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We present here a Tribute to Klaus Apel (1942-2017), a photosynthesis pioneer-an authority on plant molecular genetics-in five parts. The first section is a prologue. The second section deals with a chronological discussion of Apel's research life, prepared by the editor Govindjee; it is based on a website article at the Boyce Thompson Institute (BTI) by Patricia Waldron ( https://btiscience.org/explore-bti/news/post/bti-says-goodbye-klaus-apel/ ), as approved for use here by Keith C. Hannon and David Stern of BTI. The third section, which focuses on Apel's pioneering work on singlet oxygen-mediated EXECUTER-dependent signaling in plants, is written by two of us (J-DR and CK). The fourth section includes three selected reminiscences, one from BTI and two from ETH (Eidgenössische Technische Hochschule). This tribute ends with section five, which is a very brief presentation of Klaus Apel's personal life, by Wiebke Apel.

Abstract: Singlet oxygen (1O2) 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 1O2 generation. It has been thought that the generation of 1O2 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 1O2 significantly contributes to chloroplast‐to‐nucleus retrograde signalling that primes acclimation and cell death responses. Interestingly, recent studies reveal that chloroplasts operate two distinct 1O2‐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 1O2, downstream signalling components, and the perception and reaction mechanism of these mediators towards 1O2. In this review, we mainly discuss the molecular genetic basis of the mode of action of these two putative 1O2 sensors and their corresponding retrograde signalling pathways. In addition, we also propose the possible existence of an alternative source of 1O2, which is spatially and functionally separated from the grana core. [ABSTRACT FROM AUTHOR]