Altered oscillator function affects clock resonance and is responsible for the reduced day‐length sensitivity of CKB4 overexpressing plants

Jul 31, 2007The Plant journal : for cell and molecular biology

Changes in internal clock function reduce day-length sensitivity in plants with extra CKB4 protein

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Abstract

Overexpression of the CK2 regulatory subunit CKB4 leads to a shortened period in Arabidopsis clock outputs.

CKB4 overexpression results in a phase shift in gene expression and shorter hypocotyl elongation under short-day conditions.
Floral induction genes GIGANTEA, FLAVIN-BINDING KELCH REPEAT F-BOX1, and CONSTANS exhibit earlier expression phases in CKB4-ox plants.
Higher levels of the FLOWERING LOCUS T transcript are associated with earlier flowering in short-day conditions.
Restoring the synchronization of the CKB4-ox oscillator with light/dark cycles normalizes gene expression and flowering response.
Altered expression of core clock components TIMING OF CAB EXPRESSION 1 and CIRCADIAN CLOCK ASSOCIATED 1 is observed in CKB4-ox plants.
Oscillator dysfunction in CKB4-ox plants may lead to reduced sensitivity to day length and disrupt environmental synchronization.

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Most organisms have evolved a timing mechanism or circadian clock that is able to generate 24 h rhythmic oscillations in multiple biological events. The environmental fluctuations in light and temperature synchronize the expression and activity of key oscillator components that ultimately define the period, phase and amplitude of output rhythms. In Arabidopsis, overexpression of the casein kinase 2 (CK2) regulatory subunits, CKB3 or CKB4, alters the function of the clock under free-running conditions, and results in period-shortening of genes peaking at different phase angles. Here, we examine the effects of CKB4 overexpression (CKB4-ox) on a number of clock outputs that are modulated by day length or photoperiod. We have found a phase shift in gene expression, shortening of hypocotyl elongation and earlier than wild-type initiation of flowering under short-day conditions. Our study shows that the earlier expression phases of the floral induction genes GIGANTEA, FLAVIN-BINDING KELCH REPEAT F-BOX1 and CONSTANS correlate with higher abundance of the FLOWERING LOCUS T transcript under short-day conditions. Matching the period of the external light/dark cycles relative to the endogenous short period of the CKB4-ox oscillator restores the phase of gene expression and the flowering sensitivity to day length, indicating that a clock defect is responsible for the CKB4-ox phenotypes. Our studies suggest a function for CKB4 very close to the oscillator, as expression of the core components TIMING OF CAB EXPRESSION 1 and CIRCADIAN CLOCK ASSOCIATED 1 is also altered in CKB4-ox plants. Based on our results, we propose that oscillator dysfunction is responsible for the period defect of CKB4-ox plants that leads to clock dissonance with the environment and reduced sensitivity to day length.

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Altered oscillator function affects clock resonance and is responsible for the reduced day‐length sensitivity of CKB4 overexpressing plants

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