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Biotechnology:
A new era for plant
pathology and plant protection
Genetic
engineering:
A novel and
powerful tool to combat
plant virus diseases
Contributed by Baozhong
Meng and Augustine Gubba
Plant virus diseases pose severe constraints to the production
of a wide range of economically important crops worldwide (Agrios
1997). Diseases caused by plant viruses are difficult to manage
and their control mainly involves the use of insecticides to kill
insect vectors, the use of virus-free propagating materials, and
the selection of plants with appropriate resistance genes.
Virus-free stocks are obtained by virus elimination through heat
therapy and/or meristem tissue culture, but this approach is
ineffective for viral diseases transmitted by vectors. While
vectors can be controlled by insecticides, often the virus has
already been transmitted to the plant before the insect vector is
killed. The use of resistant cultivars has been the most effective
means of control, however plant virus resistance genes are
frequently unavailable and their introgression into some crops is
not straightforward.
Genetic engineering brings new hope for the effective control
of plant virus diseases. The concept of pathogen derived
resistance (Sanford and Johnston, 1985) has stimulated research on
obtaining virus resistance through genetic engineering.
Pathogen-derived resistance is mediated either by the protein
encoded by the transgene (protein-mediated) or by the transcript
produced from the transgene (RNA-mediated). In 1986, Powell-Abel et
al. (1986) showed that transgenic tobacco expressing the coat protein
gene of tobacco mosaic virus (TMV) was resistant to TMV and that
the resistance was due to the expressed coat protein. Recent
research indicates that pathogen-derived resistance to viruses is
mediated, in most cases, by an RNA-based post-transcriptional gene
silencing mechanism. This plant defense system results in
degradation of mRNA produced both by the transgene and the virus. In
general, protein-mediated resistance provides moderate protection
against a broad range of related viruses while RNA-mediated
resistance offers high levels of protection only against closely
related strains of a virus (Pang et al. 1993, Lomonosoff
1995, Baulcombe 1996a, Dawson 1996).
Coat protein genes have been shown to be effective in
preventing or reducing infection and disease caused by homologous
and closely related viruses (Gonsalves and Slightom 1993). Coat
protein-mediated protection has been reported for tobacco mosaic
virus, TMV, (Nelson et al. 1988), tomato mosaic virus, ToMV,
(Sanders et al. 1992), cucumber mosaic virus, CMV, (Namba et
al. 1991, Quemada et al. 1991), alfalfa mosaic virus,
AlMV, (Loeshc-Fries et al. 1987, Tumer et al. 1987),
potato virus X, PVX, (Hemenway et al.1988), potato virus Y,
PVY, (Perlak et al. 1994), and potato leaf roll virus, PLRV,
(Kaniewski et al. 1993). In addition to the coat protein
gene, sequences from the viral replicase gene (Palukaitis and
Zaitlin 1997), defective virus movement protein genes (Beck et
al. 1994, Cooper et al. 1995), satellite virus RNA
(Smith et al. 1992), ribozymes (Wilson, 1993) and virus
antisense RNA (LeClerc and AbourHaidar, 1995, Yepes et al.
1996) have been engineered into plants to obtain virus resistance.
Genetic engineering is proving to be highly effective for
controlling virus diseases in a wide range of crops grown
worldwide (Wilson, 1993; Gonsalves and Slightom, 1992). Compared
to conventional breeding for virus resistance, genetic engineering
provides a quicker and more precise technology to obtain plants
that are resistant to viruses, although most transgenic
virus-resistant plants are still under laboratory development.
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successful example of commercialization is that of
transgenic papaya resistant to papaya ringspot potyvirus (PRSV),
a virus that causes severe damage to the papaya industry
in a number of major producing countries (Lius et al.
1997). In Hawaii, papaya ranks as the second most
important fruit crop. Due to the destruction caused by
PRSV, papaya production on Oahu island came to a halt in
the 1950s. This forced a relocation of the papaya industry
in the early 1960s to the Puna district on Hawaii island.
