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Initiating polar growth in plant cells - Functions of RopGEFs during root hair development in Arabidopsis thaliana

Denzler, Anna

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Cell polarity is a prerequisite for the formation of distinct cell shapes, which allow different cell types to fulfill their specialized functions. In plants, the family of Rho-Of-Plants (ROPs) controls cellular pathways required for the initiation symmetry breaking including cytoskeleton rearrangements and targeted vesicle transport. Because of their decisive role in these fundamental processes, the spatio-temporal regulation of ROP activation is a delicate task. Efficient ROP activation is mediated by plant-specific ROP guanine nucleotide exchange factors (RopGEFs), which share the central Plant-specific Rop Nucleotide Exchanger (PRONE) domain. While the PRONE domain of RopGEFs is conserved, the flanking N- and C-termini are variable in sequence and do not contain known functional domains. In this PhD thesis, selected RopGEFs were studied for their roles in the establishment of cell polarity during root hair development in the model plant Arabidopsis thaliana. I investigated the functions and regulation of RopGEFs, which were reported to polarize at the Root Hair Initiation Domain (RHID) early (RopGEF3, RopGEF14) or during the onset of polar growth (RopGEF4). In this thesis, I showed that the early polarizing RopGEF3 is crucial for early events including ROP2 recruitment and timing of growth initiation, while the late polarizing RopGEF4 is required for root hair elongation. Furthermore, the unrelated N-termini of early and late polarizing RopGEFs were characterized by exchange of N-termini in RopGEF3. I showed that the N-termini of early polarizing RopGEFs are functionally related in distinction to the N-terminus of RopGEF4. While N-termini of RopGEF3 and RopGEF14 promote RopGEF removal from the RHID, absence of these N-termini or presence of RopGEF4 N-terminus resulted in root hair phenotypes reminiscent of ROP overexpression as well as protein stabilization at the RHID. Cross-species and pairwise RopGEF N-termini sequence alignments combined with in silico prediction for phosphorylation sites in the RopGEF3 N-terminus revealed promising candidate amino acid residues possibly being involved in RopGEF3 regulation at the RHID. Taken together, in this PhD thesis evidence is provided for a putative task-sharing mechanism of early and late polarizing RopGEFs during root hair development as well as for the involvement of RopGEF N-termini in RopGEF protein abundance regulation at the RHID most likely involving regulatory phosphorylations.

Item Type: Dissertation
Supervisor: Schumacher, Prof. Dr. Karin
Place of Publication: Heidelberg
Date of thesis defense: 12 May 2021
Date Deposited: 21 May 2021 06:44
Date: 2022
Faculties / Institutes: The Faculty of Bio Sciences > Dean's Office of the Faculty of Bio Sciences
Subjects: 500 Natural sciences and mathematics
570 Life sciences
580 Botanical sciences
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