August 11, 2005
The blossoms of cherry trees are celebrated in many places including the Jefferson Memorial in Washington, DC, or Ueno Park in Tokyo, Japan, because they tell us that spring has finally arrived. As we all know, most plants make flowers only at certain times of the year, the spring blooms of cherry trees being but one example. Plants can use several cues from the environment to choose the season that is right for flowering. For example, some plants such as tulips will not flower unless exposed for several months to winter cold, while others rely on the increase in day length that heralds the arrival of spring.
Scientists have known since the 1930s that plants detect day length with their leaves. Since flowers form typically at the tip of branches, researchers concluded that a signal that induces flowering must travel from the leaves to the site where flowers are initiated. Despite these early findings, little progress has been made in pinpointing the hypothetical flower-inducing substance, dubbed florigen. These difficulties have led many scientists to believe that florigen might be not a single entity, but a complex mixture of molecules. In the new work, two teams, led by Detlef Weigel at the Max Planck Institute in Tuebingen and Philip Wigge at the John Innes Centre, have now identified a molecule, called FT, that has all the hallmarks of florigen. The FT gene is induced in leaves within hours after plants receive a stimulus that promotes flowering, but its product, the FT protein, acts at the growing tips of the plant to activate the flowering process.
The teams had been studying the FT gene, using the small mustard plant Arabidopsis. Although they knew that FT was a potent inducer of flowering, it was unclear how it influenced genes that control the formation of flowers. The breakthrough came with the discovery that FT protein binds to another protein, FD. FD in turn directly acts on genes that turn groups of unspecialised stem cells into flower buds. The FD protein, which in contrast to FT is produced at the tips of branches, is only active when bound by FT protein. Since the FT gene is induced in leaves, but FT protein acts at a distant site, the tip of branches, the authors conclude that the small FT protein must be moving from one place to the other, making it the best known candidate for the mysterious florigen molecule. It remains to be seen whether FT travels directly all the way from leaves to the branch tips, or whether a relay mechanism is involved.
"We discovered the FT gene in the late 1990s, but couldn’t figure out for many years how this small protein controlled the activity of genes that make flowers. Once we saw that FT needs the FD protein, which is present at the growing point of a plant, it made perfect sense," explains Detlef Weigel, Director at the Max Planck Institute for Developmental Biology. "Only when FT and FD join forces in the same cell can they be active."
"The transition to flowering is one of the most important decisions made by plants. It has to be carefully controlled according to the seasons," says Philip Wigge, who recently moved from the Max Planck Institute to become a Group Leader at the John Innes Centre. "For example, plants that need to be fertilized by pollen from other members of the same species, as is the case for cherry trees, need to make sure that they produce flowers at the same time as their neighbours. Requiring two independent components to come together for activation of flowering is a neat trick. One determines the right time of year and the other specifies the right place for the formation of flowers."
The work was carried out in the model plant Arabidopsis thaliana, since this represents the most versatile experimental system for plant biology. The lessons learnt, however, have far-reaching consequences for plant biology, as the FT and FD genes are present throughout the plant kingdom, including important crops such as rice and wheat. When we hear "flowering", we normally think of colorful bouquets or tree blossoms. The most important role of flowers is, however, that they give rise to fruits and seeds, including all the cereal grains, and thus are the basis of much of our daily lives. Because plants use environmental information to determine when to flower, they are geographically limited in the area where they can be grown. Outside their normal range, they will often not flower at all, or will flower too early or too late in the year. Being able to control the flowering process better should help to breed new varieties that can flourish in places where they would normally not flower at the appropriate time.
About the Max Planck Society
The Max Planck Society for the Advancement of Science (www.mpg.de) is an independent nonprofit research organization, based in Germany and funded primarily by the Federal government as well as the 16 German States. The 78 institutes of the Max Planck Society perform fundamental research in the interest of the general public and are committed to making the results of their work accessible to the general public.
About the John Innes Centre
The John Innes Centre (www.jic.ac.uk) is an independent, international centre of excellence in plant science and microbiology. It carries out fundamental and strategic research and makes its findings available to society. The JIC wins the majority of its funding in open competition from various funding agencies in the UK and worldwide. It is further grant-aided by the UK government's Biotechnology and Biological Sciences Research Council (BBSRC).
Also participating in the study were: Min Chul Kim, Wolfgang Busch, Markus Schmid and Jan Lohmann at the Max Planck Institute in Tübingen, and Katja Jaeger at the John Innes Centre.
The study was funded by grants from the international Human Frontiers Research Organisation, the German Ministry for Education and Research and the Max Planck Society, and by postdoctoral fellowships from the British Wellcome Foundation, the Korea Science and Engineering Foundation and the European Molecular Biology Organisation.