Temperature-regulated FLOWERING LOCUS T homologs make distinct contributions to the floral transition in vernalization-dependent and vernalization-independent Taraxacum koksaghyz plants
Flowering that requires a period of cold (vernalization) is a key trait in many crops, but the underlying regulatory pathways are often poorly understood. Taraxacum koksaghyz is a rubber-producing dandelion of the family Asteraceae, which also includes other economically important crops such as chicory and lettuce. Most T. koksaghyz plants require cold exposure to induce flowering, but plants that have lost the dependence on vernalization are more suitable for domestication and breeding. To provide insight into the molecular basis of mandatory vernalization in T. koskaghyz, we identified three FLOWERING LOCUS T (FT) homologs (TkFT1-3) that are differentially expressed under varying environmental conditions. TkFT1 and TkFT2 are expressed under long-day conditions at ambient temperatures whereas only TkFT1 is weakly expressed under short-day conditions. Exposure to cold was shown to repress TkFT1 but induce TkFT3. Overexpression experiments revealed that TkFT1-3 bypass the vernalization requirement in T. koksaghyz and its close relative T. officinale, and promote early flowering in vernalization-independent T. brevicorniculatum. We also identified two FRUITFULL homologs (TkFUL1 and TkFUL2) as downstream targets, which were upregulated in TkFT overexpression lines. Our findings suggest that TkFT1 promotes vernalization-independent flowering, whereas TkFT3 expression during the cold period is needed to promote vernalization-dependent flowering. This study explicates the regulatory network controlling flowering time in T. koksaghyz, contributing to a broader understanding of flowering in the family Asteraceae and providing knowledge that can be used in the future to facilitate domestication and breeding. Key messageIn Taraxacum koksaghyz, FT homologs function as floral inducers that upregulate FRUITFULL homologs, with TkFT1 linked to vernalization-independent flowering and TkFT3 associated with the acquisition of flowering competence during vernalization.