Love and Flower Power
Scripps Research Scientists Discover Protein that Senses Daylight and Regulates Flowering

By Jason Socrates Bardi

A group of scientists at The Scripps Research Institute are publishing a paper in which they describe a new class of proteins that regulates the timing of the flower cycle in one small leafy weed, a relative of the mustard plant called Arabidopsis thaliana.

The protein, called FKF1, regulates flowering in Arabidopsis by targeting and degrading other proteins that are involved in the flower cycle. Interestingly, FKF1 is itself regulated by light.

Like many large protein complexes, FKF1 is a mixture of several different protein domains—proteins or pieces of protein that have distinct functions. FKF1 combines several domains in a unique way that has never been seen before.

FKF1 has a domain called an "F-box" that degrades other proteins, and FKF1 uses this domain to control the flowering cycle of plants. FKF1 also has another domain, known as a "LOV" domain, that it uses to sense light. These domains are common in plant, fungal, and bacterial cells, where they sense external cues such as light, oxygen, and voltage.

"We have discovered [in the FKF proteins] a completely novel photoreceptor family," says Scripps Research Cell Biology Professor Steve Kay, Ph.D., who is the lead author of the study. "These proteins are regulating the degradation of other proteins in a light-dependent fashion."

The work should have special relevance to agriculture because the appropriate seasonal control of flowering is a major determinant of crop productivity. The same technology might be used to make certain crops bear fruit faster and in larger and more nutritious yields.

The paper appears in the latest issue of the journal Nature and is featured on the cover.

Light Switches for Protein Degradation

Kay's Scripps Research laboratory has for several years been studying the way that plants use "circadian rhythms" to follow the solar day, using Arabidopsis thaliana.

Arabidopsis is a good model organism for several reasons. Its genome was solved a few years ago, and many of its genes have been identified. It is tiny and has a fast generation time, both of which fit well in the modern tight-on-space-and-time laboratory. It also produces an overabundance of seeds at the end of its reproductive cycle. Finally, Arabidopsis is easily grown.

Members of the laboratory vary the plants' environment—the amount of light, for instance—then ask how the plants adjust their own clocks to keep abreast of these changes, which genes are turned on and off, and what other molecules are persistently present.

Anticipating the seasons is but one of the strategies plants have evolved as a means to cope with the various challenges of their environment. Because of the stark seasonal differences in weather in many climates—with long days of burning sunlight in summer and wet, dark, and freezing conditions in winter—plants with the ability to flower at the best possible time have had an advantage in evolution.

Scientists have known since the 1920s, when researchers first began experimenting with growing plants under artificial light, that plants flower following a "photoperiodic response"—they flower when they detect a certain day length indicating season.

However, until recently, nobody understood the precise molecular tools plants use to accomplish this.

Kay is one of several scientists at Scripps Research and elsewhere who have been exploring how plants control their flowering cycle on the level of individual cells, and how they use circadian "clock" genes that follow the solar day. These clock genes ebb and flow throughout the day and year as they are needed, and comprise a complicated set of feedback switch "clockworks" that turn on and off other genes as needed.

One gene involved in these clockworks encodes a protein called "CONSTANS," which triggers the flowering of the plant, but only when the timing is right. The expression of CONSTANS varies throughout the day, and it must be abundant at the end of the day for flowering to occur.

Kay and his colleagues found that the FKF1 protein is a key regulator of the levels of CONSTANS and of flowering because it can control how much CONSTANS protein accumulates. If CONSTANS levels are too low at the end of the day, flowering will not occur.

Interestingly, FKF1 is itself regulated by light. If there is not enough sunlight in the late afternoon, FKF will alter the level of CONSTANS, and delay flowering. FKF1 does this by sensing how much blue light is in the environment. Blue light is a good sensor because it is not absorbed by plant chlorophyll as other types of light are.

This discovery opens the possibility that we could learn to boost food production by manipulating day length sensitivity of different crops and increasing our capacity to grow them efficiently at different latitudes at different times of the year. "The danger of running out of arable land is very real," says Kay, "and we have to solve the problem of feeding a rapidly increasing population in the next 10 years."

The discovery of the FKF1 protein is also significant to scientists, because this light-activated switch could be put into other types of cells—possibly introducing a gene-regulating switch into mammalian cells that can be activated by light, for instance. The article, "FKF1 is essential for photoperiodic-specific light signaling in Arabidopsis" was authored by Takato Imaizumi, Hien G. Tran, Trevor E. Swartz, Winslow R. Briggs, and Steve A. Kay and appears in the November 20, 2003 issue of the journal Nature.

This work was supported by the National Institutes of Health, the National Science Foundation, and by a grant from the Japan Society for the Promotion of Science Postdoctoral Fellowships for Research Abroad.



Because the leafy weed
Arabidopsis thaliana is easy to grow and has a fast generation time, it is often used by scientists studying the inner workings of plants.