Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 203

SA biosynthesis has also been investigated during ozone fumigation. The Cvi-0
Arabidopsis genotype, which accumulates SA, is ozone-sensitive, since the large
quantity of SA induces oxidative processes during ozone stress, leading to cell
death similar to that caused by the hypersensitive reaction. In NahG plants,
however, which are incapable of accumulating SA, the lack of a satisfactory
antioxidant response led to increased ozone sensitivity (Rao and Davis 1999 ).
In the Cvi-0:NahG genotype the lack of SA reduced the level of ozone-induced
cell death. Exposure of Arabidopsis to O 3 enhanced the accumulation of SA and
the activity of isochorismate synthase (ICS) but did not affect PAL activity. In sid2
mutants, which have a defect in ICS1, the level of SA and the activity of ICS did
not increase in response to O 3 exposure. These results suggest that SA is mainly
synthesized from isochorismate in Arabidopsis. Furthermore, the level of ICS1
expression and the activity of ICS during O 3 exposure were elevated in plants
deficient for SA signalling (npr1 and eds5 mutants and NahG transgenics) (Ogawa
et al. 2007 ). When C 14 -labelled BA was applied to ozone-exposed tobacco leaves,
it was efficiently metabolized to SA (Ogawa et al. 2005 ). However, there was no
increase in the activity or mRNA level of ICS. In contrast, the activity of this
enzyme was increased in ozone-exposed Arabidopsis. These results suggest that
SA is synthesized via BA from phenylalanine in ozone-exposed tobacco leaves but
via isochorismate in Arabidopsis. Ozone increased the activity of phenylalanine
ammonia-lyase and the transcript levels of the chorismate mutase and phenylal-
anine ammonialyase genes in wild-type tobacco.
2.6.2 Ultraviolet Radiation
The depletion of the stratospheric ozone layer may result in an increase in the level
of potentially harmful ultraviolet (UV) radiation reaching the surface of the earth.
UV radiation is traditionally divided into UV-A (320-400 nm), UV-B
(280-320 nm) and UV-C (200-280 nm) wavelength ranges, which have increas-
ing levels of energy and harmful effects. Plants, which use direct sunlight for
photosynthesis, are unable to avoid UV radiation, so mechanisms which may
protect them from the harmful effects of UV radiation are of particular interest
(Hollósy 2002 )
It has been shown that the foliar application of SA at 150 mg m -2 may alleviate
UV-B damage by upregulating plant defence systems in the grass species Poa
pretensis L. (Ervin et al. 2004 ). Endogenous a-tocopherol, SOD and CAT were
reduced by UV-B stress. The anthocyanin content increased in the first 5 days then
decreased during continuous UV-B irradiation. The application of SA enhanced
the photochemical efficiency after UV-B treatment. In addition, the application of
SA increased the a-tocopherol concentration, the SOD and CAT activity and the
anthocyanin content compared to the control 10 days after UV-B initiation,
leading to improved plant growth under UV-B stress. These results suggest that the
application of SA may alleviate the decline in photochemical efficiency and turf
quality associated with increased UV-B light levels during the summer. Exogenous