The word immunity comes from the Latin immunitas, which means "freedom from something."
Immunity is understood as the body's immunity to the action of pathogens and their metabolic products. For example, conifers have immunity against powdery mildew, and deciduous ones - against schutte. Spruce is absolutely immune to shoot rust, and pine is completely immune to cone rust. Spruce and pine are immune to the false fungus, etc.
I.I. Mechnikov is immune to infectious diseases understood common system phenomena due to which the body can resist the attack of pathogenic microbes. The ability of a plant to resist disease can be expressed either in the form of immunity to infection, or in the form of some kind of resistance mechanism that weakens the development of the disease.
The different resistance to diseases of a number of plants, especially agricultural ones, has been known for a long time. Selection of crops for disease resistance, along with selection for quality and productivity, has been carried out since ancient times. But only at the end of the 19th century the first works on immunity appeared, as a doctrine of plant resistance to disease. Among the many theories and hypotheses of that time, one should mention phagocytic theory of I.I. Mechnikov. According to this theory, the animal's body secretes protective substances (phagocytes) that kill pathogenic organisms. This applies mainly to animals, but also occurs in plants.
Gained greater fame mechanical theory of the Australian scientist Cobb(1880-1890), who believed that the reason for plant resistance to diseases comes down to anatomical and morphological differences in the structure of resistant and susceptible forms and species. However, as it turned out later, this cannot explain all cases of plant resistance, and, therefore, cannot recognize this theory as universal. This theory was criticized by Erikson and Ward.
Later (1905) the Englishman Massey put forward chemotropic theory, according to which the disease does not affect those plants that do not contain chemicals that have an attractive effect on the infectious principle (fungal spores, bacterial cells, etc.).
However, later this theory was also criticized by Ward, Gibson, Salmon and others, since it turned out that in some cases the infection is destroyed by the plant after it has penetrated the cells and tissues of the plant.
After the acid theory, several more hypotheses were put forward. Of these, the hypothesis of M. Ward (1905) deserves attention. According to this hypothesis, susceptibility depends on the ability of fungi to overcome plant resistance using enzymes and toxins, and resistance is determined by the ability of plants to destroy these enzymes and toxins.
From others theoretical concepts most attention deserves phytoncidal theory of immunity, extended B.P.Tokin in 1928. This position for a long time developed by D.D. Verderevsky, who established that in the cell sap of resistant plants, regardless of the attack of pathogenic organisms, there are substances - phytoncides that suppress the growth of pathogens.
And finally, of some interest theory of immunogenesis proposed by M.S. Dunin(1946), which considers immunity in dynamics, taking into account the changing state of plants and external factors. According to the theory of immunogenesis, he divides all diseases into three groups:
1. diseases affecting young plants or young plant tissues;
2. diseases affecting aging plants or tissues;
3. diseases, the development of which is not clearly associated with the development phases of the host plant.
N.I. Vavilov paid a lot of attention to immunity, mainly of agricultural plants. The works of foreign scientists I. Erikson (Sweden), E. Stackman (USA) also belong to this period.
BASICS OF PLANT IMMUNITY TO DISEASE
In the most severe epiphytotics, plants are affected by the disease unequally, which is associated with the resistance and immunity of the plants. Immunity is understood as absolute innocence in the presence of infection under conditions favorable for infection of plants and the development of diseases. Resilience is the ability of the body to withstand severe disease damage. These two properties are often identified, meaning that plants are weakly affected by diseases.
Resistance and immunity are complex dynamic states that depend on the characteristics of the plant, the pathogen and environmental conditions. Studying the causes and patterns of resistance is very important, since only in this case is successful work on breeding resistant varieties possible.
Immunity can be congenital (hereditary) or acquired. Innate immunity is passed on from parents to offspring. It changes only with changes in the genotype of the plant.
Acquired immunity is formed during the process of ontogenesis, which is quite common in medical practice. Plants do not have such a clearly defined acquired property, but there are techniques that can increase plant resistance to disease. They are being actively studied.
Passive resistance is determined by the constitutional characteristics of the plant, regardless of the action of the pathogen. For example, the thickness of the cuticle of some plant organs is a factor of passive immunity. Active immunity factors act only upon contact between the plant and the pathogen, i.e. arise (induced) during the pathological process.
The concept of specific and nonspecific immunity is distinguished. Nonspecific is the inability of some pathogens to cause infection of a certain plant species. For example, beets are not affected by pathogens of smut diseases of grain crops, potato late blight, potatoes are not affected by beet cercospora blight, grains are not affected by potato macrosporiosis, etc. Immunity that manifests itself at the variety level in relation to specialized pathogens is called specific.
Factors of plant resistance to disease
It has been established that resistance is determined by the total effect of protective factors at all stages of the pathological process. The whole variety of protective factors is divided into 2 groups: preventing the penetration of the pathogen into the plant (axenia); preventing the spread of the pathogen in plant tissues (true resistance).
