ABA Biosynthetic and Catabolic Pathways - Abscisic Acid: Metabolism, Transport and Signaling - page 22

2.1 ABA Biosynthesis—Two Distinct Routes
of ABA Biosynthesis
Abscisic acid (ABA) is a C15 sesquiterpene containing fifteen carbon atoms in its
structure that originates from isoprene known as isopentenyl pyrophosphate (IPP).
Now, ABA is known to be synthesized via two distinct pathways (Nambara and
Marion-Poll 2005 ; Oritani and Kiyota 2003 ; Schwartz and Zeevaart 2010 ). One is the
direct pathway that occurs in phytopathogenic fungi. The other is the indirect path-
way that operates in plants. IPP for the direct pathway is synthesized from the meva-
lonate (MVA) pathway that exists in prokaryotes and almost all eukaryotes (Newman
and Chappell 1999 ). On the other hand, the indirect pathway uses the methylerythri-
tol phosphate (MEP) pathway as a source of IPP. The MEP pathway seems to exist in
cyanobacteria and all photosynthetic eukaryotes (Lichtenthaler 1999 ).
2.2 Direct Pathway in Fungi
In phytopathogenic fungi such as Botrytis cinerea and Cercospora cruenta , ABA
is synthesized from MVA. When radiolabeled MVA or farnesyl diphosphate (FDP)
was fed to ABA-producing fungi, the labels were effectively incorporated into
ABA (Neill et al. 1982 ; Norman et al. 1983 ). In addition, when [1- 13 C] glucose
was fed to ABA-producing fungi or plants, the positions of the labeled carbons in
ABA were different between plants and fungi, because the labeled carbon was dif-
ferently incorporated into IPP depending on its source, the MVA or MEP pathway
(Hirai et al. 2000 ). These results strongly indicate that fungal ABA is synthesized
from IPP produced in the MVA pathway (Hirai et al. 2000 ). Since all intermedi-
ates between FDP and ABA are sesquiterpenes, the ABA biosynthetic pathway in
fungi has been referred to as the direct pathway (Nambara and Marion-Poll 2005 ;
Oritani and Kiyota 2003 ; Zeevaart and Creelman 1988 ).
The direct pathway involves several modifications of FDP to generate ABA
(Fig. 2.1 ). Isomers of ionylideneethanol and/or ionylideneacetic acid have been
identified from several fungi and are supposed to be the endogenous precursors
of ABA in fungi (Oritani and Kiyota 2003 ). As shown in Fig. 2.1 , similar inter-
mediates are utilized among diverse fungal genera, suggesting that the ABA
biosynthetic pathway might be mostly similar but distinct among these fungi.
Recently, allofarnesenes and ionylideneethanes were shown to be the endog-
enous precursors in Botrytis cinerea and Cercospora cruenta (Inomata et al.
2004a , b ). The allofarnesenes were supposed to be converted from FDP to allo-
farnesenes via 4,5-didehydrofarnesyl diphosphate or 6 E ,10 E -2,6,10-trimethyl-
2,6,10-dodecatriene (Inomata et al. 2004a , b ). Then, cyclization and isomerization
of allofarnesenes give ionylideneethanes (Fig. 2.1 ) (Inomata et al. 2004a , b ). In
addition, possible carotenoid precursors having a ʳ -ring to produce ionylideneace-
toaldehyde were not detected in C. cruenta (Inomata et al. 2004b ). These results