Molecular Regulation of the Annual Growth Cycle in Populus Trees

Adaptation to the change of seasons is essential for tree survival. Here I show that the phenology of hybrid aspen is regulated by three FLOWERING LOCUS T (FT) genes. FT1, FT2a and FT2b are the result of both a whole genome and a local duplication. All three FTs are highly similar in sequence but their expression patterns and functions have diverged over time. FT1 expression is drastically induced by cold temperatures during winter in vegetative and reproductive buds, while FT2a and FT2b are expressed in leaves during spring and summer. I used CRISPR/Cas9 gene editing tools to generate individual and specific knockout mutants of FT1 and FT2. FT1 mutants showed no defects in vegetative growth during the first year. However, their bud flush was severely delayed, indicating a role of FT1 in dormancy release during winter. In contrast, knock-out of both FT2s greatly impaired growth and lead to early growth cessation, showing their importance for vegetative growth during summer. Additionally, I investigated the regulation of FT and possible mechanisms that can fine-tune the response to seasonal changes. I show that the timing of both bud set and bud flush is regulated by the photoreceptor Phytochrome B and its interacting factor PHYTOCHROME INTERACTING FACTOR 8 trough FT2 and probably also FT1. Furthermore, I show that growth cessation is induced in response to SD by SHORT VEGETATIVE PHASE LIKE, which represses the expression of FT2 and gibberellin metabolism genes in the leaves