Stanford researchers solve plant sex cell mystery

ORIGINAL: Stanford News

BY MAX MCCLURE

Stanford Report, August 6, 2012

Graduate student Tim Kelliher and Professor Virginia Walbot examine corn tassels in the field on the Stanford campus. (Photo: L.A. Cicero)

Although farmers have been manipulating plant germlines since the Neolithic, plant 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.

1. 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.
2. 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.

Stanford University is currently seeking a patent on some of the paper's findings.

Media Contact

Virginia Walbot, Biology: (650) 723-2227, walbot@stanford.edu

Max McClure, Stanford News Service: (650) 725-6737, maxmc@stanford.edu

The Big Heat

ORIGINAL: http://www.newyorker.com/talk/comment/2012/07/23/120723taco_talk_kolbert

by Elizabeth Kolbert

JULY 23, 2012

Corn sex is complicated. As Michael Pollan observes in “The Omnivore’s Dilemma,” the whole affair is so freakishly difficult it’s hard to imagine how it ever evolved in the first place. Corn’s female organs are sheathed in a sort of vegetable chastity belt—surrounded by a tough, virtually impenetrable husk. The only way in is by means of a silk thread that each flower extends, Rapunzel-like, through a small opening. For fertilization to take place, a grain of pollen must land on the tip of the silk, then shimmy its way six to eight inches through a microscopic tube, a journey that requires several hours. The result of a successfully completed passage is a single kernel. When everything is going well, the process is repeated something like eight hundred times per ear, or roughly eighty thousand times per bushel.

It is now corn-sex season across the Midwest, and everything is not going well. High commodity prices spurred farmers to sow more acres this year, and unseasonable warmth in March prompted many to plant corn early. Just a few months ago, the United States Department of Agriculture was projecting a record corn crop of 14.79 billion bushels. But then, in June and July, came broilingly high temperatures, combined with a persistent drought across much of the midsection of the country.

“You couldn’t choreograph worse weather conditions for pollination,” Fred Below, a crop biologist at the University of Illinois at Urbana-Champaign, told Bloomberg News recently. “It’s like farming in Hell.” Last week, the U.S.D.A. officially cut its yield forecast by twelve per cent, citing a “rapid decline in crop conditions since early June and the latest weather data.” Also last week, because of the dryness, the U.S.D.A. declared more than a thousand counties in twenty-six states to be natural disaster areas. This was by far the largest such designation the agency has ever made. In the past month, as the severity of the situation has become apparent, corn prices have risen by more than forty per cent. Since so much corn is used to feed livestock, it’s likely that the increase will translate into higher prices for dairy products and beef—although, as many have pointed out, beef prices were already rising, owing to last year’s devastating drought in Texas.

Up until fairly recently, it was possible—which, of course, is not the same as advisable—to see climate change as a phenomenon that was happening somewhere else. In the Arctic, Americans were told (again and again and again), the effects were particularly dramatic. The sea ice was melting. This was bad for native Alaskans, and even worse for polar bears, who rely on the ice for survival. But in the Lower Forty-eight there always seemed to be more pressing concerns, like Barack Obama’s birth certificate. Similarly, the Antarctic Peninsula was reported to be warming fast, with unfortunate consequences for penguins and sea levels. But penguins live far away and sea-level rise is prospective, so again the issue seemed to lack “the fierce urgency of now.”

The summer of 2012 offers Americans the best chance yet to get their minds around the problem. In late June, just as a sizzling heat wave was settling across much of the country—in Evansville, Indiana, temperatures rose into the triple digits for ten days, reaching as high as a hundred and seven degrees—wildfires raged in Colorado. Hot and extremely dry conditions promoted the flames’ spread. “It’s no exaggeration to say Colorado is burning,” KDVR, the Fox station in Denver, reported. By the time the most destructive blaze was fully contained, almost three weeks later, it had scorched nearly twenty-nine square miles. Meanwhile, a “super derecho”—a long line of thunderstorms—swept from Illinois to the Atlantic Coast, killing at least thirteen people and leaving millions without power.

Referring to the fires, the drought, and the storms, Jonathan Overpeck, a professor of geosciences and atmospheric sciences at the University of Arizona, told the Associated Press, “This is certainly what I and many other climate scientists have been warning about.” He also noted, “This is what global warming looks like at the regional or personal level.”

Or, at least, what it looks like right now. One of the most salient—but also, unfortunately, most counterintuitive—aspects of global warming is that it operates on what amounts to a time delay. Behind this summer’s heat are greenhouse gases emitted decades ago. Before many effects of today’s emissions are felt, it will be time for the Summer Olympics of 2048. (Scientists refer to this as the “commitment to warming.”) What’s at stake is where things go from there. It is quite possible that by the end of the century we could, without even really trying, engineer the return of the sort of climate that hasn’t been seen on earth since the Eocene, some fifty million years ago.

Along with the heat and the drought and the super derecho, the country this summer is also enduring a Presidential campaign. So far, the words “climate change” have barely been uttered. This is not an oversight. Both President Obama and Mitt Romney have chosen to remain silent on the issue, presumably because they see it as just too big a bummer.

And so, while farmers wait for rain and this season’s corn crop withers on the stalk, the familiar disconnect continues. There’s no discussion of what could be done to avert the worst effects of climate change, even as the insanity of doing nothing becomes increasingly obvious. ♦

ILLUSTRATION: TOM BACHTELL