Signaling Role of Salicylic Acid in Abiotic Stress Responses in Plants - Salicylic Acid: Plant Growth and Development - page 252

tobacco cells and leaves (Raz and Fluhr 1992 ), and carrot cell culture (Schneider-
Müller et al. 1994 ).
The first report connecting ROS and Ca 2+ in plant SA signaling (Kawano et al.
1998 ) reported the direct measurements of the SA-induced O •- and the SA-
induced increase in [Ca 2+ ] c in aequorin-expressing tobacco BY-2 cells. Addition
of SA to tobacco BY-2 cells reportedly resulted in rapid and transient generation of
ROS chiefly O •- (Kawano et al. 1998 ) and H 2 O 2 (Kawano and Muto 2000 ), and
also a transient increase in [Ca 2+ ] c (Kawano et al. 1998 ). There, ROS actively
triggers the influx of Ca 2+ into the cells and this early oxidative burst by SA was
shown to be an extracellular event catalyzed by extracellular free and cell wall-
bound POXs (Kawano 2003 ).
Action of SA mediated by both the cell wall POX-dependent ROS production
and Ca 2+ influx was also observed in the cell suspension-cultured Arabidopsis
thaliana (Kadono et al. 2010 ) and Vicia faba epidermis (Mori et al. 2001 ). Both
SA-induced O •- and chemically generated O •- were shown to induce the closure
of stomata, which is known as a Ca 2+ -dependently regulated event studied in
Commelina communis L. (Lee 1998 ), Vicia faba L. (Manthe et al. 1992 ; Mori et al.
2001 ) and Arabidopsis thaliana (Khokon et al. 2011 ).
The SA-induced stomatal closure in Vicia faba and Arabidopsis thaliana was
reportedly inhibited by pre-treatment with ROS scavengers such as CAT and SOD,
and an inhibitor of POX suggesting the involvement of POX-mediated production
of extracellular ROS during the action of SA leading to stomatal closure (Mori
et al. 2001 ; Khokon et al. 2011 ). It is noteworthy that pharmacological inhibition
of RBOHs nor mutations of atrbohD and atrbohF showed no inhibition on the SA-
induced stomatal closures (Khokon et al. 2011 ), suggesting that the POX-depen-
dent mechanism is solely responsible for the rapid stomatal regulation by SA. As
the stomata is the major path connecting the internal environment in the plants and
the aerial environment surrounding plants through gas exchange, it is tempting to
relate the perception of air quality by plants and the action of SA leading to
stomatal closure.
3 Plant Responses to Air Pollutants
3.1 Ozone (O 3 )-Induced Cell Death
O 3 produced by a complex series of photochemical reactions via primary precursor
emissions of nitrogen oxides and volatile organic compounds, is a major secondary
air pollutant often reaching high concentrations in urban areas under strong day-
light, and studies are now suggesting that a steep increase in global background
concentrations of O 3 is in progress and thus the impact of atmospheric O 3 to plants
including valuable crops might be severer in the future world (Ashmore 2005 ).
Despite of great efforts to identify the physiological and biochemical elements of