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Paran, 2008; Sakuraba et al. , 2012).
Interestingly, a survey of heirloom tomatoes
that exhibit the gf mutant phenotype
identifi ed four additional alleles, indicating
that this colour trait has been selected on at
least fi ve separate occasions, illustrating
the novelty value of such colour variants to
gardeners, breeders and growers (Barry and
Pandey, 2009). The navel negra ( nan )
mutant of Citrus sinensis also exhibits a
stay-green phenotype and the expression of
an SGR homologue was reduced in nan
fruit compared with the wild type.
However, sequencing of SGR from the nan
mutant failed to reveal any nucleotide
changes, suggesting that nan may be the
result of a mutation in a regulatory factor
rather than in a catalytic step of the
chlorophyll degradation pathway (Alos et
al. , 2008).
In addition to simply infl uencing colour,
mutants that directly or indirectly infl uence
fruit plastids have additional pleiotropic
effects. For example, carotenoid-defi cient
mutants of tomato impact fruit fl avour
characteristics, and both the gf and cl
mutants infl uence the rate of carotenoid
biosynthesis (Ramirez and Tomes, 1964;
Roca et al. , 2006; Vogel et al. , 2010).
Similarly, the high-pigment 1 and 2
mutants that encode tomato homologues of
DNA DAMAGE BINDING PROTEIN 1 and
DETIOLATED 1, which are involved in
ubiquitin-mediated protein turnover, have
pleiotropic effects on fruit quality resulting
in an increase in chloroplast number and
altered plastid ultrastructure, leading to
higher levels of chlorophyll, carotenoids
and fl avonoids, together with altered
patterns of aroma volatile production (Yen
et al. , 1997; Mustilli et al. , 1999; Liu et al. ,
2004; Bino et al. , 2005; Kolotilin et al. ,
2007; Galpaz et al. , 2008; Kovacs et al. ,
2009).
Recent research has shown that
mutations disrupting plastid components
in tomato can also inhibit or slow the rate
of ripening. The Orange ripening ( Orr DS )
mutant encodes the M subunit of the
plastidial NADH dehydrogenase complex
and exhibits inhibition of fruit ripening,
including a delay in the onset of ethylene
biosynthesis, together with a range of
altered ripening-related phenotypes
(Nashilevitz et al. , 2010). Similarly, the
lutescent 1 and 2 ( l1 and l2 ) mutants
display a range of phenotypes indicative of
chloroplast defects and rapidly lose
chlorophyll, leading to mature fruits that
can be almost white prior to the onset of
ripening. Fruit of the l1 and l2 mutants
also show a delay in the onset of ripening
and ripening-related ethylene biosynthesis,
but once ripening is initiated, the fruit
ripen at the same rate as those of wild-type
plants (Barry et al. , 2012). Positional
cloning of the l2 locus revealed that it
encodes a chloroplast-targeted zinc metal-
loprotease related to ETHYLENE-
DEPENDENT GRAVITROPISM DEFICIENT
AND YELLOW GREEN 1 (EGY1) of
Arabidopsis (Barry et al. , 2012).
15.5 Mutants With Altered Flavonoid
Biosynthesis: The Role of MYB
Transcription Factors
Flavonoids, particularly anthocyanins,
contribute to the nutritional and visual
quality of ripe fruits, and engineering of
anthocyanin biosynthesis in fl eshy fruits to
improve nutritional content has been
achieved (Bovy et al. , 2002; Butelli et al. ,
2008). Mutations that infl uence fruit
fl avonoid or anthocyanin accumulation
have been identifi ed in several species. For
example, the colorless epidermis ( y ) locus
of tomato ( Solanum lycopersicum ) encodes
a MYB transcription factor, designated
SlMYB12 , that is required for synthesis of
the fl avonoid naringenin chalcone in fruit
peels (Adato et al. , 2009; Ballester et al. ,
2010). Mutants, genetic variants and
transgenic lines in which SlMYB12 is
inactivated have a characteristic pink
colour, whereas varieties that contain a
functional Y gene possess a more vibrant
red colour due to the presence of the
orange pigmentation associated with
naringenin chalcone. In addition, mutants
at the y locus have reduced cuticle
thickness, a lower cutin monomer content,
an altered cuticular wax profi le and altered
 
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