Unfortunately, PRSV was discovered in Puna in 1992 and by
late 1994 had spread throughout the Puna district (Gonsalves,
1998a and b). Transgenic papaya cultivars Sunrise and
Rainbow resistant to PRSV were developed in a
collaborative program of Dennis Gonsalves at Cornell
University, Richard Manshardt and Maureen Fitch at the
University of Hawaii and the USDA, and Jerry Slightom at
Upjohn Company.
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resistant cultivars were commercialized in Hawaii in 1998.
(See APSnet Feature: Transgenic
Virus Resistant Papaya). The use of these transgenic
papaya cultivars saved the papaya industry in Hawaii from
severe damage caused by PRSV Figure 2.1. Through
technology transfer, transgenic papaya cultivars that are
resistant to various strains of the virus have been
developed by Gonsalves’s program to satisfy the need of
other papaya producing areas in the world where different
strains of the virus prevail. Transgenic papaya plants are
under field trials in Jamaica, Thailand and Brazil (D.
Gonsalves, personal communication). The livelihood of
farmers in these countries could be impacted positively (Figure
2.2).
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Figure
2.1
Transgenic papaya plants show resistance (right)
while non-transgenic plants (left) are susceptible to
papaya ringspot virus under field conditions.
Photo courtesy of Dennis Gonsalves,
Cornell University.
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Figure
2.2 Farmer displaying healthy transgenic papaya fruit
in Thailand. Photo courtesy of
Dennis Gonsalves, Cornell University.
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Another virus
resistant transgenic crop that has been commercially released is
transgenic "Freedom II" squash as a result of work by
Asgrow Seed Company with collaboration by Gonsalves's program.
Freedom II is resistant to zucchini yellow mosaic and watermelon
mosaic 2 potyviruses (Fuchs and Gonsalves 1995) Figure 2.3.
Transgenic tomato engineered to resist cucumber mosaic virus (CMV),
a virus causing severe stunting and yield reductions throughout
the world, showed high levels of resistance to CMV under field
conditions (Figure 2.4) (Fuchs et al. 1996).
Research is underway to generate transgenic tomato plants that are
resistant to different strains of tomato spotted wilt virus and
different tospoviruses, a group of viruses that seriously limit
tomato production in the glasshouse and field, by combining virus
transgenes with the native plant resistance gene Sw-5 (Gubba,
2000).
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| Figure 2.3
Transgenic ‘Freedom II’ squash showing
resistance to both zucchini yellow mosaic virus and
watermelon mosaic virus 2 (right) compared to
non-transformed plants that are susceptible (left).
The yield differences between the two rows are
obvious. The transgenic, protected plants are dark
green and their fruits are large and yellow. Photo courtesy
of Dennis Gonsalves, Cornell University. |
Figure
2.4 Transgenic tomato plants show resistance
(left) while non-transformed plants are susceptible
to cucumber mosaic virus under field conditions.
Note the abundant fruit on the transgenic, protected
plant and the absence of fruit on the unprotected
plant. Photo courtesy of
Dennis Gonsalves, Cornell University. |
Unanswered questions regarding the use of this technology include gene
escape from transgenic plants to wild relatives producing "superweeds"
and recombination between the invading virus and the transgene
leading to the emergence of new viruses with novel host ranges
and more potent virulence. It is well documented that gene
transfer and virus recombination do occur in nature and they are
not restricted to transgenic plants (Trewavas 1999). The
question is whether gene transfer and virus recombination will
have a negative impact on the environment or agricultural
production. Thus, it is necessary to monitor the use of
genetic engineering in agriculture. Strategies being formulated
to minimize the possibility of gene escape and recombination
include the use of short nonfunctional gene fragments or
nontranslated versions of virus genes (Jan 1998).
The potential risks of the technology need to be balanced
against the proven or potential benefits of biotechnology. Scientists,
research institutions, and international organizations should
take an active role to promote the wise development of
biotechnology. Private corporations and research institutions
should make the technology and its products available to
developing countries at relatively low or no cost where they are
urgently needed (Conway and Toenniessen 1999).
References
Agrios, G. N. 1997.
Plant Pathology. Fourth edition. Academic Press, Inc., San Diego,
California. 635 pp.
