The development of the root system represents an important feature of plant development,
as the root is the site of water and nutrient uptake, it anchors the plant in the soil, and is a
site of interaction with other organisms, which may be pathogenic (such as nematodes) or
beneficial (such as mycorrhiza). The model organism Arabidopsis thaliana is an excellent
model to study the genetic basis of root development, and to study the control of gene
expression in response to hormones and as mediators of hormone action. A key question in
developmental biology is how genes and hormone signalling systems interact to control cell
identity, cell division, cell patterning and cell differentiation. To address this, the work
described in this thesis focused of two genes, previously identified as playing important roles
in root development - the POLARIS (PLS) gene, which encodes a 36 amino acids peptide; and
MERISTEM-DEFECTIVE (MDF), which encodes an SR protein. In the course of this work,
information on the pathways influenced by each gene was determined using high resolution
RNA profiling followed by bioinformatics analysis, and information was used to identify
pathways in which the two genes are involved. The results obtained show that PLS is
required for both correct ethylene signalling and, independently, auxin biosynthesis in
response to ethylene; and PLS exerts its effect via control of the tryptophan-independent
pathway for auxin biosynthesis. It is also shown that MDF is a likely splicing factor, required
for the regulation of auxin pathway genes and transcription factors expressed in the root
meristem via the control of alternative splicing - it likely controls the balance in the
meristem between stem cell identity and differentiation. The results provide new insights
into the genetic and molecular mechanisms by which these genes regulate hormone
signalling pathways to in turn control the development of the Arabidopsis root