The first group includes factors or mechanisms of a morphological, anatomical and physiological nature.
Anatomical and morphological factors. Barriers to the introduction of pathogens can be the thickness of the integumentary tissue, the structure of the stomata, the pubescence of the leaves, waxy coating, and structural features of plant organs. The thickness of the integumentary tissues is a protective factor against those pathogens that penetrate plants directly through these tissues. These are primarily powdery mildew mushrooms and some representatives of the Oomycetes class. The structure of stomata is important for the introduction into tissue of bacteria, pathogens of false powdery mildew, rust, etc. Usually, it is more difficult for the pathogen to penetrate through tightly covered stomata. The pubescence of leaves protects plants from viral diseases and insects that transmit viral infections. Thanks to the waxy coating on leaves, fruits and stems, drops do not linger on them, which prevents the germination of fungal pathogens.
Plant habit and leaf shape are also factors that inhibit the initial stages of infection. Thus, potato varieties with a loose bush structure are less affected by late blight, since they are better ventilated and infectious droplets on the leaves dry out faster. For narrow leaf blades Fewer spores settle.
The role of the structure of plant organs can be illustrated by the example of rye and wheat flowers. Rye is very strongly affected by ergot, while wheat is very rarely affected. This is explained by the fact that the scales of wheat flowers do not open and the spores of the pathogen practically do not penetrate into them. Open type flowering in rye does not prevent spores from entering.
Physiological factors. Rapid penetration of pathogens may be hampered by high osmotic pressure in plant cells, the speed of physiological processes leading to healing of wounds (formation of wound periderm), through which many pathogens penetrate. The speed of passage of individual phases of ontogenesis is also important. Thus, the causative agent of durum smut of wheat penetrates only into young seedlings, therefore varieties that germinate amicably and quickly are less affected.
Inhibitors. These are compounds found in plant tissue or synthesized in response to infection that inhibit the development of pathogens. These include phytoncides - substances of various chemical natures that are factors of innate passive immunity. IN large quantities phytoncides are produced by the tissues of onions, garlic, bird cherry, eucalyptus, lemon, etc.
Alkaloids are nitrogen-containing organic bases formed in plants. Plants of the legume, poppy, nightshade, asteraceae, etc. families are especially rich in them. For example, solanine in potatoes and tomatine in tomatoes are toxic to many pathogens. Thus, the development of fungi of the genus Fusarium is inhibited by solanine at a dilution of 1:105. Phenols can suppress the development of pathogens, essential oils and a number of other compounds. All of the listed groups of inhibitors are always present in intact (undamaged tissues).
Induced substances that are synthesized by the plant during the development of the pathogen are called phytoalexins. By chemical composition all of them are low molecular weight substances, many of them
are phenolic in nature. It has been established that the plant’s hypersensitive response to infection depends on the rate of induction of phytoalexins. Many phytoalexins are known and identified. Thus, rishitin, lyubin, and fituberin were isolated from potato plants infected with the causative agent of late blight, pisatin from peas, and isocoumarin from carrots. The formation of phytoalexins represents typical example active immunity.
Active immunity also includes activation of plant enzyme systems, in particular oxidative ones (peroxidase, polyphenoloxidase). This property allows you to inactivate the hydrolytic enzymes of the pathogen and neutralize toxins.
Acquired, or induced, immunity. To increase plant resistance to infectious diseases, biological and chemical immunization of plants is used.
Biological immunization is achieved by treating plants with weakened cultures of pathogens or their metabolic products (vaccination). It is used to protect plants from certain viral diseases, as well as bacterial and fungal pathogens.
Chemical immunization is based on the action of certain chemicals, including pesticides. Assimilating in plants, they change metabolism in a direction unfavorable for pathogens. An example of such chemical immunizers are phenolic compounds: hydroquinone, pyrogallol, orthonitrophenol, paranitrophenol, which are used to treat seeds or young plants. A number of systemic fungicides have immunizing properties. Thus, dichlorocyclopropane protects rice from blast disease by enhancing the synthesis of phenols and the formation of lignin.
The immunizing role of some microelements that are part of plant enzymes is also known. In addition, microelements improve the supply of essential nutrients, which has a beneficial effect on plant resistance to disease.
Genetics of resistance and pathogenicity. Types of Resilience
The resistance of plants and the pathogenicity of microorganisms, like all other properties of living organisms, are controlled by genes, one or more, qualitatively different from each other. The presence of such genes determines absolute immunity to certain races of the pathogen. The pathogens, in turn, have a virulence gene (or genes) that allows it to overcome the protective effect of resistance genes. According to X. Flor's theory, for each plant resistance gene a corresponding virulence gene can be developed. This phenomenon is called complementarity. When exposed to a pathogen that has a complementary virulence gene, the plant becomes susceptible. If the resistance and virulence genes are not complementary, plant cells localize the pathogen as a result of a hypersensitive reaction to it.
