Virus-resistant transgenic papaya helps save Hawaiian industry
tion had dropped to only 26 million
pounds as PRSV spread throughout
the region. Since then, the transgenic
varieties have enabled farmers to
reclaim infected areas and in 2001,
Puna produced 40 million pounds of
papaya. The resistance has held up
remarkably well and remains stable
after 5 years of extensive plantings.
Hawaii also exports papaya to
Canada and Japan. The transgenic
papaya was recently deregulated in
Canada, which is a relatively small
market for Hawaii. The main chal
-
lenge is deregulation of transgenic
papaya in Japan, where Hawaii sells
about 30% of its papaya. Presently,
nontransgenic papaya must also be
produced in Hawaii to satisfy the
Japanese market, but this is increas
-
ingly difficult due to the disease
pressure. Exporters face added ex
-
penses to guard against the acciden
-
tal shipment of transgenic papaya
to Japan. In December 2000, Japan’s
Ministry of Agriculture, Forestry
and Fisheries approved line 55-1,
and the Ministry of Health, Labor
and Welfare is reviewing a recently
submitted petition for deregulation.
Anticipated approval of transgenic
papaya in Japan will allow Hawai
-
gated and established
in a field trial in
Waimanalo on Oahu
in April 1992. By De
-
cember 1992, it was
evident that line 55-1
was resistant under
field conditions.
From the 1992 field
trial, two cultivars
were developed and
designated ‘SunUp’
and ‘Rainbow’. ‘Sun
-
Up’ is homozygous
for the coat protein
gene while ‘Rainbow’
is an Fl hybrid of
‘SunUp’ and the non
-
transgenic ‘Kapoho’.
Unfortunately, by Oc
-
tober 1994, PRSV had spread through
-
out much of Puna, causing HDOA to
abandon rouging efforts to slow the
spread of PRSV. The race was on to
move the transgenic papaya line to
commercialization. A 1995 field trial in
Puna conclusively showed
that ‘SunUp’ and ‘Rainbow’
were resistant under pro
-
longed and heavy disease
pressure.
The U.S. Department of
Agriculture’s Animal Plant
Health Inspection Service
(APHIS) deregulated transgenic line
55-1 in November 1996, and the U.S. En
-
vironmental Protection Agency deregu
-
lated it in August 1997. The consultation
process with the U.S. Food and Drug
Administration was completed in Sep
-
tember 1997. Licenses to commercialize
the transgenic papaya were obtained by
the Papaya Administrative Committee
in Hawaii by April 1998. A celebration
was held to mark the debut of the trans
-
genic papaya on May 1, 6 years after
PRSV was discovered in Puna and after
the first field trial of line 55-1 was initi
-
ated. The transgenic fruit is currently
sold throughout the United States.
In 1992, Puna produced 53 million
pounds of papaya, but by 1998 produc
-
The experience in Hawaii shows
that transgenic virus resistance is
an excellent approach for controlling
viral diseases in horticultural crops.
92
CALIFORNIA AGRICULTURE, VOLUME
58
, NUMBER
2
Two varieties of papaya resistant to papaya ringspot virus
have been developed using biotechnology: SunUp,
left,
and Rainbow,
right.
They have performed well for Hawai
-
ian growers, even under prolonged and heavy disease
pressure.
Courtesy of R. Manshardt
http://CaliforniaAgriculture.ucop.edu
•
APRIL- JUNE
2004
93
ian growers to expand their transgen
-
ic papaya markets and will eliminate
excessive costs associated with segre
-
gating trans-genic and nontransgenic
fruits.
The experience in Hawaii shows
that transgenic virus resistance is an
excellent approach for controlling vi
-
ral diseases in horticultural crops. This
industry was fortunate to have a po
-
tential product already under devel
-
opment when PRSV was discovered in
the main growing area of Puna. There
are many reports that virus-resistant
transgenic plants are being developed
in diverse crops, but few have been
commercialized. The papaya story
shows that this approach can provide
a stable and safe option for virus pro
-
tection that can be essential for the
success of specific horticultural crops.
D. Gonsalves is Director, Pacific Basin
Agricultural Research Center, USDA-
ARS, Hilo, Hawaii.
