Understanding epidermal cell fate specification during plant embryogenesis
Item statusRestricted Access
Embargo end date31/12/2100
Shoot epidermal identity is critical for plant survival, growth, and interaction with the environment. Epidermal identity is specified during very early embryogenesis, and maintained in the outermost cells of the plant throughout the entire life cycle. In this work I aimed to generate a model for the establishment of basal epidermal cell fate during embryogenesis based on the analysis of both known and novel regulators. Loss of function of two HD-ZIP IV transcription factors, ATML1 and PDF2 had previously been shown to lead to embryo lethality due to loss of epidermal specification. In this study I uncover dosage dependency of ATML1 and PDF2 function during embryogenesis. By expressing functional ATML1 and PDF2 fusion proteins specifically in the epidermis, I developed a novel tool allowing demonstration of homo- and heterodimerization of these two transcription factors in planta. Using genetic and proteomic analysis I provide evidence that other HD-ZIP IV proteins are involved in epidermal specification together with ATML1 and PDF2, suggesting the presence of multiple regulatory protein complexes. Based on previous published and unpublished work, I tested the hypothesis that ATML1 and PDF2 form part of a regulatory feedback loop necessary for maintenance of epidermal identity, and involving cell-cell signalling mediated by the receptor kinase ACR4. Using a genetic approach I confirm that ATML1 and PDF2 likely act together with ACR4 in the specification of embryonic epidermal identity. I show that ATML1 and PDF2 negatively regulate both ACR4, and their own expression, most likely by binding to L1 box motifs. In contrast, I provide evidence that ACR4-mediated signalling participates in maintaining expression levels of ATML1 and PDF2. Mathematical modelling of the properties of the feedback loop supported by my results, suggests that it is capable of maintaining a robust epidermal cell fate, and predicts possible changes in network interactions during the process of epidermal cell fate specification. Finally I used a combination of bioinformatics approaches to integrate in silico and experimental data with the aim of discovering potential novel epidermal regulators and targets of epidermal fate specifying pathways. This work highlighted potential roles for WOX-family transcription factors in epidermal fate specification, which were further analysed genetically. In addition, bioinformatics analysis pinpointed an intriguing overlap between the targets of epidermal specification pathways and targets of abiotic stresses signalling.