Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

320 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Ornithine
Arginine
CO 2
Arginine
decarboxylase
Ornithine
decarboxylase
CO 2
O 2 +H 2 O
Agmatine
Agamatine
iminohydrolase
N-Carbamoylputrescine
Carbamoylputrescine
amidohydrolase
NADH NAD + H 2 O 2
GABA
Δ 1 -Pyrroline
PDH
DAO Putrescine
NH N
3
N
SAM decarboxylase
Succinatee
PAO
Decarboxylated SAM
Krebs
CO
cycle
N-Acetyl Spermidine synthase
2
spermidine
β-Alanine
H 2 O 2
H 2 O 2
PAO
1,3-Diaminopropane
PAO
SSAT
O 2 +H 2 O
1-(3-Aminopropyl)-pyroline
PAO
N-Acetyl
spermine
SSAT
O 2 +H 2 O
N
N
Spermidine
Spermine synthase
Spermine
N
N
Methionine
cycle
Methylthioadenosine
N
Decarboxylated SAM
N
N
S-adenosylmethionine
(SAM)
ACC
synthase
1-Aminocyclopropane-1-
carboxylic acid (ACC)
ACC
oxidase
Ethylene
1,5-Diazabicylononane
Fig. 15.1 Biosynthetic pathway of PAs: SAM is a common precursor of both PA and ethylene biosynthesis.
Methionine cycle allows recycling of the carbon skeleton for biosynthesis of both PAs and ethylene, thereby
preventing Met or SAM from becoming limiting under normal conditions. DAO, diamine oxidase; PAO, polyamine
oxidase; PDH, pyrroline dehydrogenase; SSAT, spermidine/spermine acetyl transferase; GABA, γ -aminobutyric
acid.
Gemperlova et al., 2006). As shown in Fig. 15.1, Put is synthesized in plants through
either decarboxylation of ornithine by ornithine decarboxylase or decarboxylation of arginine
to agmatine, which is subsequently converted to Put. The latter pathway is mainly
present in bacteria and plants (Martin-Tanguy, 2001). In plants, agmatine is first converted
into N-carbamoyl-Put by agamatine iminohydrolase, and subsequently N-carbamoyl-Put
amidohydrolase converts N-carbamoyl-Put to Put (Mayer and Michael, 2003).
Ornithine-dependent Put production has been reported to be absent in Arabidopsis (Hanfrey
et al., 2001), but present in other plants including tomato, tobacco, rice, and apples
suggesting species-specific variation in PA production pathways. Put is converted to Spd
by Spd synthase, and Spd is converted to Spm by Spm synthase, through sequential addition
of aminopropyl residues derived from decarboxylated S-adenosylmethionine (dcSAM)
(Mehta et al., 2002). dcSAM is synthesized from S-adenosylmethionine (SAM) by the catalytic
activity of SAM decarboxylase. Levels of both Spd and Spm increase in transgenic
tomatoes overexpressing SAM decarboxylase, demonstrating the availability of dcSAM as