Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 184

(Rakwal et al. 2001 ). A large body of evidence indicates that SA is also required for
the development of systemic acquired resistance (SAR). The level of endogenous SA
increased in cucumber plants when acquired resistance developed (Métraux et al.
1990 ). Transgenic tobacco plants incapable of accumulating SA due to the presence
of a salicylate-hydroxylase enzyme gene (nahG) of bacterial origin were unable to
develop systemic acquired resistance (Gaffney et al. 1993 ). Nevertheless, SA does
not appear to be the signal molecule transported from the site of infection to more
distant tissues, though the accumulation of SA in the given tissues is essential if SAR
is to develop (Vernooij et al. 1994 ). Most of the SA synthesized in plants is glu-
cosylated and/or methylated. Glucose conjugation generally takes place at the
hydroxyl group of SA, resulting in the formation of SA glucoside [SA 2-O-b-D-
glucoside] (SAG), whereas the less frequent glucose conjugation at the SA carboxyl
group produces SA glucose ester. Both reactions are catalysed by cytosolic SA
glucosyl transferases (Lee and Raskin 1999 ). SAG is actively transported from the
cytosol into the vacuole, where it may function as an inactive storage form (Dean and
Mills 2004 ). SA is also converted to methyl salicylate (MeSA) by SA carboxyl
methyl transferase, and this volatile derivate is an important long-distance signal in
systemic acquired resistance (Shulaev et al. 1997 ).
There is an increasing body of evidence suggesting that SA is involved not only
in biotic stress, but also in abiotic stress, as the endogenous SA content changes
during abiotic stresses (see Chap. 3 ) and the protective effect of exogenously
applied SA has also been demonstrated (Horváth et al. 2007 ; Hayat et al. 2010 ).
Yang et al. ( 2004 ) divided plants into SA-sensitive and SA-insensitive species and
suggested that in SA-insensitive plants such as rice, although SA may play an
important role in modulating the redox balance and protecting plants from the
oxidative damage caused by various biotic and abiotic factors, it is unable to act as
an effective secondary signal for the activation of defence genes and induced
resistance. In the present chapter a summary will be given of the relationship
between SA and various abiotic stress factors in relation to biotic stress and other
plant hormones, followed by a summary of the known physiological and bio-
chemical effects of SA that may explain the change in stress tolerance.
2 Studies on the Role of Salicylic Acid During Abiotic
Stress
2.1 Toxic Metals
2.1.1 Effect of Toxic Metals
The term ''heavy metals'' is often used as a synonym of toxic metals, but other
metals may also be toxic and not all heavy metals are particularly toxic. The
definition may also include trace elements when they are present in abnormally