Circadian Clock Components Regulate Entry and Affect Exit of Seasonal Dormancy as Well as Winter Hardiness in Populus Trees    

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 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.

Real-time monitoring of PtaHMGB activity in poplar transactivation assays

“…1a). A well-marked oscillatory pattern was detected for PtaLHY2 mRNA accumulation showing a peak at dawn, similarly to the diurnal expression reported for its poplar homologs PnLHY2 and PttLHY2 , and for Arabidopsis LHY [10, 82, 83].

Fig.

…”

Clock genes and diurnal transcriptome dynamics in summer and winter in the gymnosperm Japanese cedar ( Cryptomeria japonica (L.f.) D.Don)

“…In Arabidopsis, core clock components CCA1 and LHY regulate expression of the CBF (C-REPEAT BINDING FACTOR) pathway, which is highly conserved among plants and has a major role in plant freezing tolerance [74]. Reducing the expression of LHY genes by RNA interference compromises freezing tolerance in Populus trees [16]. The expression of clock genes may be influenced by seasonal environmental changes, and consequently, may lead to activation of downstream pathways that contribute to freezing tolerance, which is important for survival of tree species in winter.…”

“…Transcriptome dynamics plays important roles for diurnal and seasonal adaptation in plants to synchronize them with environmental changes, and may be under clock control [15][16][17]. Signal transduction mechanisms due to changes in light are well studied in the model angiosperm Arabidopsis thaliana.…”

Ectopic expression of a novel peach (Prunus persica) CBF transcription factor in apple (Malus × domestica) results in short-day induced dormancy and increased cold hardiness

“…The expression of CBF genes has been shown to be gated by a circadian clock (Fowler et al 2005), and more recently Ibañez et al (2010) have shown that genes that constitute regulatory clock components in poplar (PttLHY1, PttLHY2) play a significant role in both the daylength-controlled and temperature-dependent processes of dormancy, cold hardiness, and bud burst, including the expression of poplar CBF1. These observations indicate that genes regulating dormancy and those that regulate cold hardiness may be more interactive than previously expected, especially in woody plants.…”

“…Seasonal regulation of growth and freezing tolerance is marked by phenological events such as growth cessation, bud set, the onset of dormancy, cold acclimation, deacclimation, and bud burst (Ibañez et al 2010;Kalberer et al 2006). Cold acclimation results in and is governed by a distinct set of changes in gene expression (Chinnusamy et al 2006;Welling and Palva 2006;Thomashow et al 2001).…”