Salicylic Acid Biosynthesis and Role in Modulating Terpenoid and Flavonoid Metabolism in Plant Responses to Abiotic Stress - Salicylic Acid: Plant Growth and Development - page 146

glucosyltransferases that are induced by SA application or pathogen attack in
tobacco and A. thaliana plants (Lee and Raskin 1998 ; Song 2006 ). The lack of
SAG in phloem exudates makes it an implausible candidate for the translocatable
form of SA. In addition to lowering cellular SA, SAG may function as a slow-
release storage form of SA that keeps SAR over extended periods of time (Dean
and Mills 2004 ; Dean et al. 2005 ). Another alternative is that the SAG formation is
the first step in SA catabolism. SA can also be converted to methylsalicylate
(MeSA) by SA carboxyl methyltransferase (Chen et al. 2003 ; Park et al. 2007a , b ;
Vlot et al. 2008 ). Methylation inactivates SA, while increasing its membrane
permeability, as well as its volatility, thus allowing more effective long distance
transport. It is hypothesized that volatile MeSA may function as an airborne signal
for both intra- and interplant communication (Shulaev et al. 1997 ). MeSA can be
further glucosylated to produce MeSA 2-O-b- D -glucose, but this SA-conjugated
form is not stored in the vacuole (Dean et al. 2005 ). Amino acid conjugation of SA
is less well characterized, but may be involved in SA catabolism. Additionally, SA
has been shown to be sulfonated in vitro by members of the SOT family of
sulphotransferases in A. thaliana (Baek et al. 2010 ). The characterization of
SOT12 revealed that it catalyzes the transfer of a sulphuryl group (SO 3 -1 ) to the 2-
OH position of SA in vitro (Baek et al. 2010 ). Still, sulphonated SA has not been
detected in planta so far.
3.2 Role of SA in the Response of Plants to Water and Salt
Stress
SA was initially regarded as an important signal molecule during plant-pathogen
interactions, a chemical defence against microbes, and an allelopathic compound
(Malamy and Klessig 1992 ; Raskin 1992 ). Since then, numerous new roles have
been proposed for SA, including mediation of the response of plants to a number
of abiotic stresses, such as heat stress (Dat et al. 2000 ; He et al. 2005 ; Larkindale
et al. 2005 ), chilling (Janda et al. 1999 ; Senaratna et al. 2000 ), salt and osmotic
stress (Borsani et al. 2001 ; Molina et al. 2002 ), and ozone and UV- light exposure
(Yalpani et al. 1994 ; Sharma et al. 1996 ; Rao and Davies 1999 ). It is now clear that
the role of SA during plant biotic and abiotic stress responses is coupled to the
production of ROS (Dat et al. 2000 ; Mittler 2002 ). These compounds can alter the
cellular redox homeostasis and lead to oxidative damage to lipids, proteins and
nucleic acids (Reviewed by Møller et al. 2007 ).
When applied exogenously at adequate (usually low) concentrations, SA was
found to improve the efficiency of the antioxidant system in plants (Knorzer et al.
1999 ). SA treatment was found to alleviate the oxidative stress generated by
paraquat in tobacco and cucumber (Strobel and Kuc 1995 ). SA was also found to
boost the activities of antioxidant enzymes, CAT, peroxidase (POX) and super-
oxide dismutase (SOD), when sprayed to drought-stressed tomato plants (Hayat