ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 90

In contrast to the Arabidopsis PYR/PYL/RCAR receptor for ABA, which is a
cytosolic protein with a low molecular mass (about 20 kDa) and a highly hydro-
philic nature (Ma et al. 2009 ; Park et al. 2009 ; Santiago et al. 2009a ; and also see
description below), the properties of a chloroplast-membrane ABAR/CHLH pro-
tein (Shang et al. 2010 ) with a high molecular mass and a slightly hydrophobic
nature make ABA-binding assay difficult (Wang et al. 2011 ). This may likely be
a reason why ABA binding of CHLH was not detected by other groups with the
3 H-labeled ABA-binding assay (Mller and Hansson 2009 ; Tsuzuki et al. 2011 ).
6.3.2 CHLH/ABAR Mediates ABA Signaling Positively
CHLH/ABAR is a multi-functional protein, which participates in chlorophyll bio-
synthesis (Walker and Willows 1997 ), and plastid-to-nucleus signaling (Mochizuki
et al. 2001 ; Surpin et al. 2002 ; Strand et al. 2003 ; Nott et al. 2006 ). We observed
that down- and up-expressions of the Arabidopsis ABAR gene result in ABA insen-
sitivity and hypersensitivity, respectively, in the major ABA responses including
ABA-induced inhibition of seed germination, post-germination growth arrest, and
promotion of stomatal closure and inhibition of stomatal opening, revealing that
ABAR/CHLH is a positive regulator of ABA signaling (Shen et al. 2006 ; Wu et al.
2009 ). There are five abar mutant alleles in Arabidopsis , abar - 1 , abar - 2 , abar -
3 , cch , and rtl1 . The abar - 1 mutant, a knockout mutant resulting from a T-DNA
insertion in ABAR gene, is lethal (Shen et al. 2006 ). The ABA responses are sig-
nificantly altered in all the rest four mutants. The abar - 2 , abar - 3 , cch , and rtl1
mutants all harbor a single-nucleotide substitution in ABAR gene; the mutations
of abar - 2 and abar - 3 result in a single amino acid mutation Leu348 ₒ Phe and
Ser183 ₒ Phe, respectively, in the N-terminal half of ABAR protein (Wu et al.
2009 ), and the cch and rtl1 mutations result also in a single amino acid mutation
Pro642 ₒ Leu and Leu690 ₒ Phe, respectively, in the middle of ABAR protein
(Mochizuki et al. 2001 ; Tsuzuki et al. 2011 ). The cch mutant shows ABA insen-
sitivity in all the three major ABA responses with ABA-insensitive phenotypes
relatively weak in seed germination and seedling growth but strong in stomatal
movement that leads to hypersensitivity to dehydration (Shen et al. 2006 ; Wu et al.
2009 ; Du et al. 2012 ). The rtl1 mutant is susceptible to drought with significant
defect in stomatal response to ABA and displays weak ABA-insensitive phenotype
in seed germination but wild-type phenotype in ABA-induced post-germination
growth arrest (Tsuzuki et al. 2011 ; Du et al. 2012 ). The strong ABA-insensitive
phenotype in stomatal movement of the cch and rtl1 mutants suggests that the core
function domains of ABAR for regulation of stomatal response to ABA are more
lesioned in these mutants. The abar - 2 and abar - 3 mutants both shows ABA insen-
sitivity in ABA-induced post-germination growth arrest, and seed germination of
the abar - 2 mutant is insensitive to ABA while that of the abar - 3 mutant is hyper-
sensitive to ABA (Wu et al. 2009 ). The abar - 2 and abar - 3 mutants show wild-type
phenotypes in stomatal movement in response to ABA, likely because these two