Format: ORAL

Stephanie Ruaud1, Svenja Ntzold2, James Barrett3, Manuel Waller1, Anna Neubauer1, Luke Mackinder3, Susann Wicke2, Peter Szovenyi1

1 Dept. of Systematic and Evolutionary Botany, University of Zurich, Switzerland 2 Plant Systematics and Biodiversity, Humboldt-University Berlin, Germany 3 Department of Biology, University of York, UK

Biophysical carbon concentrating mechanisms (CCMs) operating at the single-cell level have evolved independently in various lineages of eukaryotic algae and a single land plant lineage, the hornworts. An essential component for an efficient eukaryotic CCM is a pyrenoid, a specialized compartment inside the chloroplast that mainly comprises the CO2-fixing enzyme RuBisCO. Information on pyrenoid biology and CCM is primarily available for the unicellular green alga, Chlamydomonas reinhardtii, suggesting that both pyrenoid formation and CCM is highly dynamic and inducible by low CO2 concentrations. In contrast to C. reinhardtii, molecular underpinnings, inducibility and dynamics of the hornwort CCM and the pyrenoids are poorly understood. To start investigating molecular underpinnings of the CCM in hornworts we used a combination of methods including (1) protein co-IP of pyrenoid components, (2) localization of candidates homologous to CCM genes in Chlamydomonas, and (3) CO2 assimilation measurements in pyrenoid-bearing and pyrenoid-free species. While we provide evidence that the scaffolding candidate and the RuBisCO co-localize in hornworts like in Chlamydomonas, they appear to be less dynamic. We further found that the carbon anhydrase homolog (CAH3) of hornworts is localized to the pyrenoid, while the LCIB homolog is less intimately linked to the pyrenoid than in Chlamydomonas. Surprisingly, we observed that pyrenoid formation and subcellular localization of CAH3 and LCIB do not react to changing CO2 concentrations, darkness or H2O2 as it is described in C. reinhardtii. Our results imply that the pyrenoid-based CCM of hornworts is characterized by a mixture of Chlamydomonas-like as well as hornwort-specific features which is line with their independent evolutionary origin. Furthermore, our study suggests that hornwort CCM may be less dynamic than that of C. reinhardtii.