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Fig. 6.1   Overlapping hori-
zons of different functions
of SA
of trans-cinnamic acid respectively in different species. However, under conditions
of inhibition of either of the two pathways in Arabidopsis , a third route of bio-
synthesis of SA via shikimik acid pathway-derived chorismate in chloroplast, is
by means of isochorismate synthase (ICS; Hayat et al. 2010 ). Several conjugated
forms of SA have been reported after it is glycosylated, esterified (Balke and Shulz
1987 ; Popova et al. 1997 ). The formation of β (beta)-glycosidic SA is catalyzed by
the enzyme SA glucosyl transferase (GTase; Balke and Schulz 1987 ; Yalpani et al.
1992 ). Other forms identified include 2,3-dihydroxybenzoic acid (2,3-DBHA) and
2,5-dihydroxybenzoic acid (2,5-DBHA; Fig. 6.2 ).
Volatile MeSA, after infection, in tobacco is produced from SA which on hydro-
lyzing to SA, can induce defense responses via inter/intra plant signaling (Shualev
et al. 1997 ). It has been found (Chen et al. 1993 ) that binding of SA to catalase in-
hibits its activity thereby aiding H 2 O 2 production, which activates defense signaling
and gene expression (Chen et al. 1993 ; Durner et al. 1997 ). H 2 O 2 , in synergy with
ROS, may dispose of the pathogen to death by its direct antibiotic activity providing
local acquired resistance (LAR).
Though plants do have prerequisite innate immunity to withstand a biotic chal-
lenge, the activation and amplification of inducible form of defense mechanism is
more economically augmented during invasion. Mobile signals prepare distal tis-
sues for a possible future attack to induce de novo expression of primary defense
mechanism. Recent focus has been on acquisition of primed expression of defense
through long distance signaling in plants (Lam et al. 2001 ). Physical travelling of
signals since the work of Ross ( 1961a , b ) on tobacco (TMV infected) and (Green and
Ryan 1972 ) on tomato (insect infestation) till now has well established the concept.
However, volatile nature of MeSA and MeJA to prime such responses followed by
synergy of vascular transported signal is later suggested by Lam et al. ( 2001 ).
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