ABA Regulation of Plant Response to Biotic Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 405

inhibit the expression of ABA biosynthesis-related and ABA responsive genes,
in which the NPR1 protein or signaling downstream of NPR1 may participate
(Yasuda et al. 2008 ). Collectively, we can deduce that the bidirectional interac-
tion between ABA and SA can occur at multiple points of their signaling pathways
with their effect of cross talk prominently antagonistic.
20.10 ABA and JA/ET
Numerous studies have also suggested that there is a complex cross talk between
ABA and JA/ET-mediated signaling pathways. As we know that the JA sig-
nal pathway can be divided into two antagonistic branches, namely the ET co-
regulated ethylene response factor (ERF)-branch and the ABA co-regulated
MYC-branch. The ERF-branch is mainly responsible for the resistance to necro-
trophic pathogens and is controlled by the AP2/ERF domain transcription fac-
tors ERF1 and ORA59, leading to activation of the downstream marker gene
PDF1.2 (Penninckx et al. 1998 ; Lorenzo et al. 2004 ; Pr￩ et al. 2008 ),while the
MYC-branch is activated during plant-herbivore interactions and is regulated
by the basic helix-loop-helix leucine zipper proteins MYC2/3/4, resulting in the
transcription of VSP1 and VSP2 marker genes (Anderson et al. 2004 ; Thaler and
Bostock 2004 ; Lorenzo and Solano 2005 ; Fernández-Calvo et al. 2011 ; Niu et al.
2011 ). Interestingly, ABA can confer synergistic effects on the MYC-branch while
antagonistic effects on the ERF-branch, as demonstrated by the opposite effects
of ABA on two branches' master genes, with enhanced expression for MYC2 and
VSP2 but suppressed expression for ERF1 , ORA59 , and PDF1.2 (Anderson et al.
2004 ; Kazan and Manners 2013 ). Consistent with these findings, it was reported
that ABA-deficient mutants showed increased susceptibility to herbivory (Thaler
and Bostock 2004 ; Bodenhausen and Reymond 2007 ) and enhanced resistance
to necrotrophic pathogens (Anderson et al. 2004 ; Sánchez-Valletetal et al. 2012 ).
Studies also demonstrated that ABA can function as an essential signal to promote
JA biosynthesis and then lead to the activation of defense responses against the
damping-off oomycete P. irregulare (Adie et al. 2007 ). Additionally, there exist
bidirectional antagonism between ABA and ET signaling. For example, the eth-
ylene insensitive mutant ein2 had elevated ABA levels (Ghassemian et al. 2000 ),
while increased ABA levels may result in reduced ethylene levels as demonstrated
by the OsMPK5 over-expression plants or exogenous ABA treatment in rice
(Xiong and Yang 2003 ; Yang 2007 ). Together, ABA can both suppress and enhance
the JA/ET-mediated defense responses whose efficacy is determined by the combi-
nation between host plants and pathogens.
Furthermore, there may have complex cross talk between ABA, JA, and SA
during the interaction between A. brassicicola and susceptible Brassica juncea or
resistant Sinapis alba . The susceptible B. Juncea triggered an SA-mediated bio-
trophic mode of defense response upon challenge with A. Brassicicola , while the
resistant S. Alba initiated enhanced ABA response to counteract SA response and