Thesis presented April 28, 2023

Abstract:
UV is an intrinsic component of sunlight, which affects phototrophs in different ways, mainly through its UV-B component. While high doses of the UV-B irradiation are detrimental to photosynthesis, low doses of UV-B trigger specific acclimation pathways catalyzed by the UVR8 photoreceptor. In Chlamydomonas reinhardtii, we previously reported a link between UV-B detection and photoprotection, mediated by the induction of two Non Photochemical Quenching (NPQ) effector proteins: LHCSR1 and the small Photosystem II Subunit PSBS (Allorent et al., 2016). This response parallels the induction of the third NPQ effector (LHCSR3) by high visible light (HL) via the blue light phototropin (PHOT) photoreceptor (Petroutsos et al., 2016).
In this thesis work, we focus on the role of a novel target of UVR8: the LHC-like protein 4 - LHL4. LHL4 has a photoprotective role in C. reinhardtii but does not modulate directly NPQ. Instead, we hypothesise that LHL4 acts as a "S.O.S protein" that plays its function at the very beginning of light stress to prevent photoinhibition of PSII. Interestingly, LHL4 is induced by both UV-B and HL via both the UVR8 and PHOT photoreceptors. This makes it an interesting probe to study the nature and synergies between the two light signalling pathways in Chlamydomonas. By comparing its induction upon exposure to UV-B and HL in different mutants affecting photoperception, we were able to show that the two pathways converge early in their signalling cascades. Overall, our results provide new insights into the interplay between light sensing, the use of light for photosynthesis and the dissipation of excess light to avoid photodamage.

Keywords:
Chlamydomonas reinhardtii, LHL4, UV-B, Hight light, Photoprotection, Signalling

On-line thesis. ​