ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 376

cues (for example, continuous light and constant temperature) with periods close
to 24 h. Daily changes in light and temperature are able to synchronize these
rhythms every day, so the rhythmicity exactly matches the 24-h cycle (Liu and
Bell-Pedersen 2006 ; Más and Yanovsky 2009 ; McClung 2006 ). Furthermore, the
circadian rhythms remain almost unchanged over a wide range of physiologi-
cal temperatures, thus they are temperature compensated (Bruce and Pittendrigh
1956 ). Hence, the clock has an interesting duality. On the one hand, it is a flexible
mechanism capable of being synchronized by environmental changes, and on the
other hand, it is a robust and precise timekeeper maintained at different tempera-
tures (Pittendrigh 1954 ).
Classically, the clock signaling pathway has been represented as linear and uni-
directional with three functional modules: input, central oscillator, and output. The
input is the module capable of sensing and transmitting the environmental infor-
mation to the central oscillator, synchronizing it; thus, the central oscillation is
in resonance with the external environment. The central oscillator is the mecha-
nism responsible for generating and maintaining the rhythmicity that is going to
be transmitted to the output. The output is defined by those rhythmic processes
controlled by the clock. However, the present knowledge indicates that this linear
and unidirectional idea is an oversimplification and the clock signaling pathway is
more complex and branched.
19.2 Evolution of the Circadian Clock Model
The generation of circadian rhythms is based on negative feedback loops, involv-
ing repressors and activators elements that regulate each other's expression,
localization, and/or activity (Bell-Pedersen et al. 2005 ; Wijnen and Young 2006 ).
In Arabidopsis thaliana , the first model described involves the MYB transcrip-
tion factors CIRCADIAN CLOCK ASSOCIATED 1 ( CCA1 ) (Wang and Tobin
1998 ) and LATE ELONGATED HYPOCOTYL ( LHY ) (Schaffer et al. 1998 ) that
downregulate the expression of the pseudo response regulator TIMING OF CAB
EXPRESSION 1 ( TOC1 ) (Makino et al. 2002 ; Strayer et al. 2000 ). In turn, TOC1
activates CCA1 and LHY . This mutual regulation allows to generate an antiphasic
and rhythmic expression on TOC1 and CCA1 / LHY (Alabadi et al. 2001 ). While
CCA1 and LHY expression peaks early in the day, TOC1 has its maximum expres-
sion during the evening. CCA1 and LHY therefore are classified as morning genes
and TOC1 as evening gene (Fig. 19.2 ).
The functional involvement of these components in the circadian clock has
been proven in numerous studies. In plants constitutively expressing CCA1 (Wang
and Tobin 1998 ) or LHY (Schaffer et al. 1998 ), the clock is no longer functional,
whereas loss-of-function mutations of these components results in a clock period
shortening (Green and Tobin 1999 ; Mizoguchi et al. 2002 ). Although LHY and
CCA1 display high sequence homology, similar phenotypes, and the same phase
of expression, their function is only partially redundant (Mizoguchi et al. 2002 ).
Similar studies showed that the constitutive overexpression of TOC1 leads to