Agriculture Reference
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effect of ABA in callose deposition and ROS generation, ABA can also affect the
expression of defense-related genes. Accumulating evidences have demonstrated
that ABA can participate in plant biotic stress responses by connecting to the well-
documented SA and JA/ET-mediated signaling pathways. Compared to the pre-
dominantly antagonistic relation between ABA and SA, the interaction between
ABA and JA/ET can be both synergistic and antagonistic.
20.9 ABA and SA
Numerous studies have provided evidences that ABA can negatively regulate
SA-mediated pathogen responses. The production of ABA was enhanced by Pst
DC3000 infection, implying that ABA may increase susceptibility of host plant to
this bacterium (Truman et al. 2006 ; de Torres-Zabala et al. 2009 ). Coincidently,
both exogenous ABA application and addition of exogenous ABA by over-expres-
sion of ABA biosynthesis-related NCED genes result in increased growth of bac-
terial pathogens, while mutations in Arabidopsis ABA synthesis mutants, such
as aba3 - 1 and aao3 , confer enhanced resistance to Pst DC3000 infection (Mohr
and Cahill 2003 ; Fan et al. 2009 ; de Torres-Zabala et al. 2009 ). Similarly, exog-
enous ABA application renders rice hypersusceptible to the infection by both the
rice leaf blight bacteria Xanthomonas oryzae pv oryzae ( Xoo ) and the blast fun-
gus Magnaporthe grisea , and further analysis demonstrated that ABA promotes
susceptibility to the two pathogens by suppressing SA-regulated defenses (Jiang
et al. 2010 ; Xu et al. 2013 ). Furthermore, ABA suppresses the induction of SAR
by inhibition of SA-dependent gene expression (Yasuda et al. 2008 ). Studies also
showed that ABA can suppress SA accumulation. For example, exogenous ABA
application can reduce SA content in P. syringae pv. Tomato ( Pst 1065 )-infected
leaves (Mohr and Cahill 2007 ). This finding was echoed by a subsequent study
where the increased resistance in the ABA-deficient aao3 mutant was accompa-
nied by a higher basal and induced SA level, coincident with the enhanced basal
and induced expression level of the SA biosynthetic gene ICS1 , implying that
ABA can suppress SA biosynthesis through downregulation of ICS1 (de Torres-
Zabala et al. 2009 ). In ABA-deficient sitiens tomato mutants, it was showed that
the enhanced resistance to B. cinerea was correlated with the elevated expression
of the SA biosynthesis gene PHENYLALANINE AMMONIA LYASE ( PA L ) and
increased sensitivity to BTH-induced PR1 expression, indicating that ABA can
promote susceptibility to B. cinerea in tomato by negatively modulating the SA-
dependent defense pathway (Audenaert et al. 2002 ). Together, these results dem-
onstrated that ABA appears to suppress the SA-dependent signaling mechanisms.
Interestingly, SA can both enhance and suppress the ABA-mediated signaling
pathway. For example, it was showed that SA can positively regulate ABA accu-
mulation—at least in the latter stages of infection, as reflected by the reduced
accumulation of endogenous ABA in the Pst DC3000 -infected sid2 - 1 mutants
(de Torres-Zabala et al. 2009 ), while the activation of the SA-mediated SAR can
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