Ibáñez C, Kozarewa I, Johansson M, Ögren E, Rohde A, Eriksson ME
Circadian clock components regulate entry and affect exit of seasonal dormancy as well as winter hardiness in Populus trees
Plant Physiology: 2010 153:1823-1833

Abstract
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 x P. tremuloides (Ptt) LATE ELONGATED HYPOCOTYL 1 (PttLHY1), PttLHY2 and TIMING OF CAB 1 (PttTOC1) constitute regulatory clock components because down-regulation by RNA interference (RNAi) of these genes leads to altered phase and period of clock controlled gene expression as compared to the wild type. Also, both RNAi lines show about one hour shorter critical daylength (CDL) 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, down-regulation of PttLHY1 and PttLHY2 expression compromises freezing tolerance and the expression of PttCBF1, 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.

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