Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 210

different, but possibly overlapping, pathogen-defence pathways in tomato, which
may not necessarily involve ABA. Although, a high endogenous ABA level
suppressed the resistance of tomato to O. neolycopersici and B. cinerea, exogenous
application of ABA did not increase susceptibility to these pathogens.
In contrast, abiotic stress may also interact negatively with pathogen stress. An
increase in temperature may lead to a negative interaction by lowering resistance
to bacterial, viral, fungal and nematode pathogens: in wheat, higher mean tem-
peratures over a 6 year experimental period correlated with heightened
susceptibility to the fungus Cochliobolus sativus (Sharma et al. 2007 ). In tobacco
and Arabidopsis, the hypersensitive response (HR)- and R-gene-mediated defence
responses to Pseudomonas syringae and viral elicitors are compromised at high
temperatures, allowing these pathogens to multiply (Wang et al. 2009a ).
Certain gene products are crucial to both biotic and abiotic stress signalling, and
may therefore control the specificity of the response to multiple stresses (Atkinson
and Urwin 2012 ). Several transcription factors were found to be associated with
the control of both biotic and abiotic stress responses. For example, MYC2 is a
positive regulator of specifically JA-induced defence genes, but represses the
genes induced by combined JA/ET signalling. It also acts as a key repressor of the
SA pathway, and MYC2 has also been found to be activated by ABA. Therefore
MYC2 may act as a central regulator by which ABA controls biotic stress sig-
nalling pathways (Pieterse et al. 2009 ). The partial synergy between ABA and JA
signalling may explain how pathogenresistance can be enhanced by abiotic stress
(Atkinson and Urwin 2012 ), as found in barley, where abiotic stress increased
resistance to Blumeria graminis, and the resistance was in positive correlation with
the salt concentrations applied (Wiese et al. 2004 ).
Mitogen-activated protein kinase (MAPK) cascades are crucial in eukaryotes
for transducing the perception of environmental stimuli into internal signalling
pathways. A recent publication reviewed a list of MAPKs involved in abiotic
stresses signalling in various plant sources (Sinha et al. 2011 ). MAPK cascade-
mediated signalling is also an essential step in the establishment of resistance to
pathogens. The best-characterized MAPKs are MPK3, MPK4 and MPK6, all of
which are activated by various abiotic stresses, pathogens and also by oxidative
stress (Pitzschke et al. 2009 ). MPK4 and MPK6 were activated by cold, salt stress,
osmotic stress, touch and wounding (Ichimura et al. 2000 ), while Cd treatment
activated both MPK3 and MPK6 through the accumulation of ROS (Liu et al.
2010 ). mpk4 mutants were shown to constitutively express SA-dependent stress
genes, and this was found to be due to elevated levels of SA, as the dwarfed
phenotype of these mutants could be rescued by the expression of the bacterial
nahG gene (Petersen et al. 2000 ). MPK4 acts as a negative regulator of SA-
mediated defence against biotrophic pathogens, while it is an essential component
in the JA- and ET-mediated defence against necrotrophic pathogens (Brodersen
et al. 2006 ).
Another response common to biotic and abiotic stress is ROS (Fujita et al.
2006 ). ROS accumulation increases during abiotic stress conditions such as