June 29, 2001
Identification of Genes May Tell How Plants Recognize Pollen
Researchers have identified the genes that code for proteins that
coat the pollen of the flowering plant Arabidopsis thaliana. The
studies may help scientists understand how plants recognize pollen from
their own species.
Understanding the basis of species recognition by plants could
enable crop scientists to cross previously incompatible plant species
or to prevent genetically engineered plants from crossing with other
strains.
“Manipulating pollen recognition would enable crosses that are otherwise difficult to do -- such as mating an apple tree that is cold-tolerant with an otherwise incompatible variety that has delicious fruit.”
Daphne Preuss
The researchers, led by Howard Hughes Medical Institute investigator
Daphne
Preuss at the University of Chicago, reported on the identification
of the Arabidopsis pollen coat protein gene families in an
article published in the June 29, 2001, issue of the journal
Science.
Arabidopsis thaliana is a member of the mustard family that
also includes cabbage and radish. Arabidopsis is small,
prolific, easily grown and has a rapid life cycle. In December 2000, an
international consortium of scientists announced that it had sequenced
the entire Arabidopsis genome, an achievement that plant
scientists believe will lead to advances in understanding plant
physiology.
Earlier experiments by Preuss and others had shown that switching or
altering pollen coat proteins can change plants' species specificity or
eliminate pollination altogether. "Scientists have understood for quite
a while that the pollen coating is critical for launching the process
of pollination in a large number of species," said Preuss. "But a
comprehensive analysis of all of the pollen protein coat genes in one
plant had never been done before, and that's where this work is
unique.”
The search for the pollen coat protein genes began with work by lead
author Jacob A. Mayfield, who extracted and identified all of the
Arabidopsis pollen coat proteins. Researchers at Stanford
University Medical Center provided protein sequence information.
"Although we had these protein sequences, it was not until the
Arabidopsis genome was sequenced that our search revealed that
these proteins are encoded in tandem arrays in the genome," said
Preuss.
The scientists discovered that the coat protein genes existed in two
distinct clusters of the Arabidopsis genome, an exciting
finding, said Preuss, because it indicates that the plants have 'mating
clusters' of genes that are kept together. One of the gene clusters
coded for enzymes called lipases that cleave lipid molecules; and the
other cluster coded for lipid-binding proteins called oleosins.
According to Preuss, the lipid-related enzymes and proteins enable the
dry pollen coat to interact with the stigma cells of plant flowers
during pollination.
After finding that the genes were contained in two clusters, the
scientists began experiments designed to reveal the role of the
clusters. "Whenever genes are clustered, it raises important
evolutionary questions about why they are in clusters and what
maintains the cluster," Preuss said. To explore whether the clusters
are maintained in related Arabidopsis "ecotypes" — distinctive
plant strains from different geographic regions — co-author Aretha
Fiebig sequenced the clustered genes from five ecotypes.
Fiebig discovered that the gene clusters were maintained across
ecotypes. She also found that any variation within the genes did not
disrupt the functionality of the genes. "These findings argued that
there was evolutionary pressure to maintain the genes," said
Preuss.
When Mayfield compared the pollen protein coat gene clusters from
Arabidopsis to those of its relative, broccoli, he found a large
genetic divergence. "We believe that this divergence means that these
genes are important for speciation, and we would like to begin seeking
similar genes in other plants," said Preuss.
In future studies, Preuss’s team plans to mutate the
Arabidopsis pollen coat protein genes and swap gene clusters
from other plants to explore the proteins' basic function in species
recognition and their role in speciation. Understanding these functions
could lead to powerful new techniques for crossing plants, said
Preuss.
"Understanding the basic recognition molecules would allow two main
problems to be addressed that are opposite sides of the same coin," she
said. "First, manipulating pollen recognition would enable crosses that
are otherwise difficult to do — such as mating an apple tree that is
cold-tolerant with an otherwise incompatible variety that has delicious
fruit. The other side of the coin would be to engineer pollen to
inhibit crosses, for example so that the pollen from genetically
engineered crops would not be recognized by another variety."
According to Preuss, the discovery of the pollen protein coat genes
is an example of the kind of research that plant biologists can do now
that the Arabidopsis genome has been sequenced.
"Arabidopsis has all the basic parts that plants need for doing
what they do, so it's an excellent model. The sequencing of its genome
has put us into a wonderful discovery phase, in which we can race ahead
and test the function of large numbers of genes very quickly.
"The Arabidopsis genome sequence allows plant scientists to
work from just a small bit of gene sequence to obtain the entire
sequence of a gene," said Preuss. "Also, we already have many known
mutations of this plant, so when we make new mutants, we can compare
them with these previously characterized mutants. Finally, since much
work has been done relating Arabidopsis to other species, now
that we have the Arabidopsis genome nailed, it's very easy to
make comparisons."
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