ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 91

mutants involve mutations in the N-terminus of ABAR, which may function as a
regulatory region but not as a core functional region like the C-terminal half (Wu
et al. 2009 ). Together, the phenotypic analysis of the four abar mutants provides
genetic evidence that ABAR may regulate seed germination/seedling growth and
stomatal movement in response to ABA through different mechanisms by modifi-
cation of the specific domains in the ABAR molecule (Wu et al. 2009 ).
As a matter of fact, the ABAR -down-expression lines through transgenic RNAi
technique show strong ABA-insensitive phenotypes in all the three major ABA
responses (Shen et al. 2006 ), but the ABAR -RNAi construct is deleted in the T4
generation RNAi lines in which the ABAR gene expression is restored to wild-type
level (X.F. Wang and D.P. Zhang, unpublished data). Currently, there is no mutant
defective in the C-terminal half of ABAR, which is likely the reason why the
above-mentioned mutants show relatively weak ABA-related phenotypes in seed
germination and post-germination growth.
Increasing evidence verifies the essential role of ABAR in ABA signal-
ing. Du et al. ( 2012 ) showed that ABAR also regulates guard cell signaling in
response to ABA in tobacco ( Nicotiana benthamiana ) leaves. Other groups
demonstrated that CHLH/ABAR mediates ABA signaling in guard cells in both
Arabidopsis (Legnaioli et al. 2009 ; Tsuzuki et al. 2011 , 2013 ) and peach ( Prunus
persica ) leaves (Jia et al. 2011b ). Recently, Tsuzuki et al. ( 2013 ) showed that
CHLH/ABAR mediates ABA inhibition of blue light (BL)-induced phospho-
rylation of H + -ATPase in Arabidopsis guard cells, suggesting that CHLH/ABAR
regulates not only ABA-induced stomatal closure but also ABA inhibition of
BL-mediated stomatal opening. Interestingly, it has been demonstrated that
CHLH/ABAR mediates ABA signaling in fruit ripening of both peach (Jia et al.
2011b ) and strawberry ( Fragaria ananassa ) (Jia et al. 2011a ). These data demon-
strate that CHLH/ABAR is a conserved ABA signaling regulator in plant cells.
6.3.3 ABAR-mediated ABA Signaling Is Distinct
from Chlorophyll Biosynthesis
As described earlier, CHLH/ABAR was initially identified as a key enzyme of
chlorophyll biosynthesis pathway (Walker and Willows 1997 ), and a regulator of
plastid-to-nucleus signaling (Mochizuki et al. 2001 ; Surpin et al. 2002 ; Strand
et al. 2003 ; Nott et al. 2006 ). Although some ABAR -RNAi transgenic lines and cch
mutant have low chlorophyll contents and show yellow leaves (Shen et al. 2006 ;
Wu et al. 2009 ), multiple lines of evidence demonstrate that ABAR-mediated
ABA signaling is distinct from chlorophyll biosynthesis in Arabidopsis . First, the
ch1 - 1 and ch1 - 2 mutants defective in chlorophyll biosynthesis show wild-type
response to ABA (Shen et al. 2006 ; Liu et al. 2013 ). Second, many ABAR -RNAi
transgenic lines with a chlorophyll level that is not lower than in wild-type plants
show strong ABA-insensitive phenotypes; no correlation of the chlorophyll con-
tent with ABA signaling was found in the phenotypic analysis with a large number