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

By Jim Ruddock

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

Article first published in Plant Biotechnology Journal online: 5 NOV 2015 | DOI: 10.1111/pbi.12497

There is currently great interest in radical approaches to enhancing photosynthetic efficiency. Introduction of CO₂ concentrating mechanisms into C3 plants based on algal and cyanobacterial carbon concentrating mechanisms (CCMs) is an approach being addressed by several research groups. Atkinson et al. (DOI: 10.1111/pbi.12497) focus on the algal (Chlamydomonas) pyrenoid. This is a structure in chloroplasts, containing clumped together Rubisco, carbonic anhydrase and other proteins with less defined function that are required for the CCM. In addition bicarbonate transporters are needed to pump inorganic carbon (Ci) into the chloroplasts. As this system is not present in vascular plants, the authors have investigated the possibility of expressing its components in tobacco (transiently) and arabidopsis.  

The authors show that Chlamydomonas CCM components tagged with fluorescent protein are located to known/predicted positions in the cells. In general the same proteins localised correctly in plants, or did so after addition of appropriate transit peptides. The authors also show that two Ci transporters were unexpectedly targeted to mitochondria in Chlamydomonas and tobacco suggesting mitochondria might have a role in the CCM, and that two Ci transporters (LCA1 and HLA3) appear to be functional when expressed in Xenopus oocyte membranes, while Arabidopsis plants expressing LCA1 and HLA3 have normal growth and photosynthesis.  In conclusion CCM components from Chlamydomonas can be expressed both transiently (in tobacco) and stably (in Arabidopsis) and retargeted to appropriate locations in higher plant cells. The authors conclude that as the expression of individual Ci transporters did not enhance Arabidopsis growth, the stacking of further CCM components will probably be required to achieve a significant increase in photosynthetic efficiency in this species.