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 limits growth under normal and stressed conditions and (b) how plants regulate their growth rates. Current projects include the following:

Mechanisms of Cell Enlargement - Wall Loosening Proteins We have identified a highly conserved family of proteins - 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. Current work is focused on the molecular genetics and biochemistry of expansins, for 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.

Grass pollen allergens as b-expansins Group I allergens are the major allergens of grass pollen and cause seasonal asthma and related immune response 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.

Mechanisms of Gravitropism and Phototropism Plants sense gravity and light, and modulate their growth so that they bend towards or away from the stimulus. Our studies show that growth is altered by dramatic changes in the wall extensibility of the growing tissues. However, the biochemical means by which this is accomplished is largely unknown. We are pursuing the idea that transient changes in the wall environment, such as redox potential or pH, affect the activity of wall loosening and stiffening enzymes to modulate wall expansion. Expression of expansins and other wall-loosening factors may also play a role in these growth phenomena.