Cell Wall Biochemistry Related To Specificity In Host Plant Pathogen Interactions

There has been a significant surge of interest in the study of the physiology and biochemistry of plant host-parasite interactions in recent years, as evidenced by the number of research papers currently being published on the subject. The in creased interest is probably based on the evidence that effective management of many plant diseases is, for the most part, contingent upon a clear understanding of the nature of host-parasite interactions. This intensified research effort calls for a greater number of books, such as this one, designed to compile, synthesize, and evaluate widely scattered pieces of information on this subject. The study of host-parasite interactions concerns the struggle between plants and pathogens, which has been incessant throughout their coevolution. Such in teractions are often highly complex. Pathogens have developed sophisticated of fensive systems to parasitize plants, while plants have evolved diversified defen sive strategies to ward off potential pathogens. In certain cases, the outcome of a specific host-parasite interaction seems to depend upon the presence or efficacy of the plant's defense system. A plant may become diseased when a parasite manages to invade it, unhindered by preexisting defense systems and/or without eliciting the plant's induced resistance response(s). Absence of disease may re flect the inability of the invading pathogen to overcome the plant's defense sys tem(s).

Each plant-pathogen interaction involves a two-way molecular communication. On one hand, the pathogen perceives signals from the plant, secretes chemical arsenals to establish infection courts, and produces metabolites that disrupt structural integrity, alter cellular function, and circumvent host defenses. On the other hand, the plant senses the signals from the pathogen, reinforces its cell walls, and accumulates phytoalexins and pathogenesis-related proteins in an attempt to defend itself. The production of pathogenicity and virulence factors by the pathogen, the elicitation of defense mechanisms by the plant, and the dynamic interaction of the two are the focal points of this book. The book will be of interest to researchers and advanced undergraduate and graduate students in the areas of plant pathology, plant physiology, and plant biochemistry.

Plant Disease An Advanced Treatise, Volume V: How Plants Defend Themselves describes the active, passive, physical, chemical, mechanical, and physiological defense systems of plants against the pathogens. Divided into 23 chapters, this volume discusses theories, experimental approaches, and ways to help plant defend themselves. The opening chapters of this volume deal with certain general aspects of plant defense, such as the theories of “tolerance to disease and “the time sequence of defense , including a dynamic model of defense. A chapter discusses how plant populations defend themselves in natural ecosystem and the implications of disease management on agroecosystems. Considerable chapters examine the defense by the host by analogy with defense of a medieval castle, such as perimeter, internal, and chemical defenses. Discussions on the defenses triggered by the invading pathogen; recognition and compatibility phenomena; the concept of hypersensitivity; the role of phytoalexins in defense; and the metabolic detoxification done by plants to suffer less damage from toxins are provided. This volume also discusses the theory and mechanisms of hypovirulence and hyperparasitism. The concluding chapters summarize the effects of numerous nutrients on disease and the mechanisms involved. This volume is an invaluable source for plant pathologists, mycologists, advanced researches, and graduate students.

A NATO Advanced Study Institute on "Active Defence Mechanisms in Plants" was held at Cape Sounion, Greece, 21 April - 3 May 1981. It succeeded a similar Institute held at Porte Conte, Sardinia in 1975 on "Specificity in Plant Diseases. " What are active defence mechanisms in the context of plant disease in which a plant, the host, may be damaged by a pathogen? Defence mechanisms comprise properties of the host that decrease this damage. The mechanisms are passive when they are independent of the pathogen. They are active when they follow changes in the host caused by the pathogen. Thus for a fungal pathogen, cell walls of a higher plant which are lignified before infection would be a passive defence mechanism if they decreased damage by impeding growth of the fungus. Cell walls known to become lignified as a response to the pathogen would be an active defence mechanism if it were established that this response decreased damage. The papers and discussions at this Advanced Study Institute were about active defence mechanisms in higher plants, mainly econo mically important crop plants, against fungi, bacteria and viruses as pathogens. Taking the microorganisms first it is a truism but one that bears repeating that although plants almost always grow in close association with a wide variety of fungi and bacteria, often of types that can be pathogens, they rarely become diseased, at least not sufficiently so as to attract notice.

Interdependence between species is a law of nature. The degree of this interdependence is vividly evident in the plant-microbial world. Indeed, there is no axenic plant in nature and one finds various forms of interac tions between these two kingdoms ranging from completely innocuous to obligate parasitic. Most of these interactions are poorly understood at the molecular and physiological levels. Only those few cases for which a molecular picture is emerging are discussed in this volume. With the advent of recombinant DNA technology and the realization that some of these interactions are very beneficial to the host plant, a spate of activity to understand and manipulate these processes is occurring. Microbes interact with plants for nutrition. In spite of the large number of plant-microbe interactions, those microbes that cause harm to the plants (i. e. , cause disease) are very few. It is thus obvious that plants have evolved various defense mechanisms to deal with the microbial world. The mecha nisms for protection are highly diverse and poorly understood. Some pathogens have developed very sophisticated mechanisms to parasitize plants, an excellent example for this being crown gall caused by a soil bac terium, Agrobacterium tumefaciens. A remarkable ingenuity is exhibited by this bacterium to manipulate its host to provide nitrogenous compounds which only this bacterium can catabolize. This is carried out by a direct gene transfer mechanism from bacteria to plants.

In 1958, a single volume in the original series of this Encyclopedia adequately summarized the state of knowledge about plant carbohydrates. Expansion into two volumes in the New Series highlights the explosive increase in information and the heightened interest that attended this class of compounds in the interven ing years. Even now the search has just begun. Much remains to be accom plished; e.g., a full description of the plant cell wall in chemical terms. Why this growing fascination with plant carbohydrates? Clearly, much credit goes to those who pioneered the complex chemistry of polyhydroxylated compounds and to those who later sorted out the biochemical features of these molecules. But there is a second aspect, the role of carbohydrates in such biological func tions as host-parasite and pollen-pistil interactions, the mating reaction in fungi, symbiosis, and secretion to name a few. Here is ample reason for anyone concerned with the plant sciences to turn aside for a moment and consider how carbohydrates, so many years neglected in favor of the study of proteins and nucleic acids, contribute to the physiological processes of growth and devel opment in plants.