Investigations of Root Epidermal Cell Specification in Arabidopsis thaliana

Abstract

Development of multicellular organisms relies on proper specification of multiple cell types. The root epidermis of Arabidopsis thaliana consists of root-hair and non-hair cell types, and has been used as a powerful tool to study cell specification thanks to its easy accessibility and discernable morphologies. Arabidopsis root epidermis generates a position-dependent cell pattern, underlying which is a complicated network of transcription factors. Taking advantage of a wealth of previous studies on gene regulation during Arabidopsis root epidermal cell specification, my research deciphers novel root epidermis patterning mutants emerging from genetic screenings. The ultimate goal of this dissertation is to provide deeper and novel insights into root epidermal cell specification.

The first half of my dissertation research starts from a missense mutation altering one residue of the WEREWOLF (WER) protein, a central transcription factor regulating root epidermal cell specification. WER is critical for proper specification of both root-hair and non-hair cells, but little functional analysis of this protein has been performed. My research characterizes how this missense mutation alters DNA-protein interactions and the protein-protein interactions that are essential for WER function, and how expression of WER target genes, which encode important regulators for root epidermal cell specification, is affected accordingly. The importance of this specific residue in WER is further addressed through generation of a series of substitutions at this position that lead to a variety of disruptions in root epidermal cell patterning. Taken together, this part of my dissertation dissects WER protein function during root epidermal cell specification, and more importantly, reveals the necessity of a balanced production of multiple regulators for proper cell patterning.

The second half of my dissertation studies the effects of defective ribosome biogenesis on root epidermis development. The rationale of this research project is endorsed by observations that mutants in ribosome biogenesis factors cause a cell fate switch from root-hair cell to non-hair cell. Incorporating genetic and molecular approaches, my research identifies misregulated root epidermis cell specification regulators responsible for this unique mutant phenotype. Moreover, a novel regulatory module is identified as the connection between root epidermal cell specification and ribosomal stress responses. Therefore, my research provides original evidence for plants’ ability to adjust their root hair production according to ribosome biogenesis status.

Taken together, my dissertation investigates Arabidopsis root epidermal cell specification in two distinct yet related aspects: how the robustness of cell specification is achieved under normal growth conditions, and how the plasticity of cell specification is adopted under stressed conditions.