The Modified Function of a Stem Cell Regulator in Monocots and Dicots

Abstract

Within the angiosperms, numerous differences can be found between monocots and dicots, including major differences to vascular organisation. In dicots, the meristematic vascular cambium divides radially to generate populations of both xylem and phloem cells, however monocots lack a cambial layer, thus their method of vascular cell generation remains elusive. In the dicot Arabidopsis thaliana, regulation of vascular development via the leucine-rich receptor like kinase PHLOEM INTERCALATED WITH XYLEM (PXY) and its ligand TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF), encoded by CLAVATA3/EMBRYO SURROUNDING REGION (CLE) genes, has been well studied. As such, the PXY gene was selected as the subject of an investigation into the development of monocot vascular development, specifically in the crop Hordeum vulgare, to discover what role a homologue of this gene performs in this species.

The evidence included in this thesis shows that whilst the homologue of PXY in H. vulgare is capable of recovering the pxy phenotype in A. thaliana, that Hvpxy individuals do not show signs of disruption in their vascular development. Moreover, whilst Hvpxy individuals do show significant differences in gross morphology to their wild type counterparts, that these differences are not as extreme as would be expected when comparing to similar mutations in A. thaliana. Further investigation revealed that whilst no disruption to radial cell divisions or organisation were noted in the Hvpxy genotype, that Hvpxy individuals had significantly longer internode cells than the wild type. This evidence was combined with other evidence showing that Hvpxy individuals had shorter tillers than the wild type, in situ hybridisation indicating a potential location for expression in the intercalary meristem, and RNAseq data further highlighting the impact of Hvpxy mutation on cell division. Taken together, this indicates that whilst the PXY genes of A. thaliana and Hordeum vulgare both regulate the divisions of a stem cell population, that the location and orientation of divisions regulated differs – in A. thaliana, PXY regulates radial divisions in the vascular cambium, however the H. vulgare homologue instead regulates longitudinal divisions in the intercalary meristem.