Light-independent regulation of algal photoprotection by CO₂availability

Abstract

Photosynthetic algae cope with suboptimal levels of light and CO2. In low CO2 and excess light, the green alga Chlamydomonas reinhardtii activates a CO2 Concentrating Mechanism (CCM) and photoprotection; the latter is mediated by LHCSR1/3 and PSBS. How light and CO2 signals converge to regulate photoprotective responses remains unclear. Here we show that excess light activates expression of photoprotective and CCM-related genes and that depletion of CO2 drives these responses, even in total darkness. High CO2 levels, derived from respiration or impaired photosynthetic fixation, repress LHCSR3 and CCM genes while stabilizing the LHCSR1 protein. We also show that CIA5, which controls CCM genes, is a major regulator of photoprotection, elevating LHCSR3 and PSBS transcript accumulation while inhibiting LHCSR1 accumulation. Our work emphasizes the importance of CO2 in regulating photoprotection and the CCM, demonstrating that the impact of light on photoprotection is often indirect and reflects intracellular CO2 levels.

Transcriptional regulation of photoprotection in dark-to-light transition - more than just a matter of excess light energy

“…Indeed, we observed that LHCSR3 was still induced by blue light (Supplementary Figure 5) and UV-B radiation (Figure 6) in the cia5 mutant, although both the basal and induced levels of expression were lower than in WT cells (but similar fold-change). The lower LHCSR3 mRNA levels present in dark-acclimated cia5 mutant cells are in line with the work by Ruiz-Sola et al (2021) that demonstrates that CIA5-dependent LHCSR3 induction also occurred in total darkness when the availability of inorganic carbon becomes very low (Figure 6B).…”

“…We measured LHCSR3.1 transcript levels before and after transferring WT and cia5 cells to HSM and we could see that this transfer led to LHCSR3.1 mRNA accumulation (Figure 6B). This result can be explained based on the recent findings of Ruiz-Sola and collaborators who demonstrated that changes in CO2 availability can activate LHCSR3 gene expression even in the absence of light in a CIA5-dependent manner (Ruiz-Sola et al, 2021). In our case the transfer of the cells from TAP to HSM would drop CO2 levels in the culture [CO2 that accumulated in TAP medium (associated with metabolism of acetate) in the dark would decline] and therefore activate LHCSR3.…”

“…Additionally, LHCSR3 is strongly controlled by CO2 levels and the regulatory factor CIA5, which may function as an enhancer while PSBS may be affected by CO2 to a minor extent (orange). Additional discussion of the role of CO2 in regulating LHCSR3 is presented in Ruiz et al (Ruiz-Sola et al, 2021). The heatmap table summarizes the transcript fold change for each gene in the transition from dark to the indicated conditions.…”

“…It is known that the LHCSR3 transcript accumulates when inorganic carbon levels are low, and that this induction is mediated by CIA5, the main regulatory factor involved in controlling the genes associated with the CCM (Fang et al, 2012;Polukhina et al, 2016). PSBS protein levels are also elevated more in minimal medium (air levels of CO2) than in TAP medium (17 mM acetate) (Correa-Galvis et al, 2016); internal CO2 levels would increase in the presence of acetate (Ruiz Sola et al, 2021). In contrast, LHCSR1 transcript levels accumulated in the presence of high CO2, especially upon exposure to HL (Yamano et al, 2008).…”

“…The Chlamydomonas reinhardtii strains used in this study were wild-type CC-125 mt + and CC-124 mt + (137c), phot1 (CC-5392), uvr8 (CC-5442), cia5 (CC-2702), the phot1 rescued strain, designated pho1t-C, and the cia5 rescued strain, designated cia5-C. The phot1 mutant was engineered by CRISPR-CAS9 inactivation (Greiner et al, 2017) and cia5-C is described in (Ruiz-Sola et al 2021). For the complementation of phot1, resulting in the pho1t-C strain, a 2.25-kb fragment containing the PHOT1 CDS was amplified by PCR with KOD hot start DNA polymerase (Novagen) using PHOT1 FW and PHOT1 RV primers (Table 1), gel purified and cloned into phk330 (Miao et al, 2019) using the BamHI and EcoRI restriction sites for expression under control of HSP70/RBC hybrid promoter.…”