Plant cell walls.

Cell walls are important features of plant cells that perform a number of essential functions, including providing shape to the many different cell types needed to form the tissues and organs of a plant. Forming the interface between adjacent cells, plant cell walls often play important roles in intercellular communication. Because of their surface location, plant cell walls play an important role in plant-microbe interactions, including defense responses against potential pathogens. The desire to understand these and other plant functions help explain the strong interest in wall structure and biosynthesis. Plant cell walls are usually divided in textbooks into two categories: primary walls that surround growing cells or cells capable of growth and secondary walls that are thickened structures containing lignin and surrounding specialized cells such as vessel elements or fiber cells. In reality, all differentiated cells contain walls with distinct compositions, resulting in a spectrum of specialized cell walls with primary and secondary walls as two extremes. This brief prospective overview focuses mainly on issues thatmust be resolved if we are to understand the role of cell walls in plant physiology. Many outstanding reviews cover recent progress, including a series of excellent updates in a recent special issue of Plant Physiology focused on this topic (see McCann and Rose, 2010). In addition, the lignin component of secondary cell walls is covered elsewhere in this issue (Li and Chapple, 2010) as is the uses of cell walls as a source of energy (Somerville et al., 2010). The author apologizes to the many colleagues whose work could not be cited because of space limitations.