More Like this

1. Modelling the pyrenoid-based CO2-concentrating mechanism provides insights into its operating principles and a roadmap for its engineering into crops

   https://doi.org/10.1038/s41477-022-01153-7  

   Fei, Chenyi ; Wilson, Alexandra T. ; Mangan, Niall M. ; Wingreen, Ned S. ; Jonikas, Martin C. ( May 2022 , Nature Plants)

   Many eukaryotic photosynthetic organisms enhance their carbon uptake by supplying concentrated CO₂to the CO₂-fixing enzyme Rubisco in an organelle called the pyrenoid. Ongoing efforts seek to engineer this pyrenoid-based CO₂-concentrating mechanism (PCCM) into crops to increase yields. Here we develop a computational model for a PCCM on the basis of the postulated mechanism in the green algaChlamydomonas reinhardtii. Our model recapitulates allChlamydomonasPCCM-deficient mutant phenotypes and yields general biophysical principles underlying the PCCM. We show that an effective and energetically efficient PCCM requires a physical barrier to reduce pyrenoid CO₂leakage, as well as proper enzyme localization to reduce futile cycling between CO₂and HCO₃⁻. Importantly, our model demonstrates the feasibility of a purely passive CO₂uptake strategy at air-level CO₂, while active HCO₃⁻uptake proves advantageous at lower CO₂levels. We propose a four-step engineering path to increase the rate of CO₂fixation in the plant chloroplast up to threefold at a theoretical cost of only 1.3 ATP per CO₂fixed, thereby offering a framework to guide the engineering of a PCCM into land plants.

    

   more » « less
2. Prospects for Engineering Biophysical CO2 Concentrating Mechanisms into Land Plants to Enhance Yields

   https://doi.org/10.1146/annurev-arplant-081519-040100  

   Hennacy, Jessica H ; Jonikas, Martin C ( March 2020 , Annual review of plant biology)

   Although cyanobacteria and algae represent a small fraction of the biomass of all primary producers, their photosynthetic activity accounts for roughly half of the daily CO2 fixation that occurs on Earth. These microorganisms are able to accomplish this feat by enhancing the activity of the CO2-fixing enzyme Rubisco using biophysical CO2 concentrating mechanisms (CCMs). Biophysical CCMs operate by concentrating bicarbonate and converting it into CO2 in a compartment that houses Rubisco (in contrast with other CCMs that concentrate CO2 via an organic intermediate, such as malate in the case of C4 CCMs). This activity provides Rubisco with a high concentration of its substrate, thereby increasing its reaction rate. The genetic engineering of a biophysical CCM into land plants is being pursued as a strategy to increase crop yields. This review focuses on the progress toward understanding the molecular components of cyanobacterial and algal CCMs, as well as recent advances toward engineering these components into land plants. 

   more » « less
3. Atmospheric CO2 decline and the timing of CAM plant evolution

   https://doi.org/10.1093/aob/mcad122  

   Sage, Rowan F. ; Gilman, Ian S. ; Smith, J. Andrew C. ; Silvera, Katia ; Edwards, Erika J. ( August 2023 , Annals of Botany)

   CAM photosynthesis is hypothesized to have evolved in atmospheres of low CO2 concentration in recent geological time because of its ability to concentrate CO2 around Rubisco and boost water use efficiency relative to C3 photosynthesis. We assess this hypothesis by compiling estimates of when CAM clades arose using phylogenetic chronograms for 73 CAM clades. We further consider evidence of how atmospheric CO2 affects CAM relative to C3 photosynthesis.

   Where CAM origins can be inferred, strong CAM is estimated to have appeared in the past 30 million years in 46 of 48 examined clades, after atmospheric CO2 had declined from high (near 800 ppm) to lower (<450 ppm) values. In turn, 21 of 25 clades containing CAM species (but where CAM origins are less certain) also arose in the past 30 million years. In these clades, CAM is probably younger than the clade origin. We found evidence for repeated weak CAM evolution during the higher CO2 conditions before 30 million years ago, and possible strong CAM origins in the Crassulaceae during the Cretaceous period prior to atmospheric CO2 decline. Most CAM-specific clades arose in the past 15 million years, in a similar pattern observed for origins of C4 clades.

   The evidence indicates strong CAM repeatedly evolved in reduced CO2 conditions of the past 30 million years. Weaker CAM can pre-date low CO2 and, in the Crassulaceae, strong CAM may also have arisen in water-limited microsites under relatively high CO2. Experimental evidence from extant CAM species demonstrates that elevated CO2 reduces the importance of nocturnal CO2 fixation by increasing the contribution of C3 photosynthesis to daily carbon gain. Thus, the advantage of strong CAM would be reduced in high CO2, such that its evolution appears less likely and restricted to more extreme environments than possible in low CO2.

    

   more » « less
4. Assembly of the algal CO 2 -fixing organelle, the pyrenoid, is guided by a Rubisco-binding motif

   https://doi.org/10.1126/sciadv.abd2408  

   Meyer, Moritz T. ; Itakura, Alan K. ; Patena, Weronika ; Wang, Lianyong ; He, Shan ; Emrich-Mills, Tom ; Lau, Chun S. ; Yates, Gary ; Mackinder, Luke C. ; Jonikas, Martin C. ( November 2020 , Science Advances)

   null (Ed.)

   Approximately one-third of the Earth’s photosynthetic CO 2 assimilation occurs in a pyrenoid, an organelle containing the CO 2 -fixing enzyme Rubisco. How constituent proteins are recruited to the pyrenoid and how the organelle’s subcompartments—membrane tubules, a surrounding phase-separated Rubisco matrix, and a peripheral starch sheath—are held together is unknown. Using the model alga Chlamydomonas reinhardtii , we found that pyrenoid proteins share a sequence motif. We show that the motif is necessary and sufficient to target proteins to the pyrenoid and that the motif binds to Rubisco, suggesting a mechanism for targeting. The presence of the Rubisco-binding motif on proteins that localize to the tubules and on proteins that localize to the matrix–starch sheath interface suggests that the motif holds the pyrenoid’s three subcompartments together. Our findings advance our understanding of pyrenoid biogenesis and illustrate how a single protein motif can underlie the architecture of a complex multilayered phase-separated organelle. 

   more » « less
5. Towards engineering carboxysomes into C3 plants

   https://doi.org/10.1111/tpj.13139  

   Hanson, Maureen R. ; Lin, Myat T. ; Carmo‐Silva, A. Elizabete ; Parry, Martin A. J. ( June 2016 , The Plant Journal)

   Photosynthesis in C3 plants is limited by features of the carbon‐fixing enzyme Rubisco, which exhibits a low turnover rate and can react with O₂instead ofCO₂, leading to photorespiration. In cyanobacteria, bacterial microcompartments, known as carboxysomes, improve the efficiency of photosynthesis by concentratingCO₂near the enzyme Rubisco. Cyanobacterial Rubisco enzymes are faster than those of C3 plants, though they have lower specificity towardCO₂than the land plant enzyme. Replacement of land plant Rubisco by faster bacterial variants with lowerCO₂specificity will improve photosynthesis only if a microcompartment capable of concentratingCO₂can also be installed into the chloroplast. We review current information about cyanobacterial microcompartments and carbon‐concentrating mechanisms, plant transformation strategies, replacement of Rubisco in a model C3 plant with cyanobacterial Rubisco and progress toward synthesizing a carboxysome in chloroplasts.

    

   more » « less