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[SANET-MG] gene for late blight in potato
January 28, 2006
Prof. Joe Cummins
Genes from a wild plant, Solanum bulbocastanum, used to resist potato
blight fungus
The German company BASF Plant Science GmbH is planning to
test genetically modified (GM) potatoes in Ireland by deliberately
releasing the modified potatoes into the environment. The field trial is
planned to be undertaken during a five year period. The GM potatoes are
modified using a gene from a wild Mexican plant, Solanum
bulbocastanum, related to potato along with marker genes including a
gene for resistance to a herbicide. The potatoes are modified to be
resistant to the fungus causing late blight disease No environmental and
health studies appear to be planned (1). Animal feeding studies on the
GM potatoes do not appear to have been done. Release to the environment
of untested GM crops to the environment are unwise because even
related plant to related plant single gene transfers have resulted in
unexpected toxicity which appeared in the transgenic gene product
resulting from altered structure and immunogenicity of the modifying
gene product (2).When single genes from plants such as bean are used to
modify another plant, the pea, it was assumed that such transfers could
not produce toxic products in the plant being modified. , however, the
unexpected prevailed. The people and animals of the townland of
Arodstown should not be exposed to inadequately tested genetic
constructions.
Late blight is one of the most devastating plant diseases.
It is caused by the fungus ,Phytophora infestans, a pathogen of potato
and to a lesser degree tomato. In potato, Solanum tuberosum, there are
four main dominant genes for resistance to blight infection, R1 through
R4, an additional 7 genes were identified 5 of which are alleles of
the complex R3 locus (for a total of 11 dominant R genes).
Hybridization with wild Mexican species began in 1909 and continues to
the present. However, in spite of constant effort the fungus rapidly
developed strains that overcame the genetic resistance. Chemical
fungicides have been developed to control blight but these to succumbed
to the versatility of the fungus. The fungus has two mating types (A1
and A2) both of which appeared first in Mexico, however, only the A1
mating type was present in European potatoes until 1978 when the A2
mating type appeared in Britain. The presence of the two mating types
greatly enhances gene exchange leading to accelerated loss of genetic
resistance and fungicide control (3,4).
Early resistant potatoes were obtained using true sexual
hybridization with wild Mexican species but the resistant strains
soon succumbed to mutants of the blight fungus. A wild Mexican plant,
Solanum bulbocastanum, was stably resistant to blight but could not be
sexually crossed with potatoes. A process called somatic hybridization
was used to create sexual hybrids. Somatic hybridization includes
fusing cells from cell cultures of Solanum bulbocastanum and potato,
fused cells contain nuclei of both potato and Solanum bulbocastanum.
When the fused cells undergo mitosis the chromosomes of the two species
are mixed and a single hybrid nucleus is formed in the cell. The cells
can be cultured on solid media to form solid callous (tumor) which when
treated with plant growth hormones produces plantlets that produce
flowering plants . The somatic hybrids have irregular meiosis and
irregular chromosome pairing but relatively stable blight resistant
lines can be obtained (5,6,7) Along with the 11 potato blight
resistance genes that produce broad spectrum resistance are very
effective against blight, these .include the gene RB (8) along with the
genes Rpi-blb1 (9) and Rpi-blb 2 (10) which are active in both potato
and tomato. The somatic hybrids are useful in identifying resistance
genes and transmitted into potato breeding lines by crossing.
Nevertheless, genetic modification of potato breeding lines is
presently preferred because resistance can be introduced into
commercial lines with greater speed.
The BASF proposal for field testing GM potatoes (11)
involves the use of two broad spectrum resistance genes, Rpi-blb1 and
Rpi-blb2, These two genes have a structure associated with regulatory
genes called nucleotide binding site-leucine rich repeat (NBS-LRR)
class of regulatory proteins. Many disease resistance genes code for
proteins of that class. Numerous plant NBS-LRR genes are present
are present in the typical plant genome, each protein is specific for
a particular pathogen signaling a defense response frequently a
localized plant cell death called a hypersensitive response. The C
terminus of the protein containing LRR recognizes a ligand feature of a
pathogen activating the NBS signaling module to initiate the defense
response (12). The blight fungus suppresses the potato defense genes
in sensitive plants but thwarted by successful defense genes. The
NBS-LRR resistance genes in plants are localized in the cell cytoplasm
and do not span the cell membrane but are activated by pathogen signals
that penetrate the cell (13). The plant NBS-LRR proteins generally
produce antibodies when injected into mammals but the modifications of
the disease resistance proteins by glycosylation or mryistylation which
contribute to the immune response are not yet studied.
