ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 93

2011 ; Xu et al. 2012 ; Liu et al. 2013 ). Further evidence reveals that WRKY18,
WRKY40, and WRKY60 transcription factors function upstream of ABI4 and
ABI5 and cooperate to repress ABI4 and ABI5 genes (Shang et al. 2010 ; Liu et al.
2012 ). In addition, these WRKYs may downregulate expression of their own
genes in a manner of auto- and cross-repression (Yan et al. 2013 ).
Recently, a chloroplast co-chaperonin 20 (CPN20) was identified as a novel
interaction partner of ABAR in Arabidopsis , which negatively regulates ABA sign-
aling at the same node with ABAR but upstream of WRKY40 transcription repres-
sor (Zhang et al. 2013 , 2014 ).
These studies support a model that ABAR antagonizes the WRKY transcription
repressors to relieve ABA-responsive genes of inhibition (Shang et al. 2010 ; Liu
et al. 2012 ; Yan et al. 2013 ; Zhang et al. 2013 , 2014 ). The cytosolic C-terminus
of ABAR interacts with a group of WRKY transcription factors, WRKY40,
WRKY18, and WRKY60, which negatively regulate ABA signaling and inhibit
expression of ABA-responsive genes, such as ABI4 and ABI5 . High level of ABA
promotes ABAR-WRKY40 interaction, recruits WRKY40 from the nucleus to
the cytosol, and activates an unknown factor or signaling cascade to downregu-
late WRKY40 expression. Consistent with our observations (Shang et al. 2010 ),
an independent group observed that the expression level of the WRKY40 gene is
greatly enhanced in the cch mutant (Adhikari et al. 2011 ), supporting that ABAR
is required for repression of WRKY40 . Decrease of WRKY40 relieves ABI4 and
ABI5 genes of inhibition to induce physiological responses (Shang et al. 2010 ; Liu
et al. 2012 ; Yan et al. 2013 ; Fig. 6.1 ). This ABAR-WRKY40 coupled mechanism
may be inhibited by CPN20, a chloroplast interaction partner of ABAR. CPN20
interacts with ABAR tightly at low ABA level, competitively attenuating the inter-
action between ABAR and WRKY40, which is favorable to keeping a high level
of WRKY40 to repress ABA-responsive genes. High level of ABA inhibits the
ABAR-CPN20 interaction, which in turn promotes the ABAR-WRKY40 inter-
action to trigger the downstream signaling to repress WRKY40 expression and
finally to relieve ABA-responsive genes of inhibition (Zhang et al. 2013 , 2014 ;
Fig. 6.1 ). These findings describe a unique ABA signaling pathway from the early
signaling events to downstream gene expression.
6.3.5 A Pentatricopeptide Repeat Protein SOAR1:
A Hub of ABA Signaling Downstream of ABAR
To further explore the mechanism of the ABAR-mediated ABA signaling, a s up-
pressor o f the A BA R -overexpression 1D ( soar1D ) lines was identified from the
population of the ABAR -overexpression transgenic lines (Mei et al. 2014 ). The
SOAR1 gene encodes a pentatricopeptide repeat (PPR) protein, which local-
izes to both the cytosol and nucleus, but not to chloroplast or mitochondrion.
Downregulation of SOAR1 strongly enhances, but upregulation of SOAR1 almost
completely impairs, ABA responses, revealing that SOAR1 is a critical, negative,