The role of haem in plant signalling and regulation

Haem is an organo-transition metal compound containing iron (Fe) at the centre of a porphyrin macrocycle. Haem is pivotal for the functional activity of a large number of proteins: the globins, which bind small gas molecules, such as O2, for transport and storage; the cytochromes, which are important for the movement of electrons and redox chemistry; and the peroxidases, where the Fe centre in haem is the site of substrate binding in the enzymes. However, more proteins are being identified that do not require haem for their functional activity, but whose behaviour is modulated by interacting with haem. In this case, haem is acting as either a signalling or a regulatory molecule. For example, haem binding to certain transcription factors can modulate the affinity of protein binding to enhancer sites on DNA, and thus control gene transcription. Haem binding to the proteins involved in circadian timing is a likely explanation for the observation of dysregulation of rhythmicity in the presence of an excesses of intracellular haem. In this research, the genetically-encoded fluorescent sensor will be deployed for the first time in plants to measure intracellular haem. We will probe changes in haem levels in response to either different environment cues, the circadian time, or stress in plants. There has already been reports about the role of haem in retrograde signalling, which has been linked to a specific interaction of haem with the protein, GUN1, and we will also be designing a new sensor to probe haem binding to this protein in plant cells. This approach has the potential to provide novel insight into a range of plant cellular processes, such as plant biotic and abiotic stress responses, impacting on both food and nutritional security.