Long sought-after flowering signal finally tracked down
A team of German and UK scientists has shown how plants ensure that flowers are formed at the right time and in the right place. Scientists at the Max Planck Institute for Developmental Biology in Tübingen, Germany, and the John Innes Centre in Norwich, UK, have found how a small molecule that is made in leaves is able to induce the formation of flowers at the growing tip of a plant. Because flowers, in turn, make fruits and seeds, including cereal grains, this new knowledge could have important applications agriculture. The breakthrough has been reported in the journal 'Science'. Most plants make flowers only at certain times of the year, the blooms of cherry trees announcing the spring being but one example. Plants can use several cues from the environment to choose the season that is right for flowering. 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. However, the two teams have now identified a molecule, called FT, that has all the hallmarks of florigen. The FT gene (flowering locus T) 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 thaliana. Although they knew that FT was a potent inducer of flowering, it was unclear how it influences genes that control the formation of flowers. The real breakthrough came with the discovery that FT protein binds to another protein: FD. FD protein, in turn, directly affects 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, whereas FT protein acts at a distant site, the tip of branches, the researchers 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, from 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 lessons learnt have far-reaching consequences for plant biology, as the FT and FD genes are present throughout the plant kingdom, including in important crops such as rice and wheat. Because plants use environmental information to determine when to flower, they are geographically limited in terms of 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.
Countries
Germany, United Kingdom