Abstract
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.
Highlights
Plants use an internal 24‐hr clock to synchronize their metabolism and growth with predictable changes in the environment
We studied trees in which expression of the core clock genes LHY1 and LHY2 was reduced by RNAi, causing the clock period to shorten by 3–4 hr, to investigate the impact of the circadian clock in growth
To test the hypothesis that a functional clock is central for aligning daily growth processes in Populus trees, we carried out detailed investigations of gene expression and cell division and of metabolism of the growth regulators auxin and cytokinins, as well as of primary and secondary growth
Summary
Plants use an internal 24‐hr (circadian) clock to synchronize their metabolism and growth with predictable changes in the environment. The clock mechanism of Arabidopsis is composed of interlocked transcriptional–translational feedback loops (Millar, 2016) It resets to local time on a daily basis in response to light and temperature cues and by sensing sugar produced by photosynthesis (Haydon, Mielczarek, Robertson, Hubbard, & Webb, 2013; Shin et al, 2017). The key components of the clock include the morning‐expressed and light‐responsive MYB transcription factors CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), both of which repress the expression of evening genes including TIMING OF CAB2 EXPRESSION 1 (TOC1/PSEUDO‐RESPONSE REGULATOR 1 [PRR1]). To test the hypothesis that a functional clock is central for aligning daily growth processes in Populus trees, we carried out detailed investigations of gene expression and cell division and of metabolism of the growth regulators auxin and cytokinins, as well as of primary and secondary growth
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