Universität zu Köln

Pocard, Chloe (2021). Regulation of the floral transition and inflorescence development by the bZIP transcription factor FD in Arabidopsis thaliana. PhD thesis, Universität zu Köln.

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Abstract

The integration of signals in response to endogenous and exogenous stimuli at the shoot apical meristem (SAM) determines the timing of the transition from vegetative to reproductive development in Arabidopsis. Under inductive long-day (LD) photoperiods, FLOWERING LOCUS T (FT) and the bZIP transcription factor FD form the large transcriptional complex FD–(14-3-3)–FT/TSF at the SAM. Within this complex, FD is the DNA-binding component and some target genes of FD have been well characterized. The fd mutant is late flowering under LD conditions due to the improper regulation of its targets. Despite increased knowledge on the regulatory pathways that act through the FD–(14-3-3)–FT/TSF complex, the cis-regulatory elements that are required for the binding of FD to its targets remain poorly defined, and it is unclear how different targets show distinct spatiotemporal expression patterns. For example, although FD enhances FRUITFULL (FUL) transcription within the SAM, APETALA1 (AP1) transcription is promoted by FD later in development and AP1 transcripts are specific to floral primordia. Furthermore, the subset of direct targets that are involved in the floral transition before the upregulation of FUL and AP1 remain uncharacterized. During this PhD, I generated a transgenic fd mutant line in which the translocation of FD into the nucleus can be induced at different developmental time points. Induction of FD in this line promoted flowering, and showed that FD activity was required for several days to complete floral transition. I performed RNA-seq on apices of these plants following FD induction and identified putative additional components of the FD transcriptional network. The earliest targets of FD from this whole-transcriptome analysis could not be linked to floral transition, although FUL and AP1 were upregulated at later stages, confirming that floral transition occurred following FD induction. Much evidence exists to support that FUL is regulated by FD and I identified two putative conserved binding sites in the proximal promoter of FUL. However, mutation of these cis-elements did not affect flowering time nor the accumulation or pattern of FUL protein at the SAM. Under non-inductive short-day (SD) conditions, the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 15 (SPL15) protein also binds to FUL and regulates its expression. Although FD and SPL15 regulate FUL through the LD and SD pathways, respectively, I hypothesise that they do not compete at the promoter level and that activation of FUL by FD can occur indirectly through FD-mediated activation of other transcription factors or that FD binds to redundant sites at the FUL locus under LDs. Abscisic acid (ABA) regulates stress responses such as the drought-escape response, and aspects of plant development, including axillary meristem growth and meristem arrest. The FD protein is phylogenetically closely related to bZIP transcription factors involved in ABA signalling; however, evidence for the involvement of FD in ABA signalling is weak. I disrupted ABA signalling specifically within the FD expression domain, which resulted in defects in plant shoot architecture under LDs. In legumes, FD paralogues mediate the floral transition but also determine inflorescence architecture. I identified HOMEOBOX PROTEIN 21 (HB-21), which is involved in ABA signalling, to be a direct downstream target of FD. The level of HB-21 mRNA was lower in fd than in wild-type inflorescences. The hb21 and fd mutants produced taller shoots with more siliques on the main shoot compared with wild type; thus, the regulation of HB-21 by FD links FD with inflorescence development in Arabidopsis potentially through ABA signalling. This PhD focuses on the bZIP transcription factor FD and how it regulates flowering time and inflorescence development in Arabidopsis. Collectively, the results show that FD functions throughout the Arabidopsis life cycle, and provide insight into the temporal FD-mediated transcriptional network at the SAM.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Pocard, Chloe chloe.pocard@gmail.com UNSPECIFIED UNSPECIFIED
Contributors:	Contribution Name Email Corrector Coupland, George coupland@mpipz.mpg.de
URN:	urn:nbn:de:hbz:38-710641
Date:	August 2021
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Außeruniversitäre Forschungseinrichtungen > MPI for Plant Breeding Research
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language Flowering time English Branching English Shoot apical meristem English bZIP English
Date of oral exam:	4 November 2021
Referee:	Name Academic Title Coupland, George Prof.Dr. Hocker, Ute Prof. Dr. von Korff Schmising, Maria Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71064