Cross Talk of Signaling Pathways Between ABA and Other Phytohormones - Abscisic Acid: Metabolism, Transport and Signaling - page 243

disease resistance. For example, exogenous ABA treatment could suppress both
basal and JA-activated transcription from defense genes (Anderson et al. 2004 ).
By contrast, ABA deficiency as conditioned by the mutations in the ABA1 and
ABA2 genes, which encode enzymes involved in ABA biosynthesis, resulted in
upregulation of JA-responsive defense genes (Anderson et al. 2004 ). Collectively,
these results indicate that the antagonistic interactions between multiple compo-
nents of JA- and ABA-signaling pathways modulate defense and stress responsive
gene expression in response to biotic and abiotic stresses (Anderson et al. 2004 ).
The basic helix-loop-helix (bHLH) transcription factor MYC2 has been
identified as a master regulator of most aspects of the JA-signaling pathway in
Arabidopsis (Boter et al. 2004 ; Lorenzo et al. 2004 ; Dombrecht et al. 2007 ; Chen
et al. 2011 ; Fernandez-Calvo et al. 2011 ). However, several studies showed that
MYC2 is also involved in ABA signaling in Arabidopsis. Abe et al. ( 2003 ) showed
that transgenic plants overexpressing MYC2 had higher sensitivity to ABA and
several ABA-inducible genes were upregulated in the transgenic plants. By con-
trast, the mutant of the MYC2 gene was less sensitive to ABA and showed signifi-
cantly decreased ABA-induced gene expression. These results indicate that MYC2
also function as a positive regulator of ABA signaling in Arabidopsis (Abe et al.
2003 ). Taken together, we propose that MYC2 acts as an integrative hub for the
regulation of both JA and ABA signaling in Arabidopsis.
Moreover, Nakata et al. ( 2013 ) showed that a bHLH-type transcription fac-
tor, ABA-inducible bHLH-type transcription factor/JA-associated MYC2-LIKE1
(JAM1), acts as a transcriptional repressor and negatively regulates JA signaling.
Gain-of-function transgenic plants expressing the chimeric repressor for JAM1
exhibited substantial reduction of JA responses, including JA-induced inhibition
of root growth, accumulation of anthocyanin, and male fertility. Conversely, jam1
loss-of-function mutants showed enhanced JA responsiveness, including increased
resistance to insect attack. JAM1 and MYC2 competitively bind to the target
sequence of MYC2, which likely provides the mechanism for negative regula-
tion of JA signaling and suppression of MYC2 functions by JAM1 (Nakata et al.
2013 ). Considering the role of JAM1 in ABA signaling (Li et al. 2007 ), we pro-
pose that JAM1 may be a novel integrative hub between ABA- and JA-signaling
pathways.
Significantly, Chen et al. ( 2012 ) recovered the action mechanisms of the medi-
ator25 (MED25) subunit of the Arabidopsis mediator complex in regulating JA-
and ABA-triggered gene transcription. Their results showed that during jasmonate
signaling, MED25 physically associates with the basic helix-loop-helix transcrip-
tion factor MYC2 in promoter regions of MYC2 target genes and exerts a posi-
tive effect on MYC2-regulated gene transcription (Chen et al. 2012 ). By contrast,
MED25 physically associates with the basic Leu zipper transcription factor ABA-
INSENSITIVE5 (ABI5) in promoter regions of ABI5 target genes and shows a neg-
ative effect on ABI5-regulated gene transcription (Chen et al. 2012 ). This study
reveals an important role for MED25 as an integrative hub within the mediator
complex during the regulation of JA- and ABA-signaling pathways (Chen et al.
2012 ).