ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 92

of ABAR -RNAi transgenic lines (Shen et al. 2006 ). Third, the abar - 2 and abar - 3
mutants defective in ABA signaling show normal chlorophyll content in compari-
son to wild-type plants (Wu et al. 2009 ). Fourth, the transgenic lines expressing
different truncated ABAR proteins do not alter the chlorophyll content but show
significant ABA hypersensitive phenotypes in all the major ABA responses (Wu
et al. 2009 ). Fifth, the transgenic lines expressing the C-terminus linked to or
free of the N-terminal chloroplast signal peptide, which leads to localization of
the expressed C-terminus of ABAR to the chloroplast or to the cytosol only, show
the same ABA hypersensitive phenotypes; similarly, the transgenic expression of
the C-terminus free of the N-terminal chloroplast signal peptide in the cch mutant,
which leads to localization of the expressed C-terminus of ABAR to the cytosol
but not to the chloroplast, cannot rescue the slight chlorosis phenotype but show
the same ABA hypersensitive phenotypes as the transgenic lines expressing the
C-terminus to the chloroplast in the cch mutant that is rescued from the chlorosis
phenotype (Wu et al. 2009 ). And last, among the four subunits of Mg-chelatase
(CHLH, CHLI, CHLD, and GUN4), CHLH/ABAR and CHLI, but not CHLD and
GUN4, are involved in ABA signaling, suggesting that CHLH interacts with CHLI
to form a hetero-dimer, which cooperates to regulate ABA signaling, while the
function of Mg-chelatase requires all the four components/subunits CHLH, CHLI,
CHLD, and GUN4 to form a hetero-tetramer complex, which catalyzes magne-
sium chelating to protoporphyrin-IX to produce Mg-ProtoIX (Du et al. 2012 ). This
model provides an explanation why ABAR-mediated ABA signaling is distinct
from chlorophyll biosynthesis.
6.3.4 An ABAR-WRKY40 Coupled Signaling Pathway
How a chloroplast ABAR protein transduces ABA signal to downstream cytosolic
or nuclear components of ABA signaling pathway had been puzzling. Shang et al.
( 2010 ) observed that the Arabidopsis ABAR is a chloroplast protein, which spans
the chloroplast envelope with its N- and C-termini exposed to the cytosol, which
provides opportunities for a chloroplast protein to interact with cytosolic proteins
that relay ABA signal from ABAR to downstream cascade. Further, Shang et al.
( 2010 ) identified a group of WRKY transcription factors (WRKY40, WRKY18,
and WRKY60) as ABAR interaction partners, which bind ABAR through the
C-terminal half exposed to the cytosolic space. These WRKYs function down-
stream of ABAR as negative regulators of ABA signaling in seed germination
and post-germination growth, of which WRKY40 is a central negative regulator,
and inhibit expression of a subset of ABA-responsive genes, such as ABF4 , ABI4 ,
ABI5 , DREB1A , DREB2A , MYB2, and RAB18 (Shang et al. 2010 ; Liu et al. 2012 ).
Besides, WRKY40 inhibits expression of the LHCB genes encoding the light-
harvesting chlorophyll a / b -binding proteins (Liu et al. 2013 ) and the KAT2 gene
encoding 3-ketoacyl-CoA thiolase-2 (an enzyme of fatty acid beta-oxidation; Jiang
et al. 2011 ), both of which are positively involved in ABA signaling (Jiang et al.