• Addition of far-red light to artificial illumination generates an additional FT expression peak in the diurnal profile of radish. • The ultra-long-day photoperiod with far-red enriched light (ULD-FR) condition sustains elevated FT expression throughout the day and markedly accelerates flowering in radish. • The ULD-FR condition enhances FT protein transport from rootstocks to scions, thereby promoting efficient graft-mediated floral induction. • The ULD-FR condition stimulates flowering in other Brassicaceae crops, including vernalization requiring cultivars without low temperature treatment, highlighting its versatility as an approach adaptable to diverse floral-induction strategies in Brassicaceae breeding. 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.
Motoki et al. (Wed,) studied this question.