ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 387

Concretely, A. thaliana sensitivity to oomycete and Pseudomonas syringae infec-
tion shows diurnal variations with an enhanced resistance at dawn, when the
infection is more likely to occur, and the circadian clock seems to play an impor-
tant role (Wang et al. 2011 ; Bhardwaj et al. 2011 ). Additionally, recent stud-
ies showed that not only the clock may modulate biotic stress responses, but
the immune response can also input the clock and modify its pace (Zhang et al.
2013 ). Although the connections between biotic stress and the circadian clock are
unknown, recent findings indicate an intricate and complex cross talk between
these two pathways.
Many genes implicated in plant defense against pathogens show circadian regu-
lation and a large number of them also contain an EE in their promoter regions
(Wang et al. 2011 ). In agreement with this observation, resistance to infection is
compromised at dawn in cca1 mutants, whereas CCA1 overexpression results
in an increased resistance (Wang et al. 2011 ). Similarly, diurnal variation in the
resistance to Pseudomonas infection was lost after suppression of circadian
rhythms, by either CCA1 overexpression or elf3 mutation (Bhardwaj et al. 2011 ).
This induced resistance at dawn is believed to be due to the higher expression lev-
els of pathogen-associated molecular patterns (PAMPs)-triggered immunity genes.
To infect a plant, Pseudomonas syringae and other pathogens should first invade
the leaf lamina through the stomata. The first layer of defense is constituted by
the recognition of PAMPs by specific receptors and is known as pattern-triggered
immunity (PTI). Activation of PTI induces rapid stomatal closure to prevent further
access of the pathogen into the leaf intercellular space. PTI-induced stomatal closure
requires components from the ABA signaling pathways such as reactive oxygen spe-
cies (ROS) (Cho et al. 2009 ). Indeed, cellular ROS levels are enhanced by ABA in
guard cells (Pei et al. 2000 ) and required for stomata closure. Both stomata closure
and ROS levels have been shown to be clock-regulated (Lai et al. 2012 ). Circadian
modulation of ROS levels is likely to be controlled through CCA1 direct regulation
of genes implicated in ROS homeostasis. Interestingly, exogenous applications of
ROS are able to modulate the circadian clock progression (Lai et al. 2012 ), adding
another pathway whether ABA signaling can turn to regulate the clock pace.
19.8.3 Floral Transition
Flowering is a critical developmental switch in the plant life cycle. Establishing
the correct timing for this transition is essential to determine reproductive suc-
cess. The molecular mechanisms implicated in the control of flowering time have
been most extensively studied in Arabidopsis and classically divided in four main
pathways: vernalization, photoperiodic, autonomous, and gibberellin (Ream et al.
2012 ; Milec et al. 2014 ; Andres and Coupland 2012 ; Mouradov et al. 2002 ).
Particularly, the circadian clock plays a major role in the regulation of photoperi-
odic flowering pathway, acting as an internal mechanism able to discriminate the
duration of the day.