Exactly how plants know when it is time to flower is a subject that has perplexed botanists for thousands of years.
But two teams of scientists have uncovered clues that may help explain why certain plants need a cold spell to stimulate flowering. The research could lead to new ways of increasing the productivity of crop plants.
Although several genes linked to flowering have already been identified, researchers at the University of Wisconsin-Madison have found another, called VIN3, that blocks the expression of a gene known as FLC.
FLC encodes a protein that prevents flowering.
\"It is part of the system that measures the duration of cold,\" Professor Richard Amasino, a biochemist at the university, said in an interview.
The cold spell needed by many plants to stimulate flowering is called vernalization. Certain plants grow in the autumn but shut down in the winter and need to make sure they are not tricked into flowering too soon during an early warm spell.
\"VIN3 is a piece of the system designed to recognize that it has been a complete winter,\" Amasino said.
Amasino and Sibum Sung, who pinpointed VIN3, said it goes into action only after the plant has been exposed to the winter cold. They found it by studying a variety of Arabidopsis thaliana, a common weed that takes two seasons to bloom.
Mutant plants without VIN3 did not bloom at all. In research reported in the science journal Nature, Amasino and his team explained that the VIN3 protein is needed to alter proteins called histones bound to the FLC gene.
In the autumn, histones are modified so FLC is highly expressed. But in the spring, after vernalization, they silence the FLC gene and switch the plant into a flowering state.
In a second study in the journal, scientists at the John Innes Center in Norwich, England also studied Arabidopsis to identify chemical changes in plants that help them \"remember\" that they have gone through a cold period.
In previous research they showed that two genes, VRN1 and VRN2, were involved in flowering. In the latest study, they discovered that the genes caused chemical changes in a histone called H3 that makes the plants remember they have had a long period of cold to stimulate them to flower in spring.
\"In the depths of a British winter we are all looking forward to the coming of spring when we can forget the cold. But many plants will be remembering the winter cold to ensure that, come spring, they flower at the right time,\" said Professor Caroline Dean, the leader of the research team.
\"Understanding this process is of tremendous scientific importance but it is also of practical interest as flowering time can have a big effect on crop yields,\" she added.