The BASF proposal (11) indicates that the potatoes being
studied were transformed using two plasmids each containing copies of
the S. bulbocastanum resistance genes Rpi-blb1 and Rpi-blb2 both of
which contained an intron. The two genes were each driven by Rpi-blb 1
or 2 promoter with including an intron as an enhancer and accompanied
by a transcription terminator from Rpi-blb1 or 2. The plasmids also
contained a mutant acetohydroxy acid synthetase (ahas) gene from the
tiny mustard plant Arabidopsis that conferred resistance to the
herbicides of the imidazolines group
( which are not approved for use on potatoes in Ireland). The ahas gene
was driven by the nopaline synthase gene promoter of Agrobacterium and
its transcription was terminated using the nopaline terminator. The
transformed potatoes aare herbicide tolerant but the herbicide is only
used during selection of transformed potato cells and not during
cultivation of the potato. All GM lines intended for the release
contain one or two copies of the plasmid inserts. Neither the
resistance genes nor the ahas gene is expected to effect pollen or seed
dispersal of the potato. The possibility that the GM potatoes will
outcross to field potatoes was not expected to be effected by the
genetic modifications. Interestingly , the expression of the modifying
genes was not studied under extreme conditions of stress such as
drought, water logging, heat, cold, nitrogen excess or starvation in
glass house experiments. In the past, gm crops have been tested under
optimum conditions for growth prior to commercial or test release into
real environments. Certainly, stress conditions may lead to unexpected
toxicity in gm crops.
The BASF proposal (11) indicate that the resistance genes
are not expected to exert any toxic , allergenic or harmful effects on
human health arising for genetic modification. The genetic modifications
are assume to be safe because plants contain numerous NBS-LRR proteins
and cultivated potatoes contain R genes from the wild species S.
demissum. The assumptions of safety are specious. The S. demissum genes
in commercial potatoes are NBS-LRR genes but are not from the broad
spectrum NBS-LRR genes used in the BASF potatoes. Mainly, however,
observed finding that transfer of genes between related species may
actually lead to proteins with powerful (sometimes fatal) immune
responses.(2). The procedure used to scan DNA sequences for epitope
specifying codes for allergic responses (IgE) would overlook the
powerful immune responses leading to fatal or near fatal inflammation
. It is only sensible to test glass house grown GM potatoes for not
only allergenicity but for inflammation before releasing the GM potatoes
to the environment. The immune response that triggered the immune
response described in reference 2 was triggered to altered protein
modification following transfer between species . However, little
information is available on the modification of plant NBS-LRR genes. It
sems a simple matter to conduct animal experiments on glasshouse grown
GM potatoes prior to release of the potatoes to the environment yet that
does not seem to have been done. Impact of the site on non-target
organisms seems to be based on an assumption safety and does not provide
for an adequate monitoring scheme. If the GM potato proves
immunologically active the impact on both human and animals may be severe.
In the proposal the handling , release controls and
disposal of Pytopthora infestans innocula and infected plants was
alluded to but not described in detail. That should be done. The
isolation distance 20 meters to cultivated potatoes does not seem
adequate. Control of GM seeds and tuber escape from the site did not
seem to be adequately described in the proposal. Post release treatment
of the test site did not seem adequately monitored nor will it achieve a
clean post harvest site. There does not seem to be any reason that a
round the clock guard cannot be kept over the test sites. In
conclusion, the considerations of human and environmental safety seem
primarily based on wishful thinking not on serious efforts to gather or
obtain factual information on the safety of the gm constructs.
Monitoring also seems based on wishful thinking rather than serious
efforts to detect negative impacts.
BASF petitioned for field test release of resistant
potatoes modified with
Rbi-blb1 and 2 beginning 2005 in the Netherlands. The notice of
petition indicated that the GM potato would be released in Germany,
United Kingdom and Sweden. (14). In the United States five field tests
have been undertaken using GM potatoes modified with RB1 and RB2 broad
spectrum NBS-LRR blight resistance genes obtained from Solanum
bulbcastanum. The releases were undertaken in Minnesota and Wisconsin,
by USDA or the university of Minnesota (15) The isolation and
deployment of the RB genes in potato has been described (16,17)
Field testing of broad spectrum .NBS-LRR genes has begun with the
potato blight resistant strains. Broad spectrum pest resistant
strains of rice, maize, soybean, and numerous food crops will soon
follow. It is imperative that the safety of these genetic
modifications to humans and the environment be fully evaluated before
the GM crops are commercialized. The proposition that the NBS-LRR
family of plant pest resistance genes and their products provide safe
transgenes for human consumption and for environmental release
because they are found in food crops and for that reason require no
further testing is simply fool hardy. The suggestion that NBS-LRR genes
must be assumed safe until proven hazardous certainly appeals to greedy
promoters of GM crops but does not serve the public good.
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