Fruit ripening is of interest to research groups and industry
alike. The obvious phenotypic changes that occur during this process
are brought about by a multitude of factors. These complex and
highly regulated processes have been studied extensively in tomato
[1]. Cell wall modifying enzymes have been considered to be the
main contributors to the softening of the fruit and consequently,
many groups have tried to alter this aspect of ripening by genetic
constructs designed to inhibit the synthesis of these specific enzymes.
However, despite the success of these techniques at the
genetic level, phenotypically these have generally resulted in minor
alterations to the softening process. Gene silencing to reduce polygalacturonase
activity to as low as 1% of normal levels had relatively
little effect on fruit softening [2e6]. Similar studies have also
been conducted to elucidate the role of PE [7,8], endo-glucanase
[9,10] and xyloglucan endotransglycosylase [11] in fruit softening.
However, all of these studies resulted in only minor or no reduction
in fruit softening, with one particular study by Phan et al. [12]
showing an increase in softening. However, more significant effects
have been achieved through inhibition of galactanase gene
expression [13] and inhibition of the ripening specific expansin
[14]. Fruits in which polygalacturonase and expansin had been
inhibited simultaneously showed a greater effect on fruit softening
and tomato paste viscosity than fruits with either alone inhibited
[15,16]. These studies show the complexity of ripening and give
evidence for multiple enzymes working in concert. Therefore, the
alteration of softening may depend on reduction of multiple enzymes
simultaneously and this is supported by the observation that
inhibition of ethene biosynthesis [17] to simultaneously inhibit
many aspects of ripening has a more dramatic effect upon softening.
For these reasons it has been suggested that blocking the
trafficking route to the cell wall might bring about just such a
simultaneous inhibition of the effect of many enzymes and thus a
more significant effect on softening [18].
Trafficking of cell wall precursors and cell wall modifying enzymes
requires control to maintain the balance of developmental
cues, particularly for metabolically active tissue such as tomato
Fruit ripening is of interest to research groups and industry
alike. The obvious phenotypic changes that occur during this process
are brought about by a multitude of factors. These complex and
highly regulated processes have been studied extensively in tomato
[1]. Cell wall modifying enzymes have been considered to be the
main contributors to the softening of the fruit and consequently,
many groups have tried to alter this aspect of ripening by genetic
constructs designed to inhibit the synthesis of these specific enzymes.
However, despite the success of these techniques at the
genetic level, phenotypically these have generally resulted in minor
alterations to the softening process. Gene silencing to reduce polygalacturonase
activity to as low as 1% of normal levels had relatively
little effect on fruit softening [2e6]. Similar studies have also
been conducted to elucidate the role of PE [7,8], endo-glucanase
[9,10] and xyloglucan endotransglycosylase [11] in fruit softening.
However, all of these studies resulted in only minor or no reduction
in fruit softening, with one particular study by Phan et al. [12]
showing an increase in softening. However, more significant effects
have been achieved through inhibition of galactanase gene
expression [13] and inhibition of the ripening specific expansin
[14]. Fruits in which polygalacturonase and expansin had been
inhibited simultaneously showed a greater effect on fruit softening
and tomato paste viscosity than fruits with either alone inhibited
[15,16]. These studies show the complexity of ripening and give
evidence for multiple enzymes working in concert. Therefore, the
alteration of softening may depend on reduction of multiple enzymes
simultaneously and this is supported by the observation that
inhibition of ethene biosynthesis [17] to simultaneously inhibit
many aspects of ripening has a more dramatic effect upon softening.
For these reasons it has been suggested that blocking the
trafficking route to the cell wall might bring about just such a
simultaneous inhibition of the effect of many enzymes and thus a
more significant effect on softening [18].
Trafficking of cell wall precursors and cell wall modifying enzymes
requires control to maintain the balance of developmental
cues, particularly for metabolically active tissue such as tomato
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