Photosynthetic organisms produce organic matter from light energy through photosynthesis. This process also releases oxygen (O₂) through a photochemical reaction catalyzed by a dimeric protein complex called photosystem II (PSII). However, when light intensity exceeds the absorption capacity of PSII, it can be damaged or even destroyed. It must then be repaired by switching to a monomeric intermediate form, which is itself vulnerable to light stress but whose protective mechanisms have remained poorly understood until now.

Using crosslinking* approaches coupled with mass spectrometry, the researchers showed that a protein called "Light Harvesting complex-Like 4" (LHL4), induced in the presence of UV-B and under light stress, specifically binds to monomeric PSII during assembly. By combining different approaches (photophysiological and genetic analyses), they were able to demonstrate that this protein plays a crucial role in protecting this intermediate form under light stress and that its absence limits the synthesis of new functional PSII, ultimately leading to cell death.

​Photograph of liquid cultures of the wild-type (WT) and lhl4 mutant strains of the microalga Chlamydomonas reinhardtii. Under low light conditions, both cultures exhibit the characteristic green coloration. However, under high light stress, the lhl4 mutant rapidly dies, being unable to activate protective mechanisms against light-induced damage. As a result, the culture progressively becomes transparent.