Ph.D, Stanford University, 1980
B.S, University of Massachusetts, 1974
Mechanisms of Plant Cell Growth
Cell expansion plays a crucial role in shaping the form and size of plants. My research focuses on the cellular and molecular mechanisms of cell expansion. By use of biophysical, biochemical, and molecular techniques, in combination with whole-plant measurements, we are trying to determine: (a) which processes limit growth under normal and stressed conditions and (b) how plants regulate their growth rates. Current projects include the following:
Wall Loosening Proteins
We have identified a conserved family of proteins—which we named named expansins—that catalyze wall extension. For this work, we devised a novel reconstitution assay to measure the ability of extracted proteins to induce extension of isolated walls, and then purified the active fractions using HPLC, electrophoresis, and related techniques. We have found that expansins do not act by the conventional theory of wall loosening by polysaccharide hydrolysis; rather, they disrupt hydrogen bonding between wall polysaccharide in a unique way that might have commercial applications. Our current work is focused on the molecular genetics and biochemistry of expansins to gain a deeper understanding of the function and evolution of these proteins. By use of transgenic plants with alterations in the expression of expansins, we plan to decipher the role of these proteins in cell enlargement, plant morphogenesis, and wall structure. Identification of genetic mutants in which T-DNA or transposon insertions interrupt expansin genes is also underway. In our studies to date, we have found that expansins are phylogenetically widespread and may serve a common function in regulating wall yielding and cell expansion in vascular plants. Recent studies have focused on expansin structure and the molecular biophysics underlying its action to loosen cell walls.
Grass Pollen Allergens as Beta-Expansins
Group I allergens are the major allergens of grass pollen and cause seasonal asthma and related immune responses in many people. We showed these group-I allergens to be structurally related to expansins. Extracts of maize pollen possess potent expansin-like activity, as measured in wall extension and wall stress relaxation assays. This activity is selective for grass cell walls and is, at least partly, due to the action of maize group I allergens. It is likely that group I allergens facilitate invasion of the pollen tube into the maternal tissues by loosening the cell walls of the grass stigma and style. Additionally, the presence of related mRNAs in vegetative tissues of rice, Arabidopsis, and soybean implies that allergen homologs may function to loosen walls in growing vegetative tissues as well. Future work will examine the development roles and biochemical functions of this subfamily of expansins.
Growth Adaptations to Water Stress
Growing maize roots respond to mild water deficits by increasing the extensibility of cell walls in the apical growth zone, thereby permitting the roots to continue to grow. We are exploring the molecular basis of this change in wall properties. Part of this adaptive response appears to be mediated by a build up of expansins in the cell walls. Other possible changes in wall structure are also being explored.
Structure and Assembly of Plant Cell Walls
To understand how plant cells grow (enlarge), we need to know how the cell wall expands irreversibly and incorporates new structural materials into the cell wall to maintain its structural and mechanical integrity in the face of sustained expansion and thinning of the older wall layers. Our approach is to characterize the extensibility of cell walls by use of specific enzymes to modify cell wall cross-linking polymers and by the use of genetic mutants with defects in specific cell wall polymers.