Roles of LHCSR3 and STT7 in the remodeling of thylakoid supercomplexes in Chlamydomonas reinhardtii under UV-B treatment.

In the green microalga Chlamydomonas reinhardtii, UV-B radiation triggers complex photoprotective mechanisms. While Light-Harvesting Complex Stress Related 1 (LHCSR1) has been traditionally viewed as the primary UV-B responsive protein, the functional significance of LHCSR3 and its interplay with the STT7 kinase under these conditions remain largely elusive. Here, we investigated the spectral and structural dynamics of thylakoid supercomplexes under continuous long-term UV-B irradiation using a comparative approach with wild-type, npq4 (LHCSR3-deficient), stt7-9 (STT7 kinase-deficient), and npq4 stt7-9 double-mutant strains. Our results demonstrate that UV-B induces substantial accumulation of both LHCSR1 and LHCSR3, yet only LHCSR3 undergoes STT7-dependent phosphorylation. This modification is essential for the formation of high-molecular-weight PSII-LHCII megacomplexes and a specialized PSI-LHCI-LHCII-LHCSRs supercomplex during state transitions. In the absence of LHCSR3, both photosystem core subunits and peripheral antennae (CP26/CP29) undergo extensive degradation. We further identified that STT7 kinase is required for antenna detachment and facilitates stable LHCSR3 accumulation. LHCSR3 is a dual-function regulator indispensable for both energy quenching and structural stabilization of the photosynthetic apparatus. The STT7-LHCSR3 mechanism represents a vital adaptive strategy that primes the machinery for fluctuating light environments.