Impact of red and far-red light ratios on the physiological parameters and expression of light-responsive genes in...

The phytochrome-regulated responses of a etiolated M. polymorpha culture to red light (RL), far-red light (FRL) and their combinations (RL/FRL 1/1, 1/2, 2/1) were studied. The explants were exposed to continuous irradiation for 240 h. At this time, the growth, chlorophyll a fluorescence, photosynthetic pigment content, chloroplast ultrastructure and light-dependent gene expression of the explants were assessed. The phytochrome of M. polymorpha (MpPHY) serves as the only receptor for RL and FRL, combining the functions of both PhyB and PhyA. It regulates the transcription of key genes involved in the functioning of the photosynthetic apparatus, including the small subunit of Rubisco (RbcS) and PSII antenna complex proteins; however, the mechanism underlying this regulation under different spectral compositions remains unclear. When grown on FRL, M. polymorpha retained the granal structure of chloroplasts, but its photosynthetic activity, pigment biosynthesis, and biomass were significantly reduced. When the plants were grown under light at a 2/1 ratio, the greatest increase in biomass was observed. These effects correlated with the activation of genes involved in auxin and cytokinin biosynthesis, as well as the cellulose synthase gene. Both the PSII activity and the pigment content significantly decreased under FRL. It is suggested that under RL dominance, MpPHY functions similarly to PhyB, promoting photosynthesis and growth, whereas under FRL dominance, its activity resembles that of PhyA, restricting photomorphogenesis. Compared with fully etiolated explants, M. polymorpha grown under white light adapts likely more effectively to FRL than fully etiolated explants, highlighting the importance of light conditions during precultivation.