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To ensure that reproduction occurs in flowering plants the egg and the sperm
need to meet, otherwise fertilization fails. Failed fertilization is a big
problem in agriculture. To improve the situation, Doctors Dresselhaus,
Marton, Cordts and Broadhvest studied the attraction of the pollen by the
female egg apparatus and found some new insights that could help increase
fertility in agriculture, September 2005 by Shelley Jambresic,
Checkbiotech.

Due to its inability to move on its own, the pollen of flowering seed
plants needs to be transported to the egg by a pollen tube to allow double
fertilisation. Double fertilisation means that two sperms (within a single
pollen tube) are involved in the fertilisation. One sperm fertilizes the
egg, producing the embryo and thus the next generation of the plant. The
other combines with the polar nuclei of the central cell to produce the
endosperm, a tissue that nourishes the developing embryo.

After pollen grains reach the flower, each sends out a tube, which carries
its two sperms inside the flower where the egg and the central cell are
located. However, little is known about the molecules, which the female egg
apparatus produces to guide the pollen tube to the egg and the central cell.
Dr. Dresselhaus and his research team from the University of Hamburg and
Bayer Bio Science N.V., Gent, now identified one of the molecules, the Zea
mays EGG APPARATUS1 (ZmEA1).

"ZmEA1 is exclusively expressed in the maize egg apparatus before
fertilization," explained Dr. Dresselhaus. "It was no longer detectable at
later embryo stages."

Similar forms of ZmEA1 were found in rice, but not in other plants. "We
think that due to its very specific expression, genes expressed in the egg
apparatus from other grass species are not available yet in the public
database," Dr. Dresselhaus told Checkbiotech. Consequently, it has not been
possible to compare the sequence of ZmEA1 to the genes expressed by other
grass species.

"However, using the ZmEA1 sequence as bait, we identified a relatively high
number of yet non-described proteins in the database that contain a
so-called EA1-box," said Dr. Dresselhaus. "This is also the region showing
highest similarity between rice and maize proteins." The EA1-box is also
similar, but not identical, to other putative signalling proteins from
grasses.

Dr. Dresselhaus suggests that the function of ZmEA1 orthologs in other grass
species might be the same, and that the fact that the EA-1 box is not
identical throughout different species indicates that the molecules guiding
the pollen tube may be involved in the species-barrier concept. This concept
helps ensure that only plants of the same species can reproduce resulting in
fertilization by the same plant species only.

"We are planning to express the EA1 protein in the egg apparatus of barley
and other grass species to increase fertilization rates with maize pollen."
However, since pollen tube guidance is a very complex process, Dr.
Dresselhaus is cautious. "We are realistic enough, where we will not promise
that by using this single molecule we can attract pollen tubes from every
species towards the maize."

Nevertheless, further research is being carried out by Dr. Dresselhaus?
team. "Identification of the mature protein is in progress, and we will then
try to identify the ZmEA1 receptor." Through their work Dr. Dresselhaus and
his research team hope to provide a better understanding of the
fertilisation barriers between plant species, which would result in
increased inter-specific fertilisation rates and better tools to generate
genetically modified plants.

To ensure their research in this area may continue, Dr. Dresselhaus and his
research team submitted a proposal for a grant from a national funding
agency. "Future work in this field will however depend on the results and
achievement obtained during the next two years," Dr. Dresselhaus told
Checkbiotech. "I may then establish a larger collaboration with additional
partners to better understand and overcome crossing barriers."

www.checkbiotech.org

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