For example (Table 4), according to this theory, potato varieties that have the R resistance gene are affected only by race 1 of the pathogen P. infestans or a more complex, but necessarily possessing virulence gene 1 (1.2; 1.3; 1.4; 1,2,3), etc. Varieties that do not have resistance genes (d) are affected by all races without exception, including the race without virulence genes (0).
Resistance genes are most often dominant, so they are relatively easy to pass on to offspring during selection. Hypersensitivity genes, or R-genes, determine the hypersensitive type of resistance, which is also called oligogenic, monogenic, true, vertical. It provides the plant with absolute invincibility when exposed to races without complementary virulence genes. However, with the appearance of more virulent races of the pathogen in the population, resistance is lost.
Another type of resistance is polygenic, field, relative, horizontal, which depends on the combined action of many genes. Polygenic resistance is inherent to varying degrees in each plant. At a high level, the pathological process slows down, which allows the plant to grow and develop, despite being affected by the disease. Like any polygenic trait, such resistance can fluctuate under the influence of growing conditions (level and quality of mineral nutrition, moisture supply, day length and a number of other factors).
The polygenic type of resistance is inherited transgressively, so it is problematic to fix it by breeding varieties.
A common combination of hypersensitive and polygenic resistance in one variety is common. In this case, the variety will be immune until the appearance of races capable of overcoming monogenic resistance, after which protective functions determines polygenic resistance.
Methods for creating resistant varieties
In practice, directed hybridization and selection are most widely used.
Hybridization. The transfer of resistance genes from parent plants to offspring occurs during intervarietal, interspecific and intergeneric hybridization. To do this, plants with the desired economic and biological characteristics and plants with resistance are selected as parent forms. Donors of resistance are often wild species, so undesirable properties may appear in the offspring, which are eliminated by backcrossing or backcrossing. Beyer wasps repeat until all signs<<дикаря», кроме устойчивости, не поглотятся сортом.
With the help of intervarietal and interspecific hybridization, many varieties of grains, leguminous crops, potatoes, sunflowers, flax and other crops that are resistant to the most harmful and dangerous diseases have been created.
When some species do not cross with each other, they resort to the “intermediary” method, in which each type of parental form or one of them is first crossed with a third species, and then the resulting hybrids are crossed with each other or with one of the originally planned species.
In any case, the stability of hybrids is tested against a strict infectious background (natural or artificial), i.e., with a large number of pathogen infections, under conditions favorable for the development of the disease. For further propagation, plants that combine high resistance and economically valuable traits are selected.
Selection. This technique is a mandatory step in any hybridization, but it can also be an independent method for obtaining resistant varieties. By the method of gradual selection in each generation of plants with the desired characteristics (including resistance), many varieties of agricultural plants have been obtained. It is especially effective for cross-pollinating plants, since their offspring are represented by a heterozygous population.
In order to create disease-resistant varieties, artificial mutagenesis, genetic engineering, etc. are increasingly being used.
Causes of loss of stability
Over time, varieties, as a rule, lose resistance either as a result of changes in the pathogenic properties of pathogens of infectious diseases, or a violation of the immunological properties of plants during their reproduction. In varieties with a hypersensitive type of resistance, it is lost with the appearance of more virulent races or complementary genes. Varieties with monogenic resistance are affected due to the gradual accumulation of new races of the pathogen. That is why breeding varieties only with a hypersensitive type of resistance is futile.
There are several reasons contributing to the formation of new races. The first and most common are mutations. They usually pass spontaneously under the influence of various mutagenic factors and are inherent in phytopathogenic fungi, bacteria and viruses, and for the latter, mutations are the only way of variability. The second reason is the hybridization of genetically different individuals of microorganisms during the sexual process. This path is characteristic mainly of fungi. The third way is heterokaryosis, or heteronuclearity, of haploid cells. In fungi, heteronucleation can occur due to mutations of individual nuclei, the transition of nuclei from hyphae of different quality through anastomoses (fused sections of hyphae) and recombination of genes during the fusion of nuclei and their subsequent division (parasexual process). Heteronuclearity and the asexual process are of particular importance for representatives of the class of imperfect fungi, which lack the sexual process.
In bacteria, in addition to mutations, there is a transformation in which DNA isolated by one strain of bacteria is absorbed by the cells of another strain and is included in their genome. During transduction, individual chromosome segments from one bacterium are transferred to another using a bacteriophage (bacterial virus).
In microorganisms, the formation of races occurs constantly. Many of them die immediately, being uncompetitive due to a lower level of aggressiveness or lack of other important traits. As a rule, more virulent races become established in the population in the presence of plant varieties and species with genes for resistance to existing races. In such cases, a new race, even with weak aggressiveness, without encountering competition, gradually accumulates and spreads.
