In four months, when flower buds spring up from the ground, you may wonder how plants know it's time to bloom. This question has baffled plant biologists for years. Now, scientists at the University of Wisconsin-Madison have an answer: a gene that functions as an alarm clock to rouse certain plants from a vegetative state in the winter to a flowering state in the spring.

According to the researchers, the findings, published in the Jan. 8 issue of the journal Nature, could lead to new methods for manipulating the productivity of crop plants, as well as a better understanding of how organisms control the fate of their cells. 

Most people may not know that some of our favorite salad ingredients - carrots, cabbage, radishes, beets and parsley - take two seasons to flower and produce seeds because we harvest them before they have the chance to flower. These plants, called biennials, require a season of cold to flower.

"We've known that winter does something to the plant's growing tip, or meristem, and makes it competent to flower," says Richard Amasino, a UW-Madison biochemistry professor and senior author of the paper. "If biennials don't go through winter, they won't flower." But why, he adds, has remained a mystery.

This mystery started to unravel in 1999, when Amasino and his colleagues identified two genes central to the flowering of Arabidopsis thaliana, a small, flowering plant that's a member of the mustard family. The genes work together to block blossoming. As they observed, one of these genes is no longer expressed in the spring, when the plants can flower and complete their life cycle. 

How winter switches off this flower-inhibiting gene in the second growing season, says Amasino, was the next obvious question. So, the Wisconsin scientist and UW-Madison biochemistry graduate student Sibung Sung looked to a biennial variety of Arabadopsis, a plant that's widely used as a model organism in plant biology and genetics. They screened for mutants that wouldn't bud after surviving temperatures just above freezing, and they found three - all lacking a gene now called VIN3.

After further investigation, the researchers learned that an extended period of cooler temperatures prompts the VIN3 gene to turn on. Once activated, the gene starts the process of vernalization, whereby the plant becomes competent to flower after exposure to cold. As this process begins, the expression of the flower-suppressing gene identified in 1999 wanes until it is completely blocked.

The researchers report that the VIN3 gene is expressed only after plants have been exposed to conditions effective for vernalization, suggesting that the VIN3 gene functions as an alarm clock rousing biennial plants to bloom.

But how do plants know they've been exposed to the right temperature for the right amount of time? "This is an intriguing question," says Sung. "Without a nervous system, plants must have a mechanism by which they can remember they have been through the winter season." Although plants don't have a brain like humans do, they do have cellular machinery that appears to remember cold exposure, according to the new research.

The Wisconsin scientists show that the expression of VIN3, which occurs after exposure to cold, initiates a series of changes in one of the flower-suppressing genes. Specifically, VIN3 activation permanently modifies the structure of histones, a group of proteins over which DNA is wrapped. These changes block the flower-suppressing gene, switching the plant from a fixed state where it won't flower to a fixed state where it can flower.

Scientists speculate that changes in histone structure play a major role in the development of higher organisms and the formation of cancer cells. Says Sung, "Histone changes in model plants could give us the opportunity to extend our understanding of how organisms control their cell fates during development."

The findings by Amasino and Sung also could lead to improvements in agriculture.

"This new molecular understanding could provide information to help design tools to manipulate flowering," the biochemistry professor says. For example, agronomists could engineer biennial crops that lack VIN3 and never flower, potentially increasing yield. But as Amasino clarifies, he's in the business of basic science - it's up to others to use the information.

by Emily Carlson