Biology Reference
In-Depth Information
(ILs), common phenotyping platforms, transpar-
ent bioinformatic methods, and relevant environ-
mental conditions strengthened and legitimized
the findings of Schauer and colleagues (2006 and
2008). It is hoped that future studies adhere to
these examples. The resource limitations of the
Bermudez and colleagues (2008) study are no
longer relevant, now that a genome sequence and
a large set of genome-wide markers amenable
to high-throughput assays are available. These
studies set the stage for the next generation of
genomics-assisted breeding strategies in tomato.
directly related to the amount ingested through
the diet (Demmig-Adams and Adams 2002). One
such compound, lycopene, has been co-opted
by some plants to attract seed dispersal agents
through its bright red appearance. Lycopene is
responsible for the deep red hue of ripe tomato
fruit and is the most abundant carotenoid in this
organ, and since deep red fruit color is a con-
ventional tomato breeding objective, elevating
lycopene content is a parallel objective to fruit
color. The biosynthesis, and to some extent reg-
ulation, of carotenoid accumulation is fairly well
understood, although some uncertainty remains
regarding the exact mechanism that funnels
metabolites into the carotenoid pathway. These
topics have been extensively reviewed and dis-
cussed (Hirschberg 2001; Fraser and Bramley
2004; Fraser et al. 2008).
As follows from current knowledge (and
speculation) surrounding carotenoids and human
health, increasing levels or altering types of
carotenoids in tomato is of importance from
a nutritional standpoint. β-carotene is the only
carotenoid defined as a “nutrient” by the USDA,
yet a large proportion of the relevant literature
has focused on modulating lycopene content in
tomato, as lycopene is the most predominant
carotenoid in the red tomato fruit (Foolad 2007a).
Also, elevated fruit carotenoid content in tomato
can be considered a value-added trait, potentially
resulting in increased consumer desire for such
varieties or food products, although the effect
of increased carotenoids on tomato product sales
has yet to be definitively quantified. A recent sen-
sory study concluded that consumers associate
increased tomato fruit quality and better overall
taste with red color; however, under conditions in
which the fruit color was masked, the association
was abolished (Stommel et al. 2005). Regardless
of whether or not lycopene actually provides a
measurable health benefit, when faced with the
choice between a deeply red tomato and a lightly
red or yellow tomato, consumers seem to pre-
fer the former. Stommel and colleagues (2005)
claim this preference results from a visual asso-
ciation between intense red fruit color and better
Carotenoids
Carotenoids are C
40
terpenoid compounds found
abundantly in tomato fruit. Ripe tomato fruits
contain significant amounts of lycopene and β-
carotene (the precursor of vitamin A), and small
amounts of phytoene, phytofluene, lutein, and
zeaxanthin. The concentration of each of these
compounds depends on the genotype, the matu-
rity stage of the fruit, and the environmental con-
ditions in which the plants are grown. The insolu-
bility of carotenoids in aqueous solutions creates
challenges for high-throughput quantification of
these compounds, and therefore carotenoid data
was not reported in the Schauer and colleagues
(2006 and 2008) studies. This class of secondary
metabolites is ubiquitous throughout nature; the
presence of conjugated double bonds in a long
polyene chain renders carotenoids indispens-
able for several reasons, including quenching
ROS generated by electron transport mecha-
nisms and photosynthetic reactions, harvesting
photons, and, due to their distinct spectral qual-
ities, attracting pollinators and seed disper-
sal agents (Hirschberg 2001; Bramley 2002).
Throughout evolutionary time, carotenoids have
been recruited for a variety of extremely impor-
tant cellular purposes, including light harvesting,
dissipation of excess solar energy, scavenging of
ROS, strengthening sight (in animals), and the
biosynthesis of hormones (Vershinin 1999).
Since animals cannot synthesize these com-
pounds de novo, carotenoid levels in animals are
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