January 27, 2005
La Jolla, CA – Scientists at the Salk Institute for Biological Studies have filled in two significant gaps in the molecular pathway that allow plant steroid hormones to stimulate plants to be larger and more fruitful. The findings may open up the prospect of larger vegetables or increased seed yield and could have a galvanizing effect on agricultural research.
A team led by Joanne Chory, a professor in the Plant Molecular and Cellular Biology Laboratory at the Salk Institute and an investigator of the Howard Hughes Medical Institute, has shown that a protein that protrudes from the surface of a plant cell membrane is the receptor that recognizes a steroid hormone essential to the growth and development of plant cells. Once the protein receptor, which was previously cloned by Chory’s lab and named BRI1 (‘bry-one’), recognizes the hormone, a series of biochemical activities occur inside the cell. At the other end of this chemical pathway, the Salk Institute team also uncovered the transcription factors that turn on key genes that control plant cell growth. The two studies were published in Nature on January 13, 2005 and Cell on January 28, 2005.
In both animals and plants, steroid hormones control growth and development, but through vastly different mechanisms. Animal steroid hormones such as testosterone and estrogen turn on cells by binding to receptors inside the cell.
By contrast, plant steroid hormones, called brassinosteroids, switch on growth and differentiation of plant cells by binding to proteins that project to the outside of the plant cell, like a key in a car ignition. Until the publication of Chory’s research, the nature of the ‘keyhole’ was unknown, and it was also unclear how the ‘sparkplug’ worked, i.e., how the resulting signal turned on the key genes that control growth and sexual maturation.
By a combination of genetic and biochemical techniques, Chory and colleagues determined that the ‘keyhole’ is BRII, a protein that protrudes from the plant cell membrane.
“It was satisfying after all these years that BRI1 turned out to be what we thought it was – the receptor for the steroid,” said Chory. “Genetics originally led us to the right gene, and follow-up biochemical studies allowed us to show that the protein made by this gene actually bound the steroid.”
At the other end of the signalling pathway, the team revealed that one of the ‘sparkplugs’ that fires the genetic engine is a protein called BES1, a member of a completely new family of plant-specific transcription factors.
“We’re finally in the position to show at least one of the mechanisms by which plants regulate growth,” said Chory. “We now have the tools to determine the complete molecular sequence for how steroids regulate gene expression in plants.”
This research may open up new avenues for creating plants such as lettuce that grow bigger or in the case of rice have a vastly increased yield.
“Making steroids synthetically is really expensive but manipulating the biosynthetic and response pathways for brassinosteroids in plants could have significant impact on crop yield,” said Chory.
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine, proven safe and effective in 1955, all but eradicated the crippling disease poliomyelitis in, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.
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