For example, when cultivating potatoes with resistance genotypes R, R4 and R1R4, races 1 will predominate in the population of the late blight pathogen; 4 and 1.4. When varieties with genotype R2 are introduced into production instead of R4, race 4 will gradually disappear from the pathogen population, and race 2 will spread; 1.2; 1,2,4.
Immunological changes in varieties can also occur due to changes in their growing conditions. Therefore, before zoning varieties with polygenic resistance in other ecological-geographical zones, they must be immunologically tested in the zone of future zoning.
In the presence of a viable pathogen and all the necessary conditions for infection. In practice, they often talk about disease resistance, which can be characterized as the genetic property of some plants to be affected by the disease to a weak degree. Immunity is absolute, resistance is always relative. Like immunity, resistance is determined by the characteristics of the genome, and there are genes for resistance not only to pathogens, but also to unfavorable environmental factors.
The direct opposite of immunity is susceptibility—the plant’s inability to resist infection and spread of a pathogen. In some cases, a plant that is susceptible to some pathogens may be tolerant or hardy to others, i.e. it does not reduce or slightly reduces its productivity (the quantity and quality of the harvest) when infected.
There are specific and nonspecific immunity. The first manifests itself at the cultivar level in relation to certain pathogens and is also called varietal immunity. The second, or nonspecific (species) immunity can be defined as the fundamental impossibility of a given plant species to become infected with specific types of pathogens or saprotrophs. For example, a tomato is not affected by the causative agents of smut diseases of cereals, a cucumber is not affected by cabbage clubroot, pepper is not affected by the causative agent of apple scab, etc.
Immunity can be congenital or acquired. Innate, or natural, immunity is controlled genetically and is inherited. It can be passive or active. Passive immunity is determined by the constitutional characteristics of the plant only and does not depend on the characteristics. Passive immunity factors are divided into two groups:
Acquired, or artificial, immunity manifests itself in the process of ontogenesis, is not fixed in the offspring and acts during one, or less often, several growing seasons. To form acquired immunity to an infectious disease, plants are treated with biological and chemical immunizers. In biological immunization, treatment is carried out with weakened cultures of pathogens (vaccination) or their metabolites. For example, tomato plants infected with a weakly pathogenic strain of TMV are not subsequently affected by more aggressive strains of this virus.
Chemical immunization, as one of the methods of disease prevention, is based on the use of substances called resistance inducers, or immunomodulators.
They are able to activate defensive reactions. Some systemic drugs, phenol derivatives, chitosams, etc. have this effect. Registered immunomodulators also include drugs Narcissus, Immunocytophyte, etc.
The founder of the doctrine of plant immunity, N. I. Vavilov, who initiated the study of its genetic nature, believed that plant resistance to pathogens was developed in the process of thousands of years of evolution in centers of origin. If plants acquired resistance genes, pathogens could infect plants due to the emergence of new physiological races resulting from hybridization, mutation, heterokaryosis and other processes. Within the population of a microorganism, shifts in the number of races are possible due to changes in the varietal composition of plants in a particular region. The emergence of new races of the pathogen may be associated with the loss of resistance of a variety that was once resistant to this pathogen.
According to D.T. Strakhov, in tissues resistant to plant diseases, regressive changes in pathogenic microorganisms occur, associated with the action of plant enzymes and their metabolic reactions.
B. A. Rubin and his colleagues connected the plant reaction aimed at inactivating the pathogen and its toxins with the activity of oxidative systems and the energy metabolism of the cell. A variety of plant enzymes are characterized by different resistance to the waste products of pathogenic microorganisms. In immune forms of plants, the proportion of enzymes resistant to pathogen metabolites is higher than in non-immune forms. The most resistant to the effects of metabolites are oxidative systems (ceroxidases and polyphenoloxidases), as well as a number of flavone enzymes.
In plants, like invertebrate animals, the ability to produce antibodies in response to the appearance of antigens in the body has not been proven. Only vertebrates have special organs whose cells produce antibodies. In the infected tissues of immune plants, functionally complete organelles are formed, which determine the inherent ability of immune plant forms to increase the energy efficiency of respiration during infection. Respiratory impairment caused by pathogenic agents is accompanied by the formation of various compounds that act as unique chemical barriers that prevent the spread of infection.
The nature of plant responses to damage by pests (formation of chemical, mechanical and growth barriers, the ability to regenerate damaged tissue, replacement of lost organs) plays an important role in plant immunity to insect pests. Thus, a number of metabolites (alkaloids, glycosides, terpenes, saponins, etc.) have a toxic effect on the digestive system, endocrine and other systems of insects and other plant pests.
In plant breeding for resistance to diseases and pests, hybridization (intraspecific, interspecific and even intergeneric) is of great importance. Based on autopolyploids, hybrids between differently chromosomal species are obtained. Similar polyploids were created, for example, by M. F. Ternovsky when breeding tobacco varieties resistant to powdery mildew. To create resistant varieties, artificial mutagenesis can be used, and in cross-pollinated plants, selection among heterozygous populations. Thus, L.A. Zhdanov and V.S. Pustovoit obtained sunflower varieties resistant to broomrape.
