Investigating the mechanisms of sensing and signaling carbon dioxide limitation in the green microalga Chlamydomonas reinhardtii

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Adjustment of the light harvesting capacity to light and CO2 supply in phototrophs is a very complex mechanism, operating on distinct time-scales and within different cellular compartments. Prolonged exposure to stress conditions, characterized by an unbalance between light/carbon supply and light capture, triggers long-term acclimation processes, which are accompanied by a modulated gene expression and hence stoichiometric adjustments of the photosynthetic machinery, ensuring unperturbed continuation of carbon fixation. In phototrophic eukaryotes, a prime target of these adjustments are nucleus-encoded light-harvesting proteins associated with photosystem II (LHCBM). A sudden drop in CO2 availability increases excitation pressure at photosystem II and immediately activates non-photochemical quenching (NPQ) mechanisms like state transitions to relieve it quickly. Prolonged limitation, however, leads to a relaxation of NPQ and a replacement of these short-term acclimation mechanisms by long-term responses, like functional antenna size truncation. In a previous DFG project, it was demonstrated that antenna size truncation mechanistically relies on LHCBM translation repression, which efficiently relieves excitation pressure. The whole process relied on an accumulation of the translation repressor NAB1 via activation of its nuclear promoter, which was dependent on a plastid retrograde signal implicating photosynthetic electron transfer. At the centre of this novel regulatory circuit is the newly identified transcription factor LCRF (low carbon dioxide response factor), which is induced by low carbon dioxide supply and crucial for NAB1 promoter activation. Exploiting the identified model pathway, we will investigate how a microalga senses CO2 limitation and how this is signaled to the nucleus by identifying novel components involved in the process of modulating photosynthetic gene expression.