By Vicente Rubio & Xing Wang Deng
Recognizing seasonal change allows many plant species to select the
most favorable time of the year to flower, thereby increasing the chances of
their reproductive success. To recognize these transitions, a plant measures
variations in day length and compares them with its circadian clock, an
internal molecular oscillator that controls daily biological rhythms, such
as leaf movements and the opening of stomata, pores in the plant's leaves
However, the molecular mechanisms underlying this coordination are
still largely missing. Now, Sawa et al. on page 261 of this issue and a
recent study by Kim et al. bring us a big step closer toward characterizing
such mechanisms by identifying two light-sensing molecular switches that
directly control flowering and clock oscillation.

Molecular genetic studies performed in the plant Arabidopsis thaliana
have shown that photoperiodic flowering and regulation of the circadian
clock share common elements, including light receptors and proteins
comprising the circadian timekeeper. The light receptors include the
phytochromes and cryptochromes, which control both clock resetting and
flowering in response to different wave-lengths of light. Members of the
ZTL-FKF1-LKP2 protein family [consisting of ZEITLUPE (ZTL), FLAVIN-BINDING,
KELCH REPEAT, F-BOX 1 (FKF1), and LOV KELCH PROTEIN 2 (LKP2)] are also
proposed to act as receptors that mediate light input to the clock.

The ZTL-FKF1-LKP2 proteins contain a light, oxygen, or voltage (LOV)
domain, which likely functions as a blue light-sensing motif, and an F-box
domain. F-box-containing proteins are usually part of complexes that attach
ubiquitin molecules to protein targets to promote their destruction in a
structure called the proteasome. The presence of LOV and F-box domains
suggests roles for this protein family in transducing light into
intracellular signals through the degradation of key proteins. Indeed, FKF1
and ZTL regulate flowering time and circadian rhythms by controlling the
protein stability of CYCLING OF DOF FACTOR 1 (CDF1), a transcriptional
repressor of flowering, and the oscillator component TIMING OF CAB 1 (TOC1),
respectively.

Accordingly, mutations in the FKF1 and ZTL genes delay flowering under
favorable conditions (long days) and alter expression of genes controlled by
the circadian clock. Similar effects are caused by lack of GIGANTEA (GI), a
protein that controls clock oscillations and photoperiodic flowering, but
whose precise biochemical activity in these processes has remained
unknown.When to flower? The plant circadian clock controls rhythmic
expression of the GI protein, whose interaction with ZTL is stabilized by
blue light. ZTL-GI interaction controls accumulation of the clock component
TOC1, thus allowing robust circadian oscillations in gene expression. Blue
light also induces formation of an FKF1-GI protein complex, which in turn
targets CDF1, a transcriptional repressor of flowering, for degradation.
CDF1 proteolysis releases transcriptional repression of the CO gene, which
allows CO protein expression and long day-dependent accumulation to promote
flowering.Similarities in the function and rhythmical expression of FKF1 and
GI prompted Sawa et al. to analyze possible regulatory relationships between
these two proteins by looking for their physical interactions in plants.

To do so, they used Arabidopsis transgenic plants that expressed
epitope-tagged versions of GI and FKF1. Such tagging allowed them to use
epitope-specific antibodies to detect the tagged proteins in plant extracts.
The authors found that both proteins precipitated together, indicating that
FKF1 and GI associate in a complex in vivo. The interesting thing is that
their interaction occurred differentially throughout the day, peaking in the
afternoon during both long and short days, and diminishing at night.
Moreover, they found that the FKF1-GI interaction was induced by blue but
not red light, and that the LOV domain in FKF1 was responsible for blue
light absorption, demonstrating that FKF1 functions as a blue light
receptor.

In accordance with light requirement, FKF1-GI association was
disrupted in the dark and was very rapidly induced upon light exposure. The
latter response coincided with quick induction of the expression of CONSTANS
(CO), a gene encoding a positive regulator of flowering, whose transcription
is impaired by the flowering repressor CDF1. An FKF1-GI-CDF1 complex was
detected on the promoter region of the CO gene, which suggests that the
association of FKF1 and GI causes CDF1 to release its repression of CO
expression, thus promoting flowering. These results unveil the molecular
basis of how photoperiodic flowering is controlled by the coincidence of
light with circadian timing.

By analyzing the phenotype of plants with mutations in FKF1 and GI,
Sawa et al. determined that GI function in photoperiodic flowering does not
completely depend on FKF1. Thus, GI may regulate the activity of other
ZTL-FKF1-LKP2 family members or that of additional proteins controlling
circadian clock functions. The demonstration of such a possibility comes
from a complementary study by Kim et al. describing the relationship between
GI and ZTL. Kim et al. show that GI interacts with ZTL in plants and that
ZTL-GI complex formation is, as in the case of FKF1, triggered by blue
light. Interaction between GI and ZTL cooperatively stabilized both
proteins, thereby increasing their accumulation. This increase consequently
amplified and sharpened the rhythmic expression profile of the clock protein
TOC1, thus providing the clock oscillator with the robustness necessary to
maintain proper circadian rhythms.

Both Sawa et al. and Kim et al. provide mechanistic views on how
day-night cycles shape circadian clock oscillations and how light is
integrated into the clock to precisely regulate expression of a gene (CO)
that controls flowering. The studies raise many questions: What factors
control ZTL, FKF1, and GI stability? What role(s) do other light receptors
(phytochromes and cryptochromes) play in controlling light signaling to the
clock? Are there more targets for the GI-containing complexes? These
insights will help us to better understand why plants see changes in seasons
by standing on the shoulders of GIGANTEA.

[www.sciencemag.org]