Corporate Banner
Satellite Banner
Scientific Community
Become a Member | Sign in
Home>News>This Article

Stanford Researchers Solve Plant Sex Cell Mystery

Published: Wednesday, August 08, 2012
Last Updated: Wednesday, August 08, 2012
Bookmark and Share
For millennia, sex cells have stubbornly guarded the secret of their origin. The surprisingly simple answer – low oxygen levels – could change the way we breed plants.

The sex life of corn has gotten a lot of prurient attention over the years. By 5,000 B.C., agriculturalists in the Americas were already producing the first hybrid corn varieties by cross-pollinating plants to generate larger plants or colored kernels.

Today, hybrid seed production in corn is a multibillion-dollar industry, and crossbreeding is fundamental to the production of most other species as well. But despite plant reproduction's central role in agribusiness, researchers have never answered a basic question: Where do plant sex cells come from?

The answer, according to Stanford biology Professor Virginia Walbot and graduate student Timothy Kelliher, is surprisingly simple. In a set of elegant experiments – Walbot prides herself on "thinking of experiments you can do with basically no money" – the researchers demonstrated that low oxygen levels deep inside the developing flowers are all that is needed to trigger the formation of sex cells.
The discovery isn't only of academic interest.

"Controlling plant reproduction is fairly fundamental to modern agriculture," Walbot said.
In a corn industry that still detassels seed corn by hand as a way of controlling who fertilizes whom, a technique that switches sex cell production on or off could allow for dramatically increased control over plant crossbreeding.

The research paper appeared recently in the journal Science.

When two flowers love each other very much

All flowering plants produce pollen within structures called anthers, which in corn grow from the distinctive cluster of male flowers we know as the tassel. But before these anthers mature, they are arranged in a clover shape deep within the plant. The central cells within each of these clover-like lobes will turn into sex cells and, eventually, pollen.

The mechanism behind this development was unknown in plants. In animals, surrounding cells signal the germ line to begin forming from a single "founder cell." Walbot and Kelliher were leaning toward this view, having identified two promising signaling molecules, MAC1 and MSCA1. Plants that lacked the protein MAC1 developed too many germ cells. Plants that lacked MSCA1 had none at all.

Clearly, MAC1 was important for organizing the non-sexual cells around germ cells, while MSCA1 was necessary for cells to develop into sex cells. But the connection between the two, and what initially led to their expression, remained unclear.

A role for redox

Although most researchers assumed that, as in animals, sex cells were developing from a special set of cells with a predetermined predilection for the role, Walbot and Kelliher saw two clues that implied otherwise.

First, the physical arrangement of the sex cells didn't point to the existence of a single "founder." In fact, it suggested a scenario where "your position as a cell mattered more than who your parents were," Kelliher said.

Second, the way the MSCA1 enzyme operated suggested that oxygen levels might play a role in the signaling process.

The environment inside a plant can be either "oxidizing" – where oxygen is plentiful, and oxidation is favored – or "reducing" – where oxidation is prevented, usually by a lack of reactive oxygen, and the opposite process of reduction is favored. MSCA1 happened to send its signal through reduction – meaning that different oxygen levels might have different developmental effects.

To test the theory, the researchers inserted a probe deep into the immature anther tissue of corn. What they found was telling: unusually low oxygen levels – likely a side effect of the rapidly growing anthers' metabolic activity – at the precise time that cells were beginning to turn into sex cells.

Corn hose

To see if low oxygen alone was responsible for sex cell development, the researchers threaded a plastic hose into the developing anther and piped in mixtures of gases.

High concentrations of oxygen drastically decreased the number of sex cells. High concentrations of nitrogen gas, which is inert and provides a reducing environment, increased sex cell formation.

"It was a remarkably easy experiment," said Walbot. "We had the initial results in two days."

The researchers showed that low oxygen levels could even cause cells outside the anther lobes – which would never normally produce pollen – to develop into sex cells.

All together, Walbot explained, the evidence suggests that naturally occurring variations in oxygen levels inside the growing anther causes the central cells to become hypoxic first: "The cells that are most hypoxic then get to throw the switch."

Once oxygen levels drop below a certain threshold, MSCA1 is finally able to go to work and reduce its target, causing central cells to become sex cells. These cells then release MAC1, which in turn ensures that the outside cells don't become germline.

It's an inside-out differentiation pattern, utterly unlike what animal germlines do – which may explain why it took so long to be discovered.

"The plant takes advantage of its own structure to create this developmental signal," said Kelliher. "And then any cell can create the next generation as long as it's in the right place – you don't have to be specially designated. It's kind of a romantic idea."

