FLORIGEN: THE HIDDEN HORMONE
Introduction
It’s a mysterious substance with a mysterious past: intrigue and imprisonment followed the first suggestion of its existence, and a lifetime of labour could not discover its identity. Mikhail Khristoforovich Chailakhyan, a biologist in the USSR during Stalin’s reign, argued in 1936 that there must be a hormone – called florigen – that causes plants to produce flowers. Florigen, and the evidence for its existence that Chailakhyan amassed, was an exciting contribution to the era’s understanding of plant development and attracted considerable scientific attention. What is surprising is that florigen was also a politically radical notion. Chailakhyan’s idea was rejected by the Communist Party and the government of the Soviet Union, and Chailakhyan and his supporters were persecuted.
A first look at plant hormones
Plants, like people, have hormones that control growth and development. Insulin (which is important to people with diabetes) as well as estrogen and testosterone (which are the male and female sex hormones) are human hormones, but plants have hormones like auxin, gibberellin, and ethylene. Hormones are chemical messengers that control and coordinate biological processes in an organism, and they are even more important in plants than in animals. This is because plants, unlike many animals, lack a central nervous system to communicate between different parts of a plant. So everything in a plant that needs to happen at a certain time (like flowering, or leaves turning brown) or in response to an external action (like an insect attack, or sunlight) has to be coordinated by hormones.
Auxin, gibberellin, and ethylene, for example, control many developmental and defensive processes in plants. Auxin causes shoots and leaves to grow towards sunlight, and it causes roots to grow downwards with the pull of gravity. Gibberellin causes plant stems to grow and elongate, and it is sometimes also involved in flowering. Ethylene controls defenses against disease, but it also causes fruit to ripen. And florigen, the elusive flowering hormone, was thought by Chailakhyan to cause plants to flower at the correct time of year.
Chailakhyan’s discovery
As a young scientist in the USSR, Chailakhyan was excited about a new idea: that perhaps the photoperiod – the length of daylight that a plant is exposed to – controls when plants produce flowers. Working in universities with occasionally odd names (like the Trans-Caucasian Zooveterinary Institute in Erevan), he tested this idea by keeping plants in unnaturally bright or unnaturally dark rooms for months at a time. He discovered that some plants produced flowers only when they were exposed to long days and short nights, and that others produced flowers only when the days were short and the nights were long.
When Chailakhyan realised that his plants flowered according to the length of daylight, he began to wonder how plants could perceive the light they were reacting to – after all, plants don’t have eyes. So he began a series of experiments to discover what parts of a plant perceive light. He exposed one part of a plant at a time to the light and kept rest of it in the dark, and he discovered that it is the leaves that matter. Flowers are only produced if the leaves are exposed to the correct amount of light.
As you might expect, Chailakhyan became even more excited about the concept of the photoperiod when he made this discovery. If the photoperiod controls flowering, and if only leaves perceive this photoperiod, then there must be some kind of signal that the leaves send to the buds (where flowers form) so they “know” when to start flowering. What was even more exciting was that the signal – which Chailakhyan called florigen – seemed to be almost universal: lots of species use the same leaf dependent signal to cause flowering.
Chailakhyan used a process called grafting to demonstrate that florigen is, indeed, a nearly universal signal. Grafting is joining pieces of two different plants together so that they become one living plant, a Frankenstein’s monster of branches and leaves from different plants growing out of a single stem. Chailakhyan grafted a branch from a plant species that flowers when photoperiod is short onto a plant that flowers when photoperiod is long, and exposed the new plant to long, bright days. The long-photoperiod plant flowered as expected, but the short photo-period plant flowered too! This meant that the florigen signal produced by the long-photoperiod plant caused the short-photoperiod plant to flower, even under the wrong light conditions. After repeating this experiment with other species, Chailakhyan concluded that florigen is a widespread signal for flowering, and he suggested that it must be a complex group of chemical messengers travelling through the plant’s stem – that is, a hormone.
The Soviet drama
Excited by hormones as only biologists, and say teenagers, can be, Chailakhyan wrote his PhD thesis about his discoveries. He had hoped to graduate after formally presenting his work at his university (this presentation is called a thesis defense). However, Trofim Lysenko, the favourite biologist of Stalin and of the Communist Party, was strongly and strangely offended by Chailakhyan’s work. The Soviet government at the time supported a wrong-headed biological theory that Lysenko himself had popularized, “Michuran agrobiology,” which contradicted Chailakhyan’s ideas about florigen. Michuran agrobiology included a different theory of plant development that was based on Lysenko’s own results which were eventually discovered to have been falsified. Worried that Chailakhyan’s excessive excitement about photoperiod might be problematic, and – more truthfully – enraged that he did not support the Communist Party’s official biological theory, Lysenko cancelled Chailakhyan’s thesis defense.
