ELF4 is a phytochrome‐regulated component of a negative‐feedback loop involving the central oscillator components CCA1 and LHY

Oct 11, 2005The Plant journal : for cell and molecular biology

ELF4, controlled by light sensors, is part of a feedback loop with key internal clock proteins CCA1 and LHY

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Abstract

Transfer to constant red light induces a biphasic pattern of CCA1 and LHY expression within 24 hours.

A negative transcriptional feedback loop involving the genes CCA1, LHY, and TOC1 is suggested to form the core of the circadian clock in Arabidopsis.
TOC1 is necessary for the light-induced expression of CCA1 and LHY, while the absence of CCA1 and LHY has minimal effect on TOC1 expression.
ELF4 is required for the light-induced expression of both CCA1 and LHY, and CCA1 and LHY negatively regulate ELF4 expression.
The expression profile of ELF4 is counter-phased to that of CCA1 and LHY, indicating a potential negative-feedback loop among these components.
ELF3 is necessary for light-induced CCA1/LHY expression but is not directly light-induced or regulated by the circadian clock during seedling development.

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Evidence has been presented that a negative transcriptional feedback loop formed by the genes CIRCADIAN CLOCK ASSOCIATED (CCA1), LATE ELONGATED HYPOCOTYL (LHY) and TIMING OF CAB (TOC1) constitutes the core of the central oscillator of the circadian clock in Arabidopsis. Here we show that these genes are expressed at constant, basal levels in dark-grown seedlings. Transfer to constant red light (Rc) rapidly induces a biphasic pattern of CCA1 and LHY expression, and a reciprocal TOC1 expression pattern over the first 24 h, consistent with initial induction of this synchronous oscillation by the light signal. We have used this assay with wild-type and mutant seedlings to examine the role of these oscillator components, and to determine the function of ELF3 and ELF4 in their light-regulated expression. The data show that whereas TOC1 is necessary for light-induced CCA1/LHY expression, the combined absence of CCA1 and LHY has little effect on the pattern of light-induced TOC1 expression, indicating that the negative regulatory arm of the proposed oscillator is not fully functional during initial seedling de-etiolation. By contrast, ELF4 is necessary for light-induced expression of both CCA1 and LHY, and conversely, CCA1 and LHY act negatively on light-induced ELF4 expression. Together with the observation that the temporal light-induced expression profile of ELF4 is counter-phased to that of CCA1 and LHY and parallels that of TOC1, these data are consistent with a previously unrecognized negative-feedback loop formed by CCA1/LHY and ELF4 in a manner analogous to the proposed CCA1/LHY/TOC1 oscillator. ELF3 is also necessary for light-induced CCA1/LHY expression, but it is neither light-induced nor clock-regulated during de-etiolation. Taken together, the data suggest (a) that ELF3, ELF4, and TOC1 all function in the primary, phytochrome-mediated light-input pathway to the circadian oscillator in Arabidopsis; and (b) that this oscillator consists of two or more interlocking transcriptional feedback loops that may be differentially operative during initial light induction and under steady-state circadian conditions in entrained green plants.

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ELF4 is a phytochrome‐regulated component of a negative‐feedback loop involving the central oscillator components CCA1 and LHY

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