Biosynthesis of Cell Wall Polymers

The magnitude of the task of making and maintaining plant cell walls is illustrated by the estimate that these cellular activities involve the products of well over 2000 different genes. One factor contributing to this large number of required genes is the large number of genes that code for glycosyltransferases and glycosidases. Indeed, plants contain a higher number of such genes than any other organism sequenced to date ( Figure 5.1 ) due to the tremendous diversity and complexity of the carbohydrates found in the plasma membrane and cell walls. In this chapter, the assembly pathways of different cell wall polymers are organized around their sites of assembly, namely, the ER and Golgi apparatus, the plasma membrane, and the cell wall. We begin with a discussion of the assembly and processing of N- and O-linked glycans in the ER and Golgi cisternae and then progress to the synthesis of cell wall matrix polysaccharides, which involves many different types of glycosyltransferases that have also been localized to Golgi cisternae. Our discussion of cellulose synthesis focuses on the CESA proteins and their functional domains, and the role of sucrose synthase in producing UDP-glucose for cellulose synthase. The discussion of the functional organization of the catalytic subunit of callose synthases (GSL proteins) is based on the analysis of their gene sequences. We also discuss the differences between Ca²⁺-dependent and Ca²⁺-independent callose synthase systems. Finally, we address the environment of the cell wall in terms of polymer assembly and how this applies to the synthesis of lignin.