Relationship between ethylene and auxin signalling in Arabidopsis root development

The growth and development of plants is controlled by a combination of genetic programming and environmental influence. For example, the patterning of floral organs is altered very little by the environment, and can be considered to be strongly dependent on genetic factors; but other aspects of plant development, such as the timing of flowering or the growth or leaves or roots, is much more dependent on prevailing conditions of temperature, water and nutrient availability, and so on. The effects of environment on gene expression and developmental changes are coordinated by plant hormones. One such hormone, the gas ethylene, influences many developmental processes, including root growth and development. The aim of this grant is to understand better how it interacts with another key hormone, auxin, to control gene expression and cellular organization in the root. We have already found that one particular mutant of the plant Arabidopsis, the polaris mutant, is unable to regulate correctly its ethylene responses, resulting in a shortened root system typical of plants that have been over-exposed to ethylene. At the same time, this ethylene problem affects the transport and accumulation of auxin in the root, previously found to be important in regulating root development. We hope to understand better the link between ethylene, auxin and root growth, by studying different ethylene mutants for altered auxin transport and accumulation, cellular organization and root growth.

We have found that the POLARIS (PLS) gene, encoding a 36 amino acids peptide, is required for correct ethylene and auxin signalling, and for Arabidopsis root development. The pls mutant exhibits developmental abnormalities, and in particular a short root phenotype. A key discovery has been that pls seedlings exhibit a triple response phenotype when grown in air in the dark, similar to the ethylene signalling mutant ctr1 or to ethylene overproducers, such as eto1 or eto2. The pls mutant does not overproduce ethylene, strongly suggesting that the PLS gene is required for regulation of ethylene signalling. Significantly, overexpression of the PLS peptide-encoding region of the PLS cDNA completely suppresses the triple response phenotype. The mutant also shows dramatically reduced auxin transport and accumulation of free IAA. These auxin defects and the root defects are largely rescued by the inhibition of ethylene signalling, either genetically or pharmacologically. These results indicate that PLS is a negative regulator of ethylene signalling, and is a novel example of a peptide as a modulator of hormone signalling in plants. We now also have strong evidence that ethylene defects in the pls mutant affect microtubule dynamics, and a failure of correct PIN2 recycling. We hypothesize that PLS is a component of the system regulting ethylene signalling, to modulate auxin transport via cytoskeleton dynamics and PIN localization. We also hypothesize that correct ethylene signalling is required for the expression of auxin-regulated genes such as PLS, IAA2 and PLT, to permit root meristem function and cellular patterning. We aim to test these hypotheses further, by investigating auxin transport, PIN recycling, cytoskeleton dynamics and gene expression in a range of ethylene mutants other than pls.