Regulation of phenology and the juvenility-to-maturity transition in trees

Sammanfattning: The juvenile reproductive phase refers to the period when young plants are unable to respond to inductive environmental signals to induce flowering. The length of this phase varies considerably between different species. In annuals, e.g., Arabidopsis, the juvenile phase is typically very short, whereas in perennials, e.g., hybrid aspen, it is several years. Apart from juvenile and mature reproductive phases, the plant can also display juvenile and mature vegetative phases, distinguished by morphological changes in growth pattern, leaf shape, trichome distribution, etc. miR156 is a primary regulator of the juvenile phase in Arabidopsis, whereas TEMPRANILLO (TEM) plays a more minor but still important role in regulating the length of the juvenile phase in Arabidopsis. In the work described in this thesis, I investigated the function of the closest Populus homologs of both miR156 and TEM in hybrid aspen with respect to their involvement in regulating the juvenile vegetative phase in Populus as well as their effects on phenology. The results showed that miR156 regulates the juvenile vegetative phase in Populus as hybrid aspen overexpressing PttmiR156e exhibits a severely prolonged juvenile phase. In addition, both PttmiR156e and the TEM homologs PttRAV1 and PttRAV2 affect sylleptic branching, possibly by changing the dormancy of the axillary bud. Interestingly, they also affect bud set. This indicates that similar genetic pathways are involved in the control of aging and phenology in Populus. I also studied the biochemical evolution of the angiosperm FT lineage. My data show that FT-like genes are absent in gymnosperms and suggests that the FT-like function emerged at an early stage during the evolution of flowering plants as a means to regulate flowering time.

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