ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 386

19.8.1 Seed Dormancy and Germination
Seed studies in order to determine the moment during the development when
the circadian clock starts revealed that rhythmic gene expression begins on the
second day after inducing germination by hydration, even in the absence of
light (Penfield and Hall 2009 ; Salome et al. 2008 ; Zhong et al. 1998 ). These
results confirm that seed hydration is sufficient to initiate the circadian clock.
However, studies combining hydration with light cycles showed a considerable
increase in the robustness and amplitude of the circadian rhythms, suggest-
ing that a light signal is critical to the clock initiation and progression in seeds
(Kikis et al. 2005 ).
Seed dormancy allows plants to germinate only when suitable environmental
conditions are present. Environmental cues have been shown to regulate seed dor-
mancy and germination by modulating levels of two phytohormones with opposite
action. ABA establishes and maintains dormancy inhibiting germination, while
gibberellins promote the breaking of dormancy and induce germination (Finch-
Savage and Leubner-Metzger 2006 ). Taking in to account that several enzymes
involved in gibberellin and ABA metabolism are controlled by the circadian clock,
it is not surprising that mutations that alter circadian rhythms also affect germina-
tion (Penfield and Hall 2009 ). In this sense, ztl (ZEITLUPE, an F-box and blue
receptor protein that mediates proteasomal degradation of TOC1 protein) and
lux mutants exhibit reduced germination (Penfield and Hall 2009 ). Furthermore,
stratification induces germination less efficiently in these mutants than wild-type
seeds, while lhy / cca1 double mutants germinate more efficiently. Germination
defects in these mutants may be due to alterations in hormone signaling as hap-
pens in other stages of plant development (Covington and Harmer 2007 ; Dodd
et al. 2007b ; Hanano et al. 2006 ; Legnaioli et al. 2009 ; Mizuno and Yamashino
2008 ).
A possible mechanistic link connecting the circadian clock with seed dor-
mancy and germination was postulated by the observation that TOC1 interact
in yeast two hybrid with the central regulator of dormancy ABI3 (Kurup et al.
2000 ). This notion was recently been reinforced in studies showing that proper
TOC1 levels are necessary for the ABA-mediated inhibition of germination
(Castells et al. 2010 ). Thus, a proper function of the circadian clock in com-
bination with the external signals is an essential mechanism that regulates the
germination process.
19.8.2 Biotic Stress
Raising evidences have linked the circadian clock with the biotic responses,
specifically pathogen resistance (Roden and Ingle 2009 ; Walley et al. 2007 ).