Résumé
In many fruit tree species, floral transition is inhibited by the presence of fruits, which are believed to be the source of an inhibitory systemic signal that acts on adjacent meristems. This could result in biennial bearing, a phenomenon where trees produce irregular yields from one year to the next. Despite its importance, little is known about the molecular control of floral transition in fruit trees. In Arabidopsis thaliana, the floral transition is controlled by a complex gene regulatory network (GRN) involving SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors, which are post-transcriptionally down-regulated by the microRNA156 (miR156) and post-translationally modified via interaction with DELLA proteins, negative regulators of gibberellins (GA). In this study, we aim at deciphering the GRN that controls the floral transition in apple (Malus domestica) and identifying potential fruit-derived floral inhibitory signals. Our results show that apple orthologues of SPL9 (MdSPL9) are transcriptionally activated in buds during the floral transition. By small RNA sequencing we detected significant levels of an apple miR156 in buds under a floral repressive condition (high crop load), which anticorrelates with the expression profile of MdSPL9 mRNA. Remarkably, this miR156 was highly expressed in seeds and likely transported to adjacent buds. To explore the potential role of GA as negative regulator of floral transition, we made use of a yeast-two hybrid screening to identify potential interactors of the apple DELLA protein (MdRGL1a), an orthologue of the GAI DELLA in Arabidopsis. This screening revealed that MdSPL9 interacts with MdRGL1a. Moreover, using an 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 regulatory motifs as part of the GRN that controls floral transition in apple, with miR156 potentially acting as a long-distance floral inhibitory signal.