Mechanisms of Stress Signalling by the Conserved (p)ppGpp Regulatory Cascade in the Chloroplast – G4PLAST

The stringent response is a conserved signalling cascade in prokaryotes that is orchestrated by the nucleotides guanosine pentaphosphate and tetraphosphate (together referred to as G4P). Conserved enzymes necessary for a stringent-like response are also found in photosynthetic eukaryotes due to the cyanobacterial ancestry of the chloroplast. In bacteria, G4P plays a pleiotropic regulatory role in coordinating growth in response to environmental stress. G4P represses macromolecular synthesis pathways involved in proliferation (protein, RNA/DNA, and lipid synthesis), and activates stress responses. In plants, G4P has been detected and G4P synthesis enzymes (RSH for RelA/SpoT Homolog) are localized in the chloroplast. We recently showed that in Arabidopsis G4P is a potent inhibitor of chloroplast gene expression in vivo, and that RSH enzymes regulate G4P homeostasis to control chloroplast function and influence plant growth and development. However, we still know very little about how RSH enzymes are regulated, the identity of the chloroplastic targets of G4P, the role of G4P during stress, or how the G4P signalling cascade works in diverse photosynthetic eukaryotes. The objectives of the proposed project are to: 1. Decipher the molecular mechanisms and regulation of G4P synthesis in the chloroplast of plants and microalgae. 2. Understand the role of G4P in controlling chloroplast function and stress acclimation. We will use innovative molecular genetic, biochemical, and 'omic approaches to address these aims in the two representative model organisms Arabidopsis and Chlamydomonas. A solid body of preliminary findings and proven genetic tools underpin our experimental approaches, and will ensure a rapid start to the project. The project has the potential to make major new advances in the emerging field of G4P signalling in photosynthetic eukaryotes, and will reinforce a growing European cluster of expertise for G4P signalling in Marseille. It is of high importance to study growth and stress signalling pathways in plants and algae because together these organisms are responsible for ~75% of planetary primary productivity, and are key to the provision of ecosystem services and a food and fuel supply to humankind.