Singlet oxygen‐triggered chloroplast‐to‐nucleus retrograde signalling pathways: An emerging perspective

Singlet oxygen ( 1 O 2 ) is a prime cause of photo‐damage of the photosynthetic apparatus. The chlorophyll molecules in the photosystem II reaction center and in the light‐harvesting antenna complex are major sources of 1 O 2 generation. It has been thought that the generation of 1 O 2 mainly takes place in the appressed regions of the thylakoid membranes, namely, the grana core, where most of the active photosystem II complexes are localized. Apart from being a toxic molecule, new evidence suggests that 1 O 2 significantly contributes to chloroplast‐to‐nucleus retrograde signalling that primes acclimation and cell death responses. Interestingly, recent studies reveal that chloroplasts operate two distinct 1 O 2 ‐triggered retrograde signalling pathways in which β‐carotene and a nuclear‐encoded chloroplast protein EXECUTER1 play essential roles as signalling mediators. The coexistence of these mediators raises several questions: their crosstalk, source(s) of 1 O 2 , downstream signalling components, and the perception and reaction mechanism of these mediators towards 1 O 2 . In this review, we mainly discuss the molecular genetic basis of the mode of action of these two putative 1 O 2 sensors and their corresponding retrograde signalling pathways. In addition, we also propose the possible existence of an alternative source of 1 O 2 , which is spatially and functionally separated from the grana core.