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Newly Discovered Hormone Controls When Plants Bloom (July 31, 2007)

The Hd3a-GFP protein at the tip of the stem (where flowers are formed), observed under a microscope. (©NARA Institute of Science and Technology)
A group of researchers led by Professor Shimamoto Ko of Nara Institute of Science and Technology (NAIST) has identified the hormone that makes rice plants flower, while a group from the Max Planck Institute in Germany has identified the equivalent hormone in the arabidopsis plant. The teams confirmed that the two proteins, called Hd3a and FT, respectively, are produced in the plants' leaves and then transported to the tip of the stem. The theory that flower buds are formed by florigen (flowering hormone) produced in the leaves through stimulation from sunlight was first put forward in 1937. It has taken 70 years for scientists to track down the hormone.

What Is Florigen?
Flowering hormone is a chemical signal that causes flower buds to form. When a plant reaches a certain point in its growth, it begins preparing to make flowers. This stage is called anthogenesis. Florigen is the hormone that sends the order to begin these preparations to the top of the stem and activates the genes responsible for making flowers.

Hd3a protein, which is the flowering hormone of the rice plant, is produced in the fibrovascular bundle of the leaves and then transported to the meristem at the tip of the stem, where it causes flowers to form. (©NARA Institute of Science and Technology)

Plants time their flowering according to the length of the day. It is their leaves that sense how long a day is, while the flowers are actually made by the meristem at the tip of the stem. Some scientists postulated, therefore, that flowering hormone was produced in the leaves and then transported to the tip of the stem through the stem's sieve tubes.

It was previously thought that florigen was responsible for causing flowers to open, but researchers have learned that this is done by a separate hormone and that florigen is responsible for triggering the formation of buds. Countless plant hormones have been identified over the past few decades, but until recently florigen had proved elusive.

A World First
In 2005 a group at Kyoto University revealed the mechanism by which the FT gene in arabidopsis (commonly known as thale cress or mouse-ear cress) forms flower buds. The team discovered that the gene works in the fibrovascular bundle of the plant's leaves, making FT protein. FT protein joins with another protein, FD, in the bud to produce a complex, and once the functioning of this complex is adjusted, it turns on the plant's bud-formation switch. It was still not known, however, that FT protein is transported from the leaves to the buds. This is what the Japanese and German groups have now confirmed.


Professor Shimamoto and his NAIST team used rice plants for their research. By combining the rice plant’s Hd3a gene with a fluorescent material called GFP, they were able to observe the route by which the flowering hormone is transported. Introducing the gene to the rice plants caused them to flower early.

It is thought that these results could be used to control when plants flower. The main scientific significance of the work, though, is that the researchers have identified a hormone that had eluded plant biologists around the world for over 70 years.

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Copyright (c) 2007 Web Japan. Edited by Japan Echo Inc. based on domestic Japanese news sources. Articles presented here are offered for reference purposes and do not necessarily represent the policy or views of the Japanese Government.

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