Retrograde signaling in diatoms

Bernard Lepetit

Jochen Buck

Diatoms belong to a huge and diverse group of organisms whose chloroplasts evolved in a complex evolutionary scenario. Briefly, 1.5 billion years ago a heterotrophic eukaryote engulfed a cyanobacterium and made it its chloroplast, resulting in the ancestors of green and red algae. Later, another heterotrophic eukaryote engulfed a red alga and domesticated this one as a plastid. Approximately 200 million years ago this resulted in the emergence of diatoms which since then started to become one of the most successful eukaryotic groups on earth and nowadays contribute to 20 % of global carbon production. During these complex evolutionary processes most of the genes of the engulfed organisms eventually becoming the chloroplast were transferred into the host nucleus. Hence, for the majority of chloroplastidic proteins their genes are transcribed into mRNA in the nucleus, translated into amino acid sequences at the ribosomes in the cytosol, and then cross a four membrane envelope (also a consequence of the endosymbiotic events) before reaching their destination place inside the chloroplast. As photosynthetic processes are highly dynamic they need to be finely adjusted based on the environmental and physiological conditions. As a consequence, the respective abundance of each chloroplastidic protein is finely balanced which requires a sensor and trigger inside the chloroplast ‘telling’ the nucleus when to start gene expression, depending on the needs of the chloroplast. Such signaling mechanism is called ‘retrograde signaling’. Recently we found strong evidence that a component of the photosynthetic electron transport chain, the plastoquinone pool, fulfills such a sensor/trigger role in diatoms. This was the first hint at all of the existence of such mechanism in organisms with secondary endosymbiosis derived chloroplasts. Currently we are investigating this process more thoroughly. On one hand we want to identify more genes triggered by the redox state of the plastoquinone pool by using real-time PCR and next generation sequencing. On the other hand we want to locate more precisely the origin of the redox signal in the chloroplast by using sophisticated spectroscopic and fluorometric methods in combination with the use of specific chemical inhibitors and mutants impaired in the redox characteristics of the chloroplast electron transport chain.