Keywords: | Disease/pest resistance; Genetic engineering. |
Correct citation: | Stiekema, W.J. (1997), "In Defence of Vertical Resistance." Biotechnology and Development Monitor, No. 33, p. 24. |
Horizontal or polygene resistance is generally considered as a more stable resistance than vertical or monogene resistance. Since the role of genetic engineering is mainly restricted to vertical resistance strategies, its potential to contribute to durable plant resistance against pest and diseases would thus be limited. However, according to W.J. Stiekema, vertical resistance in combination with genetic engineering offers many old and new possibilities to achieve fast and costeffective forms of durable resistance.
Crop protection can be achieved by the use of agrochemicals, hygiene
measures, biological control or plant resistance. The latter is the most
attractive approach, because it is cost effective and combines benefits
for growers, consumers and the environment. A disadvantage of many resistances
is their pathogen specificity. Since usually more than one parasite is
present, several resistances have to be introduced in a cultivar separately.
This is a time consuming process and includes the risk of introducing unwanted
traits, such as toxicity to the consumer. Potentially, genetic engineering
can better control the characteristics of the newly developed variety,
since it adds a known single gene to a known variety. Besides, genetic
engineering usually is less time consuming and not limited by barriers
between species.
A second disadvantage of resistance is potential instability. Pathogens
often circumvent a specific resistance soon after its introduction. Many
people relate too simplistically durability of resistances to the amount
of genes that are involved in it. Vertical resistance, i.e. resistance
that depends on one gene and thus can be handled easily by genetic engineering,
is considered less durable than horizontal resistance. However, many examples
exist of (nontransgenic) vertical, durable resistances, such as against
potato virus A, X and Y in potato, against lettuce mosaic virus in lettuce,
against the fungi Cladosporium cucumerinum and Corynespora melonis
in
cucumber, Fusarium oxysporum in cabbage, pea and tomato and against the
nematodes Globodera rostochiensis in potato. An example of transgenic
vertical, durable resistance is the transfer of a resistance gene against
the beet cyst nematode from wild beet to sugar beet. Neglecting the potential
of vertical resistance will result in an increased need for time and funds
to obtain useful resistances.
What resistances are durable, can only be determined in retrospect.
Since huge differences exist between pathogens in the ease with which they
circumvent resistances, it is difficult to forecast the durability of (vertical)
resistance in the field. In general, specialized parasitic fungal pathogens
that are airborne or splashborne and consist of many strains are often
capable of quickly circumventing vertical resistance. An example is the
fungus Phytophthora infestans, which causes late blight in potato.
Of this fungus, more than 10 races are already known that are capable of
circumventing more than 10 potato resistance genes.
Does this imply that it is useless to target one or more single resistance
genes to obtain resistance against late blight in potato? Through breeding
and selection this is indeed the case. However, upon application of molecular
biological tools there are good perspectives for the use of such genes.
Due to worldwide research into the isolation and analysis of single resistance
genes, knowledge accumulates rapidly on pathogens' capability of circumventing
resistances. This knowledge will enable the development of new strategies
to ensure durable types of resistance based on single genes.
One new strategy might be the transfer of a single resistance gene
together with its avirulence gene. An avirulence gene is usually found
in the pathogen and triggers the resistance reaction in the host. A specific
resistance gene reacts to a specific avirulence gene. The new approach
consists of the transfer of a resistance gene together with its related
avirulence gene. This avirulence gene is introduced under the control of
a promotor gene, that becomes active after a pathogen attack. If there
is no pathogen attack, the resistance is not expressed. In such an approach
the specific relationship between the resistance gene and the avirulence
gene is beyond the control of the pathogen, and a broad and durable resistance
may be obtained. This strategy is now under study for fungal and bacterial
resistance in tomato and virus resistance in tobacco. It might also be
applied to obtain resistance to late blight in potato once the resistance
gene and avirulence gene have been isolated, and if the avirulence gene
is exclusively activated by P. infestans.
Undoubtably, knowledge on resistance genes and resistance mechanisms
will lead to further strategies to obtain durable vertical resistance.
Besides isolation and analysis of vertical resistance genes of potato,
research is being conducted on the specific location of the resistance
genes on the plant genome. The aim is to relate genetic information on
horizontal resistance with information on individual resistance genes.
If vertical resistance and horizontal resistance could be related, new
routes would emerge to obtain durable resistance against pest and diseases
using vertical resistance. It may appear that the genetic context in which
resistance genes are embedded is one of the factors that determine its
durability. The new approaches towards durable vertical resistance will
depend increasingly on the application of genetic engineering.
W.J. Stiekema
Head of the Department of Molecular Biology, Centre for Plant Breeding and Reproduction Research (CPRO-DLO), Wageningen, the Netherlands.
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