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inhibition of seedling greening. FHY3 and FAR1 transcripts are up-regulated by
ABA and abiotic stresses (Tang et al. 2013 ). Thus, HY5 and FHY3/FAR1 tran-
scription activators act upstream of ABI5 to integrate light and ABA signaling
during early seedling development. In addition, the max2 mutant seedlings are
hypersensitive to ABA (Shen et al. 2012 ).
13.4.3 Stomatal Movement
ABA is a stress signal that plays a prominent role in inducing stomatal closure to
prevent water loss in response to drought stress and thereby contributes to tolerance
for plants (Cutler et al. 2010 ; Hauser et al. 2011 ). It has been known that blue light
receptor phototropins mediate stomatal opening (Kinoshita et al. 2001 ). Studies
from our laboratory showed that the fhy3 and far1 mutants have wider stomata, lose
water faster, and are more sensitive to drought than the wild type; therefore, FHY3
and FAR1 confer increased resistance to drought (Tang et al. 2013 ). The drought-
sensitive phenotype of fhy3 may be partly caused by the reduced sensitivity of guard
cell movement under drought stress conditions, which may induce the production of
ABA. In agreement with this notion, FHY3 is highly expressed in guard cells (Tang
et al. 2013 ). MAX2 plays a similar role as FHY3/FAR1 in modulating stomatal
movement. The max2 mutant plants are less sensitive to ABA-induced stomatal clo-
sure and display increased water loss and drought-sensitive phenotypes. The expres-
sion of ABA biosynthesis, catabolism, transport, and signaling genes was impaired
in max2 , compared to wild type in response to drought stress (Bu et al. 2014 ).
Down-regulation or disruption of any member of the LHCB family, LHCB1 to
LHCB6 , reduces responsiveness of stomatal movement to ABA. By contrast, over-
expression of LHCB6 enhances stomatal sensitivity to ABA (Xu et al. 2012 ). These
results demonstrate that LHCBs play a positive role in ABA signaling in stomatal
movement and the plant response to drought. Similarly, LHCBs positively regulate
seed germination and seedling growth in response to ABA (Liu et al. 2013 ).
Mg-chelatase catalyzes the formation of Mg-protoporphyrin IX by chelating
magnesium to protoporphyrin IX in the chlorophyll biosynthesis pathway (Tanaka
and Tanaka 2007 ). The H subunit of Mg-chelatase was identified as an ABA recep-
tor, and it functions in the ABA signaling pathway (Shen et al. 2006 ; Wu et al.
2009 ). ABA specifically binds to CHLH, but not to the other Mg-chelatase sub-
units, CHLI, CHLD, and GUN4 (Du et al. 2012 ). CHLH and GUN4 are major
targets for light regulation during seedling de-etiolation (Stephenson and Terry
2008 ). Genetic studies showed that the rtl1 mutant plants (a chlh allele) display
ABA-insensitive phenotypes in stomatal movement. Interestingly, down-regulation
of CHLI also confers ABA insensitivity in stomatal response, while up-regulation
of CHLI1 results in ABA hypersensitivity in seed germination (Du et al. 2012 ).
The involvement of these chlorophyll biosynthesis and binding proteins in sto-
matal movement might coordinate internal development with external signals for
optical air exchange and maximal photosynthesis.
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