July 13, 2011
Biologists Discover An ‘Evening’ Protein Complex That Regulates Plant Growth
Farmers and other astute observers of nature have long known that crops like corn and sorghum grow taller at night. But the biochemical mechanisms that control this nightly stem elongation, common to most plants, have been something of a mystery to biologists"”until now.
In this week\'s early online publication of the journal Nature, biologists at the University of California, San Diego report their discovery of a protein complex they call the "evening complex" that regulates the rhythmic growth of plants during the night. More importantly, the biologists show how this protein complex is intricately coordinated through the biological clock with the genes that promote stem elongation in a way that could enable plant breeders to engineer new varieties of crops that grow faster, produce greater yields of food or generate more biomass per acre of land for conversion into biofuels.
While most people assume that plants grow at a slow and steady rate throughout the day and night, Charles Darwin and others more than a century ago observed that they actually grow in spurts late at night, with plant stems elongating fastest in the hours just before dawn.
"Plants actually grow rhythmically," said Kay. "Some plants, like sorghum, have the ability to elongate a centimeter or more each night."
The UCSD biologists initially focused their attention on three genes from a tiny mustard plant called Arabidopsis, which is used by geneticists as a laboratory model for plants. When they are disabled by mutations, these three genes disrupt the plant\'s biological clock and promote both stem elongation and early flowering.
"These three genes have been of intense interest because the loss of function in each one of them kills the biological clock, causes a long hypocotyl, or juvenile stem, and tends to cause early flowering," said Kay. "We thought that maybe their function was related. So this investigation was basically started to figure out what these three genes do."
The answer to that seemingly simple question took the biologists more than six years to disentangle. Their efforts were led by three postdoctoral fellows in Kay\'s lab: Dmitri Nusinow, Anne Helfer and Elizabeth Hamilton.
"Circadian clocks control the timing of an extraordinary variety of developmental and physiological processes in humans and other species, but figuring out how they do this is tough," said Laurie Tompkins, who oversees biological clock grants at the National Institutes of Health\'s National Institute of General Medical Sciences, which funded the research. "Arabidopsis is ideal for this sort of analysis, since researchers can use a variety of sophisticated genetic and biochemical tools to study molecular interactions at different times of day and then easily observe the tiny plant\'s development."
Because the three genes"”Early Flowering3 (or ELF3), ELF4 and LUX"”have biological activities that peak in the early evening, the UCSD biologists wondered if the three genes acted together in a protein complex. Through a series of experiments in yeast cells, they determined the three genes produced proteins that did interact with one another, but in a specific way. ELF3 served as a docking protein that brought together ELF4 and LUX, but the latter two did not interact with each other without ELF3\'s help.
This protein complex was dubbed the "evening complex" by the UCSD scientists, who verified in Arabidopsis that not only did the biological activities of the three components of this protein complex peak in the evening, but so did the formation of the evening complex itself.
The researchers then sought to answer the question of what the physiological role of this protein complex could be in plants. One main clue pointed them in the right direction: When any one of the three genes controlling this protein complex is disabled, plants end up with grossly elongated stems.
"This protein complex is clearly acting like the brakes on growth," said Kay. "So when we mutate any one of these genes the plants elongate much more."
In another set of experiments, the researchers demonstrated that the evening complex puts the brakes on the activity of two genes in plants"”PIF4 and PIF5"”that are important in promoting plant growth.
"What we show in our paper is that the evening complex binds to the promoters of PIF4 and PIF5 and, at the end of the