Plants can defend themselves very efficiently against phytopathogens. This resistance can be based on preformed resistance factors or it may be the result of infection-induced resistance reactions. Preformed and induced resistance mechanisms can be structural or chemical in nature. Clearly, the resistance of many plants to attack by potentially pathogenic micro-organisms is due to preformed structural properties such as an inappropriate surface hydrophobicity or topography which fails to supply the signals required for microbial ingress in addition to cell walls resilient to physical and chemical attack. Should a micro-organism overcome these preformed barriers, plants will almost invariably fall back on their second line of defense, i.e., active resistance reactions will be induced. Again, these may have structural aspects, such as local cell wall thickening and reinforcement, encapsulation of the penetrating pathogen in dead and often lignified or suberized cells, and even the formation of new meristems forming new layers of cells around the site of attempted microbial ingress. In this chapter, we will learn how plant cells build a strong cell wall as a protection against phytopathogens, and we will get to know the strategies that micro-organisms have adopted to circumvent or breach these cell walls. Finally, we shall explore the many ways plants have evolved to counteract microbial attacks - including preformed structural resistance factors and infection-induced structural resistance reactions. We will close the chapter with a discussion of the ways open to plant pathologists to unravel cause/consequence-relationships between plant defense mechanisms and disease
