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The Plant Cell, Vol. 20: 1187, May 2008, www.plantcell.org ª 2008 American Society of Plant Biologists
IN BRIEF
Newly Isolated Circadian Clock Components Conserved
across Eukaryotes
Organisms maintain circadian rhythms that
coordinate biological processes with daily
environmental changes. The circadian clock
molecular mechanism is self-sustaining, yet
is also capable of entrainment to variable
environmental conditions (reviewed in
McClung, 2006). At its simplest, the circadian clock has a central oscillator that
perpetuates a diurnal cycle and is synchronized to the environment by inputs such as
light and temperature signals. The oscillator
then produces outputs that control cellular
processes. Outputs of the plant circadian
clock include changes in gene expression,
physical changes such as leaf movements,
and the measurement of daylength to determine flowering time. Not surprisingly, the
plant circadian clock is not governed by
simplicity; instead, multiple interlocked oscillator loops are phased at different times.
Recent work published in The Plant Cell
highlights two novel and interesting components of the clock mechanism.
Martin-Tryon and Harmer (pages 1244–
1259) investigate XAP5 CIRCADIAN TIMEKEEPER (XCT), which encodes a novel
protein that may play a role in light input
into the clock for the regulation of both
circadian period and photomorphogenesis,
but not flowering time. Loss-of-function xct
mutants show a shorter circadian period
under all light conditions but do not flower
early. xct mutants also show photomorphogenesis defects, including delayed
chloroplast development. Interestingly, different mutant phenotypes are sensitive to
different wavelengths of light; for example,
xct mutants have very long hypocotyls in
red light but very short hypocotyls in blue
light. By contrast, xct mutant sensitivity of
circadian period to increasing intensities of
light is increased in red light but not blue
light. Thus, XCT appears to have both lightdependent and light-independent functions, and its light-dependent function
inhibits red light inputs to the clock. XCT
also may act on the oscillation mechanism
in a light-independent manner, as its
shortening of photoperiod phenotype appears in all light conditions, including
complete darkness.
The XCT protein is nuclear localized (see
figure) and shows good conservation
across eukaryotes, with identity between
plant and mammalian proteins at nearly
50%. Another interesting nuclear-localized
clock component that shows strong conservation across kingdoms is the FIONA1
(FIO1) protein, published in the February
issue of The Plant Cell (Kim et al., 2008). In
contrast with xct mutants, fio1 mutants
show a lengthened circadian rhythm that is
independent of light and temperature conditions, indicating that FIO1 may not function in clock input, but rather acts directly in
the oscillation mechanism. Also, fio1 mutants flower extremely early. Interestingly,
most known core clock components are
not conserved between plants and animals. Unfortunately, the protein sequences
of XCT and FIO1 do not give us a clue to
their protein functions, so determination
of the relevance of their intriguing crosskingdom sequence conservation awaits
further study.
Jennifer Mach
Science Editor
[email protected]
REFERENCES
XCT protein is localized to the nucleus. Fluorescence image of XCT-YFP fusion overlaid on a brightfield image (left) and alone (right): trichome (A); hypocotyl (B). (Figure reproduced from Martin-Tyron
and Harmer [2008].)
www.plantcell.org/cgi/doi/10.1105/tpc.108.200510
Kim, J., Kim, Y., Yeom, M., Kim, J.H., and
Nam, H.G. (2008). FIONA1 is essential for
regulating period length in the Arabidopsis
circadian clock. Plant Cell 20: 307–319.
Martin-Tryon, E.L., and Harmer, S.L. (2008).
XAP5 CIRCADIAN TIMEKEEPER coordinates
light signals for proper timing of photomorphogenesis and the circadian clock in Arabidopsis. Plant Cell 20: 1244–1259.
McClung, C.R. (2006). Plant circadian rhythms.
Plant Cell 18: 792–803.
Newly Isolated Circadian Clock Components Conserved across Eukaryotes
Jennifer Mach
Plant Cell 2008;20;1187; originally published online May 30, 2008;
DOI 10.1105/tpc.108.200510
This information is current as of August 3, 2017
References
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