Extensive system Agriculture and unjustified chemicalization greatly complicate the phytosanitary situation. Imperfect agricultural technology, monoculture, uncultivated weedy fields create exceptionally favorable conditions for the spread of infection and pests.
At all stages of ontogenesis, plants interact with many other organisms, most of which are harmful. Various diseases of plants and seeds can be caused by mushrooms , bacteria and viruses .
Diseases manifest themselves as a result of the interaction of two organisms - a plant and a pathogen that destroys plant cells, releasing toxins in them, and digests them through depolymerase enzymes. The reverse reaction of plants is to neutralize toxins, inactivate depolymerases and inhibit the growth of pathogens by means of endogenous antibiotics.
The immunity of plants to pathogens is called immunity , or phytoimmunity ... N.I. Vavilov singled out natural , or congenital , and acquired immunity. Depending on the mechanism of protective functions, immunity can be active and passive . Active, or physiological, immunity is predetermined by the active reaction of plant cells to the penetration of a pathogen into them. Passive immunity is a category of resistance, which is associated with the features of both the morphological and anatomical structure of plants.
The effectiveness of physiological immunity is mainly due to the weak development of the pathogen with a sharp manifestation of immunity - its early or late death, which is often accompanied by local death of the cells of the plant itself.
Immunity is completely dependent on the physiological reactions of the cytoplasm of the fungus and host cells. The specialization of phytopathogenic organisms is determined by the ability of their metabolites to suppress the activity of defense reactions induced by infection in the plant. If the plant cells perceive the penetrating pathogen as a foreign organism, a series of biochemical changes occurs to eliminate it, so infection does not occur. Otherwise, infection occurs.
The nature of the development of the disease depends on the characteristics of both components and conditions. environment... The presence of an infection does not mean the manifestation of the disease. In this regard, scientist J. Deveroll distinguishes two types of infection: 1) high, if the causative agent of the disease is virulent, and the plant is susceptible to this disease; 2) low, characterized by the virulent state of the pathogen and increased plant resistance to it. With low virulence and weak resistance, an intermediate type of infection is noted.
Depending on the degree of virulence of the pathogen and the resistance of the plant, the nature of the disease is not the same. Based on this, Van der Planck singles out vertical and horizontal plant resistance against diseases. Vertical stability observed in the case when the variety is more resistant to some races of the pathogen than to others. Horizontal resistance manifests itself to all races of the pathogen in the same way.
A plant's immunity to diseases is determined by its genotype and environmental conditions. NI Vavilov cites information that soft wheat varieties are very affected by brown rust, while durum wheat forms are resistant to this disease. The founder of the doctrine of phytoimmunity came to the conclusion that hereditary differences in plant varieties in immunity are constant and are little subject to variability under the influence of environmental factors. With regard to physiological immunity, NI Vavilov believes that in this case heredity is stronger than the environment. However, giving preference to genotypic characteristics, he does not deny the influence of exogenous factors on resistance against diseases. In this regard, the author points to three categories of factors of immunity, or vice versa, susceptibility: 1) hereditary properties of the variety; 2) the selective ability of the pathogen; 3) environmental conditions. As an example, data are given on the negative effect of increased soil acidity on the resistance of plants against some fungal diseases.
Stronger infection of wheat with durum smut occurs at low temperatures (at 5 ° C, the infection was 70%, at 15 ° C - 54%, at 30 ° C - 1.7%). Humidity of soil and air is often a factor initiating the development of rust, powdery mildew and other diseases. For susceptibility to fungal infection light also affects. If oat plants are kept in the dark and thereby reduce the rate of photosynthesis and the formation of carbohydrates, then they become immune to rust infection. Fertilizers and other conditions affect plant resistance to disease..
The complexity of disease prevention and control is due to objective factors. It is very difficult to breed varieties that would retain resistance to the pathogen for a long time. Resistance is often lost as a result of the emergence of new races and biotypes of pathogens against which the variety is not protected.
Disease control is further complicated by the adaptation of pathogens to chemicals protection.
These factors are the main reason that the cost of plant protection in modern agriculture is growing, outstripping the growth rate of agricultural production by 4-5 times. In the main grain-growing areas, disease is often a limiting factor in obtaining high yields grains. In this regard, for the further intensification of agricultural production, new, perfect methods of plant protection are needed.
When developing new plant protection systems, it is necessary to focus on the regulation of the number of pests in the agroecosystem. In terms of methodology, it is necessary to determine the complexes of harmful organisms that infect plants at different stages of development. It is necessary to create models that reflect the influence of certain types of pathogens and their complexes on the formation of the crop and allow to optimize these processes through agrotechnological, organizational, economic and protective measures.
