Molecular Methods in Plant Pathology

Uma. S. Singh, Rudra P. Singh

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351430210
• Edition: 1
• Topic: Scienze biologiche
• Subtopic: Botanica

Molecular Methods in Plant Pathology covers methods in phytopathology at the molecular level, including PCR techniques, electron microscopy, tissue culturing, and the cloning of disease-resistant genes. Phytopathologists, botanists, horticulturists, and anyone working in agriculture will find this a useful reference on biophysical, biochemical, biomolecular, and biotechnological methods.

Section IV

BIOTECHNOLOGICAL METHODS

Chapter 19

Present Problems in and Aspects of Breeding for Disease Resistance

J. E. Parlevliet

TABLE OF CONTENTS

I.	Introduction A. Defenses of Green Plants to Parasites B. Breeding before 1900 C. Breeding after 1900
II.	Assessment of Resistance
III.	Host-Pathogen Interactions A. Durability of Resistance B. Qualitative and Quantitative Resistance C. Genetics of Resistance D. Genetics of Virulence, the Gene-for-Gene Concept E. Genetics of Aggressiveness F. Specificity G. Possible Cause of Differences in Durability of Resistance Genes IV. Selection for Resistance A. Screening Methods B. Sources of Resistance 1. Major Resistance Genes 2. Minor Resistance Genes C. Selection Procedures
V.	Strategies to Increase Durability of Nondurable Resistance Genes A. No strategy B. Resistance Genes Used One at a Time C. Multiple Gene Use D. Multiline and Varietal Mixture Approach E. Variety Diversification F. Regional Gene Deployment
References
Further Reading

I INTRODUCTION

A DEFENSES OF GREEN PLANTS TO PARASITES

In nature, organisms can be classified as producers, the green plants; consumers, all organisms that exploit producers or other organisms; and decomposers, organisms that use dead organisms. The green plants are used by a multitude of primary consumers of practically all classes of the living world, from various types of herbivores (mammals, snails, insects) to typical parasites (insects, mites, fungi, bacteria). In order to survive, green plants have developed a broad range of defense mechanisms to ward off most of these consumers. These defense mechanisms can be classified into three groups: avoidance, resistance, and tolerance.¹ Avoidance operates before parasitic contact between host and parasite is established and decreases the frequency of incidence. After parasitic contact has been established the host can resist the parasite by decreasing its growth or tolerate its presence by suffering relatively little damage.

Avoidance is mainly active against animal parasites and includes such diverse mechanisms as volatile repellents, mimicry, and morphological features like hairs, thorns, and resin ducts. Resistance is usually of a chemical nature. Of tolerance little is known. It is very difficult to measure, and is usually confounded with quantitative forms of resistance.¹

Parasites classified as fungi, bacteria, mycoplasma, viruses, or viroids are collectively indicated as pathogens, disease-inciting parasites. Of the defense mechanisms employed by host plants against pathogens, resistance mechanisms are by far the most important. Avoidance and tolerance play a minor role here. In crops, breeders have indeed predominantly used resistance to pathogens, and the topic of this chapter therefore is breeding for resistance to pathogens.

B BREEDING BEFORE 1900

From the moment wild plant species were domesticated by man, selection played a role in the evolution of our crops. Until the 19th century the selection was a combination of natural and human selection. Natural selection occurred as the plant genotypes best adapted to the local conditions (climate, topography, soil, farming system) tended to contribute more to the next generation than those not so well adapted. Plant types favored by humans were selected by humans, the human selection component. Over the centuries, this led to local landraces adapted to the combined circumstances of cultivation, climate, soil, and human desires. Until the 19th century the importance of human selection was small compared with the natural selection component. This changed completely in that century. Halfway through the 19th century, conscious selection of improved plant types from landraces as well as crossing between selected plant types were carried out in most important crops. In the 1850s an English farmer selected a promising wheat plant from his field. Its progeny became the variety Browick, which was still highly resistant to yellow rust more than a century later. In potatoes the accumulation of viruses and the dramatic late blight epidemic in the middle of the 19th century were the major forces leading to the selection of new varieties derived from cross breeding.

At the end of this century, crossing varieties in order to create genetic variation from which new varieties were developed had become a regular procedure in many crops. Resistance to the important pathogens was right from the start a major goal of plant breeders.

C BREEDING AFTER 1900

Very soon after the rediscovery and confirmation of the Mendelian inheritance of many traits in the early years of this century, it was realized that resistance to diseases did not behave differently. As early as 1920, resistant varieties were developed through cross breeding in various crops, as in wheat against stem rust, Puccinia graminis f. sp. tritici, yellow rust, P. striiformis, and stinking smut, Tilettia caries; and in flax, cotton, cabbage, and watermelon to the Fusarium wilts, Fusarium oxysporum f. sp. lini, vasinfectum, conglutinans, and niveum, respectively.

Almost as soon as resistant varieties were grown on a larger acreage, man was faced with the problem of genetic adaptation to the resistance by the pathogen population, through new races (flax wilt in flax, stem rust in wheat). Such setbacks did not discourage breeders. On the contrary, the search for resistance and the incorporation of it in commercial varieties grew with an ever increasing rate up till the present moment.

The massive use of resistance exposed the advantages as well as the disadvantages of resistance. Once introduced, resistance is a very cheap method to control the pathogen and very easy to use. The economic value of resistance totalled over all crops and pathogen is difficult to undervalue; it is at least a multibillion-dollar affair on a yearly basis. Two disadvantages, however, became apparent as well. Resistance genes are highly pathogen specific, i.e., they are effective to only one pathogen. Since in most situations crops are affected by at least several pathogens, resistance genes to each of them have to be introduced if pesticide use should be abandoned to a large extent. The specificity of the pathogens often go much further. Many resistance genes evoke adaptation in the pathogen population, resulting in a loss of effectiveness of those resistance genes. The breeder has to search again for new resistance genes.

This led to ideas of a more durable use of these nondurable genes using strategies such as multilines, variety mixtures, gene development, variety diversification, and multiple-resistance gene barriers. It also led to ideas concerning the nature and use of resistance that cannot be overcome by the pathogen: durable resistance.

At the same time, plant breeders and pathologists developed the resistance breeding into highly efficient procedures. Initially, the screening was done in the field in dependency of all kinds of disturbing factors. To expose the entries to be screened uniformly to the right pathogen population at the right time and desired plant development stage, technologically highly advanced screening methods were developed. The sources from which resistance genes are obtained have broadened steadily from the cultivated plant species in the beginning to a wide range of related wild species at present. And the first introduction of genetic information derived from totally unrelated organisms, such as bacteria (Bt-genes) and viruses (protein coat genes) into crops to provide resistance to certain parasites has been realized in the late 1980s.

From those developments it was learned that efficient breeding for disease resistance requires...