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1. LCI1, a Chlamydomonas reinhardtii plasma membrane protein, functions in active CO 2 uptake under low CO 2

   https://doi.org/10.1111/tpj.14761  

   Kono, Alfredo ; Spalding, Martin H. ( April 2020 , The Plant Journal)

   In response to high CO₂environmental variability, green algae, such asChlamydomonas reinhardtii, have evolved multiple physiological states dictated by external CO₂concentration. Genetic and physiological studies demonstrated that at least three CO₂physiological states, a high CO₂(0.5–5% CO₂), a low CO₂(0.03–0.4% CO₂) and a very low CO₂(< 0.02% CO₂) state, exist inChlamydomonas. To acclimate in the low and very low CO₂states,Chlamydomonasinduces a sophisticated strategy known as a CO₂‐concentrating mechanism (CCM) that enables proliferation and survival in these unfavorable CO₂environments. Active uptake of C_ifrom the environment is a fundamental aspect in theChlamydomonasCCM, and consists of CO₂and HCO₃^–uptake systems that play distinct roles in low and very low CO₂acclimation states. LCI1, a putative plasma membrane C_itransporter, has been linked through conditional overexpression to active C_iuptake. However, both the role of LCI1 in various CO₂acclimation states and the species of C_i, HCO₃^–or CO₂, that LCI1 transports remain obscure. Here we report the impact of anLCI1loss‐of‐function mutant on growth and photosynthesis in different genetic backgrounds at multiple pH values. These studies show that LCI1 appears to be associated with active CO₂uptake in low CO₂, especially above air‐level CO₂, and that any LCI1 role in very low CO₂is minimal.

    

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2. A role for calcium‐dependent protein kinases in differential CO 2 ‐ and ABA‐controlled stomatal closing and low CO 2 ‐induced stomatal opening in Arabidopsis

   https://doi.org/10.1111/nph.17079  

   Schulze, Sebastian ; Dubeaux, Guillaume ; Ceciliato, Paulo H. O. ; Munemasa, Shintaro ; Nuhkat, Maris ; Yarmolinsky, Dmitry ; Aguilar, Jaimee ; Diaz, Renee ; Azoulay‐Shemer, Tamar ; Steinhorst, Leonie ; et al ( December 2020 , New Phytologist)

   Low concentrations of CO₂cause stomatal opening, whereas [CO₂] elevation leads to stomatal closure. Classical studies have suggested a role for Ca²⁺and protein phosphorylation in CO₂‐induced stomatal closing. Calcium‐dependent protein kinases (CPKs) and calcineurin‐B‐like proteins (CBLs) can sense and translate cytosolic elevation of the second messenger Ca²⁺into specific phosphorylation events. However, Ca²⁺‐binding proteins that function in the stomatal CO₂response remain unknown.

   Time‐resolved stomatal conductance measurements using intact plants, and guard cell patch‐clamp experiments were performed.

   We isolatedcpkquintuple mutants and analyzed stomatal movements in response to CO₂, light and abscisic acid (ABA). Interestingly, we found thatcpk3/5/6/11/23quintuple mutant plants, but not other analyzedcpkquadruple/quintuple mutants, were defective in high CO₂‐induced stomatal closure and, unexpectedly, also in low CO₂‐induced stomatal opening. Furthermore, K⁺‐uptake‐channel activities were reduced incpk3/5/6/11/23quintuple mutants, in correlation with the stomatal opening phenotype. However, light‐mediated stomatal opening remained unaffected, and ABA responses showed slowing in some experiments. By contrast, CO₂‐regulated stomatal movement kinetics were not clearly affected in plasma membrane‐targetedcbl1/4/5/8/9quintuple mutant plants.

   Our findings describe combinatorialcpkmutants that function in CO₂control of stomatal movements and support the results of classical studies showing a role for Ca²⁺in this response.

