Molecular and Functional Characterisation of SBP-box genes: The role of SPL3 during the floral transition of Arabidopsis thaliana

Proper developmental processes require a tight control of spatial and temporal gene regulation, since specific gene and protein expression is a prerequisite of cell differentiation. Transcription factors as well as microRNAs are major components for transcriptional and translational control of gene expression. In Arabidopsis thaliana, one of the plant specific transcription factor families is the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) gene family, which comprises 17 members, that have been shown to play important roles in several developmental processes. A decisive step in plant development is the transition from vegetative to reproductive growth, as it has to happen during favorable conditions to ensure successful reproduction and is a "one-time decision", as this phase change is not reversible in the annual plant Arabidopsis. The MIR156/157-controlled SBP-box transcription factor SPL3 has been shown to play a role during flowering in Arabidopsis, since its constitutive overexpression in a microRNA insensitive form results in early flowering plants, that nevertheless remain photoperiodically sensitive. Moreover, it has been shown that SPL3 binds in vitro to the sequence core motif CGTAC. During this thesis the role of SPL3 during the development to reproductive growth should be elucidated through identification of target genes. The results of this work suggest the floral meristem identity gene FRUITFULL (FUL; AGL8) to be a direct target of SPL3. Expression studies of two transgenes carrying the reportergene GUS in combination with genomic FUL or the FUL promoter region revealed a precocious activation of FUL in cotyledons and leaves in an SPL3OX background. Moreover, these data indicate that both, the binding motifs in the promoter as well as in the FUL first intron, are required for proper activation of FUL. A global expression analysis revealed that sugar metabolism, red light signaling and the circadian clock are affected by overexpression of SPL3. Subsequent analysis of diurnal expression of clock genes as well as of leaf movement in SPL3 overexpressing plants revealed a shortened period of the circadian clock and a precocious activation of so called "evening genes"

A reduced-function allele reveals that EARLY FLOWERING3 repressive action on the circadian clock is modulated by phytochrome signals in Arabidopsis

Arabidopsis thaliana EARLY FLOWERING3 (ELF3) is essential for the generation of circadian rhythms. ELF3 has been proposed to restrict light signals to the oscillator through phytochrome photoreceptors, but that has not been explicitly shown. Furthermore, the genetic action of ELF3 within the clock had remained elusive. Here, we report a functional characterization of ELF3 through the analysis of the elf3-12 allele, which encodes an amino acid replacement in a conserved domain. Circadian oscillations persisted, and unlike elf3 null alleles, elf3-12 resulted in a short circadian period only under ambient light. The period shortening effect of elf3-12 was enhanced by the overexpression of phytochromes phyA and phyB. We found that elf3-12 was only modestly perturbed in resetting of the oscillator and in gating light-regulated gene expression. Furthermore, elf3-12 essentially displayed wild-type development. We identified targets of ELF3 transcriptional repression in the oscillator, highlighting the action at the morning gene PSEUDO-RESPONSE REGULATOR9. Taken together, we identified two separable roles for ELF3, one affecting the circadian network and the other affecting light input to the oscillator. This is consistent with a dual function of ELF3 as both an integrator of phytochrome signals and a repressor component of the core oscillator

The microRNA regulated SBP-box genes SPL9 and SPL15 control shoot maturation in Arabidopsis

Throughout development the Arabidopsis shoot apical meristem successively undergoes several major phase transitions such as the juvenile-to-adult and floral transitions until, finally, it will produce flowers instead of leaves and shoots. Members of the Arabidopsis SBP-box gene family of transcription factors have been implicated in promoting the floral transition in dependence of miR156 and, accordingly, transgenics constitutively over-expressing this microRNA are delayed in flowering. To elaborate their roles in Arabidopsis shoot development, we analysed two of the 11 miR156 regulated Arabidopsis SBP-box genes, i.e. the likely paralogous genes SPL9 and SPL15. Single and double mutant phenotype analysis showed these genes to act redundantly in controlling the juvenile-to-adult phase transition. In addition, their loss-of-function results in a shortened plastochron during vegetative growth, altered inflorescence architecture and enhanced branching. In these aspects, the double mutant partly phenocopies constitutive MIR156b over-expressing transgenic plants and thus a major contribution to the phenotype of these transgenics as a result of the repression of SPL9 and SPL15 is strongly suggested

Genome expansion of Arabis alpina linked with retrotransposition and reduced symmetric DNA methylation

This document is the Accepted Manuscript version, made available in accordance to Springer Nature Terms of reuse of archived manuscripts.Despite evolutionary conserved mechanisms to silence transposable element activity, there are drastic differences in the abundance of transposable elements even among closely related plant species. We conducted a de novo assembly for the 375 .Mb genome of the perennial model plant, Arabis alpina. Analysing this genome revealed long-lasting and recent transposable element activity predominately driven by Gypsy long terminal repeat retrotransposons, which extended the low-recombining pericentromeres and transformed large formerly euchromatic regions into repeat-rich pericentromeric regions. This reduced capacity for long terminal repeat retrotransposon silencing and removal in A. alpina co-occurs with unexpectedly low levels of DNA methylation. Most remarkably, the striking reduction of symmetrical CG and CHG methylation suggests weakened DNA methylation maintenance in A. alpina compared with Arabidopsis thaliana. Phylogenetic analyses indicate a highly dynamic evolution of some components of methylation maintenance machinery that might be related to the unique methylation in A. alpina.Peer reviewe