Cross Talk Between Light and ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 248

Signaling, based largely on progress achieved so far using genetic and molecular
approaches in Arabidopsis as a model system. In this review, we first describe a
brief introduction of the light signaling pathway, and then summarize, and discuss
the convergence of light and ABA Signaling in regulating plant responses.
13.2 Overview of the Light Signaling Pathway
Plants have evolved an array of photoreceptors to perceive and transduce differ-
ent spectra of light that ultimately modulate the transcriptomes and trigger plant
growth and development. These photoreceptors include the red and far-red light
(600-750 nm)-absorbing phytochromes (phys), the blue/ultraviolet-A light (320-
500 nm)-absorbing cryptochromes (cry1 and cry2), phototropins (phot1 and
phot2), and three newly identified LOV/F-box/Kelch-repeat proteins ZEITLUPE
(ZTL), FLAVIN-BINDING KELCH REPEAT F-BOX (FKF), and LOV KELCH
REPEAT PROTEIN 2 (LKP2), and UV-B light (282-320 nm)-absorbing UV
RESISTANCE LOCUS8 (UVR8) (Chen et al. 2004 ; Christie 2007 ; Nagatani
2010 ; Yu et al. 2010 ; Heijde and Ulm 2012 ; Ito et al. 2012 ). Phytochromes are
unique photoreceptors because they exist as two distinct but photoreversible forms
in vivo. The biological active Pfr form absorbs far-red light, whereas the inactive
Pr form absorbs red light (Li et al. 2011 ). There are five phytochromes, designated
phyA to phyE in Arabidopsis thaliana . phyA is light labile, while phyB to phyE
are light stable (Li et al. 2011 ).
Seedling de-etiolation is a light-controlled process that has been extensively
studied in the past decades. Accumulating evidence has established that phys and
crys control two main branches of light signaling during seedling de-etiolation
(Lau and Deng 2010 ). A group of constitutive photomorphogenic/de-etiolated/fusca
(COP/DET/FUS) proteins act as repressors downstream of phys and crys that
define the first branch of the light signaling pathway (Lau and Deng 2012 ). Among
these proteins, COP1 is a central repressor that targets a number of positive fac-
tors, such as ELONGATED HYPOCOTYL5 (HY5) and LONG HYPOCOTYL IN
FAR-RED1 (HFR1), for 26S proteasome-mediated degradation, thus desensitizing
light signaling (Henriques et al. 2009 ; Lau and Deng 2012 ). HY5 encodes a basic
domain/leucine zipper transcription factor that plays a key role in promoting pho-
tomorphogenesis in all light conditions by directly regulating the transcription of
a wide range of genes (Oyama et al. 1997 ; Lee et al. 2007 ). HY5 is stabilized at
the post-translational level by light and inhibits hypocotyl growth (Osterlund et al.
2000 ). In the second branch, a class of basic helix-loop-helix transcription factors,
designated PHYTOCHROME-INTERACTING FACTORs (PIFs), accumulate
in darkness and thus regulate gene expression to promote the skotomorphogenic
response (Leivar et al. 2008 ). Under light, PIF proteins interact with photoacti-
vated phys and result in PIFs' phosphorylation and subsequent degradation in an
unknown manner (Leivar and Quail 2011 ). PIF proteins mainly regulate the phy-
tochrome pathway, although they might also effect under blue light. Increasing