To preserve the long-term stability of varieties, the following methods have been proposed:
Creation of multiline varieties by crossing economically valuable forms with varieties carrying different resistance genes, due to which the resulting hybrids cannot accumulate new races of pathogens in sufficient quantities;
Combination of R-genes with field resistance genes in one variety;
Periodic change of varietal composition on the farm, which leads to increased sustainability.
In recent years, the development of crop production in our country has been associated with a number of negative processes associated with pollution of the environment and crop products with xenobiotics, and high economic and energy costs. Maximum use of the biological potential of agricultural crops can become one of the alternative ways to develop the agronomic sector of agricultural production. Certain hopes in this regard are associated with genetic engineering - a set of methodological approaches that make it possible to change the design of a plant genome by transferring foreign genes into it, which makes it possible to obtain new forms of plants, significantly expand the process of manipulating the plant genome and reduce the time spent on obtaining new varieties of agricultural crops. crops Recently, methods for creating transgenic plants have begun to be used to produce plants resistant to viral, fungal and bacterial diseases, as well as some pests (Colorado beetle, corn stem borer, cotton moth and bollworm, tobacco budworm, etc.). In terms of its methods and objects, this direction differs sharply from traditional selection for plant immunity, but pursues the same goal - the creation of forms that are highly resistant to harmful organisms.
A brilliant justification for the role of resistant varieties in plant protection was given by N.I. Vavilov, who wrote that among measures to protect plants from various diseases caused by parasitic fungi, bacteria, viruses, as well as various insects, the most radical means of control is the introduction of immune varieties into the culture or the creation of such by crossing. For cereals, which occupy three-quarters of the entire cultivated area, replacing susceptible varieties with resistant forms is essentially the most affordable way to combat infections such as rust, powdery mildew, wheat smut, various fusariums, and blight.
Domestic and world experience in agriculture shows that plant protection should be based on complex (integrated) systems of measures, the basis of which is the presence of disease and pest resistant varieties of crops.
In subsequent chapters, the main patterns that determine the presence of resistance traits in plants, ways of their effective use in the selection process, and methods of imparting induced immunity to plants will be considered.
1. HISTORY OF THE ORIGIN AND DEVELOPMENT OF THE STUDY ABOUT PLANT IMMUNITY.
Ideas about immunity began to take shape already in ancient times. According to the historical chronicles of ancient India, China and Egypt, many centuries before our era the population of the Earth suffered from epidemics. Observing their emergence and development, people came to the conclusion that not every person is susceptible to the effects of the disease and that someone who has once had one of these terrible diseases does not become ill with it again.
By the middle of the 2nd century. BC e. the idea of the uniqueness of human diseases such as plague and others is becoming generally accepted. At the same time, survivors of the plague began to be widely used to care for those suffering from the plague. It is logical to assume that it was at this stage of the development of human society that immunology arose on the basis of data obtained from monitoring the spread of epidemiological diseases. From the very beginning of its development, it sought to use the collected observations for the practical protection of the population from contagious diseases. For many centuries, to protect people from smallpox in one way or another, they deliberately infected themselves with this disease, after which the body became immune to it. Thus, methods were developed to obtain immunity to this disease. However, with the widespread use of such methods, its main disadvantages were revealed, namely that in many of those vaccinated, smallpox was severe, often fatal. In addition, vaccinated people often became a source of infection and contributed to the maintenance of the smallpox epidemic. However, despite the obvious disadvantages, the method of deliberate infection clearly proved the possibility of artificially acquiring immunity through mild transmission of the disease.
The work of the English physician Edward Jenner (1798) was of epochal importance in the development of immunity, in which he summarized the results of 25 years of observations and showed the possibility of vaccinating people with cowpox and acquiring immunity to a similar human disease. These vaccinations are called vaccinations (from the Latin vaccinus - cow). Jenner's work was an outstanding achievement of practice, but without explaining the cause (etiology) of infectious diseases, it could not contribute to the further development of immunology. And only the classic works of Louis Pasteur (1879), which revealed the causes of infectious diseases, made it possible to take a fresh look at Jenner’s results and appreciate them, which influenced both the subsequent development of immunology and the work of Pasteur himself, who proposed the use of weakened pathogens for vaccination. Pasteur's discoveries laid the foundation for experimental immunology.
An outstanding contribution to the science of immunity was made by the Russian scientist I. I. Mechnikov (1845-1916). His works formed the basis of the theory of immunity. As the author of the phagocytic theory of protection of the animal and human body from pathogens, I. I. Mechnikov was awarded the Nobel Prize in 1908. The essence of this theory is that all animal organisms (from amoeba to humans inclusive) have the ability, with the help of special cells - phagocytes, to actively capture and intracellularly digest microorganisms. Using the circulatory system, phagocytes actively move within living tissues and concentrate in places where microbes penetrate. It has now been established that animal organisms protect against microbes using not only phagocytes, but also specific antibodies, interferon, etc.
