Bilin‐regulated LHCA1 accumulation is independent of photoreceptors PHOT, CRYs, and UVR8 in Chlamydomonas reinhardtii

Abstract Light is a critical environmental signal that is perceived by various photoreceptors and is of great significance in the photosynthetic growth of algae and plants. The phytochrome‐lacking model green alga Chlamydomonas reinhardtii possesses heme oxygenase (HMOX1) and phycocyanobilin ferredoxin oxidoreductase (PCYA1) to synthesize the linear tetrapyrrole bilin from heme in the chloroplast. The hmox1 mutant has photosynthetic growth deficiency and accumulation of photosystem I proteins such as LHCA1 is severely inhibited, and these defects could be rescued by exogenous bilin feeding in a blue light‐dependent manner. To investigate the contribution of the typical blue/ultraviolet light photoreceptors PHOT, aCRY, pCRY, and UVR8 in the process of bilin and blue light‐dependent recovery of LHCA1 protein in hmox1 , we generated double mutants of these photoreceptors in hmox1 , as well as a triple mutant of phot uvr8 hmox1 , to analyze the LHCA1 protein abundance in these mutants. Results clearly showed that PHOT, CRYs, and UVR8 do not participate in this process. In addition, transcriptome profiling analysis of the hmox1 and its genetically complemented strain ho1 C2 during dark‐to‐blue light transition revealed a total of 269 blue light‐responsive genes independent of bilin (|fold change| ≥ 2). RNA‐seq also identified a set of 249 differentially expressed genes that are dependent on both blue light and bilin. These findings provide valuable insights for elucidating the role of bilin in mediating blue light signaling pathways in Chlamydomonas.