Life cycle and flowering time control in beet

Flowering plants switch from vegetative growth to flowering at specific points in time. This biological process is triggered by the integration of endogenous stimuli and environmental cues such as changes in day length and temperature. The first sign of the flowering transition is sometimes marked by the formation and the elongation of the stem in a process known as “bolting” that precedes flower development. Flowering plants have developed different life cycles to ensure optimal reproductive success depending on their habitat. Annual species complete their life cycle in one year whereas biennial species typically fulfill their life cycle in two years and need to overwinter. Perennial species, which can exhibit long juvenile periods, typically flower for several years or even decades rather than just once. This thesis describes research in which sugar beet (Beta vulgaris ssp. vulgaris) was used as a new model for experimental studies of the floral transition. Sugar beet is an attractive organism for plant biologists studying life cycle control because of its biennial growth habit and its strict vernalization- and long-day-dependent flowering. Moreover, beets belong to the caryophyllids, which is a core-eudicot clade that is distinct from the rosids and the asterids and for which no molecular-scale investigations into flowering control have previously been reported. I isolated a pair of FLOWERING LOCUS T homologs, named BvFT1 and BvFT2, which have surprisingly evolved antagonistic transcriptional regulation capabilities and functions. I show that synchronized regulation of these two genes is essential to ensure flowering in beets. In addition, by using a map-based cloning approach, I isolated the bolting gene B – a dominant promoter of bolting and flowering that can bypass the need for vernalization in annual wild beets (Beta vulgaris ssp. maritima). I show that B encodes a pseudo-response regulator protein, BOLTING TIME CONTROL1 (BTC1), which acts upstream of the BvFT1 and BvFT2 genes, and that the biennial habit results from a partial loss of function of BvBTC1. My data illustrate how evolutionary changes at strategic molecular layers have shaped life cycle adaptation in plants.