Agriculture Reference
In-Depth Information
O
Urea
Urease
H
2
N
C
NH
2
CO
2
H
2
O
NH
3
NH
2
H
2
N
C
NH
(CH
2
)
3
CH
COOH
H
2
N
(CH
2
)
3
CH
COOH
Arginine
Arginase
N
Ornitine
N
ADC
NH
2
NH
NH
2
NH
2
N
ODC
CO
2
N
N
N
C
NH
(CH
2
)
4
H
2
N
NH
2
CH
CO
2
N
N
Agamatine
H
2
O
NH
(CHOH)
2
O
CH
+
AIH
H
2
O
CO
2
NH
3
CH
NH
3
O
(CHOH)
2
H
2
N
C
NH
(CH
2
)
4
NH
2
H
2
N
(CH
2
)
4
NH
2
NCP AH
CH
NCP
2
Putreacine
CH
O
Spermidineaynthase
AdoMetDC
+
S
CH
CH
2
3
Spermicline
H
2
N
(CH
2
)
4
NH
(CH
2
)
3
NH
2
+
(CH
2
)
2
S
CH
3
Spermineaynthase
HC
NH
2
(CH
2
)
3
NH
2
COOH
H
2
N
(CH
2
)
3
NH
(CH
2
)
4
NH
(CH
2
)
3
NH
2
Spermine
AdoMet
DCAdoMet
Fig. 5.1
PAs biosynthetic pathways from arginine and ornithine.
ODC
ornithine decarboxylase,
ADC
arginine decarboxylase,
DC AdoMet
decarboxylated AdoMet,
AdoMet DC
AdoMet decar-
boxylase,
NCP
N- carbamoyl putrescine,
NCP AH
N- carbamoyl putrescine amidohydrolase
from endosymbiotic gene transfer between the cyanobacterium precursor of chlo-
roplasts and the eukaryotic nucleus (Illingworth et al.
2000
). In animals, fungi, and
also in most plants, Put is synthesized directly from ornithine via the cytosolic or-
nithine decarboxylase (ODC; EC 4.1.1.17). Evolutionary compartmentalization of
Put biosynthesis in chloroplasts is accomplished by ADC signal sequences that im-
port this enzyme into the plastid (Borrell et al.
1995
; Illingworth et al.
2000
). Both
ODC and ADC enzymes use pyridoxal 5′-phosphate as cofactor.
The higher PAs, Spd and Spm are synthesized from Put through the succes-
sive activities of Spd synthase (SPDS; EC 2.5.1.16) and Spm synthase (SPMS;
EC 2.5.1.22) through the addition of aminopropyl groups. In addition, tSpm is also
synthesized from Spd (an asymmetric molecule that allows the formation of two
isomers, Spm or tSpm, respectively), through the activity of a thermospermine
synthase (tSPMS; Knott et al.
2007
). The aminopropyl moiety is derived from
methionine, which is first converted into S-adenosylmethionine (SAM) and then
decarboxylated via S-adenosylmethionine decarboxylase (SAMDC; EC 4.1.1.50).
SAMDC is considered the mayor regulatory enzyme involved in higher PA bio-
synthesis and plays an essential role in modulating ethylene production in plants,
since the precursor of ethylene 1-aminocyclopropane-1-carboxylic acid is also de-
rived from SAM (Bagni and Tassoni
2001
). However, Del Duca et al. (
1995
) and
Tassoni et al. (
2000
) provided data showing the occurrence of a back-conversion
pathway: Spd added to
Helianthus tuberosus
chloroplasts and
Arabidopsis
plants,
