Understanding the rules of life

Bioscience for sustainable agriculture and food

Category: Standard Studentships

Chloroplast to nucleus communication and the role of GUN1

Primary Supervisor

Professor Matthew Terry – University of Southampton


Professor Martin Warren – University of Kent


The chloroplast is home to photosynthesis in plants and is therefore a critically important organelle, both for the plant and also on a planetary scale.

This endosymbiotic organelle retains its own 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, termed retrograde signalling, can be demonstrated through mutations or treatments leading to damaged chloroplasts, which 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).

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?


Shimizu et al (2019) The retrograde signaling protein GUN1 regulates tetrapyrrole biosynthesis. Proceedings of the National Academy of Sciences USA 116, 24900-6.