492 Part V: Fruits, Vegetables, and Cereals
accelerating the respiratory climacteric and time re-
quired for ripening, in a concentration-dependent
manner. Nonclimacteric fruits show increased respi-
ration in response to increasing ethylene concen-
tration without accelerating the time required for
ripening.
Ethylene is biosynthesized through a common
pathway that uses the amino acid methionine as the
precursor (Yang 1981, Fluhr and Mattoo 1996; Fig.
21.1). The first reaction of the pathway involves the
conversion of methionine to S-adenosyl methionine
(SAM) mediated by the enzyme methionine adeno-
syl transferase. SAM is further converted into 1-
aminocyclopropane-1-carboxylic acid (ACC) by the
enzyme ACC synthase. The sulphur moiety of
methylthioribose generated during this reaction is
recycled back to methionine by the action of a num-
ber of enzymes. ACC is the immediate precursor of
ethylene and is acted upon by ACC oxidase to gen-
erate ethylene. ACC synthase and ACC oxidase are
the key control points in the biosynthesis of ethyl-
ene. ACC synthase is a soluble enzyme located in
the cytoplasm, with a relative molecular mass of 50
kDa. ACC oxidase is found to be associated with the
vacuolar or mitochondrial membrane. Using molec-
ular biology tools, a cDNA (complementary DNA
representing the coding sequences of a gene) for
ACC oxidase was isolated from tomato (Hamilton et
al. 1991) and is found to encode a protein with a rel-
ative molecular mass of 35 kDa. There are several
isoforms of ACC synthase. These are differentially
expressed in response to wounding, other stress fac-
tors, and the initiation of ripening. ACC oxidase
reaction requires Fe^2 , ascorbate, and oxygen.
Regulation of the activities of ACC synthase and
ACC oxidase is extremely important for the preser-
vation of the shelf life and quality in fruits. Inhibition
of the ACC synthase and ACC oxidase gene expres-
sion by the introduction of their respective antisense
cDNAs resulted in delayed ripening and better
preservation of the quality of tomato (Hamilton et al.
1990, Oeller et al. 1991) and apple (Hrazdina et.
al. 2000) fruits. ACC synthase, which is the rate-
limiting enzyme of the pathway, requires pyridoxal-5-
phosphate as a cofactor and is inhibited by pyridoxal
phosphate inhibitors such as aminoethoxyvinylgly-
cine (AVG) and aminooxy acetic acid (AOA). Field
application of AVG as a growth regulator (Retain,Figure 21.1.Ethylene biosynthetic pathway in plants. Key: ACC synthase, aminocyclopropane carboxylic acid syn-
thase; ACC oxidase, aminocyclopropane carboxylic acid oxidase.