High-resolution suborganellar localization of Ca2+-binding protein CAS, a novel regulator of CO2-concentrating mechanism

The pyrenoid: the eukaryotic CO2-concentrating organelle

The pyrenoid is a phase-separated organelle that enhances photosynthetic carbon assimilation in most eukaryotic algae and the land plant hornwort lineage. Pyrenoids mediate approximately one-third of global CO2 fixation, and engineering a pyrenoid into C3 crops is predicted to boost CO2 uptake and increase yields. Pyrenoids enhance the activity of the CO2-fixing enzyme Rubisco by supplying it with concentrated CO2. All pyrenoids have a dense matrix of Rubisco associated with photosynthetic thylakoid membranes that are thought to supply concentrated CO2. Many pyrenoids are also surrounded by polysaccharide structures that may slow CO2 leakage. Phylogenetic analysis and pyrenoid morphological diversity support a convergent evolutionary origin for pyrenoids. Most of the molecular understanding of pyrenoids comes from the model green alga Chlamydomonas (Chlamydomonas reinhardtii). The Chlamydomonas pyrenoid exhibits multiple liquid-like behaviors, including internal mixing, division by fission, and dissolution and condensation in response to environmental cues and during the cell cycle. Pyrenoid assembly and function are induced by CO2 availability and light, and although transcriptional regulators have been identified, posttranslational regulation remains to be characterized. Here, we summarize the current knowledge of pyrenoid function, structure, components, and dynamic regulation in Chlamydomonas and extrapolate to pyrenoids in other species.

A pyrenoid-localized protein SAGA1 is necessary for Ca2+-binding protein CAS-dependent expression of nuclear genes encoding inorganic carbon transporters in Chlamydomonas reinhardtii

Microalgae induce a CO₂-concentrating mechanism (CCM) to maintain photosynthetic affinity for dissolved inorganic carbon (Ci) under CO₂-limiting conditions. In the model alga Chlamydomonas reinhardtii, the pyrenoid-localized Ca²⁺-binding protein CAS is required to express genes encoding the Ci-transporters, high-light activated 3 (HLA3), and low-CO₂-inducible protein A (LCIA). To identify new factors related to the regulation or components of the CCM, we isolated CO₂-requiring mutants KO-60 and KO-62. These mutants had insertions of a hygromycin-resistant cartridge in the StArch Granules Abnormal 1 (SAGA1) gene, which is necessary to maintain the number of pyrenoids and the structure of pyrenoid tubules in the chloroplast. In both KO-60 and the previously identified saga1 mutant, expression levels of 532 genes were significantly reduced. Among them, 10 CAS-dependent genes, including HLA3 and LCIA, were not expressed in the saga1 mutants. While CAS was expressed normally at the protein levels, the localization of CAS was dispersed through the chloroplast rather than in the pyrenoid, even under CO₂-limiting conditions. These results suggest that SAGA1 is necessary not only for maintenance of the pyrenoid structure but also for regulation of the nuclear genes encoding Ci-transporters through CAS-dependent retrograde signaling under CO₂-limiting stress.

Adapting from Low to High: An Update to CO2-Concentrating Mechanisms of Cyanobacteria and Microalgae

The intracellular accumulation of inorganic carbon (C_i) by microalgae and cyanobacteria under ambient atmospheric CO₂ levels was first documented in the 80s of the 20th Century. Hence, a third variety of the CO₂-concentrating mechanism (CCM), acting in aquatic photoautotrophs with the C₃ photosynthetic pathway, was revealed in addition to the then-known schemes of CCM, functioning in CAM and C₄ higher plants. Despite the low affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) of microalgae and cyanobacteria for the CO₂ substrate and low CO₂/O₂ specificity, CCM allows them to perform efficient CO₂ fixation in the reductive pentose phosphate (RPP) cycle. CCM is based on the coordinated operation of strategically located carbonic anhydrases and CO₂/HCO₃^- uptake systems. This cooperation enables the intracellular accumulation of HCO₃^-, which is then employed to generate a high concentration of CO₂ molecules in the vicinity of Rubisco's active centers compensating up for the shortcomings of enzyme features. CCM functions as an add-on to the RPP cycle while also acting as an important regulatory link in the interaction of dark and light reactions of photosynthesis. This review summarizes recent advances in the study of CCM molecular and cellular organization in microalgae and cyanobacteria, as well as the fundamental principles of its functioning and regulation.

New horizons for building pyrenoid-based CO2-concentrating mechanisms in plants to improve yields

Many photosynthetic species have evolved CO2-concentrating mechanisms (CCMs) to improve the efficiency of CO2 assimilation by Rubisco and reduce the negative impacts of photorespiration. However, the majority of plants (i.e. C3 plants) lack an active CCM. Thus, engineering a functional heterologous CCM into important C3 crops, such as rice (Oryza sativa) and wheat (Triticum aestivum), has become a key strategic ambition to enhance yield potential. Here, we review recent advances in our understanding of the pyrenoid-based CCM in the model green alga Chlamydomonas reinhardtii and engineering progress in C3 plants. We also discuss recent modeling work that has provided insights into the potential advantages of Rubisco condensation within the pyrenoid and the energetic costs of the Chlamydomonas CCM, which, together, will help to better guide future engineering approaches. Key findings include the potential benefits of Rubisco condensation for carboxylation efficiency and the need for a diffusional barrier around the pyrenoid matrix. We discuss a minimal set of components for the CCM to function and that active bicarbonate import into the chloroplast stroma may not be necessary for a functional pyrenoid-based CCM in planta. Thus, the roadmap for building a pyrenoid-based CCM into plant chloroplasts to enhance the efficiency of photosynthesis now appears clearer with new challenges and opportunities.

