Signaling Role of Salicylic Acid in Abiotic Stress Responses in Plants - Salicylic Acid: Plant Growth and Development - page 249

environmental constraints and biotic interactions, both pathogenic and symbiotic,
has increased considerably in recent years (Marino et al. 2012 ).
Early reports from the Doke's team also provided temporal profile of oxidative
burst after inoculation with an avirulent isolate of P. infestans onto potato leaves,
which was biphasic oxidative bursts, consisted of earlier phase peaking at initial
3 h followed by a massive continuous phase of ROS production (Chai and Doke
1987a ). In the same year, the data correlating the systemic induction of resistance
to P. infestans and the systemic regulation of ROS, i.e. activation of both the pro-
oxidative (O •- generating reaction) and anti-oxidative mechanisms involving
superoxide dismutase (SOD) and peroxidases (POXs) in potato plants were
reported (Chai and Doke 1987b , c ). The oxidative burst induced by pathogen-
derived signals may play a key role in development of systemic immunity known
as systemic acquired resistance (SAR) by activation of defense responses
throughout the plants (Fobert and Després 2005 ). To date, multiple roles of ROS
have been proposed in direct microbicidal actions, strengthening of cell wall
through oxidative cross-linking of glycoproteins, induction of intracellular sig-
naling pathway such as the synthesis of SA and activation of mitogen-activated
protein kinase (MAPK) cascade, or activation of SAR associated with systemic
propagation of the oxidative burst (Yoshioka et al. 2008 ; Swanson et al. 2011 ).
Nowadays, a number of teams working on plant ROS biology are distributed
worldwide and their studies concern numerous aspects of the plant physiology
throughout the plants' life cycle (Yoshioka et al. 2008 ). ROS production is actually
recognized as common denominator not only to biotic stress but also abiotic
environmental stressful conditions such as high salinity, drought, high intensity
light and low or high temperature stresses that cause major crop losses worldwide.
ROS are in fact, inevitably produced as by-products from a consequence of normal
metabolic reactions including mitochondrial respiration, photosynthetic processes
and fatty acid metabolism (Møller 2001 ; Baker et al. 2006 ; Noctor et al. 2007 ). A
common property of all ROS types is that they can cause oxidative damage to
cellular components such as proteins, DNA, and membranes (Møller et al. 2007 ).
However, they have the potential to be beneficial to living organisms in addition to
their harmful action, depending on the conditions (Apel and Hirt 2004 ). The
specificity of the biological response of living plant cells to ROS depends on the
chemical identity of ROS, intensity of the signal, sites of production and devel-
opmental stages (Del Río et al. 2002 ).
Exposures to environmental stresses increase intra- and intercellular levels of
H 2 O 2 by modulating the finely elaborated ROS-detoxification and regeneration
networks, composed of ROS-producing enzymes, antioxidant enzymes, and bio-
synthetic pathways for low molecular antioxidants, all responsible for maintaining
the homeostasis of ROS levels under tight control (Yoshioka et al. 2008 ; Kawano
2003 ; Del Río et al. 2002 ; Kotchoni and Gachomo 2006 ; Bolwell et al. 2002 ). This
allows ROS to serve as signaling molecules in regulation of plant metabolism and
cellular signal transduction pathways activated in response to environmental
stresses (Gechev et al. 2006 ; Mittler et al. 2011 ). Accumulated pieces of evidence
suggested that hormonal signaling pathways leading to development of SAR are