Plant Cell Wall Biosynthesis
The plant cell wall provides shape and mechanical strength to the plant cell as well as modulating its interaction with the environment. While cellulose often comprises more than half of the mass of the cell wall many other compounds are important in the functioning of this structure. Many of these non-cellulosic components are complex carbohydrate molecules such as hemicelluloses and pectin. The exact nature of the contributions of each of these carbohydrate compounds to the functioning of the cell wall is not fully understood and is an area of active research. The biosynthesis of these carbohydrate molecules requires a large number of genes. This is due to the fact that unlike proteins and nucleic acids the synthesis of these polymers does not use a template. It is currently assumed that each type of glycosidic bond requires a separate enzyme; therefore, it is likely that hundreds of proteins are required for the synthesis of cell wall carbohydrates. An additionally complication is that while cellulose is made at the plasma membrane other carbohydrate components of the cell wall are synthesized in the Golgi apparatus. One class of cell wall carbohydrates synthesized in the Golgi apparatus is the hemicelluloses.
These molecules consist of a linear backbone made up of Β-glycosidic linkages decorated with side chains of various linkage types. The role of hemicelluloses may be to prevent the direct interaction of cellulose fibrils as the hemicelluloses coat the cellulose fibrils. The side chains of hemicelluloses prevent the tight interaction between hemicelluloses, which results in a soluble fiber. The alteration of these side chains after deposition in the wall allows the plant to control the properties of the wall. Different tissues within a plant have different hemcelluloses and various species of plants also differ in the hemicelluloses present in their cell walls. Currently, it is not clear what purpose this diversity of hemicellulose plays in wall function. In addition to wall function, hemicelluloses play important roles in human health and are an important consideration in the use of biomass to produce energy.
Our laboratory is interested in understanding the biosynthesis of hemicelluloses and their function in the cell wall. We are interested in three types of hemicelluloses; namely, xyloglucan, galactomannan and xylan. For each hemicellulose we have identified a plant tissue that produces a large quantity of one these hemicelluloses. These tissues are shown in figure 1. We are using deep sequencing of these tissues at multiple times points during the period of time that these tissues are synthesizing the particular hemicellulose. We are using the new generation of sequencing technologies to sequence millions of ESTs. This large data set has produce a number of candidates for genes involved in the biosynthesis of these hemicelluloses as well as genes involved in regulating the synthesis of these biosynthetic genes. We are in the process of using reverse genetics in arabidopsis to explore the function of these genes. We are also using heterologous expression of the biosynthetic gene candidates in yeast and bacterial to determine their function.