Variability of the CO2 concentrating mechanism in hornworts (Anthocerotae)

Abstract

Hornworts (Anthocerotae) are the only group of land plants with pyrenoid-containing chloroplasts. The hornwort pyrenoid is an aggregation of Rubisco, and is analogous to algal pyrenoids. CO2 exchange and ?13C values have demonstrated the presence of a CO2 concentrating mechanism (CCM) in pyrenoid-containing species of the genus Anthoceros. However, the need for large amounts of tissue and difficulties in culturing hornworts have hampered more detailed studies. We have circumvented these problems by developing a mass spectrometric system that can use less than 2 cm2 of tissue per assay. In addition, our combined fluorometer/mass spectrometer based technique can simultaneously measure net CO2 exchange, 18O2 uptake, 16O2 evolution, chlorophyll fluorescence and P700 absorbance, which provides several new tools for assessing CCM function. We analyzed field-grown and tissue-cultured representatives of pyrenoid-containing and pyrenoid-lacking hornwort genera (Anthoceros, Megaceros, and Notothylas). These data were compared with the liverwort Marchantia polymorpha that has standard C3 photosynthesis and a thalloid growth form similar to hornworts. We found that O2 and CO2 exchange characteristics support the presence of a CCM only in pyrenoid-containing hornworts. In addition, the ratio of FPSI to FPSII was higher among all hornworts than in Marchantia. We also present Rubisco kinetic properties from all genera.