Baulcombe, D. C.
1996a. Mechanisms of pathogen-derived resistance to viruses in
transgenic plants. Plant Cell 8: 1833-1844.
Baulcombe, D. C.
1996b. RNA as a target and an initiator of post-transcriptional
gene silencing in transgenic plants. Plant Mol. Biol. 32: 79-88.
Beachy, R. N. 1993.
Transgenic resistance to plant viruses. Seminars in Virology 4:
327-416.
Beck, D. L., Van
Dolleweerd, C. J., Lough, T. J., Balmori, E., Voot, D. M.,
Anderson, M. T., O’Brien, I. E. W., Forster, R. L. S. 1994.
Disruption of virus movement confers broad-spectrum resistance
against systemic infection by plant viruses with a triple gene
block. Proceedings of the National Academy of Sciences, USA.
91:10310-10314.
Conway, G. and
Toenniessen, G. 1999. Feeding the world in the twenty-first
century. Nature 402: C55-58.
Cooper, B., Lapidot,
M., Heick, J. A., Dodds, J. A. and Beachy, R. N. 1995. A defective
movement protein of TMV in transgenic plants confers resistance to
multiple viruses whereas the functional analog increases
susceptibility. Virology 206: 307-313.
Dawson, W.D. 1996.
Gene silencing and virus resistance: A common mechanism. Trends in
Plants Science 1:107-108.
Fuchs, M. and
Gonsalves, D. 1995. Resistance of transgenic hybrid squash ZW-20
expressing the coat protein genes of zucchini yellow mosaic virus
and watermelon mosaic virus 2 to mixed infections by both
potyviruses. Bio/Technology 13: 1466-1473.
Fuchs, M,
Provvidenti, R., Slightom, J. L., and Gonsalves, D. 1996.
Evaluation of transgenic tomato plants expressing the coat protein
gene of cucumber mosaic virus WL under field conditions. Plant
Disease 80: 270-275.
Gonsalves, D. 1998a.
Transgenic virus resistant papaya: new hope for controlling papaya
ringspot virus in Hawaii. APSnet Feature article, September 1
through September 30, 1998.
Gonsalves, D. 1998b.
Resistance to papaya ringspot virus. Annual Review of Phytopathology 36:415-437.
Gonsalves, D. and
Slightom, J. L. 1993. Coat-protein mediated protection: analysis
of transgenic plants resistance in a variety of crops. Sem. Virol.
4: 397-406.
Gubba, A. 2000.
Transgenic and natural resistance: Towards development of tomato
and pepper plants with broad resistance to virus infection. PhD
Dissertation, Cornell University, 196 pp.
Hemenway, C., Fang,
R-X., Kaniewski, W., Chua, N-H. and Tumer, N. 1988. Analysis of
the mechanism of protection in transgenic plants expressing the
potato virus X coat protein or its antisense RNA. EMBO J 7:
1273-1280.
Jan, F. J. 1998.
Role of nontarget DNA and viral gene length in influencing
multiple resistance through homology-dependent gene silencing. PhD
Dissertation, Cornell University, 1998.
Kaniewski, W.,
Lawson, C. and Thomas, P. 1993. Agronomically useful resistance in
Russet Burbank potato containing a PLRV CP gene. Abstract. IX
International Congress of Virology, Glasgow, Scotland.
LeClerc, D. and Abou
Haidar, M. G. 1995. Transgenic tobacco plants expressing a
truncated form of the PAMV capsid protein (CP) gene show CP-mediated
resistance to potato aucuba mosaic virus. Mol Plant-Microbe
Interact 8: 58-65.
Lius, S., Manshardt,
R. M., Fitch, M. M. M., Slightom, J. L., Sanford, J. C. and
Gonsalves, D. 1997. Pathogen-derived resistance provides papaya
with effective protection against papaya ringspot virus. Mol.
Breed. 3: 161-168.
Loesch-Fries, L. S.,
Merlo, D., Sinnen, T., Burhop, L., Hill, K., Krahn, K., Jarvis,
N., Nelson, S. and Halk, E. 1987. Expression of alfalfa mosaic
virus RNA 4 in transgenic plants confers virus resistance. EMBO J
6: 1845-1852.
