The Evolution of Flowering Time Regulation in Beta Species
The species Beta vulgaris encompasses wild and cultivated members with a broad range of phenological development. The annual life cycle is commonly found in wild beets, which germinate, bolt and flower within one season. Biennials such as sugar beet require prolonged exposure to cold temperature over winter to acquire floral competence. Sugar beet is the only sucrose-storing crop grown for sugar production in Europe. Flowering time strongly affects yield and is a trait of high agronomic relevance. From Arabidopsis, we learned about the prominent module of CONSTANS (CO) and the FLOWERING LOCUS T (FT), which induces flowering and appears conserved among various plant species. In beet, two FT paralogs, BvFT1 and BvFT2, have been detected, having evolved antagonistic functions. Despite, BOLTING TIME CONTROL 1 (BTC1) and BvBBX19, encoding a DOUBLE B-BOX TYPE ZINC FINGER protein, were identified to control life cycle switch in beet. However, a true orthologue of CO has not yet been found. This study aimed at extending the knowledge of the genetic factors behind bolting in Beta species. Mapping of a new bolting locus led to the identification of the btc1l allele which caused life cycle switch in beet, due to minor sequence variation in the coding region. Phenotyping of a diverse Beta panel revealed that geographical origin has a strong impact on life cycle adaptation. Haplotype analysis of BTC1, BvBBX19, BvFT1 and BvFT2 uncovered a high conservation of the protein-domains with regard to SNP occurrence and indicate that domestication of cultivated sugar beets is mainly attributed to BTC1. The results of this work notably contribute to the knowledge about the evolution of life cycle variation in Beta species under natural conditions and under breeding selection. Moreover, the identified btc1l allele may provide a future resource for breeding winter beets.