PARENT SESSION
Posters P4C Controling CO₂: Stomates and carbon concentrating mechanisms. Abstracts (631-642)

Induction of the hornwort carbon concentrating mechanism. David Hanson^*,1, Camille Puronen¹, Nate McDowell², ¹ Department of Biology, Albuquerque, New Mexico, USA² Earth and Environmental Sciences Division, Los Alamos, New Mexico, USA

ABSTRACT- Hornworts (Anthocerotophyta) are early divergent land plants closely related to mosses and liverworts. The photosynthetic apparatus of hornworts is unique among land plants due to the presence of a pyrenoid (an electron dense body in each chloroplast). This structure is an aggregation of all the Rubisco (ribulose-1, 5-bisphosphate carboxylase/oxygenase) in the cell, along with a few other enzymes, and is often traversed by thylakoid membrane regions lacking photosystem II. Despite their scarcity among land plants, pyrenoids are common in algae and are generally believed to be an integral part of a carbon concentrating mechanism (CCM). The involvement of the pyrenoid in CCMs has been clearly demonstrated for Chlamydomonas reinhardtii and several species of pyrenoid-containing hornworts, however, its precise function is unclear. The pyrenoid-based CCM of C. reinhardtii is known to be inducible under conditions of carbon limitation which results in a greater affinity of the cell for inorganic carbon in the form of bicarbonate and carbon dioxide. The presence of a CCM also correlates with less negative carbon isotope values which are often used to screen for CCM function. The inducibility of the hornwort pyrenoid-based CCM is unknown and is examined in this study. We grew hornworts at 1% and ambient CO₂ and then transferred plants from high to low CO₂ conditions to examine evidence of CCM induction. We report measurements of pyrenoid size, Rubisco content and activation, carbon isotope ratios of whole plant tissues, and gas exchange properties. In addition, we used a tunable diode laser to monitor carbon isotope discrimination of intact hornwort tissues in real-time using an open flow gas exchange system.

KEY WORDS: CCM, hornwort, pyrenoid, isotope