Agriculture Reference
In-Depth Information
although they produce a small amount of
ethylene during ripening (Iannetta et al. ,
2006). Different studies have tried to relate
this hormone to the ripening process.
However, in spite of many efforts, no
results have been obtained that can
demonstrate a clear relationship. In pepper
fruit, some cultivars seem to be ethylene
insensitive, whilst other cultivars, as well
as grape berries treated with exogenous
ethylene, are able to stimulate the expres-
sion of ripening-specifi c genes (Armitage
1989; Ferrarese et al. , 1995; Harpster
et al. , 1997; El-Kereamy et al. , 2003). In
strawberry, the situation is even more
complex, because molecular data about the
possible relations between ethylene and
the expression of genes during ripening are
not in agreement. The expression of two
genes involved in softening of strawberries
(expansin and cellulose) seems to be
ethylene insensitive (Civello et al. , 1999).
However, the expression of other ripening-
related genes in strawberry (pectin methyl
esterase and E -galactosidase) were modifi ed
by treatments with ethylene (Trainotti
et al. , 2001; Castillejo et al. , 2004).
Interestingly, this dual effect of ethylene
has also been found in climacteric peach
fruit where the role of this hormone can
either be positive or negative according to
the different genes (Trainotti et al. , 2003).
Currently, no single growth regulator
appears to play a positive role analogous to
the role played by ethylene in the ripening
of climacteric fruits. However, it has been
observed for some time that auxin can
negatively control the ripening of some
non-climacteric fruits. In strawberry, it has
been shown that the expression of many
ripening-specifi c genes can be down-
regulated by treatments with an exogenous
auxin. By contrast, the expression of
ripening-specifi c genes is accelerated fol-
lowing the removal of the achenes, which
are a source of endogenous auxin (Harpster
et al. , 1998; Aharoni et al. , 2002). Also, in
grape, auxin seems to play a negative role
in the regulation of ripening. In fact, it
has been shown that treatments with a
synthetic auxin are able to delay the
expression of a number of ripening-related
genes (Davies et al. , 1997). However,
detailed studies on the content, synthesis
and signalling of this hormone in different
fruit parts at different developmental stages
are lacking. As a consequence of the
prominent role of auxin in the develop-
ment and ripening of some non-climateric
fruits such as strawberry ( Fragaria
ananassa ), little attention has been paid to
possible roles of other plant hormones in
these processes such as gibberellins (GAs).
It has been reported that application of
GA3 to ripening fruit caused a signifi cant
delay in the development of the red colour
(Martinez et al. , 1996). In addition,
external application of GA3 was able to
modify the expression of fruit genes such
as FaGAST , which encodes a protein
involved in cell enlargement and fi nal fruit
size (de la Fuente et al. , 2006), and FaXyl ,
encoding a E -xylosidase (Bustamante et al. ,
2009).
2.3 Central Carbon Metabolism
Metabolism in the fruit involves the
conversion of high-molecular-weight pre-
cursors to smaller compounds that help to
produce viable seeds and to attract seed-
dispersing species. The fl avour of fruit is
generally determined by tens to hundreds
of constituents, most generated during the
ripening phase of the fruit growth and
development process. Any study on the
metabolic pathways leading to their
synthesis must be considered in the con-
text of this developmental process. Thus, it
is known that, during the rapid growth
phase, the fruits act as strong sinks that
import massive amounts of photo-
assimilates from photosynthesizing organs.
Translocation occurs in the phloem, with
sucrose being the main compound trans-
located, although in some species there are
other predominant compounds such as
polyalcohols (e.g. mannitol or sorbitol) and
even oligosaccharides. These translocated
compounds, which are products of primary
metabolism, are the precursors of most of
the metabolites that account for the fruit
fl avour, generally classifi ed as secondary
 
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