One of the most important prerequisites for obtaining seeds with high biological properties is the absence of pathogenic microflora. Diseases cause great harm to seeds at all stages of their life - during formation, storage and germination.
Through seeds, pathogens can be transmitted in three ways: 1) as mechanical impurities (sclerotia in rye seeds); 2) in the form of spores on the surface of seeds (hard smut of cereals); 3) in the form of mycelium in the middle of the seeds, for example, dust smut.
The microflora of seeds is divided into several groups. Epiphytic microflora are microorganisms that colonize the surface of seeds and feed on the waste products of plant cells. Under normal conditions, such pathogens do not invade the internal tissue and do not cause significant harm ( Alternaria, Mucor, Dematium, Cladosporium and etc.). Endophytic (phytopathogenic) microflora consists of microorganisms that can penetrate into the inner parts of plants, develop there, cause disease of seeds and plants growing from them ( Fusarium, Helmintosporium, Septoria and etc.). Microorganisms that get on seeds accidentally upon contact with contaminated surfaces of warehouse equipment, containers, soil particles, plant residues with dust and rain drops ( Рnісіllium, Aspergillus, Mucor and etc.). Storage mold that develops as a result of the vital activity of fungi ( Рnісіllium, Aspergillus, Mucor and etc.).
Distinguish embryonic infection when pathogens are found in any of component parts embryo and extraembryonic infection when pathogens are in the endosperm, membrane, pericarp and bracts. The placement of a pathogen in seeds depends on the anatomy of the seeds and the site of entry specific to each microorganism.
FUNDAMENTALS OF PLANT IMMUNITY TO DISEASE
With the most severe epiphytotics, plants are affected by the disease differently, which is associated with the resistance and immunity of plants. Immunity is understood as absolute immunity in the presence of infection in conditions favorable for infection of plants and the development of diseases. Resilience is the property of the body to withstand severe disease damage. These two properties are often equated, meaning that plants are weakly affected by diseases.
Resistance and immunity are complex dynamic conditions that depend on the characteristics of the plant, the causative agent of the disease and environmental conditions. The study of the causes and patterns of resistance is very important, since only in this case successful work on the breeding of resistant varieties is possible.
Immunity is congenital (hereditary) and acquired. Inborn immunity is passed from parents to offspring. It changes only with a change in the genotype of the plant.
Acquired immunity is formed during ontogenesis, which is quite common in medical practice. Plants do not have such a clearly expressed acquired property, but there are techniques that make it possible to increase the resistance of plants to diseases. 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. The factors of active immunity act only upon contact between the plant and the pathogen, i.e. arise (induced) during the pathological process.
The concept of specific and non-specific immunity is distinguished. Nonspecific is the inability of some pathogens to infect a particular plant species. For example, beets are not affected by pathogens of cereal smut diseases, potato late blight, potatoes are not affected by cercosporosis of beets, cereals are not affected by macrosporosis of potatoes, etc. Immunity manifested at the level of a variety in relation to specialized pathogens is called specific.
It was found that resistance is determined by the total action of protective factors at all stages of the pathological process. All the variety of protective factors is subdivided into 2 groups: preventing the introduction of a pathogen into a 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. An obstacle to the introduction of pathogens can be the thickness of the integumentary tissues, the structure of the stomata, the pubescence of the leaves, a waxy coating, and the structural features of plant organs. The thickness of the integumentary tissues is a protective factor against those pathogens that penetrate into plants directly through these tissues. These are primarily powdery mildew fungi and some representatives of the class Oomycetes. The structure of the stomata is important for the penetration of bacteria, pathogens of downy mildew, rust, etc. into the tissue. Usually, it is more difficult for the pathogen to penetrate through tightly covering stomata. Leaf pubescence protects plants from viral diseases, insects that transmit a viral infection. Due to the wax coating on the 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 prevent initial stages infection. So, potato varieties with a loose structure of the bush are less affected by late blight, since they are better ventilated and infectious drops on the leaves dry out faster. Less spores settle on narrow leaf blades.
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 rare. This is due to the fact that the flower 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. The rapid introduction of pathogens can be hindered by the high osmotic pressure in plant cells, the speed of physiological processes leading to the tightening of wounds (the formation of wound peridermis), through which many pathogens penetrate. The speed of passage of individual phases of ontogenesis is also important. So, the causative agent of durum smut of wheat is introduced only into young seedlings, therefore, varieties that sprout amicably and quickly are less affected.
