Perspectives Online

What Tells a Plant to Grow? College scientists map the molecular activity that affects plant size. By Anton Zuiker

A healthy Arabidopsis plant and a plant stunted by a genetic inability to respond to growth-regulating hormones.
(Photo by Daniel Kim)
Steven Clouse was still tan from his September trip Down Under to speak to the 18th International Conference on Plant Growth Substances when he was asked to take a moment to explain the role of brassinosteroids, naturally occurring plant hormones that regulate plant growth.

Clouse, a professor of horticultural science in the College of Agriculture and Life Sciences, is an expert on brassinosteroids; he went to Australia to present his research into the cellular signaling pathways that are turned on when these steroids bind to proteins in a cell's membrane.

Clouse and Dr. Michael B. Goshe, NCSU assistant professor of biochemistry, recently received a $350,000 grant from the U.S. Department of Agriculture to study the precise molecular mechanisms initiated by brassinosteroids, including binding of these molecules to a membrane protein called BRI1, which normally recognizes brassinosteroids at the cell surface.

One visual is all it takes, says Clouse, showing a healthy Arabidopsis thaliana next to a stunted version of that same plant. This dwarf plant had a mutated BRI1 gene, so its cells couldn't respond to the hormones that initiate and control cell elongation and seed generation. Without an ability to respond to brassinosteroids, a plant can't grow.

Dr. Steven Clouse displays a healthy Arabidopsis plant and a plant stunted by a genetic inability to respond to growth-regulating hormones.
(Photo by Daniel Kim)
But, just like an athlete pumped up on anabolic steroids, plants treated with brassinosteroids can produce gold-medal yields. Crops treated with brassinosteroids aren't harmed by the steroids, nor are they harmful to humans. Since brassinosteroids were first reported in 1970, researchers have sprayed the plant steroids on wheat and rice crops and orange and pear trees; the steroids can improve the freezing tolerances of plants and prevent premature fruit drop.

Similarly, a barley variety from Japan that includes a partially mutated form of the BRI1 gene produces a shorter and stronger stalk - one that hasn't grown to its full length. This strength can support heavier loads of barley seeds without toppling. "Mutations in a similar set of genes responding to the plant hormone gibberellin were the basis of the green revolution," says Clouse, referring to the technological developments and farming improvements in the mid-1900s that dramatically increased crop production in wheat and rice and helped alleviate world hunger.

Clouse studies Arabidopsis, a small flowering plant that is widely used as a model organism in plant biology, because there is a growing collection of bioinformatics resources on the plant, including a fully sequenced genome. "The biological resources for studying Arabidopsis are extensive, and that's turned Arabidopsis into the best-known plant in the world at the molecular level," says Clouse. Clouse and Goshe recently received a $2 million grant from the National Science Foundation to study BRI1 and a large family of related receptors in Arabidopsis that regulate many aspects of plant growth. The grant to Clouse and Goshe is N.C. State's first through the 2010 Project, an NSF initiative to determine the function of 25,000 Arabidopsis genes by the year 2010.

Clouse, Goshe and their collaborators at the Universities of Illinois and Oklahoma are undertaking a functional analysis and phosphorylation site mapping of 217 leucine-rich repeat receptor-like kinases (LRR RLKs) in Arabidopsis. These kinases are enzymes that are anchored inside the cell membrane. When a brassinosteroid or other small molecule connects with the portion of the receptor on the cell exterior, it starts a chain reaction of molecular activity that eventually leads to specific genes being turned on or off. Clouse uses a variety of genetics methods to map this activity, and already he's been able to determine how various kinases react through autophosphorylation (a chemical reaction) to create modified proteins with different activities.

"This is not easy to do," he says. "That's why my teaming up with Michael is so important. Our collaborative approach benefits from Michael's biochemistry background and my molecular genetics background." Goshe is skilled with mass spectrometry technology. Using an isotope-coded affinity tagging technique he co-developed to measure the precise changes in the protein phosphorylation, Goshe helped Clouse identify 11 specific kinase locations that are triggered by the brassinosteroid. Clouse says this is the first time such detail has been shown in plant receptor kinases; he's submitted their research for publication.