A key gene for vernalization-independent bolting under extra-long days in biennial sugar beet

A BvFT2 cis-haplotype is strongly associated with vernalization-independent bolting under extra-long days, providing a novel genetic resource for speed breeding and yield improvement in sugar beet. Sugar beet (Beta vulgaris) is a biennial crop that requires prolonged cold (vernalization) to induce bolting; this limits breeding speed. Recently, bolting by longer than natural daylength (BLOND) lines was reported to bolt and flower without vernalization under extra-long-day conditions. To identify the genetic basis of this trait, we conducted phenotypic, QTL, QTL-seq, and RNA-seq analyses using F₂ and F_2:3 populations derived from crosses between BLOND and biennial lines. Bolting rates exhibited continuous distribution consistent with monogenic dominant inheritance. QTL and QTL-seq analyses identified a major locus at the terminal region of chromosome 4 (60.1-63.0 Mbp) containing BvFT2, an ortholog of FLOWERING LOCUS T (FT). RNA-seq revealed that BvFT2 expression was significantly higher in BLOND lines than in biennial lines under long-day conditions (FDR < 0.01), whereas BvBTC1 and BvFT1 showed no significant differences. Targeted Sanger sequencing of the BvFT2 locus detected several sequence variations, including insertions and SNPs in the 5' UTR and intron, as well as one nonsynonymous substitution in the coding region. Collectively, variation in BvFT2 defines a cis-haplotype strongly associated with BLOND-type bolting, providing a genetic target for speed breeding and yield-preserving strategies in sugar beet. We further validated that lines carrying the BvFT2 variant associated with high expression bolted under extended photoperiods in 29 additional breeding lines, including lines related to the mapping population, generally recapitulating the high bolting rates observed in the mapping population.