Plants precisely time the onset of flowering to ensure reproductive success. A major factor in seasonal control of flowering time is the photoperiod. The length of the daily light period is measured by the circadian clock in leaves, and a signal is conveyed to the shoot apex to initiate floral transition accordingly. In the last two decades, the molecular players in the photoperiodic pathway have been identified in Arabidopsis thaliana. Moreover, the intricate connections between the circadian clockwork and components of the photoperiodic pathway have been unravelled. In particular, the molecular basis of time-of-day-dependent sensitivity to floral stimuli, as predicted by Bünning and Pittendrigh, has been elucidated. This review covers recent insights into the molecular mechanisms underlying clock regulation of photoperiodic responses and the integration of the photoperiodic pathway into the flowering time network in Arabidopsis. Furthermore, examples of conservation and divergence in photoperiodic flower induction in other plant species are discussed.

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 × 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, down-regulation 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.

Map based cloning in Arabidopsis thaliana can be a difficult and time-consuming process, specifically if the phenotype is subtle and scoring labour intensive. Here, we have re-sequenced the 120-Mb genome of a novel Arabidopsis clock mutant early bird (ebi-1) in Wassilewskija (Ws-2). We demonstrate the utility of sequencing a backcrossed line in limiting the number of SNPs considered. We identify a SNP in the gene AtNFXL-2 as the likely cause of the ebi-1 phenotype.

Large-scale biology among plant species, as well as comparative genomics of circadian clock architecture and clock-regulated output processes, have greatly advanced our understanding of the endogenous timing system in plants.

Trees are carbon dioxide sinks and major producers of terrestrial biomass with distinct seasonal growth patterns. Circadian clocks enable the coordination of physiological and biochemical temporal activities, optimally regulating multiple traits including growth. To dissect the clock's role in growth, we analysed Populus tremula × P. tremuloides trees with impaired clock function due to down‐regulation of central clock components. late elongated hypocotyl (lhy‐10) trees, in which expression of LHY1 and LHY2 is reduced by RNAi, have a short free‐running period and show disrupted temporal regulation of gene expression and reduced growth, producing 30–40% less biomass than wild‐type trees. Genes important in growth regulation were expressed with an earlier phase in lhy‐10, and CYCLIN D3 expression was misaligned and arrhythmic. Levels of cytokinins were lower in lhy‐10 trees, which also showed a change in the time of peak expression of genes associated with cell division and growth. However, auxin levels were not altered in lhy‐10 trees, and the size of the lignification zone in the stem showed a relative increase. The reduced growth rate and anatomical features of lhy‐10 trees were mainly caused by misregulation of cell division, which may have resulted from impaired clock function.