New Bolting Genes From Sugar Beet (B. vulgaris) Which Control Early Bolting Independent Of BvBTC1

The species Beta 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 induction of flowering in biennial accessions such as sugar beet requires prolonged exposure to cold temperatures (vernalization)  followed by LD for induction of flowering. The recently isolated pseudo-response regulator gene BvBTC1 at the bolting locus B encodes a master switch distinguishing annual from biennial beets. Besides BvBTC1, two additional loci B2 and B4 have been identified in populations derived from crosses between an annual sea beet accession and biennial genotypes, isolated after EMS mutagenesis. Dominant alleles at these loci promote annual bolting. While BvBTC1 and B4 were mapped on chromosome II, the locus B2 was mapped on chromosome IX. We cloned the B2 gene from its position on chromosome IX using a large F2 population of 6000 plants segregating for early bolting. To improve the quality and resolution of the map, phenotypic data from F3 families were recorded. Sequence comparison of molecular markers to the draft sugar beet genome sequence identified one sequence scaffold of ~1.5 Mb. Based on this scaffold we generated new markers flanking and tightly linked to B2. Within this region, we identified a putative candidate gene which is completely linked to B2. We analysed its expression together with the expression of putative up- and downstream regulators in beet. A model is suggested how B2 is involved in the flowering time pathway in biennial Beta species.