; Li and Kim, 2021). Now we reveal that the EX1-like protein, namely EX2, also encounters 1O2-dependent Trp530 oxidations, which decelerates 1O2 signaling by mitigating the oxidation level of EX1-Trp643 and FtsH-dependent EX1 degradation. The loss of EX2 reinforces the EX1-dependent retrograde signaling by increasing the EX1-Trp643 oxidation level. In contrast, overexpression of EX2 attenuates EX1-dependent chloroplast-nucleus communication by reducing the oxidation level of EX1-Trp643, providing a new mechanistic insight into how EX2 modulates EX1-mediated 1O2 signaling.
One would ask why chloroplasts need a system in which a closely related protein (EX2) antagonizes its homolog's (EX1) function. Given that EX1-mediated 1O2 signaling induces cell death, EX2 seems to modulate the sensitivity of EX1 towards 1O2, preventing an overly sensitive reaction of chloroplasts to 1O2 (e.g., growth inhibition and cell death) (Kim et al., 2012; Dogra et al., 2019; Li et al., 2020). Moreover, a phylogenetic analysis provides an intriguing clue that EX2 seems to be evolutionarily emerged to attenuate the sensitivity of EX1 towards 1O2.We believe that this finding advances our current understanding of the regulation of 1O2 signaling.
Oxidative post-translational modifications of Trp residues in EX1 and EX2 modulate 1O2 signaling: EX1 and EX2 proteins have long been considered major players in mediating singlet oxygen (1O2)-triggered chloroplast-to-nucleus retrograde signaling. However, it has been vague how these two proteins regulate 1O2 signaling. This study demonstrates that the 1O2-driven oxidative modification of EX2-Trp530 modulates the EX1-mediated 1O2 signaling by hindering the oxidation of EX1-Trp643 that is essential for initiating 1O2 signaling.    
Article Link: https://www.cell.com/molecular-plant/fulltext/S1674-2052(21)00486-X
DOI: https://doi.org/10.1016/j.molp.2021.12.016
Prof. Chanhong Kim form CAS Center of Excellence in Molecular Plant Sciences, is the corresponding author, and Dr. Vivek Dogra (currently Senior Scientist of Institute of Himalayan Bioresource Technology), Dr. Rahul Singh and Dr. Mengping Li form CAS Center of Excellence in Molecular Plant Sciences, are co-first authors. This research was supported by the Strategic Priority Research Program from the Chinese Academy of Sciences, and the National Natural Science Foundation of China.
Dogra, V., Li, M., Singh, S., Li, M., and Kim, C. (2019). Oxidative post-translational modification of EXECUTER1 is required for singlet oxygen sensing in plastids. Nature communications 10:2834.
Kim, C., Meskauskiene, R., Zhang, S., Lee, K.P., Ashok, M.L., Blajecka, K., Herrfurth, C., Feussner, I., and Apel, K. (2012). Chloroplasts of Arabidopsis Are the Source and a Primary Target of a Plant-Specific Programmed Cell Death Signaling Pathway. Plant Cell 24:3026-3039.
Li, M., and Kim, C. (2021). Chloroplast ROS and stress signaling. Plant Communications.
Li, B., Fang, J., Singh, R.M., Zi, H., Lv, S., Liu, R., Dogra, V., and Kim, C. (2020). FATTY ACID DESATURASE5 Is Required to Induce Autoimmune Responses in Gigantic Chloroplast Mutants of Arabidopsis. Plant Cell 32:3240-3255.
Prof. Chanhong Kim
Center for Excellence in Molecular Plant Sciences (CEMPS), Chinese Academic of Sciences
Email: chanhongkim@cemps.ac.cn