From Proust's madeleine-sparked reminiscences on his youth to a bridge player trying to count out her opponents' hands, we're all familiar with human memory. Some plants, including biennials that require two growing seasons to flower and set seed, have a kind of memory too, in that they can recall a prolonged cold spell. Only then will they flower in spring--an agronomically important adaptation known as vernalization that prevents premature flowering. How plants do this has been a mystery, but researchers are getting a handle on the genes involved.
Researchers have already identified five genes involved in vernalization in the model plant Arabidopsis thaliana. Now they are beginning to crack the puzzle in commercially valuable cereals. About a year ago, Jorge Dubcovsky of the University of California, Davis, and colleagues found the first wheat vernalization gene, called VRN1. Prolonged cold exposure turns up its activity in winter wheat, which requires vernalization, but not in spring wheat, which can flower without a long cold exposure.
Now the Dubcovsky team has cloned VRN2, which encodes a suppressor of flowering. Consistent with that, its activity pattern is opposite to that of VRN1--going down, not up, during cold exposure. Mutations that inactivate it produce spring wheat lines--evidence that the team has found the right gene. Furthermore, introducing an RNA that inhibits VRN2 activity into a line of winter wheat accelerated flowering, as expected for a gene that suppresses flowering, they report in the 12 March issue of Science .
The model emerging from this work and previous genetic studies suggests that the activities of VRN1 and VRN2 are connected: Prolonged cold puts a chill on VRN2 activity, leading to up-regulation of VRN1 and flowering. "We know that these genes talk with each other somehow," Dubcovsky says. Now, the challenge is to learn how they communicate and to find the pathway by which cold turns down VRN2 expression.
Finding the wheat VRN genes was difficult, given that that they do not resemble those found so far in Arabidopsis and that the unwieldy wheat genome has yet to be sequenced. The Dubcovsky team's work is "outstanding," says Arabidopsis researcher Richard Amasino of the University of Wisconsin, Madison. "It's not trivial to do what they did in wheat."