Loss-of-expression FLC alleles derived from weedy radish facilitate the development of highly florigenic rootstocks for graft-mediated floral induction

Graft-mediated floral induction is a classical yet promising breeding technique that accelerates seed production and shortens generation time in crops with long life cycles. However, its application is still limited to a few species due to the instability or failure of floral induction. Although genetic variation in floral induction ability of rootstocks has been reported, the underlying natural genetic factors remain unclear. Here, using a Brassicaceae grafting model, we investigated two early-bolting radish (Raphanus sativus) accessions differing in their ability to induce flowering. We focused on a significant difference in FLOWERING LOCUS T (FT) expression between these accessions, leading to the identification of two quantitative trait loci (QTLs) associated with FT levels. Transcriptomic and genomic analyses revealed that both QTLs harbored previously unreported loss-of-expression alleles of FLOWERING LOCUS C (FLC) homologs, key floral repressors involved in vernalization. Grafting experiments with F2 populations demonstrated that these FLC loss-of-expression alleles enhanced floral induction additively, with RsFLC1 having a stronger effect. Our results confirmed that successful graft-mediated floral induction requires FT expression exceeding the threshold for rootstock flowering itself. This elevated FT expression level can be accomplished by the substantially reduced repressive activity of FLC homologs. Further analysis revealed that these alleles originated from weedy radish (Raphanus raphanistrum), likely contributing to early flowering of these weeds in disturbed environments. This study demonstrates that identifying natural genetic variants that markedly enhance FT expression is an effective strategy to develop highly florigenic rootstocks, thereby broadening the application of graft-mediated floral induction in crop breeding.