The initiation of floral structures is a crucial step in the lifecycle of plants. In model species, the various endogenous and environmental cues that coordinate flowering generally converge in the expression of FLOWERING LOCUS T (FT), the major flowering activator. FT encodes a mobile protein that is expressed in the leaves and migrates towards the shoot apical meristem, where it activates the expression of the floral integrators SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1), LEAFY and APETALA1 (AP1), thus leading to the conversion of a vegetative into a reproductive state. Further downstream, the identity of each floral organ is specified by the combined expression of distinct classes of MADS-box transcription factors, with different combinations leading to the formation of each organ, as outlined by the ABCDE model. In the Fagaceae family, the dynamics of floral induction and development have been studied in Quercus suber and Castanea sativa, revealing contrasting induction schedules between the two species. However, despite the great economic and ecological importance of this family, studies on the mechanisms that regulate flowering in other species remain scarce.

In this work, the expression of floral integrators was analysed in Quercus ilex, an evergreen oak species that plays a fundamental role in mediterranean environments. Throughout the year, two FT expression peaks were detected, each followed by the upregulation of SOC1 and floral homeotic genes within the buds. This suggests the occurrence of two separate flowering induction events, in similarity to previous works with Q. suber. Furthermore, the expression of B- and C-class genes in male and female flowers was found to be sex-biased, with B-class genes being predominantly expressed in the male flowers. In particular, the B-class gene PISTILLATA was found to be exclusively expressed in male flowers, suggesting a pivotal role of this gene in unisexual flower identity. Overall, the results here presented constitute an important step forward in understanding the mechanisms of unisexual flower development in Q. ilex.