Diverse Activities of Carotenoid Cleavage Oxygenases - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 397

the chloroplasts (Iuchi et al. 2000). Disruptions or deletions of the VP14 N-terminus interfered with
thylakoid associations (Tan et al. 2001). Following cleavage of plastidal 9- cis -epoxy carotenoids to
xanthoxin, the C 15 apocarotenoid is transported to the cytosol where it is oxidized and reduced to
make ABA (Seo and Koshiba 2002, Auldridge et al. 2006). The transport mechanisms of xanthoxin
are unknown at this time.
The NCEDs represent a multigene family with spatial and temporal regulation (Chernys and
Zeevaart 2000, Iuchi et al. 2000, Lefebvre et al. 2006, Rodrigo et al. 2006, Kato et al. 2007).
Many plants contain multiple NCEDs and they are thought to act during different developmental
and growth phases. Of the nine CCO paralogs found in Arabidopsis , i ve are classii ed as NCEDs
and contain signal peptides that target them to the thylakoid membranes. NCED 2 and 3 from
A. thaliana are expressed in the roots, whereas NCEDs 6 and 9 have seed specii c expression (Tan
et al. 2003, Lefebvre et al. 2006). In Phaseolus vulgaris , NCEDs 1 and 3 are expressed during fruit
ripening and NCED1 is present in the leaves (Chernys and Zeevaart 2000). Expression of NCEDs
can be induced by water (Tan et al. 1997, Qin and Zeevaart 1999, Chernys and Zeevaart 2000,
Tan et al. 2003, Rodrigo et al. 2006) or salt stress (Iuchi et al. 2000). Overexpression of the NCED
enzymes in plants leads to an increase in ABA production and concurrent increase in drought toler-
ance (Thompson et al. 2000a,b, Qin and Zeevaart 2002).

19.3.1.2 CCDS
A second class of plant CCOs was discovered that had a higher afi nity for carotenes than xanthophylls.
These enzymes, named CCD1s, cleave cyclic carotenoids symmetrically at the 9,10- and 9

-double
bonds forming a C 14 dialdehyde and two volatile C 13 cyclohexone derivatives (e.g., b-ionone) (Figure
19.1). CCD1s are thought to play a role in carotenoid turnover; although the extent of this activity is not
well characterized (Simkin et al. 2004). These enzymes have been described from a number of differ-
ent sources (Table 19.2) (Schwartz et al. 2001, Simkin et al. 2004a,b, Cao et al. 2005). The b-ionone
cleavage product produced by CCD1s can rearrange and be modii ed to a number of different mol-
ecules producing many of the aromas associated with fruits and plants (Lefi ngwell 2003).
The substrate specii city of these enzymes is not stringent; for example, CCD1 from tomato was
also shown to cleave at the 9,10- and 9

′ ,10 ′

-positions of b-carotene, zeaxanthin, lutein, violaxan-
thin and neoxanthin all of which have different ionone ring modii cations. Unlike NCEDs, CCD1
enzymes have no plastid-targeting sequences and are localized in the cytosol. It is postulated that
they access the carotenoids in the plastids through a monotopic membrane association (Kloer et al.
2005). Similar to the NCEDs, the CCD1s have differential expression under different environmental
conditions. The transcripts of Ph CCD1 (Table 19.2) in leaves oscillate according to light levels and
circadian mechanisms (Simkin et al. 2004). Light exposure increases the transcript levels in leaves
and transcription is diurnally regulated directly by light. In corollas, the oscillation of transcript
levels is circadian in nature.
In addition to CCD1, there are three more CCO paralogs from Arabidopsis that preferentially
cleave carotenes over 9- cis -epoxycarotenoids. These enzymes are targeted to the plastid and have
been named CCD 4, 7, and 8. CCD7 is 9, 10 specii c and like CCD1 accepts a variety of carotenoid
substrates. However, unlike CCD1, this enzyme cleaves carotenoids asymmetrically: b-carotene is
cleaved by CCD7 at the 9,10-position to yield b-ionone and 10

′ ,10 ′

-apocarotenal. The C 27 product is
then cleaved by another CCD1 paralog, CCD8, into the C 18 compound 13-apo-b-carotenone and
a C 9 dialdehyde (Booker et al. 2004, Schwartz et al. 2004, Auldridge et al. 2006). The sequential
activities of CCD7 and CCD8 synthesize a plant hormone that regulates apical dominance. The bio-
active dialdehyde product has not been identii ed or characterized, but it is known to promote shoot
branching because a loss of CCD7 or CCD8 results in a bushy growth phenotype (Auldridge et al.
2006). CCD7 and CCD8 orthologs have been found in many plant genomes suggesting the hormone
has widespread importance in plant development. Another CCO paralog in the Arabidopsis genome
is named CCD4, which cleaves carotenoids when expressed in E. coli , but it is not well character-
ized at this time (see Table 19.2 for a list of identii ed CCDs).

′