Introducing an algal carbon‐concentrating mechanism into higher plants: location and incorporation of key components

Abstract

SummaryMany eukaryotic green algae possess biophysical carbon‐concentrating mechanisms (CCMs) that enhance photosynthetic efficiency and thus permit high growth rates at low CO 2 concentrations. They are thus an attractive option for improving productivity in higher plants. In this study, the intracellular locations of ten CCM components in the unicellular green alga Chlamydomonas reinhardtii were confirmed. When expressed in tobacco, all of these components except chloroplastic carbonic anhydrases CAH3 and CAH6 had the same intracellular locations as in Chlamydomonas. CAH6 could be directed to the chloroplast by fusion to an Arabidopsis chloroplast transit peptide. Similarly, the putative inorganic carbon (Ci) transporter LCI1 was directed to the chloroplast from its native location on the plasma membrane. CCP1 and CCP2 proteins, putative Ci transporters previously reported to be in the chloroplast envelope, localized to mitochondria in both Chlamydomonas and tobacco, suggesting that the algal CCM model requires expansion to include a role for mitochondria. For the Ci transporters LCIA and HLA3, membrane location and Ci transport capacity were confirmed by heterologous expression and H14 CO 3 ‐ uptake assays in Xenopus oocytes. Both were expressed in Arabidopsis resulting in growth comparable with that of wild‐type plants. We conclude that CCM components from Chlamydomonas can be expressed both transiently (in tobacco) and stably (in Arabidopsis) and retargeted to appropriate locations in higher plant cells. As expression of individual Ci transporters did not enhance Arabidopsis growth, stacking of further CCM components will probably be required to achieve a significant increase in photosynthetic efficiency in this species.

A Spatial Interactome Reveals the Protein Organization of the Algal CO2-Concentrating Mechanism

“…HLA3 is an ABC transporter, and its absence under low CO 2 conditions results in a reduced uptake of inorganic carbon by Chlamydomonas cells (Yamano et al, 2015). HLA3 expressed in Xenopus oocytes showed moderate uptake of HCO 3 − (Atkinson et al, 2016). LCI1 lacks any conserved functional or structural domains and contains four predicted transmembrane regions.…”

“…One potential approach for improving yields is the transfer of a CCM into higher plants to increase CO 2 fixation rates (Long et al., 2015). Recent work has found that nearly all algal CCM proteins localize correctly in higher plants with no changes to their protein sequence, suggesting that the transfer of algal components could be relatively straightforward (Atkinson et al, 2016). However, engineering efforts were constrained by our limited knowledge of the components of the algal CCM.…”

“…The AP-MS data produce a spatially resolved protein-protein interaction map of the CCM and pyrenoid, identifying novel protein complexes including a complex between inorganic carbon transporters LCI1 and HLA3, and suggesting CCM functions for multiple proteins. These results transform our basic knowledge of the eukaryotic CCM and advance the prospects of transferring this system into higher plants to improve crop production (Atkinson et al, 2016; Long et al, 2015). …”

A repeat protein links Rubisco to form the eukaryotic carbon-concentrating organelle

“…There is great interest in introducing a CCM into C 3 plants, given that this enhancement is predicted to increase yields by up to 60% and to improve the efficiency of nitrogen and water use (34). Although much remains to be done to improve our understanding of the algal CCM, recent work suggests that algal components may be relatively easy to engineer into higher plants (35). Our discovery of a possible mechanism for Rubisco assembly to form the pyrenoid is a key step toward engineering an algal CCM into crops.…”

The Chlamydomonas CO2‐concentrating mechanism and its potential for engineering photosynthesis in plants

“…Transferring the Chlamydomonas CCM into a higher plant will involve the introduction of components into multiple compartments and membranes as well as the potential removal of native carbonic anhydrase and aquaporins to prevent the CCM from short‐circuiting. Early work has been promising, with core Chlamydomonas CCM proteins – including HLA3, LCIA and LCIB – localizing correctly in higher plants (Atkinson et al ., ), and the stable functional expression of Chlamydomonas Rubisco in Arabidopsis (Atkinson et al ., ). Modeling has indicated that a partial CCM effect may be achieved by the addition of HCO 3 − transporters in the chloroplast envelope (McGrath & Long, ).…”