Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 195

contrast, increased SOD and CAT activities were observed in Kentucky blue grass
during heat stress after SA application (He et al. 2005 ). The efficacy of heat
acclimation and SA, applied as a 0.1 mM foliar spray, in inducing thermotolerance
was also tested in Cicer arietinum L. plants (Chakraborty and Tongden 2005 ).
Both SA treatment and heat-acclimation resulted in an increase in the protein and
proline contents over the control seedlings, and led to the induction of POD and
APX, while there was a reduction in CAT activity.
Other studies investigate the effect of a lack of SA in transgenic plants or in
plants with a mutation affecting SA biosynthesis. NahG transgenic plants carry a
salicylate-hydroxylase gene of bacterial (Pseudomonas putida) origin, which
prevents them from accumulating SA, as the salicylate-hydroxylase enzyme
converts SA into catechol (Gaffney et al. 1993 ). It was found that after heat stress
NahG Arabidopsis plants became more sensitive to the oxidative damage caused
by high temperature than non-transformed plants with a normal level of SA.
Furthermore, exogenous SA pre-treatment for 1 h enhanced the survival of Ara-
bidopsis plants after heat treatment and reduced the levels of thiobarbituric acid-
reactive substances (TBARS), the indicator of oxidative damage to membranes,
(Larkindale and Knight 2002 ). However, it has also been shown that catechol, the
product of SA degradation in NahG plants, induces susceptibility to pathogens in
wild-type Arabidopsis plants, most probably due to catechol-mediated H 2 O 2
production (van Wees and Glazebrook 2003 ). When evaluating the experiments it
is thus important to check, that the observed changes were not caused by the
catechol produced during the decomposition of SA.
Later it was found that SA pretreatment at 10 or 20 lM for 2 h prior to heat shock
was just as effective in imparting thermoprotection at the seedling stage in Brassica
species as heat acclimation for 3 h at sublethal temperature. SA pretreatment helped
the seedlings to recover from heat stress by increasing total soluble sugars and fresh/
dry weight and by stimulating the activity of the enzymes invertase, CAT and POD.
Furthermore SDS-PAGE revealed the enhanced expression of new proteins
including heat shock proteins (HSPs) after both pre-treatments (Kaur et al. 2009 ).
In a recent study it was found that spraying 100 lM SA onto the leaves of grapevine
did not affect the level of HSP21before heat stress, but the HSP21 immune signal
increased in both SA-treated and control plants during heat stress. During the
recovery period, the HSP21 levels remained high until the end of the experiment in
the SA-treated leaves, but decreased in the control (Wang et al. 2011 ).
2.4.3 Involvement of Endogenous Salicylic Acid in Heat Tolerance
The role of SA in the signal transduction process involved in heat tolerance
development is also confirmed by the increase in the level of endogenous bound
and free SA during the heat acclimatisation of mustard plants (Dat et al. 1998b ).
The endogenous glucosylated SA content was enhanced in the shoots of plants
grown on 0.01 or 0.1 mM SA, while free SA was also enhanced in those grown on
0.1 mM SA (Dat et al. 2000 ). In heat-treated grape plants, both the endogenous SA