Lysenko convened a meeting with Chailakhyan and the other major biologists at his university to denounce Chailakhyan’s theories, forbid further research, and threaten any dissident who might dare oppose official Soviet biological theory. He ranted incoherently and madly until he was interrupted by the famous academician Nikolai Vavilov, who defended Chailakhyan and suggested that an edited version of his ideas might be resubmitted for his thesis defense.
Nikolai Vavilov was critical of Lysenko’s theories, and encouraged Chailakhyan to pursue his research into florigen and the role of hormones in plant development. Vavilov was himself a celebrated researcher who, in a series of dangerous secret trips to Iran and Afghanistan, discovered that the Middle East is the origin of many agricultural crops. But for supporting Chailakhyan’s research, and for his continued criticism of Lysenko’s pseudoscientific theories, Vavilov was imprisoned and starved to death. Years later, a star was named in Vavilov’s honour, commemorating his scientific work and his suffering.
Through the 1930s and 1940s, it became increasingly dangerous to criticise Lysenko’s theories, until in 1948 dissent was formally outlawed. Chailakhyan’s ideas were ridiculed and denounced by Lysenko and his supporters, who invoked political principles to justify their strange biological theories. Chailakhyan was twice dismissed from his position as head of the laboratory at the Institute of Plant Physiology, and the laboratory itself was shut down. In each incident, Chailakhyan was rescued by the daring director of the Institute, who hired him to work a low-level job in a different laboratory. Chailakhyan, who had by this time graduated and defended his PhD thesis, was also forbidden from teaching graduate students. Many of his colleagues and supporters were also ill-treated and imprisoned. But Chailakhyan survived: by the 1950s, Lysenko had fallen out of favour, Stalin had died, and Chailakhyan continued his research on florigen in safety.
The elusive identity of florigen
Chailakhyan’s early work had established that a chemical messenger produced in the leaves of plants travels to their buds and causes them to flower. But after two decades of covert research (interrupted by World War II), Chailakhyan had no idea what specific chemicals and mechanisms were involved in the process. Florigen was not an easy hormone to pin down: it was still just a hypothesis that Chailakhyan had suggested to explain communication between leaves and flowers.
After three more decades of research, in the 1980s, Chailakhyan and his lab still had not discovered the identity of florigen. By then, Chailakhyan had become a famous and respected biologist worldwide, and he had made important discoveries about hormones and plant development – but florigen was elusive and complex. Chailakhyan postulated that there might be two parts to florigen: one part that involved the hormone gibberellin (which also makes stems grow), and another part involving an unknown hormone. He postulated that one of these would be a florigen suppressor, a compound produced by leaves that stops flowering, unless florigen is present in the correct concentration.
But evidence for Chailakhyan’s ideas remained scarce, and competing theories of flowering appeared – that flowering might be caused by gibberellin in combination with known substances like sugars. Although nothing was proven either way, the problem was so old and intractable that interest faded. Chailakhyan died in 1991, without having identified the hormone he had named.
Florigen discovered?
In 2005, seventy years after Chailakhyan suggested the existence of florigen, a protein was discovered that seemed to fit the part. Further evidence published in the following years has shown that a gene called Flowering Locus T (or FT for short) produces a protein in response to photoperiod that causes plants to flower. The mechanism that causes flowering is somewhat surprising: A large protein, and possibly also an RNA fragment (a short mobile copy of a DNA sequence), are produced from the FT gene in leaves when the photoperiod is right for flowering. The protein and the RNA then travel in the sap through the plant’s stem to its buds, where they enter the living cells and create another protein that activates a complex cascade of genes and proteins that cause the creation of flowers. Other substances are also involved (including gibberellin and antiflorigens), but FT seemed to be the essential florigen.
In retrospect, it is unsurprising that Chailakhyan could not discover the chemical identity of his florigen. Scientists at the time were not aware that plant hormones could be a complex mix of protein and RNA, so Chailakhyan was searching for the wrong kind of hormone. And the actual mechanism that causes plants to flower is exceedingly complex, possibly too complex to be characterised using the technology of the time. But the recent discovery of the identity of the FT gene has revived interest in the control of flowering in plants, and new research is exploring other components of fascinating process. The rediscovery of “florigen” in 2005 was named one of the top three breakthroughs of the year by the appropriately-named scientific journal Science, and Chailakhyan’s ideas are – finally – proven right.