Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 208

10-fold increase in IAA compared with the control. External damage also caused JA
accumulation, which in turn induced the accumulation of SA as a defence mecha-
nism (De Diego et al. 2012 ). ABA may indirectly affect SA signalling via its effect
on JA signalling and/or vice versa (Adie et al. 2007 ). In contrast, investigations
on ET- and JA-insensitive mutants showed that the suppressive effect of ABA on
SAR is regulated by unknown mechanisms, independent of the JA/ET-mediated
signalling pathway (Yasuda et al. 2008 ).
Not only are synergistic and antagonistic effects demonstrated between plant
hormones, but each also has an impact on the biosynthesis of the other. SA inhibits
JA biosynthesis and/or JA-dependent gene expression (Gupta et al. 2000 ). Con-
versely, JA inhibits the SA pathway in response to wounding (Niki et al. 1998 ).
It has been discovered that IAA promotes GA biosynthesis; on the other hand, GA 3
application enhances the catabolism of ABA (Javid et al. 2011 ).GA 3 stimulates SA
biosynthesis by inducing the SID2 gene. SA also induces genes encoding the
enzymes involved in GA 3 biosynthesis (Lee and Park 2010 ). In another study it
was found that ABA negatively regulated SA synthesis by the transcriptional
regulation of at least ICS (de Torres Zabala et al. 2009 ).
Based on these results, there is clearly a relationship between SA and other
plant hormones. SA is primarily antagonistic to JA/ET and to ABA signalling. JA/
ET and ABA, in turn, repress SA signalling, while SA and GA 3 are in close
positive correlation with each other.
4 Relationship Between Biotic and Abiotic Stress Factors
One of the earliest responses observed after pathogen attack is the oxidative burst,
which involves the generation of ROS (Torres 2010 ). The relationship between SA
and ROS is complicated and bidirectional (Vlot et al. 2009 ), but the role of SA in
signalling disease resistance is well documented, both by studies on SA-induced
pathogen resistance and by reports on altered endogenous SA levels after pathogen
attack. There is more and more evidence showing that biotic and abiotic signalling
pathways cross-communicate by each other (Fujita et al. 2006 ; Atkinson and
Urwin 2012 ), but it is not yet clear how they interact if plants are exposed
simultaneously to both biotic and abiotic stress (Mittler 2002 ).
Phytohormones have a central role in abiotic and biotic stress signalling. SA
is essential for the activation of defence responses against biotrophic and hemi-
biotrophic pathogens as well as for the establishment of systemic acquired resis-
tance. By contrast, JA and ET are usually associated with defence against
necrotrophic pathogens and herbivorous insects. However, in the last decade
evidence has accumulated to suggest that ABA is also involved in biotic stress
signalling, In Arabidopsis bacterial effectors rapidly activate ABA biosynthesis (de
Torres-Zabala et al. 2009 ). It is becoming increasingly clear that ET, JA and SA
are also involved in the response to abiotic stress (see Sect. 3 ).