Cross Talk Between Light and ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 254

IMB1 (for imbibition-inducible 1) defines as a putative bromodomain transcrip-
tion factor. The imb1 loss-of-function mutant is hypersensitive to ABA-mediated
inhibition of cotyledon expansion and greening, and is deficient in the phyA-medi-
ated VLFR of seed germination (Duque and Chua 2003 ). IMB1 transcript level is
elevated during seed imbibition. This study implicates that IMB1 might link phyA
to ABA signaling in seed germination (Duque and Chua 2003 ). Interestingly, ABI5
transcript level was up-regulated in imb1 seed germination in ABA when com-
pared to the wild type, suggesting that IMB1 acts upstream of ABI5 in the ABA
pathway (Duque and Chua 2003 ).
In addition to transcription factors, two JmjC domain-containing proteins,
JMJ20 and JMJ22, have been shown as positive regulators of seed germination
(Cho et al. 2012 ). JMJ20 and JMJ22 encode histone arginine demethylases, and
their expression is directly repressed by SOM. Upon phyB activation by red light,
JMJ20 and JMJ22 are derepressed, resulting in increased GA levels through the
removal of repressive histone arginine methylation at GA3ox1 and GA3ox2 loci,
which in turn promote germination (Cho et al. 2012 ). However, the ABA meta-
bolic genes are not regulated by JMJ20/JMJ22. This study adds an additional layer
that involves repressive epigenetic mechanism during seed germination.
The F-box protein MORE AXILLARY BRANCHES2 (MAX2) plays an
important role in promoting photomorphogenesis through modulating GA and
ABA biosynthetic pathways (Shen et al. 2007 , 2012 ). The max2 mutant seeds
are hyposensitive to light-induced seed germination and hypersensitive to ABA.
Surprisingly, expression of ABA biosynthetic and catabolic genes and ABA-
regulated genes is up-regulated by max2 mutation (Shen et al. 2012 ; Bu et al.
2014 ). Though a genetic study indicated that the seed germination phenotype of
max2 is epistatic to pil5 (Shen et al. 2012 ), the molecular mechanism between
MAX2 and PIL5 remains to be elucidated.
13.4.2 Seedling Growth and Development
After seed germination, seedling growth and development are also regulated by
light and ABA. It has been shown that disruption of HY5 confers tolerance to the
inhibitory effect of ABA on lateral root growth and seedling growth. The hy5 seed-
lings were also more susceptible to salt and osmotic stresses than the wild-type
plants (Chen et al. 2008 ). ABI5::GUS promoter activity was detected in cotyle-
dons, hypocotyls, roots, flowers, and siliques. However, this activity was greatly
reduced in the hy5 mutant background. Furthermore, light promotes ABI5 expres-
sion in a HY5-dependent manner (Chen et al. 2008 ). This is because HY5 protein
is tightly controlled by the COP1-mediated 26S proteasome degradation pathway
in the dark (Osterlund et al. 2000 ). As a consequence, overexpression of ABI5
restores ABA sensitivity in hy5 and enhances light response (hypocotyl elonga-
tion) in the wild type (Chen et al. 2008 ). Since FHY3/FAR1 also bind to ABI5
promoter sequence, fhy3 and/or far1 mutants are hyposensitive to ABA-mediated