Children of the corn research

Keeping a close watch over this entire plant fertility process is crucial for the hybrid seed industry. Fields are usually planted with two varieties of seed corn that are going to be crossbred. In order to keep plants from fertilizing themselves – which results in inferior-quality plants – all of the tassels of one species need to be removed.

This is an enormous task that requires specialized detasseling machines, followed up by people who check for plants that were missed.

"Currently they remove the tassels on 1 million acres of corn each year, at 20,000 plants per acre," said Walbot. "That's billions of hand-detasseled plants."

Sterile varieties of corn have been developed that don't require detasseling, but self-perpetuating versions have proven difficult to perfect. A low-oxygen sterilization method could make automated hybridization much simpler, allowing it to be applied to a large number of varieties.

"We leave those applications to industry," said Walbot. But the effects of the research could be wide-ranging. Assuming that the findings hold true for all flowering plants, as a number of research groups are now seeking to confirm, the discovery could open up a new level of fertility control for a huge array of crops.

Further Information
Access to this exclusive content is for Technology Networks Premium members only.

Join Technology Networks Premium for free access to:

  • Exclusive articles
  • Presentations from international conferences
  • Over 2,800+ scientific posters on ePosters
  • More than 4,000+ scientific videos on LabTube
  • 35 community eNewsletters

Sign In

Forgotten your details? Click Here
If you are not a member you can join here

*Please note: By logging into you agree to accept the use of cookies. To find out more about the cookies we use and how to delete them, see our privacy policy.

Related Content

Scientists Home In On Origin Of Human, Chimpanzee Facial Differences
A study of species-specific regulation of gene expression in chimps and humans has identified regions important in human facial development and variation.
Monday, September 14, 2015
Research Shows Importance of European Farmers Adapting to Climate Change
New Stanford research reveals that farmers in Europe will see crop yields affected as global temperatures rise, but that adaptation can help slow the decline for some crops.
Tuesday, May 20, 2014
Stanford Scientists Discover a Novel Way to Make Ethanol Without Corn or Other Plants
Stanford scientists have created a copper-based catalyst that produces large quantities of ethanol from carbon monoxide gas at room temperature.
Saturday, April 12, 2014
Scientific News
Turning up the Tap on Microbes Leads to Better Protein Patenting
Mining millions of proteins could become faster and easier with a new technique that may also transform the enzyme-catalyst industry, according to University of California, Davis, researchers.
Tardigrade's Are DNA Master Thieves
Tardigrades, nearly microscopic animals that can survive the harshest of environments, including outer space, hold the record for the animal that has the most foreign DNA.
GMO Food Animals Should be Judged by Product, Not Process
In a world with a burgeoning demand for meat, milk and eggs, regulatory policies around the use of biotechnologies in agriculture need to be based on the safety and attributes of those foods rather than on the methods used to produce them, says a UC Davis animal scientist.
Cancer-Fighting Tomato Component Traced
The metabolic pathway associated with lycopene, the bioactive red pigment found in tomatoes, has been traced by researchers at the University of Illinois.
TGAC Announces Milestone in Wheat Research
A more complete and accurate wheat genome assembly is being made available to researchers, by The Genome Analysis Centre (TGAC) on 12 November 2015.
Shedding Light on the Origin of the Date Palm
Researchers also find ‘genetic mutation’ that is responsible for dates’ color.
New Way to Find DNA Damage
University of Utah chemists devised a new way to detect chemical damage to DNA that sometimes leads to genetic mutations responsible for many diseases, including various cancers and neurological disorders.
Speeding Up Potato Breeding
A joint project is investigating the potential of drones for speeding up the development of new potato varieties.
Gene Editing Could Enable Pig-To-Human Organ Transplant
The largest number of simultaneous gene edits ever accomplished in the genome could help bridge the gap between organ transplant scarcity and the countless patients who need them.
Ancestors of Land Plants Were Wired to Make the Leap to Shore
When the algal ancestor of modern land plants made the transition from aquatic environments to an inhospitable shore 450 million years ago, it changed the world by dramatically altering climate and setting the stage for the vast array of terrestrial life.
Skyscraper Banner

Skyscraper Banner
Go to LabTube
Go to eposters
Access to the latest scientific news
Exclusive articles
Upload and share your posters on ePosters
Latest presentations and webinars
View a library of 1,800+ scientific and medical posters
2,800+ scientific and medical posters
A library of 2,500+ scientific videos on LabTube
4,000+ scientific videos