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endosperm pollution

August 30, 2003
Prof. Joe Cummins
e-mail: jcummins@uwo.ca
?Endosperm and food standards for transgenic crops?
The European Union (EU) has developed criteria for the transgenic DNA content
of seeds for planting and the plant based foods sold for human consumption.
Grain crops such as maize and soy along with vegetable oil such as canola are
derived from seed. Seed and grain crops differ from crops such as potatoes or
beets which are modified plant organs. For example normal beets, potatoes or
maize fodder is not effected by pollen from polluting transgenic plants , none
of the plant organs are genetically modified in the crop except the seed. In
contrast, grain crops are altered by transgenic pollen the first season of
their planting. Pollination and double fertilization are fundamental to the
production of grain crops. Double fertilization means that one haploid nucleus
from the pollen grain fertilizes the ovule to produce and embryo while the
other haploid nucleus fertilizes a pair of haploid nuclei from the egg
reduction divisions to produce the triploid endosperm. (1,2). Endosperm
material makes up from 70% to 90% of the final grain or oil seed mass (2). Thus
the grain crop is immediately and strongly effected by pollen from transgenic
polluting plants so that a transgene free seeding of a grain or oil seed crop
can be extensively polluted by transgenic pollen the season of its planting.
Transgene polluted endosperm normally consist of one copy of transgene per
triploid nucleus (the transgenes are hemizygous). In maize the triploid nuclei
undergo mitotoic divions following nuclear replication beginning about 8 days
after pollination and undergoes replication and mitotic divisions for about 6
days then mitosis stops but endoreduplication of nuclei begins and continues
for about 10 days. The number of cycles of endoredublication is influenced by
environmental factors but under optimum conditions the final endosperm nuclei
have many multiples of the transgene copies. The endoreduplicated endosperm
nuclei reach 24 to 48 times the DNA levels in a haploid nucleus (1). Each maize
kernel on a cob results from a single pollen fertilization of an egg cell so
each corn cob may have none, one or many transgenic kernels. The effect of the
pollen parent on the corn crop is called Xenia, the pollen effects the size of
the kernel but is not the sole determinant (3). The actual size of the grain ,
which is mainly endosperm, is governed by genetic considerations (4), by high
temperature during endosperm cell division (5) and by water availability (6).
The implications of the studies described above is that the transgenic DNA
level in crop and in the seeds giving rise to the transgenic pollution of the
crop is strongly highly variable because it is influenced by environmental
genetic and environmental factors. The size of the embryo in the seed seems to
be fairly invariable while the endosperm which dominates the mass of the grain
is highly variable.
The European Union (EU) has been trying to thrash out acceptable levels of
transgenic pollution of seeds and food. The problem of setting acceptable
levels of transgenic pollution seems to have been approached in a manner more
arbitrary and bureaucratic than based on fundamental aspects of plant biology.
According to EU foods may contain up to 0.9% transgenic contamination while a
draft regulation sets thresholds of 0.3% trans gene pollution in oil seed rape
(canola) ;0.5% transgene pollution in
tomato, beet, cotton, chicory, maize ,potato and cotton ; 0.7% transgene
pollution in soya; before having to be labeled (7,8,9,10). The differences in
acceptable seed pollution were based on on the degree of out crossing of the
crop ( canola widely disperses pollen while soy is mainly, but not entirely,
self pollinated).In maize, for example, 5 trangenic maize seeds in a batch of
1000 seeds would be deemed acceptable, while 3 trangenic oilseed rape seeds in
a batch of 1000 seeds would be deemed acceptable.
In contrast, the transgenic pollution allowed in food does not mean that the
0.9% pollution of food meant that the food was manufactured from , for example,
maize containing 9 transgenic corn kernels per 1000 corn kernels milled. In
processed foods 0.9% transgenic pollution means that transgenes were measured
directly in the processes foods and there were 9 such transgenes for 1000 maize
nuclear marker genes, from an enzyme essential for the survival of the corn.
The processed foods can be identified as maize containing from the enzymes and
the transgene level can be determined relative to the corn enzyme genes. The
publications listed below show that the available methods for detecting
transgenic DNA in food refer to the processed food and not the proportion of
transgenic grains or seeds in the raw materials for the food were processed
(11,12, 13, 14). The essential point is that the 0.9% transgene content in
processed food is a very different quantification from the proportion of
transgenic seeds in a crop planted in the field.