    

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3. Temperature sensitivity of carbon concentrating mechanisms in the diatom Phaeodactylum tricornutum

   https://doi.org/10.1007/s11120-023-01004-2  

   Li, Meng ; Young, Jodi N. ( March 2023 , Photosynthesis Research)

   Marine diatoms are key primary producers across diverse habitats in the global ocean. Diatoms rely on a biophysical carbon concentrating mechanism (CCM) to supply high concentrations of CO₂around their carboxylating enzyme, RuBisCO. The necessity and energetic cost of the CCM are likely to be highly sensitive to temperature, as temperature impacts CO₂concentration, diffusivity, and the kinetics of CCM components. Here, we used membrane inlet mass spectrometry (MIMS) and modeling to capture temperature regulation of the CCM in the diatomPhaeodactylum tricornutum (Pt). We found that enhanced carbon fixation rates byPtat elevated temperatures were accompanied by increased CCM activity capable of maintaining RuBisCO close to CO₂saturation but that the mechanism varied. At 10 and 18 °C, diffusion of CO₂into the cell, driven byPt’s ‘chloroplast pump’ was the major inorganic carbon source. However, at 18 °C, upregulation of the chloroplast pump enhanced (while retaining the proportion of) both diffusive CO₂and active HCO₃⁻uptake into the cytosol, and significantly increased chloroplast HCO₃⁻concentrations. In contrast, at 25 °C, compared to 18 °C, the chloroplast pump had only a slight increase in activity. While diffusive uptake of CO₂into the cell remained constant, active HCO₃⁻uptake across the cell membrane increased resulting inPtdepending equally on both CO₂and HCO₃⁻as inorganic carbon sources. Despite changes in the CCM, the overall rate of active carbon transport remained double that of carbon fixation across all temperatures tested. The implication of the energetic cost of thePtCCM in response to increasing temperatures was discussed.

    

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4. Influence of Temperature and CO 2 On Plasma‐membrane Permeability to CO 2 and HCO 3 − in the Marine Haptophytes Emiliania huxleyi and Calcidiscus leptoporus (Prymnesiophyceae)

   https://doi.org/10.1111/jpy.13017  

   Blanco‐Ameijeiras, Sonia ; Stoll, Heather M. ; Zhang, Hongrui ; Hopkinson, Brian M. ; Raven, ed., J. ( June 2020 , Journal of Phycology)

   Membrane permeabilities to CO₂and HCO₃⁻constrain the function of CO₂concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO₂but effectively impermeable to HCO₃⁻. Here, CO₂and HCO₃⁻membrane permeabilities were measured using an¹⁸O‐exchange technique on two species of haptophyte algae,Emiliania huxleyiandCalcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO₂(0.006–0.02 cm · s⁻¹) but minimally permeable to HCO₃⁻. Increased temperature and CO₂generally increased CO₂membrane permeabilities in both species, possibly due to changes in lipid composition or CO₂channel proteins. Changes in CO₂membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO₂were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret¹⁸O‐exchange data assumes that carbonic anhydrase, which catalyzes¹⁸O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate¹⁸O‐exchange data and then inferring plasma‐membrane CO₂permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO₂permeabilities are minimal estimates but should be quite accurate under most conditions.

    

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5. Using multirate rapid A / C i curves as a tool to explore new questions in the photosynthetic physiology of plants

   https://doi.org/10.1111/nph.15657  

   Stinziano, Joseph R. ; Adamson, Rachael K. ; Hanson, David T. ( January 2019 , New Phytologist)

   Steady‐state photosyntheticCO₂responses (A/C_icurves) are used to assess environmental responses of photosynthetic traits and to predict future vegetative carbon uptake through modeling. The recent development of rapidA/C_icurves (RACiRs) permits faster assessment of these traits by continuously changing [CO₂] around the leaf, and may reveal additional photosynthetic properties beyond what is practical or possible with steady‐state methods.

   Gas exchange necessarily incorporates photosynthesis and (photo)respiration. Each process was expected to respond on different timescales due to differences in metabolite compartmentation, biochemistry and diffusive pathways. We hypothesized that metabolic lags in photorespiration relative to photosynthesis/respiration andCO₂diffusional limitations can be detected by varying the rate of change in [CO₂] duringRACiR assays. We tested these hypotheses through modeling and experiments at ambient and 2% oxygen.

   Our data show that photorespiratory delays cause offsets in predictedCO₂compensation points that are dependent on the rate of change in [CO₂]. Diffusional limitations may reduce the rate of change in chloroplastic [CO₂], causing a reduction in apparentRACiR slopes under highCO₂ramp rates.

   MultirateRACiRs may prove useful in assessing diffusional limitations to gas exchange and photorespiratory rates.

    

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