Plant parasitic nematodes: Digesting a page from the microbe book

It is somewhat ironic that an important finding concerned with cellulose biosynthesis in higher plants (1) appeared just before the paper in this issue of the Proceedings by Smant et al. (2) describing genes from a plant parasitic nematode that encode proteins that degrade cellulose. Cellulose is the most abundant polymer on the planet and is an essential component of the plant cell wall. Although the cell wall or extracellular matrix, as it is sometimes called, is complex (3), crystalline cellulose is the structural linchpin. Given this situation, it is not surprising that hungry pathogens commonly attack components of the plant cell wall, including cellulose. Genetic evidence has established that cellulases that depolymerize cellulose often are required for high virulence (the quantitative degree of pathogenicity) by microbial pathogens (e.g., refs. 4 and 5). These entrepreneurs damage the plant cell wall to facilitate their movement through plant tissue and to obtain nutrients from the digested cell wall components or the contents of unclothed plant protoplasts. Curiosities abound, including questions such as: Where and when did cell wall degrading genes originate? How were such genes recruited by pathogens? What do plants do about this mode of attack? How can human activity intervene and prevent such plant diseases without secondary downsides? Answers are not abundant, but we are improving our understanding of plant pathogenesis and what to do about it so that humans reap more of the harvest. First, pathogens are turning out to be copycats. For example, microbial pathogens of plants and animals share clusters of highly homologous genes involved in pathogenicity and virulence. These have come to be …