Salicylic Acid Biosynthesis and Role in Modulating Terpenoid and Flavonoid Metabolism in Plant Responses to Abiotic Stress - Salicylic Acid: Plant Growth and Development - page 142

Fig. 1 Biosynthesis of salicylic acid in Arabidopsis thaliana. Key enzymes are shown in red
(AAO Arabidopsis aldehyde oxidase; BZL benzoyl-CoA ligase; BA2H benzoic acid 2-
hydroxylase; 4CL 4-coumaroyl:CoA ligase; CM chorismate mutase; ICS isochorismate synthase;
IPL isochorismate pyruvate lyase; PAL phenylalanine ammonia lyase). Enzymes the identity of
which has not been identified so far are marked with a question tag. Single arrows indicate
enzymatic reactions; green arrows indicate reactions (or sets of reactions) leading to other
pathways competing for the same substrate. Tags in bold letters indicate subsets of reactions.
Adapted from Dempsey et al. ( 2011 )
inoculated tobacco synthesized [ 14 C]SA after being fed with [ 14 C]BA or [ 14 C]CA
(Yalpani et al. 1993 ) via the activation of a 2-benzoate hydroxylase (León et al.
1993 ). The formation of [ 14 C]BA from [ 14 C]Phe through [ 14 C]CA was also shown
in Tsuga canadiensis (Zenk and Muller 1964 ), in young Glautheria procumbens
tissue (Ellis and Amrhein 1971 ), and in uninfected tomato seedlings (Chadha and
Brown 1974 ). In addition, the direct conversion of [ 14 C]BA to [ 14 C]SA was also
reported in etiolated Helianthus annuus hypocotyls, Solanum tuberosum tubers,
and Pisum sativum internodes (Klämbt 1962 ). Labeled [ 14 C]SA in infected
cucumber plants was found not only after feeding [ 14 C]BA but also after using
[ 14 C]Phe as precursor (Meuwly et al. 1995 ). The enzyme that catalyzes the
transformation of CA to BA has been identified in Quercus pedunculata (Alibert
et al. 1972 ). The third route for the biosynthesis of SA (Route 2b) involves a 2-
hydroxylation of CA to O-coumaric acid which is then decarboxylated to SA and
the reaction is catalyzed by the enzyme trans-cinnamate-4-hydroxylase (Alibert
and Ranjeva 1972 ), which was first detected in pea seedlings (Russell and Conn
1967 ). This enzyme was also identified in Quercus pedunculata (Alibert and
Ranjeva 1972 ) and in Melilotus alba (Gestetner and Conn 1974 ). In support of the
O-coumaric acid pathway, leaves of G. procumbens or Primula acaulis accumu-
lated both labeled O-coumaric acid and SA after feeding with [ 14 C]CA or Phe
(Grisebach and Vollmer 1963 ; El-Basyouni et al. 1964 ). The growth conditions
could favour one of the aforesaid SA biosynthesis routes in the plant. It was