Agriculture Reference
In-Depth Information
Ornithine
Arginine
Arginine
decarboxylase
CO
2
Ornithine
decarboxylase
Agmatine
Agamatine
iminohydrolase
N
-Carbamoylputrescine
CO
2
Carbamoylputrescine
amidohydrolase
H
2
O
2
O
2
+
H
2
O
NAD
+
NADH
1
-Pyrroline
Δ
GABA
Putrescine
PDH
DAO
N
NH
3
N
SAM decarboxylase
Succinate
Succinat e
S
-adenosylmethionine
(SAM)
PAO
Decarboxylated SAM
Krebs
cycle
CO
2
N
-Acetyl
spermidine
Spermidine synthase
Methionine
cycle
SSAT
PAO
Methylthioadenosine
Spermidine
H
2
O
2
O
2
+
H
2
O
N
N
ACC
synthase
N
PAO
β
-Alanine
1,3-Diaminopropane
N
-Acetyl
spermine
Spermine synthase
1-Aminocyclopropane-1-
carboxylic acid (ACC)
Decarboxylated SAM
SSAT
ACC
oxidase
PAO
Spermine
N
N
H
2
O
2
O
2
+
H
2
O
N
N
Ethylene
1-(3-Aminopropyl)-pyroline
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 argi-
nine 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
(Han-
frey 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 addi-
tion of aminopropyl residues derived from decarboxylated
S
-adenosylmethionine (dcSAM)
(Mehta et al., 2002). dcSAM is synthesized from
S
-adenosylmethionine (SAM) by the cat-
alytic activity of SAM decarboxylase. Levels of both Spd and Spm increase in transgenic
tomatoes overexpressing SAM decarboxylase, demonstrating the availability of dcSAM as
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