Though a rose, carnation or tulip each has its own distinguishing feel, look and smell, they all share one common trait: the flower's petals adorn its perimeter, while the reproductive organs sit in the flower's center.
In fact, just about every flowering plant ever catalogued follows this same pattern. Till recently, scientists didn't understand how this occurred.
Now, plant biologists led by Detlef Weigel, an associate professor at The Salk Institute, have gotten to the root of this mystery.
In a study published in today's issue of the journal Cell, Weigel and his colleagues report that floral patterning takes advantage of the same gene, called WUSCHEL or WUS, which scientists already knew was essential for patterning in shoots that form stems and leaves.
The finding not only helps solve a long-sought question about the evolution of flowers, it also could have a practical application -- the production of larger fruit.
"Fruits come from the center of a flower, so if you can adjust the interplay between the genes involved, one certainly could make larger fruit in the center of the flower," Weigel said.
The Weigel-led study builds on previous work in his lab that resulted in the identification of the LEAFY gene as a "master switch" for flower development. Salk investigators had shown that LEAFY itself was sufficient to convert shoots to flowers. The finding not only suggested a plethora of potential agricultural applications, including accelerating flower and fruit production in crop plants, it also showed that flowers were modified leaves.
What remained to be discovered was what triggered the common pattern seen in virtually all flowers, so that its major organ types of sepals, petals, stamens and carpels are arranged in four concentric rings or whorls.
"People had a hard time figuring this out," said Weigel. "Most everyone thought this patterning evolved from scratch. We discovered that was wrong."
In their study, Weigel's team showed that WUS, which helps determine center from periphery in shoots, also works to control patterning in flowers. Here, it works with LEAFY to activate yet another gene called AGAMOUS. This latter gene specifies the development of the inner section of the flower, the reproductive organs.
"So basically, the way it works is you have one factor, LEAFY, which makes flowers different from shoots," said Weigel. "And then you have another factor, WUS, which makes center different from periphery but has nothing to do with flower specificity. But then you add both together, and you specifically mark the center of a flower."
Interestingly, the scientists also discovered why stems and branches can keep growing for years, while flowers have a limited life expectancy. That's because AGAMOUS shuts down WUS during the latter stages of floral development.
"WUS induces its own repressor and therefore is involved in specifying this very important difference between flowers and shoots to make sure flowers don't grow forever," said Weigel.
Theoretically, he added, one could adjust the interplay between these two genes, somewhat like the way a faucet mixes cold and hot water, to engineer plants that continually flower. The result, for example, could be the production of more carpels.
"Big tomatoes come about when you make many more carpels than normal tomatoes," said Weigel. "So you could presumably play with the size of the fruit if you somehow can play with the interaction of these two genes."
There's an historical footnote to all this, Weigel notes. More than 200 years ago, the German poet and philosopher Johann Wolfgang Goethe first proposed that floral organs are modified leaves. (See this web page.)
"This was confirmed in the 1990s," Weigel said. "What wasn't known was how floral pattern evolved.
"Now, we show that the pattern arose through clever recycling of another pattern that plants had previously used in shoots, for other purposes. This is a big step toward solving the 'abominable mystery,' as Darwin put it -- the evolutionary origin of flowers and flowering plants.
"I think this all goes a long way toward solving this problem of where flowers came from."
Also participating in the study were: Jan U. Lohmann, Ray Hong, Maximilian A. Busch, and Francois Parcy, all from Salk; and Martin Hobe and Rudiger Simon, from the University of Cologne.
The study, called "A Molecular Link Between Stem Cell Regulation and Floral Patterning in Arabidopsis", was funded by the Human Frontiers Science Program Organization, the Studienstiftung des Deutschen Volkes and BASF, the National Institutes of Health and the U.S. Department of Energy.
(Editor's Note: As reported in the same Issue of Cell, Thomas Laux and colleagues at the University of Freiberg in Germany created transgenic plants with reduced levels of AG. These plants had more flowers than normal, with interior petals within the flowers where other organs such as the sepals would normally be found. Taken together, these findings suggest that AG in turn negatively regulates WUS to limit flower production. The results show that flowers are developmentally similar to other plant organs like carpels, shoots and leaves. This is evidence for the hypothesis that flowers, which first appeared on Earth abruptly around 130 million years ago, co-opted a pre-existing protein patterning system present in shoots.)
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation. - By Suzanne Clancy
The Salk Institute for Biological Studies
[Contact: Warren R. Froelich, Suzanne Clancy]