Advances in the Functions and Molecular Mechanisms of SPL Transcription Factors in Plant Flower Development

SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors constitute a plant-specific family defined by a conserved SBP DNA-binding domain. They recognize cis-elements containing the core GTAC motif and cooperate with various co-regulators to govern downstream gene expression, thereby participating in leaf morphogenesis, phase transition, fruit development, anthocyanin accumulation and abiotic stress adaptation. Notably, the abbreviation SPL is also used for the unrelated SPOROCYTELESS/NOZZLE (SPL/NZZ) protein, which belongs to a separate family lacking the SBP-box domain; these two families should not be confused. Flower development, a central reproductive program in angiosperms, encompasses the acquisition of flowering competence, the vegetative-to-reproductive transition, floral meristem specification, organ patterning, and gametophyte formation. This review summarizes structural characteristics and classification of SPL proteins, distinguishes miR156/miR157-targeted paralogs from non-targeted ones, and synthesizes current understanding of their molecular functions across successive floral stages. Acting as signaling hubs, SPLs integrate photoperiod, gibberellin-DELLA, age-dependent miR156/miR157, temperature, sugar and cytokinin signals to fine-tune flowering time. They drive the reproductive transition via direct activation of LFY, FUL and AP1, and later remodel inflorescence architecture and floral organ size by balancing cell proliferation and expansion. In addition, they maintain fertility by modulating gametogenesis. Based largely on findings from Arabidopsis thaliana, we construct an updated regulatory framework for SPL-dependent floral development, compare conserved and lineage-divergent functions across herbaceous and woody species, identify unresolved questions, and discuss potential applications for crop improvement.