Reference
Gonsalves D. 1998. Control of papaya
ringspot virus in papaya: A case study.
Annu Rev Phytopathol 36:415–37.
ies (see sidebar, page 96). The latter ap
-
proach avoids the task of transforming
many varieties of a particular tree crop
and in the future may be used to regu
-
late quality and productivity traits.
Nutrients, consumer qualities.
Al
-
though more difficult technically and
therefore not close to market, there
are many potential opportunities for
enhancing the nutritional value or con
-
sumer appeal of horticultural products
through biotechnology. In addition to
modification of ripening, projects to
increase the content of vitamins, miner
-
als or nutraceuticals in horticultural
products are in progress (Grusak and
Della Penna 1999). The development of
Golden Rice with enhanced beta
-
carotene (pro-vitamin A) in the grain (Ye
et al. 2000) demonstrated the potential
for biotechnology to increase nutritional
value. Whether such products will have
sufficient consumer appeal in fully de
-
veloped markets to drive their commer
-
cialization remains to be seen.
Floriculture, ornamental plants
Since floricultural and ornamental
plants are grown for aesthetic or other
nonedible purposes, there may be less
potential for public concern about GE
varieties than there has been with bio
-
tech food crops.
Flower color.
Several ornamental
plants, including carnation, rose and
gerbera, have been engineered for mod
-
ified flower color. Research has focused
on the manipulation of either anthocya
-
nins (red and blue colors) or carotenoids
(yellow and orange colors), with the in
-
tent of creating a wider range of flower
colors than occurs naturally, as well as
to produce natural dyes for industrial
purposes (Lu et al. 2003). Florigene is
selling Transgenic Moon
series carnations engineered for dark vi
-
olet-purple color around the world. The
varieties are developed in Australia and
flowers are produced primarily in South
America for marketing in the United
States and Japan.
Floral scent.
Putting the scent back
into flowers that have “lost” this trait
over years of traditional hybridization
and selection, or creating new fragrances
in plants, has considerable potential and
appeal. Research on genes controlling the
different biochemical pathways for vari
-
ous floral fragrances is being conducted
on wild plants and on crops such as snap
-
dragon, petunia and rose (Vainstein et al.
2001).
Plant size.
Currently, growth-
regulating chemicals are applied to
ornamental plants to inhibit gibberel
-
lic acid (GA) synthesis and reduce plant
height during crop production. Many
newly introduced ornamental species are
receiving particular attention via conven
-
tional breeding for dwarf plants because
their natural habits do not fit into market
-
ing systems requiring compact plants.
The manipulation of GA metabolism via
biotechnology has the potential to pro
-
duce ornamental and flowering plants
with reduced-height phenotypes (Clark et
al. 2003). The development of lawn grass
-
es that require significantly less frequent
mowing is another obvious application.
Early experiments suggest that expression
of genes controlling height can be applied
to many plant species.
Leaf life.
Engineering of plants to
delay leaf senescence (yellowing) is also
being pursued in ornamental crops. For
years, ornamental breeders have selected
new cultivars of plants with more attrac
-
tive “stay green” phenotypes. Cytokinins
are plant hormones well known to delay
the loss of chlorophyll in leaves; using
biotechnology, targeted expression of
genes involved in cytokinin synthesis is
now possible. When a gene promoting cy
-
tokinin biosynthesis is inserted into plants
in conjunction with a regulator (promot
-
er) that turns the gene on only when the
leaf starts to senesce, leaf life is extended
in transgenic plants exposed to drought,
nutrition and pathogen stress (Gan and
Amasino 1995; Clark et al. 2004).
Ethylene sensitivity
. As in fruit ripen
-
ing, manipulation of ethylene synthesis
or sensitivity has applications in the orna
-
mental plant industry. Ethylene accelerates
floral and foliar senescence, and chemical
methods have been developed to miti
-
Simply the diversity of crops utilized in horticulture slows
the adoption of new technologies. For example, as many
as 60 distinct cultivars of iceberg lettuce alone may be
grown throughout the year.
D. Gonsalves and S. Ferreira
Papaya ringspot virus causes small, darkened
rings on the surface of fruit, as well as foliar
damage.
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