Salicylic Acid Biosynthesis and Role in Modulating Terpenoid and Flavonoid Metabolism in Plant Responses to Abiotic Stress - Salicylic Acid: Plant Growth and Development - page 143

suggested, for example, that tomato seedlings infected with A. tumefaciens syn-
thesized SA via O-coumaric acid whereas the BA- involving routes (Route 2c-1
and Route 2c-2) operated in noninfected plants (Chadha and Brown 1974 ). Using
pathogen-inoculated tobacco and cucumber, elicitor-treated potato, or healthy rice
seedlings, radiolabelling studies suggested that SA was synthesized from Phe via
BA (Yalpani et al. 1993 ; Silverman et al. 1995 ; Coquoz et al. 1998 ). Further
support to the PAL role in SA biosynthesis came from the combined findings that
tobacco and A. thaliana resisting pathogen infection showed increases in PAL
expression (Mauch-Mani and Slusarenko 1996 ) and endogenous SA levels
(Dempsey et al. 1999 ). Moreover, loss of PAL activity, due to sense-suppression or
treatment with the PAL inhibitor 2-aminoindan-2-phosphonic acid (AIP), reduced
pathogen-induced SA accumulation in tobacco, cucumber, and A. thaliana (Me-
uwly et al. 1995 ; Pallas et al. 1996 ).
2.2 The IC Pathway
Genetic studies in A. thaliana showed that SA was also formed when the SA
biosynthetic routes mentioned before were either inhibited or the specific activity
of radiolabeled SA in feeding tests was lower than expected. In this additional
pathway (the IC pathway), SA is synthesized from chorismate by means of is-
ochorismate synthase (ICS) in chloroplasts. The SA synthesized by this pathway is
responsible for providing local and SAR in plants (Wildermuth et al. 2001 ) and
was also shown to be implicated in several physiological processes including
modulation of flowering time (Martínez et al. 2004 ) and leaf senescence (Abreu
and Munné-Bosch 2008 ). Furthermore, the importance of this pathway for SA
synthesis has been explored in response to biotic and abiotic stresses. The IC
pathway was found to be the major route for SA synthesis in both basal and
induced thermotolerance in A. thaliana, although it should be kept in mind that the
PAL pathway was also operational in this study (Garcion et al. 2008 ).
The IC pathway for SA biosynthesis was firstly studied in bacteria. In fact
several bacterial genera have been shown to synthesize SA, which is employed in
the production of iron-chelating siderophores (Garcion and Métraux 2006 ). In the
bacterial pathway, chorismate is converted to SA through IC (Verberne et al.
1999 ). In some bacterial species a unifunctional enzyme, ICS, isomerizes
chorismate to IC which is then transformed to SA and pyruvate by another uni-
functional enzyme, isochorismate pyruvate lyase (IPL; Serino et al. 1995 ; Mer-
cado-Blanco et al. 2001 ). However, the SA synthesis in Yersinia enterocolitica and
Micobacterium tuberculosis is mediated by a sole, bifunctional enzyme termed SA
synthase (SAS). This enzyme directly converts chorismate to SA via an IC
intermediate (Kerbarh et al. 2005 ; Harrison et al. 2006 ). The ICS and SAS
enzymes are structurally very similar and contain preserved active sites (Parsons
et al. 2008 ).