Agriculture Reference
In-Depth Information
1977; Fang et al. , 1997; Maliepaard et al. ,
1998). In tomato, fruit acidity was shown
to be polygenic, but malic and citric acids
are each governed by a major gene (Fulton
et al. , 2000; Saliba-Colombani et al. , 2001).
Peach fruits can be grouped into two types
according to their pH value: the normal-
acid (pH below 4.0) and the low-acid
phenotypes (pH above 4.0) (Yoshida 1970;
Dirlewanger et al. , 1998). A major
dominant allele, D , is responsible for low
acidity (Monet, 1979). Boudehri et al.
(2009) generated a high-resolution genetic
map of the D locus, currently delimited at a
genetic interval of 0.4 cM. In contrast, the
low-acid characteristic was found to be
recessive in apple and citrus (Visser and
Verhaegh, 1978; Maliepaard et al. , 1998). In
apple, the position of the Ma gene, con-
trolling malic acid accumulation, was
determined by genetic mapping (Maliepaard
et al. , 1998). More recently, Yao et al. (2009)
isolated three genes encoding key enzymes
involved in malic acid metabolism and
transportation. Their expression pattern
and the corresponding enzyme activity
equated to the difference in fruit acidity
between low- and high-acid genotypes. In
contrast, in peach, Etienne et al. (2002) did
not fi nd any clear-cut difference between
normal-acid and low-acid fruits for gene
expression of six genes implicated in
organic acid metabolism (mitochondrial
citrate synthase, cytosolic NAD-dependent
malate dehydrogenase and cytosolic NADP-
dependent isocitrate dehydrogenase) and
storage (vacuolar proton translocating
pumps: one vacuolar H + -ATPase and two
vacuolar H + -pyrophosphatases).
hydrogenase is involved in the ratio of
hexanal to hexanol in the tomato fruit
(Speirs et al. , 1998). TomloxC , a gene
encoding a fruit-specifi c lipoxygenase, has
been shown to be related to the generation
of volatile C6 aldehyde and alcohol
compounds including hexanal, hexenal
and hexenol (Chen et al. , 2004). Two genes,
LeAADC1 and LeAADC2 , are responsible
for the decarboxylation of phenylalanine
and subsequent synthesis of phenylethanol
and related compounds in tomato (Tieman
et al. , 2006b). The gene coding for the
carotenoid cleavage dioxygenase 1 enzyme
(CCD1) is involved in the synthesis of
several aroma volatiles derived from
carotenoid cleavage (Vogel et al. , 2008).
Tieman et al. (2007) showed that
phenylacetaldehyde reductases catalyse
the last step in the synthesis of the aroma
volatile 2-phenylethanol. A salycylic acid
methyl transferase has been shown to be
involved in the synthesis of methyl
salicylate (Tieman et al. , 2010).
14.6.5 Genes involved in fruit texture
Peach cultivars can be grouped into two
main types according to the fl esh texture:
melting (M) or non-melting types. This
trait, known to be Mendelian (Monet,
1989), is highly linked to the Freestone
locus ( F ) mapped in linkage group 4
(Dettori et al. , 2001). Peace et al. (2005)
showed evidence of a single locus con-
taining at least one gene for endo-
polygalacturonase, and controlling both F
and M with at least three effective alleles.
It is thus possible to differentiate by a PCR
test the three major phenotypes of peach:
freestone melting fl esh, clingstone melting
fl esh, and clingstone non-melting fl esh.
14.6.4 Genes involved in volatile
compounds
Volatiles are derived from the degradation
of amino acids, fatty acids, carotenoids or
phenolic compounds (Klee, 2010). Due to
the diversity of compounds, little is known
about the genes controlling their accumu-
lation, but a few genes have been identifi ed
as responsible for their accumulation. The
ADH gene encoding an alcohol de-
14.6.6 Genes involved in fruit ripening
Our current understanding of ripening
mechanisms and the molecular basis of
fruit texture evolution in fl eshy fruits is
largely due to tomato and relies mainly on
transgenic or mutant plant analysis
 
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