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 agriculture.
"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 sexually.
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 asexual reproduction.
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," Vielle-Calzada says.
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 fiddling."
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.
Andrea Widener | EurekAlert!
Plasma-zapping process could yield trans fat-free soybean oil product
02.12.2016 | Purdue University
New findings about the deformed wing virus, a major factor in honey bee colony mortality
11.11.2016 | Veterinärmedizinische Universität Wien
In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.
Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...
Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
07.12.2016 | Health and Medicine
07.12.2016 | Life Sciences
07.12.2016 | Health and Medicine