Shaohua Zhou, Tiffany Langewisch, Tapasree Roy Sarkar, Khaldoun Al-Hadid.
Katie Wusik and Maria Mills
Molecular architecture and structural properties of
Arabidopsis cell walls
The plant cell wall is a highly organized composite of many different polysaccharides, proteins, and aromatic substances that undergo dynamic changes during cell division, expansion and differentiation. 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. As part of an NSF-funded genomics project, we are using a high-through-put infrared spectroscopic screen to identify a broad-based collection of mutant plants with variation in cell wall amount, composition or molecular architecture. Our aim is to relate changes in the molecular architecture of the cell wall to its structural properties and to the downstream consequences for plant cell growth and differentiation.
The role of pectin in cell elongation
Both final cell size and rate of cell expansion are constrained by the cell wall. Our hypothesis is that the wall-loosening mechanisms that permit cell growth to occur are regulated by the biophysical properties of a network of pectic polysaccharides. Pectins (loosely defined as polymers rich in galacturonic acid residues) comprise 20 to 40% of mass in dicotyledonous cell walls and are cross-linked in a variety of ways. They are some of the most heterogeneous and structurally complex biomolecules in nature. In order to elucidate the role of pectin in cell elongation, we are using mutants and transgenic approaches to generate Arabidopsis plants of altered pectic composition in order to study the impact on growth of the hypocotyl, an organ in which growth occurs by cell expansion rather than cell division.
Building a tracheary element
The primary xylem of plants contains several cell types including xylem parenchyma, fibers and tracheary elements. The latter form a series of connected tubes that transport water and dissolved minerals from the roots to all parts of the plant and are characterized by distinctive patterns of secondary wall deposition. Because of the complexity of vascular tissues in plants, it is difficult to analyze in planta the molecular mechanisms involved in differentiation of the different cell types of the vascular system: only a few cells at any time are actively engaged in the process of forming xylem. We have used a remarkable system, the zinnia mesophyll cell system, to study this entire developmental pathway in vitro . Intact, single cells are obtained aseptically and then incubated in a medium containing a 1:1 ratio of cytokinin and auxin. Over a time-course of 48 h, about 70% of the cells undergo xylogenesis synchronously. Thus, one cell type can be reproducibly and synchronously switched, by known external signals, into a different cell type in 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, involving many hundreds of genes. We have applied cDNA-AFLP (complementary DNA-Amplified Fragment Length Polymorphism) technology in order to identify new genes involved in xylem formation by exploiting the model system of zinnia. We identified over 600 partial cDNA sequences that showed altered transcription at specific times during the 48 h of cell commitment and differentiation. We are studying the roles of some of these differentially expressed 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.
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Carpita, N.C., Defernez, M., Findlay, K., Wells, B., Shoue, D.A., Catchpole, G.C., Wilson, R.H., and McCann, M.C. (2001) Cell wall architecture of the elongating maize coleoptile. Plant Physiology 127, 551-565.
Milioni, D., Sado, P., Stacey, N.J., Roberts, K., and McCann, M.C. (2002) Early gene expression associated with the commitment and differentiation of a plant tracheary element is revealed by cDNA-Amplified Fragment Length Polymorphism analysis. The Plant Cell 14, 2813-2824.
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Ryden, P., Sugimoto-Shirasu, K., Smith, A.C., Findlay, K., Reiter, W-D. and McCann, M.C. (2003) Tensile properties of Arabidopsis cell walls depend on both a xyloglucan cross-linked microfibrillar network and rhamnogalacturonan II-borate complexes. Plant Physiology 132, 1033-1040.
Mourelatou, M., Doonan, J.H. and McCann, M.C. (2004) Transition of G1 to early S phase may be required for Zinnia mesophyll cells to trans-differentiate to tracheary elements. Planta, 220, 172-176.