Antagonistic regulation of Lavender (Lavandula angustifolia) flowering through competitive interaction of LaFT8 and LaTFL1-1 with LaFD3

Lavender (Lavandula angustifolia) is an economically and ornamentally important species with unexplored flowering mechanisms. Genomic analysis revealed a significant expansion of key flowering gene families in lavender, including those encoding phosphatidylethanolamine-binding protein (PEBP) family (which harbors both FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1) genes, collectively termed LaPEBPs), bZIP transcription factor FD (LaFDs), and GENERAL REGULATOR FACTORs (LaGRFs). This expansion, particularly in TFL1-like and Group 4 GRF genes, was likely caused by polyploidization and segmental duplication events. Expression profiling demonstrated that these genes exhibit distinct tissue-specific patterns during floral development. Heterologous overexpression of LaFT8 in Arabidopsis accelerated flowering in a dose-dependent manner and upregulate the expression of key flowering promoters SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (AtSOC1), LEAFY (AtLFY), and APETALA1 (AtAP1) by approximately 2- to 15-fold. In contrast, flowering was delayed and AtAP1 and AtSOC1 expression was suppressed by LaTFL1-1 overexpression. However, no significant phenotypic changes were observed in Arabidopsis overexpressed LaFT4 or LaTFL1-3. Protein interaction assays confirmed the formation of a flowering activation complex between LaFT8, LaFD3 and LaGRFs (e.g., LaGRF3-3). Antagonistically, LaTFL1-1 was shown to compete by binding LaFD3 with higher affinity, thereby inhibiting LaFT8 function. This competitive inhibition mechanism provides a precise rheostat for fine-tuning floral transition signals, representing a novel regulatory layer beyond the canonical FT-FD activation paradigm. These results illustrate both conserved and species-specific characteristics of the expanded "FT-TFL1-FD/GRF" module in lavender flowering regulation. The findings not only improve the understanding of flowering mechanisms in non-model perennial plants and also provide a foundation for molecular breeding in lavender.