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

Nicky Atkinson,Doreen Feike,Howard Griffiths,Luke C M Mackinder,Alistair J Mccormick,Alison M Smith,Martin C Jonikas,Moritz T Meyer

doi:10.1111/pbi.12497

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.

Highlights

Most plants, including the major grain crops rice and wheat, assimilate carbon using the C3 photosynthetic pathway
Our results show that concentrating mechanisms (CCMs) components from Chlamydomonas can be expressed in appropriate locations in higher plant cells without compromising growth, – consistent with modelling predictions – additional elements of the algal CCM will need to be co-expressed to achieve enhanced productivity
The locations of CCM components in Chlamydomonas were investigated by transforming Chlamydomonas cells with constructs encoding these proteins fused to a fluorescent tag (Venus) at the C-terminus (Figures 1a and S1a)

Summary

Introduction

Most plants, including the major grain crops rice and wheat, assimilate carbon using the C3 photosynthetic pathway. 250 lM at 25 °C) competes at the RuBisCO active sites, resulting in both loss of assimilated carbon and nitrogen and energy consumption in the photorespiratory pathway that recycles the product of RuBP oxygenation (Sharkey, 1988). The productivity of C3 crops is limited by the efficiency of CO2 photoassimilation, even when grown under elevated CO2 levels (up to 650 ppm) (Long et al, 2006). Generating C3 crop plants with increased photosynthetic efficiencies is a major target for improving yields and safeguarding future food security. Strategies under consideration and development include modifying canopies to increase light interception, enhancing repair mechanisms to overcome lags associated with photoprotection, increasing the efficiency of RuBisCO and eliminating photorespiration by introducing molecular components of microbial carbon-concentrating mechanisms (CCM) (Lin et al, 2014a,b; Long et al, 2015; Parry et al, 2013; Whitney et al, 2011; Zhu et al, 2010)

Results

Discussion

Conclusion