Salicylic Acid Biosynthesis and Role in Modulating Terpenoid and Flavonoid Metabolism in Plant Responses to Abiotic Stress - Salicylic Acid: Plant Growth and Development - page 141

salinity (Rivas-San Vicente and Plasencia 2011 ; Peñuelas and Munné-Bosch 2005 ;
Hernández et al. 2009 ; Agati and Tattini 2010 ). Here, we will focus on the role of
SA in modulating the response of plants to abiotic stress, including salinity and
drought stress. Emphasis will be put on discussing the SA signalling pathways that
affect flavonoid and terpenoid metabolism.
2 SA Biosynthesis
SA is synthesized through two distinct and differentially compartmentalized
pathways that employ different precursors: the phenylpropanoid route starting in
the cytoplasm from phenylalanine (phenylalanine ammonia-lyase (PAL) pathway),
and the isochorismate (IC) pathway operative in the chloroplast of the cell. Both
pathways derive ultimately from chorismate, the end product of the shikimate
pathway. Although up to date neither of these biosynthetic routes have been
entirely defined, it is becoming clearer that both IC and PAL pathways contribute
to SA synthesis.
2.1 The PAL Pathway
It was often accepted that SA synthesis occurred via PAL pathway (Raskin 1992 ;
Lee et al. 1995 ; Coquoz et al. 1998 ). Early radiolabelling studies with phenylal-
anine (Phe), trans-cinnamic acid (t-CA), or benzoic acid (BA) suggested that SA is
synthesized from Phe via t-CA, which is then transformed to SA via two possible
intermediates: ortho-coumaric acid or BA, depending on the plant species and the
plant growth conditions (El-Basyouni et al. 1964 ; Chadha and Brown 1974 ).
Through the PAL pathway, plants can potentially develop three biosynthetic routes
to BA, including a b-oxidation route from cinnamoyl Co-A (Fig. 1 , route 2c-1), a
non-oxidative route from cinnamoyl Co-A (Fig. 1 , route 2c-2), and a non-oxida-
tive route from t-CA to BA (Route 2b; Wildermuth 2006 ). Radiolabelling studies
using Phe or putative pathway intermediates performed in TMV-infected tobacco
or cucumber detected incorporation of radiolabelled C onto BA and SA but not
benzaldehyde, suggesting that SA is synthesized by the cinnamoyl-CoA b-oxi-
dation route (Fig. 1 , route 2c-1; Ribnicky et al. 1998 ; Jarvis et al. 2000 ). However,
a study of BA production in developing seeds identified an Arabidopsis thaliana
aldehyde oxidase 4 (AAO4) that catalyzes the conversion of benzaldehyde to BA,
which is then incorporated into benzoyl glucosinolates (Ibdah et al. 2009 ). The
conversion of BA to SA has been suggested to take place via an inducible benzoic
acid 2-hydroxylase (BA2H). However, there has not been any report describing a
BA2H-encoding gene in plants (Ford et al. 2010 ). The first two biosynthetic routes
of SA (Route 2c-1 and Route 2c-2), have been reported in tobacco (Yalpani et al.
1993 ) and in rice (Silverman et al. 1995 ). For instance, healthy and virus-