Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 213

5 Possible Salicylic Acid Action Mechanisms
and Signalling
5.1 Action Mechanisms and Signalling at the Molecular
Level
5.1.1 SA in Relation To Oxidative Stress
It is a fact that most stress factors, whether biotic or abiotic, are usually associated
with the oxidative burst, leading to oxidative stress, a secondary phenomenon
which is a common component of several types of stress. The generation of ROS
causes rapid cell damage by triggering chain reactions. Cells have evolved a
complex system of enzymatic and non-enzymatic antioxidants to scavenge these
harmful molecules (Ahmad et al. 2010 ; Kocsy et al. 2011 ). In many cases
increased tolerance can be attributed to the efficiency with which the plant is
capable of neutralising ROS (Gill and Tuteja 2010 ). The rapidity of ROS pro-
duction and the ability of H 2 O 2 to freely diffuse across membranes suggested that
at low concentration ROS may play a central role in the complex signalling
network of cells under normal and stress conditions (Sharma et al. 2012 ). The level
of ROS is controlled both by production and by removal through various scav-
engers. SA may influence both of these processes, as being involved in the fine-
tuning of signalling pathways.
In most studies on the role of exogenous SA during abiotic stress, it was
demonstrated that the protective effect of SA treatment was accompanied by
transient oxidative stress, which induced the antioxidant defence system and the
synthesis of certain stabilising substances (see Sect. 2 ). Thus, the application of SA
can act as a hardening process, leading to a decrease in oxidative injury. Recent
results also show that while NaCl induces cell death mainly by ET-induced ROS
production, but ROS generated by SA was not controlled by ET in tomato cell
suspension (Poór et al. 2012b ).
As a stress acclimatisation process, SA treatment at a suitable concentration
may decrease the catalase activity and in turn increase the level of H 2 O 2 in several
plant species (Dat et al. 1998a ; Janda et al. 1999 ; Tas gin et al. 2006 ). Furthermore,
SA has been shown to bind to the catalase enzyme, resulting in the inhibition of its
activity (Chen et al. 1993 ; Conrath et al. 1995 ; Horváth et al. 2002 ). SA treatment
decreased the catalase activity in wheat (Tas gín et al. 2006 ) and in tomato (Szepesi
et al. 2005 ), but had no effect in Brassica napus L. (Haddadchi and Gerivani,
2009 ). The decrease in catalase activity in relation to an increase in SA content
was also investigated in rice, wheat and cucumber seedlings exposed to oxidative
stress (Shim et al. 2003 ). Although, SA initially decreased the catalase activity in
bermudagrass, its inhibitory effect disappeared 12 days after the treatment (Zhang
et al. 2009 ). In contrast, some authors reported on SA-induced increase in catalase
activity, for example in wheat (Agarwal et al. 2005a,b), maize (Ahmad et al. 2012 )