Modification of Bolting Behavior in Biennial Beets by Constitutive Expression of BvBTC1

The knowledge of flowering time regulation in the long day crop plant sugar beet (Beta vulgaris L. ssp. vulgaris) is compared to the model species Arabidopsis still in its infancy. The species B. vulgaris includes annual and biennial accessions. Annual accessions such as sea beet (B. vulgaris L. ssp. maritima) flower under long day conditions (LD) within one season, whereas biennial accessions like the cultivated sugar beet require vernalization followed by LD for induction of flowering. Vernalization response in biennial forms is mediated by BvFT1 (FLOWERING LOCUS 1), which acts as a floral repressor and is downregulated by vernalization, thus enabling induction of the floral inducer gene BvFT2 (Pin et al. 2010, Science 330:1397). The recently isolated pseudo-response regulator gene BvBTC1 (BOLTING TIME CONTROL 1) at the bolting locus B is a master switch that distinguishes annual from biennial beets and acts upstream of BvFT1 and BvFT2 (Pin et al. 2012, Curr. Biol. 22:1095). Furthermore it was suggested that cultivated beets carry a partial loss-of-function mutation in BvBTC1 that causes reduced sensitivity to inductive photoperiods before winter. Here we present the results of overexpressing the annual and biennial BvBTC1 allele from the constitutive 35S promoter in biennial beets. Overexpression of BvBTC1 leads to changes of its diurnal expression profile. Furthermore, transgenic events possessing three or more transgene inserts failed to bolt even after vernalization as a result of sense-silencing of BvBTC1. Taken together our data indicate the necessity of fine-tuning the expression of BvBTC1 for induction of bolting in beet.