Pectin as a barrier and nutrient source for fungal plant pathogens

Fungi that interact with plants gain access to host tissues by actively passing the surface through the cuticle and/or cell wall. Cell walls provide plant tissue strength and structure, and form a barrier against microbial invasion. Plants invest substantial resources in constructing the cell wall and maintaining its integrity. On the other hand, carbon deposited in cell walls offers opportunities for fungi to utilise them as nutrients. This chapter discusses the opposite roles of plant cell walls, both as a barrier for penetration and a food source for fungi, with focus on pectin. We discuss the chemical structures of plant cell-wall polysaccharides, the cell-wall-associated mechanisms that confer resistance against pathogens, and the microbial enzymes involved in cell-wall decomposition. We then focus on plant cell-wall-degrading enzymes of pathogenic fungi, and illustrate with case studies how Botrytis cinerea decomposes pectin present in plant cell walls, and utilizes the breakdown products as nutrients.