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The transition from vegetative to reproductive development in plants is tightly controlled to ensure their reproductive success. Plants integrate many different environmental signals to flower at the appropriate time, and complex regulatory networks underlie this decision. In Arabidopsis thaliana, the model organism for plant molecular research, the timing of floral transition is influenced by environmental cues, which include temperature and day length, and by internal factors, including the age of the plant and the levels of the phytohormone gibberellin. During the transition, Arabidopsis switches from producing leaves to producing flowers, a process that includes morphological and identity changes in the shoot apical meristem (SAM). In favourable environmental conditions, such as floral inductive long-days, Arabidopsis accelerates the floral transition and quickly bolts and flowers. In the absence of floral inductive signals, these plants still undergo the floral transition, but do so later after producing many more leaves. The transcription factor SQUAMOSA PROMOTER BINDING PROTEIN LIKE 15 (SPL15) promotes flowering in non-inductive conditions. SPL15 integrates signals from multiple floral induction pathways at the shoot apical meristem and is proposed to directly activate transcription of two other genes with a prominent function in flowering: FRUITFULL (FUL), which encodes a MADS box transcription factor, and MICRORNA172b (MIR172B), which encodes a short non-coding RNA. However, the precise role of SPL15 in floral induction remains unknown. To gain understanding of the importance of SPL15 targets and other downstream components, I genetically assessed their contribution to floral induction. I found that FUL and MIR172B were important for SPL15 function during floral induction. However, in their absence, increased expression of SPL15 still induced early bolting of the inflorescence, but could not induce floral development. These analyses suggested that SPL15 regulates more target genes than FUL and MIR172B during the floral transition. Subsequently, I identified the binding sites of SPL15 in the FUL promoter, and studied the effect of mutating them. This revealed that SPL15 is not the only SPL protein that recognises these sites to regulate floral transition. I therefore propose that during vegetative growth, other SPLs bind there to repress the expression of FUL. Lastly, I set out to identify additional putative target genes of SPL15 by two complementary transcriptome analyses. The resulting high confidence list of putative target genes of SPL15 showed that SPL15 likely regulates several other genes with described functions in floral induction. In addition, SPL15 regulates a set of genes with functions in cell proliferation, which might be relevant for the morphological changes occurring in the SAM during the floral transition. Altogether this thesis has contributed to a better understanding of how SPL15 regulates different stages of the floral transition in A. thaliana under non-inductive conditions.

MicroRNAs (miRNAs) play important roles in regulating flowering and reproduction of angiosperms. Mature miRNAs are encoded by multiple MIRNA genes that can differ in their spatiotemporal activities and their contributions to gene regulatory networks, but the functions of individual MIRNA genes are poorly defined. We functionally analyzed the activity of all 5 Arabidopsis thaliana MIR172 genes, which encode miR172 and promote the floral transition by inhibiting the accumulation of APETALA2 (AP2) and APETALA2-LIKE (AP2-LIKE) transcription factors (TFs). Through genome editing and detailed confocal microscopy, we show that the activity of miR172 at the shoot apex is encoded by 3 MIR172 genes, is critical for floral transition of the shoot meristem under noninductive photoperiods, and reduces accumulation of AP2 and TARGET OF EAT2 (TOE2), an AP2-LIKE TF, at the shoot meristem. Utilizing the genetic resources generated here, we show that the promotion of flowering by miR172 is enhanced by the MADS-domain TF FRUITFULL, which may facilitate long-term silencing of AP2-LIKE transcription, and that their activities are partially coordinated by the TF SQUAMOSA PROMOTER-BINDING-LIKE PROTEIN 15. Thus, we present a genetic framework for the depletion of AP2 and AP2-LIKE TFs at the shoot apex during floral transition and demonstrate that this plays a central role in floral induction., Flowering of plants is repressed by microRNA 172; this study shows that three genes encoding miR172 act redundantly at the shoot apex to reduce expression of the transcription factor APETALA2, a repressor of flowering. Dual repression of APETALA2 at the transcriptional level by FRUITFULL and the post-transcriptional level by miR172 enables rapid and stable transition to the reproductive state.