Cell Fate Acquisition in the Early Embryo by MAP Kinase-Mediated Phosphorylation of Transcriptional Regulators

Abstract

In the process of plant embryogenesis, the fundamental framework for the future plant is established. During this process the initial differences between cells are set in place after the polarized zygote divides and subsequent assignment of cell fate of the apical and basal cell. Critical for these decisions are transcription factors (TF), which are subject to control by modification. One pathway that has been demonstrated to play a critical role in this process is the ERECTA/YODA (ER/YDA) pathway. The TF SCREAM/INDUCER OF CBF EXPRESSION 1 (SCRM/ICE1) has been characterized thoroughly in the context of stomata as critical for correct entry into the cell lineage and subsequent steps. In stomata SCRM is phosphorylated by Mitogen-Activated Protein Kinase 6 (MPK6), a downstream component of the ER/YDA pathway. Phosphorylation of SCRM in stomata leads to decreased stability and in turn ER/YDA activity leads to suppression of stomatal cell fate. However, the role of SCRM hasn’t been characterized thoroughly in embryo development. Examination of SCRM mutants revealed that scrm loss-of-function and scrm-D, a stabilized mutant, show similar phenotype. Yet, in stomata these mutations produce opposite effects on development. Investigation of the role of SCRM in embryo development revealed that it is critical for correct polarization of the embryo. Additionally, SCRM has been shown to function in parallel with WRKY2 to regulate the activation of WUSCHEL HOMEOBOX 8 (WOX8), a TF important for cell fate assignment. Furthermore, we found evidence that phosphorylation of SCRM by MPK6 is critical for zygote elongation and polarization, as non-phosphorylatable SCRM mutants are unable to rescue the scrm phenotype. The phytohormone auxin has been described as critical during the process of assigning cell fate in the 1-cell embryo. The distribution of auxin following zygote division has been extrapolated based on auxin responses. However, analysis of auxin distribution using more direct methods revealed that auxin was distributed evenly, rather than differentially in the 1-cell embryo. The gap in the auxin model opened by these findings and was subsequently filled by IAA8. This INDOLE-3-ACETIC ACID INDUCIBLE (Aux/IAA) has previously been shown to be phosphorylated and in turn stabilized in stress by MPK6. Further analysis of IAA8 in the early embryo revealed that it undergoes differential stabilization in the early embryo, in a pattern corresponding with ER/YDA activity. Consequently, this results in the suppression of the auxin response machinery essentially, thereby establishing an auxin response gradient before a gradient in auxin concentration is established. Additionally, interference with differential stabilization of IAA8 has been shown to result in stochasticity in auxin responses during early embryogenesis. In summary, our findings in my thesis indicate that cellular context plays a pivotal role in the regulation of TFs. Secondly, we expanded the understanding of how the early auxin responses are regulated and established how receptor signaling can directly impact regulatory components of the auxin response machinery.

Die Dissertation ist gesperrt bis zum 21. Mai 2027 !