Scientists at the Scripps Research Institute discover how a plant times its flowering cycle

Two scientists at The Scripps Research Institute (TSRI) have described how a plant grown in their laboratory uses two sets of proteins to detect the seasons so that it can flower at the right time. And by tinkering with those proteins, the scientists were able to make the plant flower at will.

“We have demonstrated, for the first time, how plants can anticipate the seasons so that they can flower appropriately,” says Marcelo Yanovsky, Ph.D., who is a research associate at TSRI and the lead author of the study.

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.

Yanovsky, and TSRI Cell Biology Professor Steve Kay, Ph.D., describe this work in the latest issue of the journal Nature.

How Plants Keep Their Own Appointment Books

Anticipating the seasons is but one of the strategies plants have evolved as a means to cope with the various challenges of their environment.

One of the most obvious changes in the environment is the seasonal variations in weather. Because of the stark seasonal differences in weather in most climates–with long days of burning sunlight in summer and wet, dark, and freezing conditions in winter–plants that have adapted the ability to flower at the best possible time would 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 in seasons when they detect the correct day length. However, until the recent study by Yanovsky and Kay, nobody knew precisely how the plants accomplished this timing.

The timing takes place on the level of individual cells, where circadian “clock” genes have expression levels 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.

For instance, plants use two photoreceptor proteins–the blue light photoreceptor, “cryptochrome,” and the red light photoreceptor “phytochrome”–to control other genes.

Like solar-powered clocks, phytochromes and cryptochromes are activated when exposed to daylight and transported to the nucleus of the plant cell where they alter the expression of a number of genes that must be timed. As the sun varies throughout the day, the number and ratio of phytochromes and cryptochromes reaching the nucleus varies, and these changes allow them to delicately control many other genes.

One gene they influence encodes a protein called “CONSTANS.” CONSTANS triggers the flowering of the plant, but only when the timing is right–it must first be activated by the right number of cryptochrome and phytochrome molecules.

The expression of CONSTANS also varies throughout the day. It is only expressed (in an inactivate form) in the late afternoon each day. If there is not enough sunlight in the late afternoon, there will not be enough cryptochromes and phytochromes around, CONSTANS will remain inactive, and flowering will not start.

Yanovsky and Kay demonstrated this control mechanism by shifting the expression of CONSTANS towards the morning. By doing so, they were able to make the plants flower earlier during short days, as if they were growing in the long days of spring.

A Good Model Plant

The work was carried out in Kay’s TSRI laboratory, which has for several years been studying the way that plants use “circadian rhythms” to follow the solar day.

The laboratory uses one small, leafy, weed-like relative of the mustard plant, Arabidopsis thaliana. Arabidopsis is a good model organism for several reasons. 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, as a weed, Arabidopsis is easily grown.

Members of the laboratory vary the plants’ environments–the amount of light these test plants receive, for instance–then ask how the plants adjust their own clocks to keep abreast of these changes, looking for which genes are turned on and off when and determining what other molecules are persistently present.

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.”