Studies on the POLARIS gene of Arabidopsis

Abstract

The POLARIS (PLS) gene of Arabidopsis encodes a 36 amino acid peptide required for correct root growth and vascular patterning. Previous work indicates that PLS acts as a negative regulator of ethylene signalling. The pls mutant has enhanced ethylene signalling, which is the key determinant of its short root phenotype and results in a ‘triple response’ phenotype when seedlings are grown in the dark in air. The defects can be rescued by inhibition of ethylene signalling but not by inhibitors of ethylene biosynthesis, and pls does not over-produce ethylene, indicating a role for the peptide in ethylene signalling. Inhibition of ethylene signalling either by genetic or pharmacological methods restores pls mutant root growth. These data implicate ethylene as an important regulator of root development and PLS is hypothesised as a component of an ethylene regulatory mechanism to modulate root growth. This work establishes a direct interaction between ETR1, an ethylene receptor protein, and PLS by using both Yeast Two Hybrid and Bimolecular Fluorescence Complementation assays. In addition, the pls mutant has defective auxin transport and accumulation. The present work shows that PLS is required for correct gravitropic response, gene expression in the quiescent centre and columella patterning (each mediated in part at least by auxin) and also for ACC-mediated auxin synthesis. It is therefore proposed that PLS has two principal roles in root growth and development. First, PLS may act as a negative regulator of the ethylene signalling pathway by interacting with ETR1. Second, PLS is required for ethylene-mediated auxin synthesis; evidence is presented that suggests that PLS acts downstream of WEI2, an enzyme required for ethylene-mediated auxin synthesis. These roles function independently to regulate correct auxin distribution and concentration in the root tip, to control root growth and development.