The AP2 transcription factor LF1 coordinates auxin signaling and LFY/FLO pathway to control leaflet number determination in soybean

Compound leaves initiate as simple primordia, with LFY/FLO orthologs orchestrating subsequent leaflet initiation. Although several LFY/FLO repressors have been identified, the activating mechanisms remain unknown. Here, we investigate the soybean dominant mutant Lf1, which exhibits five palmately arranged leaflets. LF1 encodes an AP2 transcription factor homologous to Arabidopsis DRN/DRNL and is specifically expressed in incipient and developing leaflet primordia. LF1 binds to GCC-box motifs, enabling autoregulation and activating downstream targets GmLFYa&b-key regulators of leaflet initiation. The Lf1 mutation substitutes Asp with Gly in the AP2 domain, enhancing its regulatory activity. Consequently, Lf1 and GmLFYa&b expression is upregulated and ectopic in early leaflet primordia, triggering extra leaflet formation. Auxin upregulates the endogenous LF1 transcription but suppresses the activity of the exogenous LF1 promoter in ProLF1:GUS reporter lines, suggesting a context-dependent role for auxin in modulating LF1 expression. Therefore, LF1 mechanistically links auxin signaling with the LFY/FLO pathway during compound leaf development.