such as "Introduction", "Conclusion"..etc
Farmers throughout the world spend an estimated $36 billion a year to
buy seeds for crops, especially those with sought after traits such as
hardiness and pest-resistance. They can't grow these seeds themselves
because the very act of sexual reproduction erases many of those
carefully selected traits. So year after year, farmers must purchase new
supplies of specially-produced seeds.
This problem is sidestepped by some plants—such as dandelions
and poplar trees—that reproduce asexually by essentially cloning
themselves. Jean-Philippe Vielle-Calzada, a Howard Hughes Medical
Institute (HHMI) international research scholar, wondered whether he
could learn enough about the genetics of asexual reproduction to apply
it to plants that produce sexually. In an advance online publication in Nature
on March 7, 2010, Vielle-Calzada and his colleagues report that they
have moved a step closer to turning sexually-reproducing plants into
asexual reproducers, a finding that could have profound implications for
"Agricultural companies and farmers around the world have a
tremendous interest in this method," says Vielle-Calzada, a plant
researcher at the Center for Research and Advanced Studies of the
National Polytechnic Institute in Irapuato, Mexico. "It would allow them
to simplify the labor-intensive cross-hybridization methods they now
use to produce hearty seeds with desirable traits."
As with animals, sexually-reproduction in plants involves the
generation of male and female gametes that each carry half of the
organism's genes. Flowering plants exhibit the most advanced form of
sexual plant reproduction, producing pollen-derived sperm cells that
join with egg cells to produce seeds. Each seed, then, is genetically
unique. There are several types of asexual reproduction in plants, but
all produce the same result: genetically identical daughter plants.
Vielle-Calzada's quest to develop an asexual seed began a decade
ago, when he decided to investigate apomixis, a specific type of
asexual reproduction. Many species of plants use apomixis to generate
viable seeds without the fusion of sperm and egg. This method of asexual
reproduction results in the formation of seeds that are essentially
clones of the main plant and has great potential for crop improvement.
In apomixis, reproductive cells retain the full complement of
chromosomes, rather than losing half their genes via meiosis, as happens
in sexual reproduction. About 350 families of flowering plants rely on
apomixis to reproduce, but nearly all plants used for food reproduce
Vielle-Calzada studied apomixis in Arabidopsis thaliana, a
small flowering mustard plant with a compact and well understood
genome. Arabidopsis was also selected because it does not
reproduce asexually. "We've been trying to induce apomixis in a species
that doesn't practice it," he says.
In the research reported in Nature, Vielle-Calzada and
scientists from Mexico, France, and the United States homed in on a
reproductive structure of Arabidopsis called the ovule. Each tiny
ovule produces a single female gamete, which, when fertilized, grows
into a seed. The team used a genetic screen to identify genes that are
active in the ovule – reasoning that measuring gene activity would lead
to important insights into which proteins are essential for guiding
The researchers netted a number of interesting genes in their
screen, but one in particular, Argonaute 9, caught their attention
immediately. The large family of Argonaute proteins has gained
widespread attention among researchers because the proteins control
which gene products—either RNA or proteins—a cell makes. Argonautes do
this by slicing up messenger RNA before it can be translated into
proteins. The identification of Argonaute activity in the ovule was all
the more interesting, says Vielle-Calzada, because Argonaute proteins
had never been seen in Arabidopsis reproductive cells before.
Next, Vielle-Calzada and his colleagues mutated the Argonaute 9
gene and watched what happened next. The results were swift and
provocative. Instead of producing a single gamete, most of the ovules
with the disrupted Argonaute gene produced several gametes, which were
abnormal. Instead of carrying half of the species' chromosomes, they
carried the full complement of genetic material— implying that they had
not undergone meiosis.
"By cutting off the function of Argonaute, we caused a
'schizophrenic' reaction of the cells in the ovule, which were not
supposed to become gametes," Vielle-Calzada says. "It looks like
Argonaute normally prevents those cells from being transformed into
gamete precursors." That suggested that Argonaute 9 prevents the
initiation of apomixis in Arabidopsis.
The finding raises the possibility that many—or maybe even
all—plants have the ability to reproduce through apomixis, but that
potential is suppressed by Argonaute 9. "It's possible that plants have a
very old memory that allows them to reproduce asexually,"
The team then searched inside the ovule to look for the pieces
of RNA that Argonaute 9 degraded. They found that Argonaute chewed up
2,600 snippets of RNA. The experiment "was a complete tour de force for
the lab," Vielle-Calzada says. "It required a lot of ovules and a lot of
After mapping those RNA sequences back to the Arabidopsis
genome, the team discovered that more than half were produced by
transposons. Transposons, also called "jumping genes," are mobile
genetic elements that copy and insert themselves throughout the genome.
Their function remains somewhat mysterious, although some evidence
suggest they are important in controlling gene expression.
"It seems that Argonaute 9 silences transposons in the ovule of
Arabidopsis," Vielle-Calzada says. "The open question now is, 'Why?'"
His working hypothesis is that squelching the transposons prevents
apomixis, but his lab is working to prove the connection. "These results
are exciting because they suggest for the first time that transposons
could be controlling early development in plants," he says.
Though he has made great progress, Vielle-Calzada is still
working toward creating a fully asexual Arabidopsis plant. His
current mutants do not develop completely asexual seeds. But by
highlighting the role of Argonaute 9 in plant reproduction,
Vielle-Calzada has moved a step closer to a slew of agricultural
possibilities. "Now we just need to discover how to trigger the second
and final step of making sexual plants asexual," he says.
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