Ethylene Perception and the Fruit bowl
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Rebecca Gunston.
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In Biblical times, Amos was the
‘piercer of fig trees’ (Amos 7:14) [1],
which was an extremely responsible
job. This required him to make an
incision in the fruit a few days before
harvest to encourage ripening. King
David even appointed an overseer for
this tedious, yet vitally important task.
Now more than 2,000 years later, we
are beginning to understand the
science behind it.
Ethylene, a gaseous phytohormone, is
a major signalling molecule in plants.
It controls pathways causing seed
germination, abscission, ripening,
senescence, and wound healing.
Control of this pathway is a billiondollar business.
Previous research has identified four
separate genes responsible for
ethylene perception. This included the
etr1 gene which, when mutated,
causes an 80% reduction in ethylene
perception and response. In 1997,
Wilkinson et al [2] reported that the
mutant gene etr1-1 protein, initially
discovered by Schaller and Bleeker [3],
confers ethylene-insensitivity in
Caption
heterologous plants. This was observed
phenotypically by the lack of the socalled ‘triple response’. Seedlings
grown in dark conditions and in the
presence of ethylene normally have
short, radially expanded roots and
hypocotyls, and exaggerated curvature
on the apical hook. In etr1-1 mutants,
this response is decreased.
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Wilkinson and co-workers [2]
engineered two forms of the
Arabidopsis thaliana etr1-1 sequence,
and transformed them into Kanomycinresistant tomato and petunia plants,
mediated by Agrobacterium. The
vectors used allowed the exchange of
the N-terminus of the tomato ethylene
receptor for the mutant Arabidopsis
protein. A similar process occurred in
petunia. They grew the plants, plus
wild type controls, in dark conditions,
on ACC-containing agar plates. The
conversion of ACC to ethylene by ACC
Oxidase results in the triple response
phenotype, but the transformed plants
failed to demonstrate this. The
Crosspresence of the transgene was
demonstrated by PCR analysis. Table 1 reference
shows the resultant phenotypes.
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Table 1. Two of the most biotechnologically-important phenotypes, and the differences
between the Wild Type and the etr1-1 mutant, as observed in experiments by Wilkinson
et al (1997).
Tomato-specific characteristic
Petunia-specific characteristic
Wild Type
Mutant
Wild Type
Mutant
Flower
Senescence
Dry and
detached after
3-5 days
Remained
expanded and
attached for
more than 10
days. Removed
when physically
separated by
the ovary.
Collapsed by day
3
Turgid and
intact for at
least 8 days
Fruit ripening
Turned deep
red, soft and
started to rot
Remained
golden yellow
after 3 months
N/A
N/A
Phenotype
Formatted
table
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The conclusions of this research have
several applications for biotechnology.
The ethylene pathway is highly
conserved between species and the
success of Wilkinson et al [2]
transgenic experiments indicate that
genetic manipulation of the ethylene
signalling pathway may be possible in
many species, including monocots. It
may not be effective at eliminating
responses in all plants, but paves the
way for the production of crops with a
longer shelf life and of better quality,
therefore more desirable for
consumers, with less wastage for
farmers. Its applications in the
floriculture industry are also extremely
beneficial. Currently, the treatment of
flowers with sodium thiosulfate
extends their lifetime because it
interferes with the ethylene signal
pathway. However, it is an
environmentally unfriendly method
and genetic manipulation will increase
flowering time, reducing the cost of
cut flower storage, making it more
beneficial for consumers.
Crossreference
Advances in this field of research have
been made due to the increased use of
genetic manipulation and the use of
epistatic analysis to deduce the
pathway, as shown in Figure 1.
Many questions remain unanswered,
the majority relating to specific
component-component interactions, or
particular component functions,
including EIN5, EIN6, EIN7, and the EILfamily.
Genetically modified crops are
becoming increasingly common-place,
despite the continuing debate over
their safety. Monsanto recently
revealed plans to increase the
worldwide area farming transgenic
crops by 44%. However, it has been
observed that the etr1-1 gene is
expressed in a cell-autonomous
manner [4]. Nonuniform expression of
the transgenic gene will initiate
uneven ripening, causing a “distinct
ripening sector within an otherwise
non-ripening fruit.”[2]. This must be
borne in mind when considering the
breadth of this technique.
Figure 1 taken from Chang and Shockey
(1999) [5], demonstrating the current
knowledge of the ethylene perception
pathway. EIN2 is membrane bound, and
thus may associate with the nuclear
membrane, although this remains
elusive.
Caption
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The proteins in the ethylene pathway
exhibit a high level of redundancy. If it
is shown that they are expressed in a
tissue-specific manner then mutation
of one gene may knockout ethylene
perception in one tissue, rather than
the whole plant. This may be
beneficial for the prevention of
ethylene-controlled post-harvest
disorders in many crops. It may also be
extended to floriculture, crop viability
and storage length, and a practicable
way of controlling climacteric fruit
ripening. Current methods for
controlling ethylene activity, such as
controlled greenhouse environment,
could be successfully combined with
gene manipulation to create a
reduction in ethylene sensitivity,
providing greater control over
ethylene-managed processes. This
would be more effective than
producing completely insensitive
mutants that could not be controlled.
Nevertheless, ethylene-producing
tomatoes, for example, must be
physically segregated from crops like
lettuces and cucumbers because their
ethylene mutation can be reversed in
the presence of ethylene.
2
Another cautionary note must be
heeded, as deleterious alterations in
disease susceptibility to usually nonpathogenic soil fungi, and secondary
responses to infection, have been
observed in experiments conducted by
Chang and Shockey [5]. Other changes
to growth, fertility and response to
environmental stresses must also be
carefully analysed before the use of
such GM crops can be considered.
After more than two millennia, it
appears that Amos’ highly respected
job may no longer be necessary.
References
1.
2.
Theologis, A., One rotten apple
spoils the whole bushel: the
role of ethylene in fruit
ripening. Cell, 1992. 70: p.
181-184.
Wilkinson, J.Q., et al., A
dominant mutant receptor
3.
4.
5.
from Arabidopsis confers
ethylene insensitivity in
heterologous plants. Nature
EndNote
Biotechnology, 1997. 15: p.
references
444-447.
and
Schaller, G.E. and A.B.
bibliography
Bleecker, Ethylene-binding
in Numbered
sites generated in yeast
style
expressing the Arabidopsis
ETR1 gene. Science, 1995. 270:
p. 1809-1811.
Tiemann, D.M. and H.J. Klee,
Differential expression of two
novel members of the tomato
ethylene-receptor family. Plant
Physiology, 1999. 120: p. 165172.
Chang, C. and J.A. Shockey,
The ethylene-response
pathway: signal perception to
gene regulation. Current
Opinion in Plant Biology, 1999.
2: p. 352-358.
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