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This study addresses the role of the circadian clock in the seasonal growth cycle of trees: growth cessation, bud set, freezing tolerance, and bud burst. Populus tremula 3 Populus tremuloides (Ptt) LATE ELONGATED HYPOCOTYL1 (PttLHY1), PttLHY2, and TIMING OF CAB EXPRESSION1 constitute regulatory clock components because down-regulation by RNA interference of these genes leads to altered phase and period of clock-controlled gene expression as compared to the wild type. Also, both RNA interference lines show about 1-h-shorter critical daylength for growth cessation as compared to the wild type, extending their period of growth. During winter dormancy, when the diurnal variation in clock gene expression stops altogether, downregulation of PttLHY1 and PttLHY2 expression compromises freezing tolerance and the expression of C-REPEAT BINDING FACTOR1, suggesting a role of these genes in cold hardiness. Moreover, down-regulation of PttLHY1 and PttLHY2 causes a delay in bud burst. This evidence shows that in addition to a role in daylength-controlled processes, PttLHY plays a role in the temperature-dependent processes of dormancy in Populus such as cold hardiness and bud burst.
In plants, timekeeping of daily and seasonal processes relies on a circadian clock that uses negativefeedback loops at the transcript/protein level to sustain a rhythmicity of approximately 24 h in the absence of time-giving cues. Daylength is a reliable environmental cue that numerous organisms use to control growth and reproduction cycles in relation to seasonal change. Prominent examples are flowering in many plants, seasonal growth in trees, and diapause in insects (Carré, 2001; Tauber and Kyriacou, 2001; Rohde and Bhalerao, 2007). Temperate tree species, including Populus spp., cease growing and set buds when daylength is shortened below a critical value, the critical daylength (CDL; Junttila, 1982; Howe et al., 2003; Böhlenius et al., 2006). Daylength is one environmental cue that resets the clock to local time. Perception of light via photoreceptors entrains the circadian clock and allows the plant to adjust itself to the 24-h cycle. The central clock homologs in the genome of the sequenced deciduous tree Populus trichocarpa (Pt) comprise LATE ELONGATED HYPOCOTYL1 (PtLHY1), PtLHY2, andTIMINGOF CAB EXPRESSION1 (PtTOC1; Ramírez-Carvajal et al., 2008; Takata et al., 2009).
Also, light received during the day modulates photoperiod- dependent processes downstream of the circadian clock such as flowering. In the...