A significant contribution to the development of immunology was made by the works of N. F. Gamaley (1859-1949) and D. K. Zabolotny (1866-1929).
Despite the successful development of the doctrine of animal immunity, ideas about plant immunity developed extremely slowly. One of the founders of plant immunity was the Australian researcher Cobb, the author of the theory of mechanical protection of plants from pathogens. The author considered mechanical protective devices to include such plant features as a thickened cuticle, the unique structure of flowers, the ability to quickly form wound periderm at the site of damage, etc. Subsequently, this method of protection was called passive immunity. However, the mechanical theory could not exhaustively explain such a complex, diverse phenomenon as immunity.
Another theory of immunity, proposed by the Italian scientist Comes (1900), is based on the fact that plant immunity depends on the acidity of the cell sap and the sugar content in it. The higher the content of organic acids, tannins and anthocyanins in the cell sap of plants of a particular variety, the more resistant it is to the diseases that affect it. Varieties with a high content of sugars and relatively low acids and tannins are more susceptible to diseases. Thus, in grape varieties resistant to mildew and powdery mildew, the acidity (% of dry matter) is 6.2...10.3, and in susceptible ones - from 0.5...1.9. However, Comes's theory is not universal and cannot explain all cases of immunity. Thus, a study of many varieties of wheat and rye, which have different susceptibility to rust and smut, did not reveal a clear correlation between immunity and acid content in leaf tissues. Similar results were obtained for many other crop plants and their pathogens.
At the beginning of the 20th century. New hypotheses appeared, the authors of which tried to explain the causes of plant immunity. Thus, the English researcher Massey proposed the chemotropic theory, according to which plants that lack the substances necessary to attract parasites have immunity. Studying pathogens of cucumber and tomato, he showed that the juice of susceptible varieties promoted the germination of pathogen spores, while the juice of resistant varieties inhibited this process. The chemotropic theory has been seriously criticized by a number of researchers. The most thorough criticism of this theory was given by N.I. Vavilov, who considered it unlikely that the cell sap contained in the vacuoles could act remotely on fungal hyphae and that some substances released from the tissues to the outside cannot be identified with the cell sap obtained by squeezing out substrates , on which the mushroom was grown.
Protecting plants from diseases by creating and cultivating resistant varieties has been known since ancient times. Spontaneously carried out in places favorable for the development of pathogens of certain diseases, artificial selection for resistance to them led to the creation of varieties of agricultural plants with increased resistance to these diseases. Natural disasters caused by the spread of particularly dangerous diseases (grain rust, potato late blight, oidium and grape mildew) stimulated the emergence of scientifically based plant selection for immunity to diseases. In 1911, the First Congress on Breeding was held, where A. A. Yachevsky (1863-1932) made a general report “On the importance of selection in the fight against fungal diseases of cultivated plants.” The data presented in the report indicated that successful work on creating disease-resistant varieties is impossible without developing a theory of plant immunity to infectious diseases.
In our country, the founder of the doctrine of plant immunity is N.I. Vavilov. His first works on plant immunity were published in 1913 and 1918, and the monograph “Plant Immunity to Infectious Diseases,” published in 1919, was the first attempt at a broad generalization and theoretical justification of all the material that had accumulated by that time in the field of immunity studies . In the same years, the works of N. I. Litvinov (1912) on assessing the resistance of cereals to rust and E. N. Iretskaya (1912) on methods of selecting cereals for rust resistance appeared. However, these works remained only episodes in the scientific activities of the authors.
Works by N. I. Vavilov “The Doctrine of Plant Immunity to Infectious Diseases” (1935), reports at the I All-Union Conference on the Control of Rust of Cereals in 1937 and at the Biological Department of the USSR Academy of Sciences in 1940, a number of his articles and speeches at different times played a huge role in the development of theoretical ideas about the genetic characteristics of plants as decisive factors determining varietal and species resistance. N.I. Vavilov substantiated the position that plant immunity is inextricably linked with their genetic characteristics. Therefore, N.I. Vavilov considered the main task of breeding for resistance to be the search for plant species differences based on immunity. The world collection of cultivated plant varieties collected by him and VIR employees still serves as a source for obtaining immune forms. Of great importance in the search for immune forms of plants is his concept of the parallel biological evolution of plants and their pathogens, which was subsequently developed in the theory of conjugate evolution of parasites and their hosts, developed by P.M. Zhukovsky (1888-1975). The patterns of manifestation of immunity, determined by the result of the interaction between a plant and a pathogen, were attributed by N. I. Vavilov to the field of physiological immunity.
