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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. 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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3. Structure and function of LCI1: a plasma membrane CO 2 channel in the Chlamydomonas CO 2 concentrating mechanism

   https://doi.org/10.1111/tpj.14745  

   Kono, Alfredo ; Chou, Tsung‐Han ; Radhakrishnan, Abhijith ; Bolla, Jani Reddy ; Sankar, Kannan ; Shome, Sayane ; Su, Chih‐Chia ; Jernigan, Robert L. ; Robinson, Carol V. ; Yu, Edward W. ; et al ( April 2020 , The Plant Journal)

   Microalgae and cyanobacteria contribute roughly half of the global photosynthetic carbon assimilation. Faced with limited access to CO₂in aquatic environments, which can vary daily or hourly, these microorganisms have evolved use of an efficient CO₂concentrating mechanism (CCM) to accumulate high internal concentrations of inorganic carbon (C_i) to maintain photosynthetic performance. For eukaryotic algae, a combination of molecular, genetic and physiological studies using the model organismChlamydomonas reinhardtii, have revealed the function and molecular characteristics of many CCM components, including active C_iuptake systems. Fundamental to eukaryotic C_iuptake systems are C_itransporters/channels located in membranes of various cell compartments, which together facilitate the movement of C_ifrom the environment into the chloroplast, where primary CO₂assimilation occurs. Two putative plasma membrane C_itransporters, HLA3 and LCI1, are reportedly involved in active C_iuptake. Based on previous studies, HLA3 clearly plays a meaningful role in HCO₃⁻transport, but the function of LCI1 has not yet been thoroughly investigated so remains somewhat obscure. Here we report a crystal structure of the full‐length LCI1 membrane protein to reveal LCI1 structural characteristics, as well asin vivophysiological studies in an LCI1 loss‐of‐function mutant to reveal the C_ispecies preference for LCI1. Together, these new studies demonstrate LCI1 plays an important role in active CO₂uptake and that LCI1 likely functions as a plasma membrane CO₂channel, possibly a gated channel.

    

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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. 

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5. MPK12 in stomatal CO2 signaling: function beyond its kinase activity

   https://doi.org/10.1111/nph.18913  

   Yeh, Chung‐Yueh ; Wang, Yuh‐Shuh ; Takahashi, Yohei ; Kuusk, Katarina ; Paul, Karnelia ; Arjus, Triinu ; Yadlos, Oleksii ; Schroeder, Julian I. ; Ilves, Ivar ; Garcia‐Sosa, Alfonso T. ; et al ( April 2023 , New Phytologist)

   Protein phosphorylation is a major molecular switch involved in the regulation of stomatal opening and closure. Previous research defined interaction between MAP kinase 12 and Raf‐like kinase HT1 as a required step for stomatal movements caused by changes in CO₂concentration. However, whether MPK12 kinase activity is required for regulation of CO₂‐induced stomatal responses warrants in‐depth investigation.

   We apply genetic, biochemical, and structural modeling approaches to examining the noncatalytic role of MPK12 in guard cell CO₂signaling that relies on allosteric inhibition of HT1.

   We show that CO₂/HCO₃⁻‐enhanced MPK12 interaction with HT1 is independent of its kinase activity. By analyzing gas exchange of plant lines expressing various kinase‐dead and constitutively active versions of MPK12 in a plant line whereMPK12is deleted, we confirmed that CO₂‐dependent stomatal responses rely on MPK12's ability to bind to HT1, but not its kinase activity. We also demonstrate that purified MPK12 and HT1 proteins form a heterodimer in the presence of CO₂/HCO₃⁻and present structural modeling that explains the MPK12:HT1 interaction interface.

   These data add to the model that MPK12 kinase‐activity‐independent interaction with HT1 functions as a molecular switch by which guard cells sense changes in atmospheric CO₂concentration.

    

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