The significance of calcium-sensing receptor in sustaining photosynthesis and ameliorating stress responses in plants

Calcium ions (Ca²⁺) regulate plant growth and development during exposure to multiple biotic and abiotic stresses as the second signaling messenger in cells. The extracellular calcium-sensing receptor (CAS) is a specific protein spatially located on the thylakoid membrane. It regulates the intracellular Ca²⁺ responses by sensing changes in extracellular Ca²⁺ concentration, thereby affecting a series of downstream signal transduction processes and making plants more resilient to respond to stresses. Here, we summarized the discovery process, structure, and location of CAS in plants and the effects of Ca²⁺ and CAS on stomatal functionality, photosynthesis, and various environmental adaptations. Under changing environmental conditions and global climate, our study enhances the mechanistic understanding of calcium-sensing receptors in sustaining photosynthesis and mediating abiotic stress responses in plants. A better understanding of the fundamental mechanisms of Ca²⁺ and CAS in regulating stress responses in plants may provide novel mitigation strategies for improving crop yield in a world facing more extreme climate-changed linked weather events with multiple stresses during cultivation.

Chlamydomonas reinhardtii cellular compartments and their contribution to intracellular calcium signalling

Calcium (Ca2+)-dependent signalling plays a well-characterized role in the response to different environmental stimuli, in both plant and animal cells. In the model organism for green algae, Chlamydomonas reinhardtii, Ca2+ signals were reported to have a crucial role in different physiological processes, such as stress responses, photosynthesis, and flagella functions. Recent reports identified the underlying components of the Ca2+ signalling machinery at the level of specific subcellular compartments and reported in vivo imaging of cytosolic Ca2+ concentration in response to environmental stimuli. The characterization of these Ca2+-related mechanisms and proteins in C. reinhardtii is providing knowledge on how microalgae can perceive and respond to environmental stimuli, but also on how this Ca2+ signalling machinery has evolved. Here, we review current knowledge on the cellular mechanisms underlying the generation, shaping, and decoding of Ca2+ signals in C. reinhardtii, providing an overview of the known and possible molecular players involved in the Ca2+ signalling of its different subcellular compartments. The advanced toolkits recently developed to measure time-resolved Ca2+ signalling in living C. reinhardtii cells are also discussed, suggesting how they can improve the study of the role of Ca2+ signals in the cellular response of microalgae to environmental stimuli.

Orchestral manoeuvres in the light: crosstalk needed for regulation of the Chlamydomonas carbon concentration mechanism

The inducible carbon concentration mechanism (CCM) in Chlamydomonas reinhardtii has been well defined from a molecular and ultrastructural perspective. Inorganic carbon transport proteins, and strategically located carbonic anhydrases deliver CO2 within the chloroplast pyrenoid matrix where Rubisco is packaged. However, there is little understanding of the fundamental signalling and sensing processes leading to CCM induction. While external CO2 limitation has been believed to be the primary cue, the coupling between energetic supply and inorganic carbon demand through regulatory feedback from light harvesting and photorespiration signals could provide the original CCM trigger. Key questions regarding the integration of these processes are addressed in this review. We consider how the chloroplast functions as a crucible for photosynthesis, importing and integrating nuclear-encoded components from the cytoplasm, and sending retrograde signals to the nucleus to regulate CCM induction. We hypothesize that induction of the CCM is associated with retrograde signals associated with photorespiration and/or light stress. We have also examined the significance of common evolutionary pressures for origins of two co-regulated processes, namely the CCM and photorespiration, in addition to identifying genes of interest involved in transcription, protein folding, and regulatory processes which are needed to fully understand the processes leading to CCM induction.

Pyrenoids: CO2-fixing phase separated liquid organelles

Pyrenoids are non-membrane bound organelles found in chloroplasts of algae and hornwort plants that can be seen by light-microscopy. Pyrenoids are formed by liquid-liquid phase separation (LLPS) of Rubisco, the primary CO₂ fixing enzyme, with an intrinsically disordered multivalent Rubisco-binding protein. Pyrenoids are the heart of algal and hornwort biophysical CO₂ concentrating mechanisms, which accelerate photosynthesis and mediate about 30% of global carbon fixation. Even though LLPS may underlie the apparent convergent evolution of pyrenoids, our current molecular understanding of pyrenoid formation comes from a single example, the model alga Chlamydomonas reinhardtii. In this review, we summarise current knowledge about pyrenoid assembly, regulation and structural organization in Chlamydomonas and highlight evidence that LLPS is the general principle underlying pyrenoid formation across algal lineages and hornworts. Detailed understanding of the principles behind pyrenoid assembly, regulation and structural organization within diverse lineages will provide a fundamental understanding of this biogeochemically important organelle and help guide ongoing efforts to engineer pyrenoids into crops to increase photosynthetic performance and yields.².

The Significance of Calcium in Photosynthesis

As a secondary messenger, calcium participates in various physiological and biochemical reactions in plants. Photosynthesis is the most extensive biosynthesis process on Earth. To date, researchers have found that some chloroplast proteins have Ca²⁺-binding sites, and the structure and function of some of these proteins have been discussed in detail. Although the roles of Ca²⁺ signal transduction related to photosynthesis have been discussed, the relationship between calcium and photosynthesis is seldom systematically summarized. In this review, we provide an overview of current knowledge of calcium’s role in photosynthesis.