<i>Flowering Promoting Factor-Like</i> Genes <i>:</i> Emerging yet Mechanistically Unresolved Regulators of Floral Transition and Plant Development

Flowering marks a pivotal transition in a plant's life cycle, signalling the shift from vegetative growth to reproductive development. Over the years, extensive research has uncovered key genes and regulatory networks governing this process. Central to this regulation is the Florigen Activation Complex (FAC), along with its interacting partners and upstream and downstream components, which have been well-characterized across numerous plant species. More recently, attention has turned to a lesser-known gene, FLOWERING PROMOTING FACTOR 1 (FPF1). Initially identified in Arabidopsis thaliana, FPF1 is a plant-specific gene lacking known functional domains, yet it plays a conserved and critical role in floral induction across diverse species. Despite its discovery in 1997, the molecular mechanism of FPF1 remained elusive until recent studies began to unravel the function of FPF and its homologs. One such study revealed that FPF1-Like Protein 1 (FLP1) in Arabidopsis is expressed in phloem companion cells sites of FLOWERING LOCUS T (FT) production. Like AtFT, AtFLP1 acts as a mobile florigenic signal, though it operates independently of the canonical AtFT pathway. AtFLP1 promotes flowering by activating the floral homeotic gene SEP3, suggesting an alternative regulatory route also influenced by photoperiod. Interestingly, studies in Brachypodium distachyon have highlighted a contrasting role for FLP-like genes, where they negatively regulate flowering by interfering with the FAC, underscoring species-specific diversity in its function. While initial studies have been majorly focused on their role in flowering, in recent years FPF1 family genes have also been implicated in other developmental processes, including stem and root elongation and shade avoidance responses. In this review, we explore these emerging insights into FPF1-like proteins, examining their multifaceted roles in flowering regulation and broader developmental functions, with a special emphasis on the most recent and impactful studies.