Dynamics of chromatin remodelling during plant cell differentiation

Abstract

Cell differentiation is the process by which a pluripotent cell acquires a determined, specialised state. This process relies on the precise spatiotemporal control of gene expression, which is partly regulated at the level of chromatin architecture. Chromatin is a complex of nucleic acids and histone proteins which compresses DNA to fit in the cell nucleus. The structure of chromatin modulates the ability of eukaryotic cells to respond to developmental cues. It does so by regulating the accessibility of DNA to transcription factors and the RNA polymerase transcriptional machinery. Histone post-translational modifications are key contributors to the regulation of chromatin architecture during development.

This thesis investigates the key histone modifications associated with plant cell differentiation, and the role of auxin, cytokinin and brassinosteroids in guiding chromatin changes during plant cell differentiation. This entailed conducting numerous experiments on the model plant Arabidopsis thaliana, including observing the effect of loss of function of histone-modifying enzymes on root cell differentiation and cytokinin response. In addition, this thesis examined the effect of disturbing the balance of phytohormones on VASCULAR-RELATED NAC-DOMAIN 7 (VND7)- induced xylem transdifferentiation. Finally, protein-protein interaction assays were conducted to identify molecular interactions between hormone signalling genes and histone-modifying enzymes.

These experiments revealed that some loss-of-function mutants had a significantly different root meristem size to the wildtype and an impeded cytokinin response. This highlights the regulatory role of chromatin architecture in cell differentiation and indicates that hormone signals may guide histone-modifying enzymes during cell differentiation. Furthermore, disrupting the phytohormone balance resulted in defective VND7-induced xylem transdifferentiation, suggesting that a specific hormonal framework may be necessary to promote VND7 activity. Finally, the negative regulator of brassinosteroid signalling, BR-INSENSITIVE 2 (BIN2), was found to interact with histone methyltransferases SU(V AR)3–9-RELA TED 5 (SUVR5), SU(VAR)3–9 HOMOLOG 5 (SUVH5) and CURLY LEAF (CLF).