The development of theoretical issues of the doctrine of plant immunity, begun by N.I. Vavilov, was continued in our country in subsequent years. Research was carried out in various directions, which was reflected in different explanations of the nature of plant immunity. Thus, the hypothesis of B. A. Rubin, based on the teachings of A. N. Bach, connects plant resistance to infectious diseases with the activity of plant oxidative systems, mainly peroxidases, as well as a number of flavone enzymes. Activation of plant oxidative systems leads, on the one hand, to an increase in the energy efficiency of respiration, and on the other, to disruption of its normal course, which is accompanied by the formation of various compounds that play the role of chemical barriers. E. A. Artsikhovskaya, V. A. Aksenova and others also participated in the development of this hypothesis.
The phytoncidal theory, developed in 1928 by B. P. Tokin based on the discovery of bactericidal substances in plants - phytoncides, was developed by D. D. Verderevsky (1904-1974), as well as by employees of the Moldavian Plant Protection Station and the Chisinau Agricultural Institute (1944-1976 ).
In the 80s of the last century, L.V. Metlitsky, O.L. Ozeretskovskaya and others developed a theory of immunity associated with the formation of special substances in plants - phytoalexins, which arise in response to infection by incompatible species or races of pathogens. They discovered a new potato phytoalexin - lyubin.
A number of interesting provisions of the theory of immunity were developed by K. T. Sukhorukoe, who worked in the Main Botanical Garden of the USSR Academy of Sciences, as well as a group of employees led by L. N. Andreev, who were developing various aspects of the doctrine of plant immunity to rust diseases, peronospora and verticillium wilt.
In 1935 T.I. Fedotova (VIZR) was the first to discover the affinity of host and pathogen proteins. All the previously listed hypotheses about the nature of plant immunity associated it with only one or a group of similar protective properties of plants. However, N.I. Vavilov emphasized that the nature of immunity is complex and cannot be associated with any one group of factors, because the nature of the relationships between plants and different categories of pathogens is too diverse.
In the first half of the 20th century. in our country, they only assessed the resistance of plant varieties and species to diseases and parasites (grain crops to rust and smut, sunflower to broomrape, etc.). Later they began to select for immunity. This is how the sunflower varieties bred by E. M. Pluchek (Saratovsky 169 and others) resistant to broomrape (Orobanche sitapa) race A and sunflower moth appeared. The problem of combating broomrape of race B “Evil” was solved for many years thanks to the work of V. S. Pustovoit, who created a series of varieties resistant to broomrape and moths. V. S. Pustovoit developed a seed production system that makes it possible to maintain the stability of sunflower at the proper level for a long time. During the same period, oat varieties resistant to crown rust were created (Verkhnyachsky 339, Lgovsky, etc.), which have retained resistance to this disease to this day. Since the mid-1930s, P. P. Lukyanenko and others began breeding for wheat resistance to leaf rust, and M. F. Ternovsky began work on creating tobacco varieties resistant to a complex of diseases. Using interspecific hybridization, he developed tobacco varieties resistant to tobacco mosaic, powdery mildew and downy mildew. Selection for the immunity of sugar beets to a number of diseases was successfully carried out.
Varieties have been obtained that are resistant to powdery mildew (Hybrid 18, Kirgizskaya odnosemyanka, etc.), cercospora (Pervomaisky polyhybrid, Kuban polyhybrid 9), downy mildew (MO 80, MO 70), root beetle and black rot (Verkhneyachskaya 031, Belotserkovskaya TsG 19).
A. R. Rogash and others successfully worked on the selection of flax for immunity. Varieties P 39, Orshansky 2, Tvertsa with increased resistance to fusarium and rust were created.
In the mid-30s, K.N. Yatsynina developed tomato varieties resistant to bacterial cancer.
A number of interesting and important works on the creation of varieties of vegetable crops resistant to clubroot and vascular bacteriosis were carried out under the leadership of B.V. Kvasnikov and N.I. Karganova.
Cotton selection for immunity to Verticillium wilt has been carried out with varying success. The variety 108 f, bred in the mid-30s of the last century, remained stable for about 30 years, but then lost it. The varieties of the Tashkent series that replaced it also began to lose resistance to wilt due to the emergence of new races of Verticillium dahliae (0, 1, 2, etc.).
In 1973, it was decided to create laboratories and departments for plant immunity to diseases and pests in breeding centers and plant protection institutes. An important role in the search for sources of sustainability was played by the Institute of Plant Growing named after. N. I. Vavilova. World collections of cultivated plant samples collected at this institute still serve as a fund of donors for the resistance of various crops necessary for breeding for immunity.
