Abstract
In many fruit tree species, the floral transition is repressed by the presence of fruits, which are believed to be the source of an inhibitory systemic signal. This signal represses the floral induction in buds adjacent to the source (Guitton et al., 2016; Belhassine et al., 2020). The fruit-mediated inhibition of the floral transition can result in a phenomenon known as biennial bearing, where trees produce an irregular crop load from one year to the next. Thus, understanding how the floral transition is genetically and molecularly controlled is crucial to ensure fruit security and improve fruit production. In the model plant species Arabidopsis thaliana, the floral transition is controlled by an intricate gene regulatory network (GRN). This network relies on a fine-tuned interplay of different transcription factors, such as members of the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) gene family. SPLs are post-transcriptionally down-regulated by members of the microRNA156 (miR156) family and the SPL proteins are post-translationally modified by the interaction to DELLAs, a group of transcription co-activators that act as negative regulators of the plant hormone gibberellin (GA). To date, whether this genetic network operates in fruit tree species remains unknown. In this study, we aim at deciphering the GRN that controls the floral transition in apple (Malus domestica) and identifying potential fruit-derived systemic signals that inhibit floral transition. Our results show that orthologues of SPL9 (MdSPL9) are transcriptionally activated in buds of apple trees during the floral transition. Notably, RNA in situ hybridisation assays confirmed the presence of MdSPL9 transcripts in shoot apical meristems undergoing floral transition. In addition, a small RNA sequencing approach detected significant levels of a miR156 family member in buds under a floral repressive condition (high crop load), which anticorrelate with the expression profile of MdSPL9 mRNA. Remarkably, this miR156 family member was highly expressed in seeds, enabling the possibility of seeds being the source of the miR156 found in adjacent buds. We also explored the potential role of GA as negative regulator of floral transition. To this end, we made use of a Yeast Two Hybrid screening to identify potential interactors of the apple DELLA protein named MdRGL1a, an orthologue of the GAI DELLA in Arabidopsis. This screening revealed that MdSPL9 interacts with MdRGL1a. Moreover, using a RNAseq approach, we identified several transcriptional targets of MdSPL9, some of which are potentially regulated by the MdSPL9-DELLA complex. Altogether, our results support the involvement of the DELLA-SPL-miR156 floral regulatory motifs as part of the GRN that controls floral transition in apple. Moreover, we propose that the miR156 is a potential fruit-derived floral repressive signal that contributes to the alternate bearing phenomenon in fruit tree species.