ROC75 is an Attenuator for the Circadian Clock that Controls LHCSR3 Expression

Abstract

Strong light intensity leads to harmful overexcitation of the photosystems in green algae. In Chlamydomonas reinhardtii, LHCSR3 is required for the rapid protective response known as energy-dependent quenching (qE). Because the majority of photoacclimation analysis has been conducted under controlled laboratory conditions, physiological responses to natural environmental changes such as light/dark cycles have not been examined in detail. Regarding fitness in higher plants and microalgae, light-dark cycles represent a major Zeitgeber for synchronizing the circadian clock to multiple physiological responses, yet there is little consensus with respect to the clock response to high-intensity light in photosynthetic organisms. In a previous study, 105 circadian rhythm insertional mutants were isolated as rhythm of chloroplast (roc) mutants. Here, we report our characterization of the roc75 mutant, which exhibited a significantly higher qE value and LHCSR3 protein accumulation when grown under red light. We performed transcript analysis of ROC75 in the pcry (plant-cryptochrome) and phot mutants and found that only the former accumulated lower levels of ROC75 mRNA, suggesting that the blue light photoreceptor pCRY positively regulates ROC75. However, the degradation of pCRY by high-light exposure contributes to prevent over-accumulation of ROC75, which in turn facilitates the PHOT mediated main activation pathway for LHCSR3. Furthermore, LHCSR3 mRNA exhibited a circadian rhythm, though its basal expression level in the roc75 mutant was higher than that in WT. We therefore conclude that ROC75 acts as an attenuator of the circadian clock to control LHCSR3 expression with blue and red light as stimuli for attenuation.

Widening the landscape of transcriptional regulation of green algal photoprotection

“…4 ). In addition, two previously reported regulators of qE, ROC75 28 and CrCO 26, 27 , are also included in this list of PHOT-dependent regulators. These observations are in line with an existing hypothesis 24 suggesting that a CUL4-dependent E3-ligase targeting CrCO, 26 acts downstream of PHOT.…”

Abiotic stress through time

“…There is evidence to support that algal ancestors utilize circadian-regulated mechanisms to respond to abiotic stress. The C. reinhardtii clock is postulated to regulate photoprotection against harmful photosystem overexcitation from strong red light via the ROC75-mediated regulation of photoprotective effector genes like LIGHT-HARVESTING COMPLEX STRESS-RELATED3 (LHCSR3) (Kamrani et al, 2018). P. patens and the latest streptophyte algae (Zygnematales, Coleochaetales, and Charales) that diverged from land plants are also postulated to rely on LHCSRs for photoprotection (Gerotto & Morosinotto, 2013;Pinnola, 2019), though whether they are circadianregulated as CrLHCSR3 is unknown.…”

“…(LHCSR3) (Kamrani et al, 2018). P. patens and the latest streptophyte algae (Zygnematales, Coleochaetales and Charales) that diverged from land plants are also postulated to rely on LHCSRs for photoprotection (Gerotto & Morosinotto, 2013;Pinnola, 2019), although whether they are circadian-regulated as CrLHCSR3 is unknown.…”

“…The C . reinhardtii clock is postulated to regulate photoprotection against harmful photosystem overexcitation from strong red light via the ROC75‐mediated regulation of photoprotective effector genes like LIGHT‐HARVESTING COMPLEX STRESS‐RELATED3 ( LHCSR3 ) (Kamrani et al ., 2018). P .…”

The World of Algae Reveals a Broad Variety of Cryptochrome Properties and Functions

“…Moreover, the Cr pCRY knockdown line exerts a significantly lengthened period of about 27.9h compared to wild type (24.5h in average); it becomes arrhythmic after a few days under free-running conditions that are used to analyze clock properties. It was also suggested that pCRY positively regulates the clock component, ROC75, a putative transcription factor ( Kamrani et al, 2018 ). These data support the model that Cr pCRY is not only involved in circadian input but is also linked to the central oscillator.…”