Plant Gene Research – serie

The co-evaluation of plants and microbes has led to an elaborate system of genes involved in recognitions, attack and defence. This volume explores the genes and the regulation of their expression. Topics covered range from considerations of population genetics to the identification of defence-related genes and their regulation. The book provides a concise review of the latest developments in this rapidly developing field of agronomic importance.

Tropical crops such as cowpea, yam, plantain, and cassava are heavily underresearched but, in addition to rice, maize, wheat, and potato, are important as primary or secondary food staples in the developing countries. The modern tools of molecular and cellular technology offer the opportunity not only to make substantial gains in knowledge of these crops, but also they overcome some of the obstacles which presently restrain both the genetic improvement and the productivity of these crops in tropical farming systems. Increased nutritional value of these crops, reduced post-harvest perishability, and lower costs of production are some of the advantages taken from biotechnology. Engineered genetic resistance would also allow to drastically reduce employment of pesticides, which at present are expensive or unavailable for farmers in developing countries and may create environmental and health hazards. In this book experts present opportunities to improve the efficiency of conventional plant breeding programs also taking into account the ethical and sociopolitical aspects of these technologies.

Recent advances in gene technology, plant transformation, and the growing knowledge of DNA sequences of plants as well as of their most important parasites and symbionts offer many interesting prospects for the breeding of new crop varieties. This was not only recognized by the major seed companies, but also by the governments of developing countries and by worldwide foundations supporting their agriculture. The know-how gained by the seed companies on crops important for the agricultural industry in developed countries could easily be provided for free to the international and national organizations dedicated to development of crops important in the third world. Results obtained worldwide become easily available to everybody through the scientific literature. Likewise, agricultural research in, e.g., the USA or Europe profits from the natural plant gene pool available in the third world. All this definitely provides for the possibility of fast change, new prosperity and security of food supply in the whole world, if properly applied. The fast development also asks for ethical and sociopolitical considerations, whereby not doing the right can be as much a mistake as doing the wrong.

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.

First attempts to isolate plant genes were for those genes that are abun­ dantly expressed in a particular plant organ at a specific stage of devel­ opment. However, many important gene products are produced in a very minute quantity and in specialized cell types. Such genes can now be isolated using a variety of approaches, some of which are described in this volume. The rapid progress during the last decade in regeneration of a number of crop plants and the availability of molecular tools to introduce foreign genes in plants is allowing the engineering of specific traits of agri­ cultural importance. These genes must, however, be regulated in a spatial and temporal manner in order to have desired effects on plant devel­ opment and productivity. The habitat of plants necessitate adaptive responses with respect to the environmental changes. Starting from germination of the seed, the plant begins to sense environmental cues such as moisture, light, temperature and the presence of pathogens, and begins to respond to them. Little is known about various signal transduction pathways that lead to biochemical and morphogenetic responses, in particular, transition from vegetative to reproductive phase. With the availability of tools to generate specific mutations via transposon tagging, identification and isolation of genes affecting these processes may be facilitated. Transfer of these genes into heterologous environments will allow understanding of the complex processes that control plant development.

There has been recent rapid progress in the transformation of plants with foreign DNA, making use either of the natural routes of genetic invasion that viruses and bacteria have developed, or of chemical, mechanical and electrical tricks to make plant protoplast membranes permeable to nucleic acids. Genes integrated into plant virus genomes can be carried systemi­ cally from the initial site of infection into the rest of the plant. Genes placed between the borders of Agrobacterium tumefaciens T-DNA can be transferred into single cells or plant tissue, which then divides to produce wound calli, or as in the case of an Agrobacterium rhizogenes infection, grow out into new roots. Calli and roots can be grown into whole plants. If virus genomes are placed between the T-DNA borders, a very effective infectious route, termed "agroinfection", is established. Once inside a pro­ toplast, DNA finds its way into the nucleus where it can finally integrate into the resident chromosome and be expressed. Whether it can also find its way into chloroplasts is not yet clear, but at least translation products can be targeted into this organelle. Regeneration of whole organisms from single cells is a special feature of plants and offers a unique tool to study genes in a multicellular organism. In addition, as in animal cells, transcription and translation of trans­ forming genes can be studied in plant cells during "transient expression".

Biologists ask how the growth, development and behaviour of organisms happen, how these processes are co-ordinated and how they are regulated by the environment. Today the questions are phrased in terms of the genes involved, their structure and the control of their expression. Mutations (recognised by a change in phenotype) label genes and can be used to study gene structure, gene function and the organisation of the genome. This is "Genetics". Study of phenotypes down to the level of the enzymes and structural proteins coded for by genes is "Biochemistry". It is self evident that only by studying phenotype ("Biochemistry") can we do "Ge­ netics" and that "Genetics" (perturbation of the phenotype) is the key to understanding the "Biochemistry". There can surely be no better argu­ ments for a more holistic approach to biology than the massive output of knowledge from microbial "Biochemical Genetics" and the more recent revelations from "Molecular Genetic" studies of development in Droso­ phila.

Plant growth and development is controlled by various environmental cues that are sensed by the plant via various signal transduction pathways coupled to specific response. Some of these pathways are conserved from yeast to plants being regulated by various kinases and phosphatases. In addition, plants have many unique pathways that transduce to specific signals such as light, phytohormones and oligosaccharides. This volume highlights some of the examples of the plant signal transduction machinery opening new vistas in research on plant growth and development. The new technologies including the use of bacteria, yeast and Arabidopsis as functional complementation systems are providing proof of function of many of the proteins that show homology to those from other organisms. These studies will eventually lead to improvement of crop plants and use of plants as a new resource for producing desirable products to meet the growing needs of mankind.

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.

Genetic material is in flux: this is one of the most exciting recent concepts in molecular biology. This volume of "Plant Gene Research" describes changes that occur in the genetic material of plants. It is worthwhile re­ membering that the first examples of unstable genomes were described for maize before DNA was known to be the genetic material. Now trans­ posable elements like the ones found in maize have been described in almost all organisms and have become incorporated into our thinking about genome structure. Flux in the plant genome is not restricted to transposable elements or to nuclear genes. Exchanges of genetic material have been demonstrated within organelle DNA, between organelle DNAs or between organelle and nuclear DNAs. Such exchanges may only occur over evolutionary times or may be a continuing process. Also the environment alters the plant genome. Stress, either viral, nutri­ tional or tissue-culture induced causes heritable changes in the genome. Infection with the crown gall bacterium Agrobacterium tumefaciens results in the transfer of bacterial DNA into the plant genome.

Although plant genes were first isolated only some twelve years ago and transfer of foreign DNA into tobacco cells first demonstrated some eight years ago, the application and extension of biotechnology to agricultural problems has already led to the field-testing of genetically modified crop plants. The promise of tailor-made plants containing resistance to pests or diseases as well as many other desirable characteristics has led to the almost compulsory incorporation of molecular biology into the research programs of chemical and seed companies as well as Governmental agricultural agencies. With the routine transformation of rice and the early evidence of transformation of maize the possibility of the world's major cereal crops being modified for improved nutritional value or resistance characteristics is now likely in the next few years. The increasing number of cloned plant genes and the increasing sophistication of our knowledge of the major developmental and biochemi­ cal pathways in plants should eventually allow us to engineer crop plants with higher yields and with less detrimental impact on the environment than now occurs in our current high input agricultural systems. This book draws together many of the expanding areas of plant molecular biology and genetic engineering that will make a substantial contribution to the development of the more productive and efficient crop plants that the world's farmers will be planting in the next decade.