Agriculture Reference
In-Depth Information
ripening fruit. During fruit development
and ripening, fruit softening, primarily
due to cell-wall degradation and re-
duction of intercellular adhesion, is a
prerequisite for a desirable eating quality
of a fruit. Ethylene plays a main role in
promoting fruit ripening at the level of
fl esh softening and lignifi cation (Yin et al. ,
2008; Johnston et al. , 2009; Wang et al. ,
2010). In tomato, ethylene is required for
normal fruit softening. Its fruit fi rmness
can be maintained through transgenic
plants carrying an antisense RNA to the
1-aminocyclopropane-1-carboxylic acid
synthase gene and subsequent reversal of
such an inhibitory effect following
exogenous ethylene treatment (Oeller
et al. , 1991). Although ripening of non-
climacteric fruit may be ethylene inde-
pendent, exogenous ethylene treatment
increases fruit fi rmness in non-climacteric
fruits like loquat; an inhibitor of ethylene
responses, 1-methylcyclopropene, sig-
nifi cantly delays postharvest fruit fi rmness,
thus suggesting that ethylene plays an
important role in regulating fl esh lig-
nifi cation (Cai et al. , 2006; Shan et al. ,
2008). Ethylene response factors are a
family of transcription factors (potential
regulators of ethylene response), and
further support the important role of
ethylene signalling in regulating fruit
softening. In addition to studies in tomato,
ethylene signalling components have also
been isolated and characterized from
several other fruits (Yin et al. , 2008; Wang
et al. , 2010). Besides ethylene, expansins
are implicated in fruit ripening/softening,
which were fi rst investigated using the
LeEXP1 gene in tomato (Rose et al. , 1997).
MiExpA1 , an D -expansin gene, has been
isolated and characterized from a variety of
mango ( Mangifera indica cv. 'Dashehari')
and is correlated with softening in mango
(Sane et al. , 2005).
in several biosynthesis pathways. These
epigenetic variations do not affect the
primary DNA sequence but consist of DNA
methylation or histone modifi cations that
affect gene expression generally at the level
of chromatin organization. In fruit, the Cnr
mutation is the only well-characterized,
natural and stably inherited epigenetic
mutation (Seymour et al. , 2007). In this
mutation, a region of the LeSPLCNR
promoter is highly methylated and the
gene expression responsible for ethylene
production is suppressed (Manning et al. ,
2006). A study of epigenetic variation in
Arabidopsis using tiling microarrays
showed that at least one-third of expressed
genes were methylated in parts of their
coding regions, while about 5% of genes
were methylated within promoter regions
(Zhang et al. , 2006; Vaughn et al. , 2007).
However, the promoter-methylated genes
had a higher degree of tissue-specifi c
expression (Zhang et al. , 2006; Zilberman
et al. , 2007), suggesting these as
preferential sites for selection of subtle cis -
regulation during fruit development and
ripening. Moreover, methylation dif-
ferences among ecotypes have also been
reported, which are common, heritable and
stable (Vaughn et al. , 2007).
13.5 Quantitative Trait Loci (QTLs)
Mapping of Fruit-ripening Traits
The advent of advanced molecular markers
like QTLs opened up new prospects for
genetic improvement of agronomic traits.
Indeed, most fruit quality traits, including
fruit development and ripening, are
controlled by multigenic families. Thus,
the QTL approach is more useful for
localization of loci on genetic maps
responsible for, at least, part of the
phenotypic variation, and enables quan-
tifi cation of their individual effects. The
molecular markers found in the proximity
of these QTLs are now being used in
marker-assisted selection to create parent
lines with increased potential, or to avoid
certain unfavourable traits (Fulton et al. ,
2002). A QTL for fruit weight has been
13.4.2 Epigenetic mechanisms
Epigenetic factors are important genetic
determinants for plant improvement,
affecting the regulation of gene expression
 
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