Inhibitors. These are compounds found in plant tissues or synthesized in response to infection, which inhibit the development of pathogens. These include phytoncides - substances of various chemical nature, which are factors of innate passive immunity. In large quantities, phytoncides are produced by tissues of onions, garlic, bird cherry, eucalyptus, lemon, etc.
Alkaloids are nitrogen-containing organic bases formed in plants. Plants of the legume, poppy, nightshade, aster family, and others are especially rich in them. For example, potato solanine and tomato tomato 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 (intact tissues).
Induced substances that are synthesized by a plant during the development of a pathogen are called phytoalexins. By chemical composition they are all low molecular weight substances, many of them
are phenolic in nature. It was found that the plant's hypersensitive response to infection depends on the rate of phytoalexin induction. Many phytoalexins are known and identified. Thus, rishitin, lyubimin, phytuberin were isolated from potato plants infected with the causative agent of late blight, pisatin from peas, and isocoumarin from carrots. The formation of phytoalexins is a typical example of active immunity.
Active immunity also includes the activation of plant enzyme systems, in particular oxidative ones (peroxidase, polyphenol oxidase). 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 plant immunization 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. Examples of such chemical immunizers are phenolic compounds: hydroquinone, pyrogallol, ortonitrophenol, paranitrophenol, which are used to treat seeds or young plants. A number of systemic fungicides have immunizing properties. So, dichlorocyclopropane protects rice from blast by increasing the synthesis of phenols and the formation of lignin.
The immunizing role of some trace elements that make up plant enzymes is also known. In addition, trace elements improve the supply of basic nutrients, which has a beneficial effect on plant resistance to diseases.
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 one or several genes that are qualitatively different from each other. The presence of such genes provides absolute immunity to certain races of the pathogen. The causative agents of the disease, in turn, have a virulence gene (or genes) that allows it to overcome the protective effect of resistance genes. According to the theory of H. Flora, a corresponding virulence gene can be developed for each plant resistance gene. This phenomenon is called complementarity. When exposed to a pathogen with a complementary virulence gene, the plant becomes susceptible. If the genes for resistance and virulence are incomplete, plant cells localize the pathogen as a result of a hypersensitive reaction to it.
For example (Table 4), according to this theory, potato cultivars with the R resistance gene are affected only by race 1 of the P. infestans pathogen or more complex, but necessarily possessing the 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 a 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 immunity 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 in each plant to varying degrees. At its high level, the pathological process slows down, which makes it possible for the plant to grow and develop, despite being affected by the disease. Like any polygenic trait, this resistance can fluctuate under the influence of growing conditions (level and quality mineral nutrition, moisture availability, day length and a number of other factors).
The polygenic type of resistance is inherited transgressively; therefore, it is problematic to fix it by breeding varieties.
A combination of hypersensitive and polygenic resistance in one cultivar is widespread. In this case, the variety will be immune until the appearance of races capable of overcoming monogenic resistance, after which the protective functions are determined by 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. For this, plants with the desired economic and biological characteristics and plants with resistance are selected as parental forms. Wild species are more often donors of resistance, therefore, 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 cereals, legumes, potatoes, sunflowers, flax and other crops have been created that are resistant to the most harmful and dangerous diseases.
If some species do not cross with each other, they resort to the “intermediary” method, in which each type of parental forms or one of them is crossed first 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 resistance of the hybrids is checked against a severe infectious background (natural or artificial), that is, with a large amount of infection of the pathogen, under conditions favorable for the development of the disease. For further reproduction, plants are selected that combine high resistance and economically valuable traits.
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 necessary traits (including resistance), many varieties of agricultural plants have been obtained. It is especially effective for cross-pollinated 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.
Reasons for loss of stability
Over time, varieties, as a rule, lose their 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 by the gradual accumulation of new races of the pathogen. That is why the selection of varieties with only the hypersensitive type of resistance is futile.
There are several reasons for 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 mode of variation. The second reason is the hybridization of genetically different microorganisms during the sexual process. This path is typical mainly for mushrooms. The third way is heterokaryosis, or heteronucleus, of haploid cells. In fungi, heteronucleus can occur due to mutations of individual nuclei, the transition of nuclei from different-quality hyphae along anastomoses (fused sections of hyphae) and recombination of genes during the fusion of nuclei and their subsequent division (parasexual process). Multinucleation and asexual process steam are of particular importance for representatives of the class of imperfect fungi, which do not have a 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 included in their genome. During transduction, individual segments of the chromosome from one bacterium are transferred to another using a bacteriophage (bacterial virus).
