The role of the POLARIS peptide in ethylene signalling and root development in Arabidopsis thaliana

Abstract

The plant hormones ethylene, auxin and cytokinin have a pivotal role in plant growth, including differential cell elongation and division, tissue patterning, root development and apical hook formation. The POLARIS (PLS) gene in Arabidopsis thaliana is critical for correct signalling and crosstalk between these hormones and encodes a 36 amino acid PLS peptide which acts to negatively regulate the ethylene signalling pathway, subsequently mediating root growth and development. PLS is expressed in the tips of primary and lateral roots, and it has been previously shown to bind to the ethylene receptor ETR1 in yeast and onion cells. ETR1 has been previously characterised and was shown to be localised to the endoplasmic reticulum (ER) membrane, requiring a copper ion for correct ethylene binding function and transduction of the ethylene signal.
In addition to previous work revealing the ethylene-mediated downregulation of the PLS gene, work in this thesis demonstrates that the expression and localisation of the PLS peptide are regulated by ethylene in the A. thaliana root. It is revealed that the PLS peptide localises to the ER in root cells, where it interacts with the A. thaliana ETR1 protein. Evidence is presented which highlights the importance of copper ions in the role of the PLS peptide. The PLS N-terminus is essential for correct peptide activity in A. thaliana seedlings and notably requires the presence of two cysteine residues that have the potential to coordinate a metal ion. Interestingly, the PLS/ETR1 interaction is evidently enhanced in the presence of copper ions. Moreover, the loss-of-function pls mutant exhibits altered responses to copper perturbations and there is strong evidence that the PLS peptide can coordinate copper ions in vitro. Therefore, it is proposed here that the PLS peptide regulates copper ion availability to the ETR1 receptor protein at the ER, mediating ethylene receptor function and downstream ethylene responses, and consequently acting to regulate root development and growth.