Isoprenoid Biosynthesis in Fruits and Vegetables - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 284

GAP + Pyruvate

DXS

1-Deoxy- D -xylulose-5-P

DMAPP
+IPP

IPP (C 5 )

γ-Carotene

GGPP (C 20 )

2x

β-Carotene

PSY

Zeaxanthin

Phytoene (C 40 )

β -Cyclase

PDS

δ-Carotene

ζ-Carotene

α-Carotene

ζ CDS

ε -Cyclase

Lutein

Lycopene

Fig. 13.1 Plastid-localized DXP/isoprenoid pathway in plants. Some key enzymes (bold face) and interme-
diates involved in isoprenoid biosynthesis are abbreviated: -carotene desaturase;

DMAPP, dimethylallyl pyrophosphate; DXS, 1-deoxy- D -xylulose-5-phosphate synthase; GA-3P, glyceraldehyde-
3-phosphate; GGPP, geranylgeranyl pyrophosphate; IPP, isopentenyl pyrophosphate; PDS, phytoene desaturase;
PSY, phytoene synthase.

β -cyclase, ε -cyclase; ζ CDS, ζ

tissue (i.e., the leaves): chlorophyll accounts for the green color that masks the effects of
most other chromophores. The major role of the carotenoids in these tissues is to act as light
receptors and protectors of the photosynthetic apparatus.
The major carotenoids that accumulate in ripe tomato fruits are lycopene (

∼ 90%),

β 1%) of other carotenoids

(Ronen et al., 1999). Changes from immature green to firm red tomato fruit (4 days post-
breaker, d.p.b.) are characterized by a loss of chlorophyll and a 6.7-fold increase in total
carotenoids (Fraser et al., 1994). This is attributed to a 184-fold increase in lycopene and a
7.7-fold increase in

-carotene (5-10%), and lutein (1-5%), with trace amounts ( <

-carotene. The trend continues, and overripe fruits (14 d.p.b) show 12-
fold and 282-fold increases in total carotenoids and lycopene, respectively. These changes
in the carotenoid profile result from the net upregulation of lycopene biosynthetic genes
and downregulation of the cyclase genes that convert lycopene to other carotenoids.

β

13.5 Carotenoid biosynthesis

As previously mentioned, the ripening phase of tomato fruit is marked by the loss of
chlorophyll and the biosynthesis and accumulation of carotenoids, particularly lycopene
(C 40 ). As these changes occur within the plastid organelles, the green chloroplasts become
red chromoplasts. All plastid carotenoids are formed via the plastid-located isoprenoid
pathway (Lichtenthaler, 1999) (Figs 13.1 and 13.2). The basic C 5 isoprenoid units are
derived from common C 3 intermediates via another plastid-localized pathway: the 1-deoxy-
D -xylulose-5-phosphate (DXP) pathway (Rohmer, 1999). This pathway is distinguished
from the cytosolic pathway that generates C 5 isoprenoid units which is regulated by 3-
hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) (Bach et al., 1999; Newman
and Chappell, 1999). In plants, compartmentalization of the chloroplasts prevents mixing