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Casal and Sanchez 1998 ). phyA mediates very low-fluence response (VLFR), while
phyB acts via photoreversible low-fluence response (LFR) to promote seed germi-
nation. However, continuous far-red light inhibits germination via high-irradiance
response in many plant species (Botto et al. 1996 ).
Light controls seed germination predominantly through regulating the endog-
enous levels of GA and ABA. ABA inhibits germination of lettuce seeds induced
by red light, whereas active GA mimics the effect of red light (Kahn et al. 1957 ;
Sankhla and Sankhla 1968 ). Extensive studies have identified a number of factors
that involve in light-controlled seed germination.
Giltu Choi's laboratory firstly reported that a basic helix-loop-helix transcrip-
tion factor PIL5 acts as a key negative regulator in phytochrome-mediated seed
germination (Oh et al. 2004 ). PIF5 preferentially interacts with the Pfr forms of
phyA and phyB. When activated by light, phytochromes bind to and accelerate the
degradation of PIL5 in both seeds and seedlings (Oh et al. 2006 ; Shen et al. 2005 ).
The destabilization of PIL5 thus releases its repression of seed germination and
allows seeds to germinate. As a result, loss-of-function mutant of pil5 germinates
well regardless of far-red light treatment mediated by LFR and VLFR, whereas
PIL5 overexpression transgenic lines fail to germinate under relative low inten-
sity of red light (Oh et al. 2004 ). It was showed that PIL5 directly binds to the
promoters of two GA repressor (DELLA) genes, REPRESSOR OF GA1 - 3 ( RGA )
and GA - INSENSITIVE ( GAI ), and activates their expression (Oh et al. 2007 ).
Furthermore, chromatin immunoprecipitation (ChIP) chip and microarray analyses
helped to identify large amount of PIL5 direct target genes involved in hormone
signaling and cell wall modification (Oh et al. 2009 ). Therefore, PIL5 regulates
seed germination not only by mediating GA signaling and coordinating GA and
ABA metabolism, but also by modulating cell wall properties in imbibed seeds.
Since pil5 could not fully restore the germination deficiency of phyB in the pil-
5phyB double mutant, other factors must be involved in the phyB-mediated germi-
nation process (Oh et al. 2004 ).
SOM was identified as another negative factor in regulating light-dependent
seed germination (Kim et al. 2008 ). The SOM gene encodes a CCCH-type zinc
finger protein that probably acts as an RNA-binding factor. The som mutants have
lower levels of ABA and elevated levels of GA and germinate in darkness inde-
pendently of various light regimens (Kim et al. 2008 ). PIL5 directly promotes the
expression of SOM through binding to its promoter sequence, and the reduced ger-
mination rate of a PIL5 overexpression line is rescued by the som mutation (Kim
et al. 2008 ). Thus, SOM functions downstream of PIL5 and the PIL5-SOM regu-
latory pathway likely defines an essential step in integrating ABA and light sign-
aling to control seed germination. In addition to PIL5, ABI3 was also found to
be targeted to the RY motifs present in the SOM promoter. ABI3 and PIL5 inter-
act and collaboratively activate the expression of SOM mRNA in Arabidopsis
imbibed seeds, but independently induce SOM expression in maturing seeds (Park
et al. 2011 ). However, HFR1 plays a negative role on PIL5 transcriptional activity
by interacting with PIL5 and preventing its binding to target DNA. Through the
HFR1-PIL5 heterdimer, light regulates expression of numerous genes involved in
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