Molecular Evolution of Root Epidermal Cell Patterns in Eudicot Plants

Abstract

In most vascular plants, the root epidermis is composed of root hair cells and non-hair cells. Its simple composition makes the root epidermis an ideal model to study cell fate specification and pattern formation. Three root epidermal cell patterns have been identified from vascular plants, namely Type I, Type II, and Type III. The model organism Arabidopsis (Arabidopsis thaliana) adopts the Type III pattern, and the underlying molecular mechanism has been intensively investigated. However, the molecular basis and evolution of root epidermal cell patterns in diverse non-model plants remain poorly understood. The biological significance of the two types of cells, particularly the non-hair cells, also remains to be investigated. My dissertation research aimed to understand the molecular mechanism and evolution of the Type III and Type I patterns in eudicots and also to explore the cytological features of Arabidopsis root epidermal non-hair cells which may contribute to specific functions of these cells. The first part of my dissertation focused on superrosids. I identified three species exhibiting Type I or Type III patterns in this group and analyzed homologs of Arabidopsis root epidermal patterning genes from these species. I discovered similarity in structure, expression, and functions of these genes in the Type III species and identified changes in the Type I species. My results suggest that the Type III pattern found in superrosid species is controlled by a conserved gene regulatory network (GRN), whereas the Type I pattern in extant superrosids arose independently via disruption of this conserved GRN. In the second part of my dissertation, I studied root epidermal cell patterns in superasterids. I identified multiple distantly related Type III and Type I species in this group. I also discovered a novel subtype of the Type III pattern. By analyzing homologs of Arabidopsis root epidermal patterning genes from these species, I discovered that the conserved GRN also regulates the Type III pattern in Fagopyrum esculentum and Impatiens balsamina. However, I conclude that alternative mechanisms evolved in Myosotis sylvatica and Beta vulgaris to determine their Type III features, as suggested by alteration of the conserved GRN. Specifically, I hypothesized that a member(s) of a different subgroup of MYB genes substitutes for the WEREWOLF gene that was lost in these species, and I acquired preliminary experimental results supporting this hypothesis. My analyses of Type I superasterid species suggest that they arose from Type III species through disruption of expression or function of Type III regulators. Together, my studies demonstrate both conservation and divergence of the molecular mechanisms underlying root epidermal cell patterns in superasterids. The third part of my dissertation focused on cytological features of the Arabidopsis root epidermis. I discovered expression of an entire set of cuticular wax biosynthesis and transport genes in the non-hair cells. Among these genes, I discovered that expression of LTPG1 is controlled by Arabidopsis root epidermal patterning genes. However, my genetic analyses of selected cuticular wax biosynthesis and transport genes suggest that they are not required for root epidermal patterning. My results indicate that these genes contribute to cuticle deposition on the non-hair cells, which may represent a special feature of these cells. Together, my dissertation provides understanding of the molecular basis and evolution of root epidermal cell patterns in eudicots and new insights into the cytological features of the Arabidopsis root epidermal non-hair cells.