Far-red-enriched ultra-long-day conditions induce constitutive FT expression and rapid flowering in radish rootstocks, promoting graft-mediated floral induction in Brassicaceae crops

Efficient floral induction is essential for breeding and seed production in Brassicaceae crops, particularly for late-bolting cultivars and plant-vernalization–type species such as cabbage (Brassica oleracea L.), which require substantial time and labor for artificial flower induction. A graft-mediated floral induction method was recently developed for cabbage, enabling flowering without vernalization treatment by grafting cabbage scions onto radish (Raphanus sativus L.) rootstocks. Although high expression of florigen gene FLOWERING LOCUS T (FT) in the rootstocks is a key determinant of success, environmental conditions capable of inducing strong FT expression in radish have remained unclear. Here, we demonstrate that a far-red-enriched ultra-long-day photoperiod (ULD-FR) markedly upregulates expression of radish FT homolog RsFTa and greatly enhances graft-mediated floral induction in cabbage. Under the ULD-FR condition, RsFTa expression remained constitutively high throughout the day, with daily transcript abundance increasing more than tenfold compared with standard high red/far-red (R/FR) ratio long-day conditions that employed fluorescent lamps. FT protein accumulation in cabbage scions grafted onto radish rootstocks was also strongly elevated, resulting in rapid flowering approximately 30 days after grafting. ULD-FR also promoted flowering in rapid-cycling Brassica rapa and B. oleracea accessions, and induced flowering in a vernalization-requiring R. sativus cultivar without low temperature treatment, suggesting that the response may be broadly conserved across Brassicaceae. Because ULD-FR can be implemented using standard lighting equipment by adding an FR light source, it presents potential utility as a versatile tool for breeding-related applications, including generation advancement and flowering synchronization among divergent accessions.