Critical Role for CCA1 and LHY in Maintaining Circadian Rhythmicity in Arabidopsis

Abstract

Circadian clocks are autoregulatory, endogenous mechanisms that allow organisms, from bacteria to humans, to advantageously time a wide range of activities within 24-hr environmental cycles. CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) are thought to be important components of the circadian clock in the model plant Arabidopsis. The similar circadian phenotypes of lines overexpressing either CCA1 or LHY have suggested that the functions of these two transcription factors are largely overlapping. cca1-1 plants, which lack CCA1 protein, show a short-period phenotype for the expression of several genes when assayed under constant light conditions. This suggests that LHY function is able to only partially compensate for the lack of CCA1 protein, resulting in a clock with a faster pace in cca1-1 plants. We have obtained plants lacking CCA1 and with LHY function strongly reduced, cca1-1 lhy-R, and show that these plants are unable to maintain sustained oscillations in both constant light and constant darkness. However, these plants exhibit some circadian function in light/dark cycles, showing that the Arabidopsis circadian clock is not entirely dependent on CCA1 and LHY activities.

CIRCADIAN CLOCK ASSOCIATED1 and LATE ELONGATED HYPOCOTYL Function Synergistically in the Circadian Clock of Arabidopsis

“…Alternatively, CCA1 and LHY may have similar functions, but either the dosage of their combined gene products or the interactions between them are crucial for their function. cca1 and lhy single mutants have similar phenotypes (Green and Tobin, 1999;Alabadí et al, 2002;Mizoguchi et al, 2002), and overexpression of each gene has similar effects on circadian rhythms (Schaffer et al, 1998;Wang and Tobin, 1998). Together with the evidence presented here that CCA1 and LHY have similar expression patterns (Fig.…”

“…A cca1 lhy Double Mutant Has More Severe Phenotypes Than a cca1 Single Mutant cca1 and lhy single loss-of-function mutants confer short periods (Green and Tobin, 1999;Alabadí et al, 2002;Mizoguchi et al, 2002) and an initial study showed that a cca1 lhy double loss-of-function mutant (cca1-1 lhy-11) is unable to maintain rhythms of clockcontrolled RNAs for more than a few cycles under constant light (LL; Alabadí et al, 2002;Mizoguchi et al, 2002). However, the lhy allele used in that study was not truly null, because a truncated LHY protein is still expressed (Supplemental Fig.…”

“…They both have circadian rhythms of expression peaking soon after dawn. cca1 and lhy single mutants have short-period phenotypes, and overexpression of either gene causes arrhythmicity in expression of clock-regulated genes, leaf movement, and hypocotyl elongation and leads to dramatically reduced levels of endogenous CCA1 and LHY transcripts (Schaffer et al, 1998;Wang and Tobin, 1998;Green and Tobin, 1999;Alabadí et al, 2002;Mizoguchi et al, 2002). Therefore, CCA1 and LHY negatively regulate their own and each other's expression.…”

Leaf Starch Turnover Occurs in Long Days and in Falling Light at the End of the Day

“…normal clock function is important for the timing of onset of starch degradation in continuous light, we compared starch accumulation patterns following transfer to continuous light in wild-type plants and clock mutants. The clock mutants were the short-period mutant cca1 lhy (Alabadí et al, 2002;Mizoguchi et al, 2002) and the long-period mutant prr7 prr9 (Farré et al, 2005;Salomé and McClung, 2005). These mutants are in the Ws-2 and Col-0 backgrounds, respectively.…”

Unraveling the circadian clock inArabidopsis

“…15,16 Conversely, the accumulation of TOC1 in the evening was initially thought to activate CCA1/LHY through an unknown mechanism. 17,18 This central loop interlocks with the "morning" and "evening" loops, forming the basic architecture of the plant circadian clock. In the morning-phased loop, CCA1 and LHY expression increases the expression of PSEUDO-RESPONSE REGULATOR 7 (PRR7), PSEUDO-RESPONSE REGULATOR 9 (PRR9) and PSEUDO-RESPONSE REGULATOR 5/NIGHT INHIBITOR (PRR5/ NI); PRR5/NI, PRR7 and PRR9 subsequently downregulate the expression of CCA1 and LHY by binding to their promoters.…”