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endosperm



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.
References
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
9. Proposal for a regulation REGULATION OF THE EUROPEAN PARLIAMENT AND
OF THE 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)
10. Proposal for the regulation REGULATION OF THE EUROPEAN PARLIAMENT
AND OF 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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