Biology Reference
In-Depth Information
overall quality and taste. In addition, for process-
ing tomatoes, the raw tomatoes used for red paste
must meet a certain grade of redness in order
to be accepted by processing plants. Following
this line of thought, a better understanding of
the genetic factors affecting lycopene accumu-
lation in tomato will allow for more successful
traditional breeding and metabolic engineering
of this important crop, with the eventual goals
of increased economic benefit for breeders and
growers (by elevating demand, not necessarily
price) as well as potentially increased nutritional
benefit to consumers.
Due to the nascent status of omics-based
research to identify novel carotenoid regulatory
targets in tomato and the relative ease of selecting
tomatoes of the desired color, commercial breed-
ers continue to employ a series of mutations iden-
tified in the mid-20
th
century in order to mod-
ulate carotenoid type and accumulation level.
Several naturally occurring tomato mutations in
structural enzymes of the carotenoid biosynthetic
pathway have been observed, reported in the lit-
erature, and are actively employed for breeding
purposes. Of these,
r
(null mutation in
PSY
),
β (
LCY-B
not down-regulated in response to
ripening),
Delta
(lycopene-
the elucidation of how carotenoid accumulation
is regulated in tomato fruit and the harness-
ing of such knowledge for breeding purposes
(Fraser and Bramley 2004). Unlike most plants,
tomato hyper-accumulates lycopene during fruit
ripening by converting chloroplasts into chro-
moplasts, and has evolved a specialized sys-
tem to specifically produce this compound in
comparatively large amounts. Intuitively, this
would require a constant supply of metabolic
precursors, a consistently high amount of active
biosynthetic machinery, and a tightly controlled
metabolic flux specifically favoring carotenoid
biosynthesis over other competing pathways.
Thus, there are two main questions that have yet
to be answered in this regard: What is the spe-
cific regulatory mechanism utilized by tomato to
convert fruit from a photosynthetic tissue into a
highly specialized carotenoid factory? How does
this mechanism differ between low-lycopene
S.
lycopersicum
varieties and high-lycopene wild
tomato accessions, such as some
S. pimpinelli-
folium
accessions, assuming carotenoid biosyn-
thetic genes are functioning normally in both
cases?
It is generally hypothesized that lycopene
accumulation in ripe tomato fruit is influenced
by the action of many regulatory genes (the spe-
cific natures of most are unknown), and in most
segregating populations, variation in lycopene
content is continuous, which indicates quanti-
tative inheritance of the trait. Also of note, it
has been observed that environmental condi-
tions, as well as fruit size (which has been nega-
tively correlated with lycopene content), highly
influence the final concentration of lycopene in
the fruit. So, intuitively, one would conclude
that lycopene accumulation in tomato fruit must
depend not only on the action of biosynthetic
genes, which have been the major focus of basic
researchers until recently, but also on as-of-yet
unidentified regulatory interactions (which may
be involved in metabolic flux, transcriptional and
post-transcriptional regulation, response to light,
or a combination of these), and indirect mecha-
nisms present in the plant and the environment
-cyclase not down-
regulated in response to ripening),
tangerine
(
t
;
mutation in
CRTISO
) and
old-gold
(
og
)or
crim-
son
(
og
c
; mutation in
LCY-B
), are most signif-
icant, owing to their drastic fruit phenotypes
(Ronen et al. 1999; Ronen et al. 2000; Liu et al.
2003). Among these,
og
/
og
c
is the only mutant
that results in increased lycopene accumulation,
albeit at the expense of β-carotene. The
og
c
mutation is widely used in commercial vari-
eties to increase fruit color intensity (Faria et al.
2003). Commercial varieties harboring some of
the other mutations have been developed and
used as ingredients in an expanding array of
processed food products or sold for fresh mar-
ket consumption. Reports of QTLs that modu-
late carotenoid content have been published and
reviewed by Foolad (2007a).
While much is known about the synthesis
of carotenoids in plants, the “next frontier” is
δ
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