Lomonossoff, G. P.
1995. Pathogen-derived resistance to plant viruses. Annual Review
of Phytopathology 33: 323-343.
Namba, S., Ling, K.,
Gonsalves, C., Gonslaves, D. and Slightom, J. L. 1991. Expression
of the gene encoding the coat protein of cucumber mosaic virus (CMV)
strain WL appears to provide protection to tobacco plants against
infection by several different CMV strains. Gene 107: 181-188.
Nelson, R. S.,
McCormick, S. M., Delanney, X., Dube, P., Layton, J., Anderson, E.
J., Kaniewska, M., Prosksch, R. K., Horsch, R. B., Rogers, S. G.,
Fraley, R. T. and Beachy, R. N. 1988. Virus tolerance, plant
growth, and field performance of transgenic tomato plants
expressing coat protein from tobacco mosaic virus. Bio/Technology
6: 403-409.
Palukaitis, P. and
Zaitlin, M. 1997. Replicase-mediated resistance to plant virus
disease. Advances in Virus Research 48: 349-377.
Pang, S-Z., Slightom,
J. L., and Gonsalves, D. 1993. Different mechanisms protect
transgenic tobacco against tomato spotted wilt and Impatiens
necrotic spot tospoviruses. Bio/Technology 11: 819-824.
Perlak, F.,
Kaniewski, W., Lawson, C., Vincent, M. and Feldman, J. 1994.
Genetically improved potatoes: Their potential role in integrated
pest management. In: M. Manka (ed). Proceed. 3rd EFPP Conference.
J Phytopath pp 451-454.
Powell-Abel, P.,
Nelson, R. S., De, B., Hoffmann, N., Rogers, S. G., Fraley, R. T.
and Beachy, R. N. 1986. Delay of disease development in transgenic
plants that express the tobacco mosaic virus coat protein gene.
Science 232: 738-743.
Quemada, H. D.,
Gonsalves, D. and Slightom, J. L. 1991. Expression of coat protein
gene from cucumber mosaic virus strain C in tobacco: Protection
against infections by CMV strains transmitted mechanically or by
aphids. Phytopathology 81: 794-802.
Sanders, P. R.,
Sammons, B., Kaniewski, W., Haley, L., Layton, J., Lavallee, B.
J., Delannay, X. and Tumer, N. E. 1992. Field resistance of
transgenic tomatoes expressing the tobacco mosaic virus or tomato
mosaic virus coat protein genes. Phytopathology 82: 683-690.
Sanford, J. C and
Johnston, S. A. 1985. The concept of parasite-derived resistance–deriving
reistance genes from the parasite's own genome. J. Theor. Biol.
113: 395-405.
Smith, C. R.,
Tousignant, M. E., Geletka, L. M. and Kaper, J. M. 1992.
Competition between cucumber mosaic virus satellite RNAs in tomato
seedlings and protoplasts: A model for satellite-mediated control
of tomato necrosis. Plant Disease 76:1270-1274.
Tumer, N. E.,
Hemenway, C., O ‘Connell, K., Cuozzo, M., Fang, R-X, Kaniewski,
W. and Chua, N-H. 1987. Expression of coat protein genes in
transgenic plants confers protection against alfalfa mosaic virus,
cucumber mosaic virus and potato virus X. Pages 351-356 In: Plant
Molecular Biology. D. von Wettstein and N-H. Chua (eds). Plenum
Publishing Corporation.
Trewavas A. 1999.
Gene flow and GM questions. Trends in Plant Science, 4: 339.
Wilson, T. M. A.
1993. Strategies to protect crop plants against viruses:
pathogen-derived resistance blossoms. Proceedings of National
Academy of Sciences (USA) 90: 3134-3141.
Yepes, L. M., Fusch,
M., Slightom, J. L. and Gonsalves, D. 1996. Sense and antisense
coat protein gene constructs confer high levels of resistance to
tomato ringspot nepovirus in transgenic Nicotiana species.
Phytopathology 86: 417-424.
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