How to build a chloroplast: Unravelling chloroplast communication with the nucleus.

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The chloroplast is home to photosynthesis in plants and is therefore a critically important organelle, both at the level of plant function and in terms of the global environment. This endosymbiotic organelle retains its own South Coast Biosciences Doctoral Training Partnership (SoCoBio DTP) Email: socobio@soton.ac.uk Website: www.southcoastbiosciencesdtp potential impact, and any candidate qualities that would be particularly suitable for the project (300 words max.) reduced genome and capacity to synthesise proteins, but the majority of its ~3000 proteins are encoded in the nucleus and transported to the developing chloroplast. The development of the chloroplast therefore requires communication between the chloroplast and the nucleus, but the mechanism by which the chloroplast signals to the nucleus is essentially unknown. Chloroplast-to-nucleus communication is termed retrograde signalling and we do know that it happens as mutations or treatments leading to damaged chloroplasts results in the down-regulation of ~1000 nuclear-encoded genes. Analysis of Arabidopsis mutants in which this down regulation is abrogated, called genomes uncoupled or gun mutants, have led to the current proposal that the tetrapyrrole, heme, synthesised in the chloroplast, acts as a positive signal to promote nuclear gene expression. The strongest mutant in terms of its effect on nuclear gene expression is gun1. GUN1 is a chloroplast-localised protein of unknown function, but we have recently shown that it also regulates tetrapyrrole synthesis in keeping with the other gun mutants (Shimizu et al, 2019, PNAS, 116, 24900). The aim of this project will be to determine how GUN1 regulates the tetrapyrrole pathway and test the hypothesis that GUN1 functions as a receptor for the retrograde heme signal. In doing so the project will address the following questions: 1. How does GUN1 alter the flow through through the tetrapyrrole pathway? 2. As GUN1 is also implicated in the regulation of chloroplast protein homeostasis, what is the relationship between chloroplast protein synthesis and the tetrapyrrole pathway? 3. What is the molecular basis of GUN1 interaction with tetrapyrrole biosynthesis?