We recognize the 40 or so different plant cell types by their characteristic shapes and sizes. However, without its cell wall, the naked protoplast is spherical and is not capable of dividing, growing, or becoming specialized for function. Molecules of the cell wall provide mechanical strength, regulate porosity, and control cell-cell adhesion. The functions of the wall are not only mechanical but also biological. Like the animal extracellular matrix, plant cell walls are a source of signaling molecules that elicit a range of cell behaviors, committing the cell to particular developmental programs. The goal of our research is to understand how the molecular machinery of the plant cell wall contributes to cell growth and specialization, and thus to the final stature and form of plants.

The cell wall is a highly organized composite of many different polysaccharides, proteins, and aromatic substances. However, it has been difficult to ascribe specific functions to these molecules. Using Arabidopsis, we hope to define the relationship between genetically-defined changes in plant cell wall-related proteins (biosynthetic, hydrolytic, and structural) and cell wall molecular architecture and structural properties. We are particularly interested in the functions of pectic polysaccharides, some of the most complex biomolecules in nature, in regulating cell expansion.One of the most remarkable examples of cellular differentiation in plants is the construction of tracheary elements, that form a series of connected tubes to transport water and dissolved minerals from the roots to all parts of the plant. These cells are functional corpses, built of highly specialized cell walls. The zinnia mesophyll cell system is a remarkable model system in which we can induce photosynthetic cells harvested from leaves to change cell fate and become tracheary elements in liquid culture. The formation of tracheary elements involves several processes fundamental to plant development, including cell division, signal transduction, cell wall synthesis and deposition, lignification and programmed cell death. Using the zinnia system, we have identified over 600 genes that are involved in tracheary element formation. We are studying the roles of some of these genes in determining cell fate, in coordinating cell proliferation and differentiation, and in building the thick hoops of secondary wall material that are characteristic of tracheary elements, using both zinnia, the engine for gene discovery, and Arabidopsis, the model genetic system.