Cross Talk Between Light and ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 249

studies demonstrated the broad function of PIFs as integrators in mediating plant
development (Leivar and Monte 2014 ).
Extensive studies have identified dozens of intermediates in the light signal-
ing pathway and revealed the importance of transcriptional regulatory networks
in controlling photomorphogenesis (Jiao et al. 2007 ; Chory 2010 ). For exam-
ple, FA R - RED ELONGATED HYPOCOTYL3 ( FHY3 ) and FA R - RED IMPAIRED
RESPONSE1 ( FAR1 ) are two positive transcription factors transducing signals in
the far-red light pathway (Hudson et al. 1999 ; Wang and Deng 2002 ; Lin et al.
2007 ). For more information on light signaling regulation, readers may go through
some recent reviewer articles (Bou-Torrent et al. 2007 ; Jiao et al. 2007 ; Demarsy
and Fankhauser 2009 ; Li et al. 2011 ).
13.3 Inter-regulation Between Light and ABA
13.3.1 Light Regulates ABA Biosynthesis
The ABA metabolic pathway has been described in detail in Chap. 20 . Most of the
genes involved in ABA biosynthesis and catabolism have been identified geneti-
cally. The oxidative cleavage of cis-epoxycarotenoid to xanthoxin is catalyzed by
9-cis-epoxycarotenoid dioxygenase (NCED) and represents the key regulatory step
of ABA biosynthesis in plants. ABA 8′-hydroxylases encoded by cytochrome P450
CYP707A genes catalyze the first committed step in the predominant ABA catabolic
pathway (Kushiro et al. 2004 ; Nambara and Marion-Poll 2005 ). The endogenous
ABA level is modulated by the precise balance between its biosynthesis and catabo-
lism. Regulation of NCED and CYP707A has thus been proposed to significantly
determine endogenous ABA level in plants (Nambara and Marion-Poll 2005 ).
ABA is increasingly accumulated in seeds during their maturation. Studies
from seed germination have well demonstrated that light plays a crucial role in
regulating ABA metabolic gene expression and subsequent ABA level. Red light
decreases, whereas far-red light increases endogenous ABA level in Arabidopsis
and lettuce ( Lactuca sativa L.) seeds (Toyomasu et al. 1994 ; Seo et al. 2006 ;
Sawada et al. 2008 ). Among AtNCED genes, AtNCED6 and AtNCED9 have been
shown to play key roles in ABA biosynthesis in developing seeds (Lefebvre et al.
2006 ). The transcript level of AtNCED6 remains high after pulse of far-red light
irradiation and is reduced by a subsequent red light pulse in Arabidopsis seeds.
In agreement with this notion, the nced6 - 1 mutant showed enhanced germination
ability relative to wild type when treated with FR light (Seo et al. 2006 ). Similarly,
red light down-regulates LsNCED2 and LsNCED4 and increases LsABA8ox4
(encoding ABA 8′-hydroxylase) expression (Sawada et al. 2008 ). As a catabolic
gene, the expression pattern of CYP707A2 undergoes an opposite manner to that
of AtNCED6 (Seo et al. 2006 ). However, photoreversible expression of CYP707A1
and CYP707A3 appears to be regulated indirectly by light (Seo et al. 2006 ). Thus,
ABA biosynthesis is likely regulated through the photoreversible expression of
AtNCED6 and CYP707A2 in an opposite manner.