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Furthermore, over-expression of DAM genes causes delayed bud-break similar to how FLC overexpression leads to delayed flowering 29, 30, 31. Indeed, DAM genes and MADS-box genes like FLOWERING LOCUS C ( FLC) are repressed during vernalization, which by low temperatures, and expression changes are correlated with changes in activating/repressive histone modifications 27, 28. The discovery of the Dormancy Associated MADS-box ( DAM) genes from the evergreen peach mutant has led to speculation that dormancy release and bud-break may share similarities with vernalization 26. How temperature controls dormancy release and bud-break is poorly understood at the molecular level. Therefore, in contrast to growth cessation and dormancy establishment phases, which are photoperiod-regulated processes, dormancy release and reactivation of growth are primarily thermo-regulated processes 24, 25. Although both LDs and warm temperatures are required for a return to active growth, the dominant triggering signal is warm temperatures 23. This phase, known as dormancy release, ensures that plants resume growth only after the stable return of favorable growth conditions in essence, it represents a clock measuring the length of winter 23. Once dormancy is established, resumption of active growth requires prolonged exposure of the bud to low temperatures 5, 20, 21, 22. These symplastic blockages isolate shoot apical meristem (SAM) from growth-promoting signals 14, 19. Specifically, under SDs, ABA concentration and signaling increase and promote the biosynthesis and deposition of callose at the plasmodesmata (PD) to develop obstructions known as PD sphincters 15, 16, 17, 18. It was recently shown that abscisic acid (ABA) plays a major role in the establishment of bud dormancy 14. Thus, growth cessation and bud-set prior to dormancy establishment have co-opted genes and signaling cascades that regulate the photoperiodic floral initiation pathway.įollowing growth cessation and bud set, continual exposure to SD results in the establishment of a dormant state during which buds are insensitive to growth-promoting signals 13.
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The signaling downstream of FT2 involves Like-APETALA1 (LAP1), which directly and positively regulates AINTEGUMENTA-Like 1 ( AIL1) gene 11, while AIL1 directly regulates D-type cyclins, an important cell cycle progression check point 12. High FT2 expression promotes active growth, while FT2 repression leads to early growth cessation and bud set 6, 10.
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In poplar, the clock daylength/nightlength sensing components LHY1 (LATE ELONGATED HYPOCOTYL 1), LHY2, GI (GIGANTEA), and CO1/2 (CONSTANS) regulate FT2 (one of two FT paralogs) expression in accordance with the day length 4, 7, 8, 9. The integration of photoperiod detection and growth inhibition involves the convergence of the regulatory activities of the circadian clock machinery (involved in photoperiod sensing) on the FT (FLOWERING LOCUS T) regulatory hub 3, 4, 5, 6. In most woody plants, including Populus, cessation of shoot growth and the induction of dormancy are either induced or accelerated by short days (SDs), and prevented or delayed by long days (LDs) 1, 2. In boreal and temperate woody perennials, winter bud dormancy is developed in the fall and involves, in chronological order: cessation of shoot elongation, the formation of buds (bud-set), and establishment of dormancy. The alteration of periods of active growth and dormancy is a widespread adaptive strategy in plants from seasonal climates that enable them to survive unfavorable conditions associated with prolonged periods of low temperature and/or moisture stress.