Many fungi and bacteria that associate with plants are potentially harmful and can cause disease, while others enter into mutually beneficial sym bioses. Co-evolution of plants with pathogenic and symbiotic microbes has lead to refined mechanisms of reciprocal recognition, defense and counter defense. Genes in both partners determine and regulate these mechanisms. A detailed understanding of these genes provides basic biological insights as well as a starting point for developing novel methods of crop protection against pathogens. This volume deals with defense-related genes of plants and their regulation as well as with the genes of microbes involved in their interaction with plants. Our discussion begins at the level of populations and addresses the complex interaction of plant and microbial genes in multigenic disease resistance and its significance for crop protection as compared to mono genic resistance (Chap. 1). Although monogenic disease resistance may have its problems in the practice of crop protection, it is appealing to the experimentalist: in the so-called gene-for-gene systems, single genes in the plant and in the pathogen specify the compatibility or incompatibility of an interaction providing an ideal experimental system for studying events at the molecular level (Chaps. 2 and 4). Good progress has been made in identifying viral, bacterial, and fungal genes important in virulence and host range (Chaps. 3-6). An important aspect of plant-microbe interactions is the exchange of chemical signals. Microbes can respond to chemical signals of plant origin.

The use of microbial plant protection products is growing and their importance will strongly increase due to political and public pressure. World population is growing and the amount of food needed by 2050 will be double of what is produced now whereas the area of agricultural land is decreasing. We must increase crop yield in a sustainable way. Chemical plant growth promoters must be replaced by microbiological products. Also here, the use of microbial products is growing and their importance will strongly increase. A growing area of agricultural land is salinated. Global warming will increase this process. Plants growth is inhibited by salt or even made impossible and farmers tend to disuse the most salinated lands. Microbes have been very successfully used to alleviate salt stress of plants. Chemical pollution of land can make plant growth difficult and crops grown are often polluted and not suitable for consumption. Microbes have been used to degrade these chemical pollutants.

In this volume expert authors review current research on diverse aspects of the interactions which occur in the rhizosphere between the host plant and the microorganisms. The chapters focus on specific phenomena, from the biochemical and genetical level to complex inter-organism communication.

This book presents a detailed discussion on the direct interactions of plants and microorganisms in the rhizosphere environment. It includes fifteen chapters, each focusing on a specific component of plant-microbe interactions, such as the influence of plants on the root microbiome, and the downstream effects of rhizosphere microbial dynamics on carbon and nutrient fluxes in the surroundings. As such, the book helps readers gain a better understanding of diversity above the ground, and its effect on the microbiome and its functionality.

The Rhizobiaceae, Molecular Biology of Model Plant-Associated Bacteria. This book gives a comprehensive overview on our present molecular biological knowledge about the Rhizobiaceae, which currently can be called the best-studied family of soil bacteria. For many centuries they have attracted the attention of scientists because of their capacity to associate with plants and as a consequence also to specifically modify plant development. Some of these associations are beneficial for the plant, as is the case for the Rhizobiaceae subgroups collectively called rhizobia, which are able to fix nitrogen in a symbiosis with the plant hosts. This symbiosis results in the fonnation of root or stem nodules, as illustrated on the front cover. In contrast, several Rhizobiaceae subgroups can negatively affect plant development and evoke plant diseases. Examples are Agrobacterium tumefaciens andA. rhizogenes which induce the formation of crown galls or hairy roots on the stems of their host plants, respectively (bottom panels on front cover). In addition to the obvious importance of studies on the Rhizobiaceae for agronomy, this research field has resulted in the discovery of many fundamental scientific principles of general interest, which are highlighted in this book. To mention three examples: (i) the discovery of DNA transfer of A.

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.

Plant pathology is an applied science that deals with the nature, causes and control of plant diseases in agriculture and forestry. The vital role of plant pathology in attaining food security and food safety for the world cannot be overemphasized.