ABA Biosynthetic and Catabolic Pathways - Abscisic Acid: Metabolism, Transport and Signaling - page 24

Bcaba2 and Bcaba4 . The authors speculated that the Bcaba2 and Bcaba4 mutants
likely accumulated ABA precursors that could not be distinguished from ABA by
their immunological ABA detection system (Siewers et al. 2006 ). Biochemical
analysis of these proteins or feeding experiments combining several known pre-
cursors with these mutants will be powerful tools to investigate the direct pathway
in detail.
2.3 Indirect Pathway in Higher Plants
ABA was discovered in the 1960s (Addicott et al. 1968 ; Ohkuma et al. 1965 ;
Schwartz and Zeevaart 2010 ) and subsequently xanthoxin (Xan) was isolated as
a plant growth inhibitor like ABA (Taylor and Burden 1970b ; Taylor and Smith
1967 ). Xan had structural similarity to part of an epoxycarotenoid (Fig. 2.2 ).
Indeed, the cleavage of all- trans - or 9- cis -epoxycarotenoids by light or lipoxyge-
nase successfully generated 2- trans ,4- trans - or 2- cis, 4- tran s-Xan (Firn and Friend
1972 ; Taylor and Burden 1970a ). A conversion experiment from Xan to ABA
using cell-free extracts from various plant species indicated that 2- cis, 4- tran s-Xan
was the possible precursor of ABA that originates from 9- cis -epoxycarotenoids
(Sindhu and Walton 1987 ). In addition, most carotenoid deficient mutants were
also ABA deficient (Moore and Smith 1985 ; Neill et al. 1986 ). These facts sug-
gested that epoxycarotenoids could be the precursors of ABA in higher plants. On
the other hand, terpenoids such as carotenoids, plastoquinone, sterol, and phytol
are synthesized from IPP produced in the MEP pathway in plastids (Lichtenthaler
1999 ). The Arabidopsis chloroplasts altered 1 ( cla1 ) mutant has a defect in the
synthesis of 1-deoxy-D-xylulose-5-phosphate in the MEP pathway and presents
carotenoid deficiency resulting in decreased levels of ABA (Estevez et al. 2001 ).
These facts support that C15 ABA is synthesized via C40 epoxycarotenoids that
are composed of IPP from the MEP pathway in plants. Therefore, this ABA bio-
synthetic pathway is referred to as the indirect pathway since C40 epoxycarot-
enoids are the intermediates of ABA in contrast to the direct pathway in fungi
(Figs. 2.1 and 2.2 ). The indirect pathway has been revealed by biochemical and
molecular genetic studies of ABA-deficient mutants as described below. Mutants
impaired in carotenoid biosynthesis or molybdenum cofactor (MoCo) synthesis
present ABA deficiency as part of a pleiotropic phenotype. We will not deal with
these biosynthetic pathways or their corresponding mutants in this chapter since
they are not specifically involved in ABA biosynthesis. The epoxidation steps of
zeaxanthin are set as the starting point of the indirect pathway.
Zeaxanthin epoxidase (ZEP) converts zeaxanthin into violaxanthin via anthe-
raxanthin by a two-step epoxidation (Fig. 2.2 ). Zeaxanthin can be also produced
from violaxanthin by violaxanthin de-epoxidase (VDE) (Fig. 2.2 ). This cyclic
reaction is called the xanthophyll cycle and is involved in nonphotochemi-
cal quenching for photoprotection (Li et al. 2009 ). The ZEP gene was first iden-
tified in the study of a tobacco ABA-deficient mutant, Npaba2 , in Nicotiana