After E. Steckman’s discovery of physiological races in the causative agent of cereal stem rust, similar work was launched in our country. Since 1930, the All-Union Selection and Genetics Institute (E. E. Geshele) began the study of physiological races brown and stem rust, smut. In the post-war years, the All-Russian Research Institute of Phytopathology began to deal with this problem. Back in the 1930s, A. S. Burmenkov, using a standard set of differentiating varieties, showed the heterogeneity of races of rust fungi. In subsequent years, especially in the 60s, these works began to develop intensively (A. A. Voronkova, M. P. Lesovoy, etc.), which made it possible to reveal the reasons for the loss of resistance by some varieties with a seemingly unchanged racial composition of the fungus. Thus, it was revealed that race 77 of the causative agent of brown rust of wheat, predominant in the 70s of the 20th century. in the Northern Caucasus and southern Ukraine, consists of a series of biotypes differing in virulence, formed not on wheat, but on susceptible cereals. Research on races of smut fungi, begun at the All-Russian Institute of Rectification by S.P. Zybina and L.S. Gutner, as well as K.E. Murashkinsky in Omsk, was continued at the All-Russian Research Institute by V.I. Krivchenko, and on dusty smut of wheat - by L.F. Tymchenko at the Institute of Agriculture of the Non-Black Earth Zone.
N. A. Dorozhkin, Z. I. Remneva, Yu. V. Vorobyova, K. V. Popkova were very productive in studying the races of Phytophthora infestans. In 1973, Yu.T.Dyakov, together with T.A. Kuzovnikova and others, discovered the phenomenon of heterokaryosis and parasexual process in Ph. infestans, which allows us to some extent explain the mechanism of variability of this fungus.
In 1962 P.AKhizhnyak and V.I. Yakovlev discovered aggressive races of the potato canker pathogen Synchythrium endobioticum. It was found that at least three races of S. endobioticum are widespread in our country, affecting potato varieties resistant to the common race.
In the late 70s - early 80s of the last century, A. G. Kasyanenko studied the physiological races of the fungus Verticillium dahliae, Cladosporium fulvum - L. M. Levkin, the causative agent of powdery mildew of wheat - M. N. Rodigin and others, peronosporosis tobacco - A. A. Babayan.
Thus, the study of plant immunity to infectious diseases was carried out in our country in three main areas:
Study of the race formation of pathogens and analysis of population structure. This led to the need to study the population composition within species, population mobility, patterns of appearance, disappearance or regrouping of individual members of the population. The doctrine of races arose: taking into account races, forecasting and patterns of the appearance of some races and (or) the disappearance of others;
assessment of disease resistance of existing varieties, search for donors of resistance and, finally, the creation of resistant varieties.
Innate, or natural, immunity is the property of plants not to be affected (damaged) by a particular disease (pest). Innate immunity is inherited from generation to generation.
Within innate immunity, passive and active immunity are distinguished. However, the results of numerous studies lead to the conclusion that the division of plant immunity into active and passive is very arbitrary. At one time this was emphasized by N.I. Vavilov (1935).
An increase in plant resistance under the influence of external factors, which occurs without changing the genome, is called acquired or induced resistance. Factors whose influence on seeds or plants leads to increased plant resistance are called inducers.
Acquired immunity is the property of plants not to be affected by one or another pathogen that arose in plants after suffering a disease or under the influence of external influences, especially plant cultivation conditions.
Plant resistance can be increased by various methods: applying microfertilizers, changing planting (sowing) dates, seeding depth, etc. Methods for achieving resistance depend on the type of inducers, which can be of a biotic or abiotic nature. Techniques that promote the manifestation of acquired resistance are widely used in agricultural practice. Thus, the resistance of grain crops to root rot can be increased by sowing spring grain crops at optimal early dates, and winter grain crops at optimal late dates; The resistance of wheat to smut, which infects plants during seed germination, can be increased by observing the optimal sowing time.
Plant immunity may be due to the inability of the pathogen to cause infection of plants of a given species. Thus, grain crops are not affected by late blight and potato scab, cabbage by smut diseases, potatoes by rust diseases of grain crops, etc. In this case, immunity is manifested by the plant species as a whole. Immunity based on the inability of pathogens to cause infection of plants of a certain species is called nonspecific.
In some cases, immunity may not be manifested by a plant species as a whole, but only by an individual variety within that species. In this case, some varieties are immune and are not affected by the disease, while others are susceptible and are severely affected by it. Thus, the causative agent of potato cancer Synchytrium endobioticum infects the Solanum species, but within it there are varieties (Kameraz, Stoilovy 19, etc.) that are not affected by this disease. This kind of immunity is called varietal specific. It is of great importance in breeding resistant varieties of agricultural plants.
In some cases, plants may be immune to pathogens of various diseases. For example, a winter wheat variety may be immune to both powdery mildew and brown stem rust. The resistance of a plant variety or species to several pathogens is called complex or group immunity. The creation of varieties with complex immunity is the most promising way to reduce crop losses from diseases. For example, wheat Triticum timopheevi is immune to smut, rust, and powdery mildew. There are known varieties of tobacco that are resistant to the tobacco mosaic virus and the downy mildew pathogen. By zoning such varieties in production, it is possible to solve the problem of protecting a particular crop from major diseases.