Grain crops such as maize or oil crops such as canola are the products of
double fertilization, as are all higher plants, but the grain crop harvested in
the season of the planting are made up for the most part of endosperm and to a
lesser extent of embryo cells. In a maize plot originating from 5 transgenic
plants among one thousand normal plants, will the final grain crop contain
0 .5% transgenic maize kernels? The answer is not likely! On the corn cob each
kernel results from pollination of one egg cell and one endosperm cell. The
maize plants are usually hemizygous so each transgenic plant releases pollen
made up of equal number transgenic and normal pollen grains. In theory the
transgenic pollen could fertilize some kernels on each and every plant in the
plot of 1000 maize plants but a more reasonable expectation is that about 100
or more corn plants would contain cobs with some transgenic kernels. For the
crop planted with 0.5 % transgenic maize seeds the pollution can vary from 0.5%
to as much as 10% transgene pollution of the final maize food. If the
transgenic pollution of the seeds is measured after milling seed samples and
the pollution measured as in finished foods the number of transgenic seeds per
1000 seeds could exceed 50 or more thus ensuring that the 0.9 % food standard
was greatly exceeded by pollen pollution of the corn crop.
Why do EU bureaucrats believe that an arbitrary allowance of 0.5% transgenic
maize seed or 0.3% transgenic oil rape seed will prevent an excess of 0.9%
transgenic pollution in the final food? The bureaucrats have not come forward
with a rational basis for the proposed regulation on transgene pollution of
seed , indeed, it seems to be just a policy, groundless but a pleasing
compromise for the benefit of politicians and industry.
Of course, the clear variability of the sources of transgenic pollution and
their dependence on genetic and environmental factors that are not entirely
defined indicates that experimentation is the only reasonable means of
establishing sound criteria for establishing acceptable transgene pollution of
seeds. Not surprisingly, the bureaucrats have argued the number for the
criteria for several years but there has been little or no interest in using
experiments to establish the criteria. Finally, those same bureaucrats recently
adamantly decried that the established criteria were fixed and could not be
altered. Any scientific criterion should be subject to regular review and
should meet experimental justification.
In conclusion, food crops such as maize or oil rapeseed consist mainly of
endosperm. A few transgenic seeds in a field can extensively spread transgenic
pollen that significantly pollutes the crop harvested the year of the initial
planting. Criteria set to determine acceptable levels of transgenic pollution
of food crops seem to be inadequate and misleading. The sole means of
establishing rational criteria is by field experimentation. In the interim, the
only reasonable seed criteria should be lowest detectable level of transgenes.
1. Dilkes,B.,Dante,R,Coelho,C and Larkins,B Genetic analysis of
endoreduplication in Zea mays endosperm: Evidence of sporophyte and maternal
control? 2002 Genetics 160,1163-77
2. Engelen-Eigles,G.,Jones,R, and Phillips,R ?DNA endoreduplication in maize
endosperm cells is reduced by high temperature during the mitotic phase? 2001
CropSci 41,1114-1121
3. Bulant,C,Gallais,A, Matthys-Rochon,E and Prioul,J. ?Xenia effects in maize
with normal endosperm: ii. Kernel growth and enzyme activities during grain
filling? 2000 Crop Sci. 40, 182-9
4. Maitz,M,Santandrea,G, Zhang,Z,Lal,S,Hannah,L,Salamini,F and
Thompson,R. ?rgf1, a mutant reducing grain filling in maize through effects on
basal endosperm and pedicel development? 2000 The Plant Journal 23,29-42
5. Commuri,P and Jones,R ?High temperature during endosperm cell division in
maize: A genotypic comparison under In Vitro and field conditions? 2001
Crop Sci 41,1122-30
6. Yu,L and Setter,T. ? Comparative transcriptional profiling of placenta and
endosperm in developing maize kernels in response to water deficit? 2003
Plant Physiology 131,568-82
7..Royal Society of Edinburgh ?The adventitious presence of GM seeds in seeds
of conventional varieties? 2001
8.Friends of the Earth ?Press release : Governments wildlife advisors slam EU
proposal? 2002
COUNCIL concerning traceability and labelling of genetically modified organisms
and traceability of food and feed products produced from genetically modified
organisms and amending Directive 2001/18/EC
(presented by the Commission)
THE COUNCIL on genetically modified food and feed 2001
(presented by the Commission)
11. Petit,L,Bairage,F,Balois,A.,Bertheau,Y. and Fach,P. ?Screening of
genetically modified organisms and specific detection of Bt176 maize in flours
and starches by PCR ?enzyme linked immunosorbant assay? 2003 Eur Food Research
Technology 217,83-9
12. Holk,A,Va-LucDiderjerjean,M and Rudi,K. ?5?-Nuclease PCR for quantitation
event-specific detection of the genetically modified Mon 810 MaisGard maize?
2002 Eur Food Research Technology 214,449-53
13. Hernandez,M.,Rio,A.,Esteve,T,Prat,S and Pia,M. ?A rapeseed specific gene ,
acetyl-coA carboxylase, can be used as a refernce for quantitative and real
time quantitative PCR detection of transgenes from mixed food samples? 2001 J.
Agric Food Chem 49,3622-7
14. Rudi,K,Rud,I and Holck,A. ?A novel multiplex quantitiative DNA assay based
on PCR (MQDA-PCR) of transgenic maize in food and feed? 2003 Nucleic Acid
Research 31, DOI: 10.1093/nar/gng061 ,pp 1-8


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