In microorganisms, the formation of races is ongoing. Many of them die immediately, being uncompetitive due to a lower level of aggressiveness or the absence of other important signs. As a rule, more virulent races are fixed in the population in the presence of varieties and species of plants with genes of resistance to existing races. In such cases, a new race, even with a weak aggressiveness, without encountering competition, gradually accumulates and spreads.
For example, when cultivating potatoes with resistance genotypes R, R4 and R1R4, races 1 will prevail in the population of the late blight pathogen; 4 and 1.4. With the introduction into production of varieties with the R2 genotype instead of R4, race 4 will gradually disappear from the pathogen population, and races 2 will spread; 1.2; 1,2,4.
Immunological changes in varieties can also occur in connection with changes in the conditions of their growth. Therefore, before zoning varieties with polygenic resistance in other ecological-geographical zones, they must be immunologically tested in the zone of future zoning.
The word immunity comes from the Latin immunitas, which means "liberation from something."
Immunity is understood as the body's immunity to the action of pathogens and their metabolic products. For example, conifers are immune to powdery mildew, and deciduous ones are immune to shute. Spruce is absolutely immune to shoot rust, while pine is completely immune to cone rust. Spruce and pine are immune to false tinder fungus, etc.
II Mechnikov understood immunity to infectious diseases as a general system of 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.
Various resistance to diseases of a number of plants, especially agricultural ones, has been known for a long time. The selection of crops for disease resistance, along with selection for quality and productivity, has been carried out since ancient times. But it was only at the end of the 19th century that the first works on immunity appeared, as the doctrine of plant resistance to diseases. Among the many theories and hypotheses of that time, one should name 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 to plants.
Received great fame mechanical theory by Australian scientist Cobb(1880-1890), who believed that the reason for the resistance of plants to diseases is reduced 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, recognize this theory as universal. This theory has met criticism from Erickson and Ward.
Later (1905) the Englishman Massy put forward chemotropic theory, by which the disease does not affect those plants in which there are no chemicals that have an attractive effect on the infectious agent (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, M. Ward's hypothesis (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 due to the ability of plants to destroy these enzymes and toxins.
Of the other theoretical concepts, the most noteworthy is phytoncidal theory of immunity nominated by B.P. Tokin in 1928 this position was developed for a long time by D. D. Verderevsky, who established that in the cell sap of resistant plants, regardless of the attack of pathogens, there are substances - phytoncides, which suppress the growth of pathogens.
Finally, of some interest the 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 does not have a clear confinement to the phases of development of the host plant.
NI Vavilov paid much attention to immunity, mainly of agricultural plants. The works of foreign scientists I. Erickson (Sweden), E. Stackman (USA) also belong to this period.
The founder of the doctrine of plant immunity, N.I. Vavilov, who laid the foundation for the study of its genetic nature, believed that plant resistance to pathogens developed in the process of millennial evolution in the centers of origin. In the case of plants acquiring resistance genes, pathogens could infect plants due to the emergence of new physiological races arising as a result of hybridization, mutation, heterokaryosis, and other processes. Within the population of a microorganism, shifts in the number of races are possible due to a change 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 DT Strakhov, in tissues resistant to plant diseases, there are regressive changes in pathogenic microorganisms associated with the action of plant enzymes, its metabolic reactions.
BA Rubin and his co-workers linked the reaction of plants aimed at inactivating the causative agent of the disease and its toxins with the activity of oxidative systems and energy metabolism of the cell. Various plant enzymes are characterized by different resistance to waste products of pathogenic microorganisms. In immune forms of plants, the proportion of participation of enzymes resistant to pathogen metabolites is higher than in non-immune ones. The most resistant to the effects of metabolites are oxidative systems (ceroxidase and polyphenol oxidase), as well as a number of flavone enzymes.
In plants, as in invertebrates, 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 infected tissues of immune plants, functionally complete organelles are formed, which determine the inherent ability of immune forms of plants to increase the energy efficiency of respiration during infection. Respiratory distress caused by pathogenic agents is accompanied by the formation of various compounds that act as a kind of chemical barriers that prevent the spread of infection.
The nature of plant responses to pest damage (formation of chemical, mechanical and growth barriers, the ability to regenerate damaged tissues, 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. On the basis of autopolyploids, hybrids between different 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. So L. A. Zhdanov and V. S. Pustovoit obtained sunflower varieties resistant to broomrape.
For long-term preservation of the resistance of varieties, the following methods are proposed:
Creation of multilinear 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 numbers;
Combination of R-genes with field resistance genes in one variety;
Periodic change of the varietal composition on the farm, which leads to an increase in resistance.
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 production by xenobiotics, high economic and energy costs. The maximum use of the biological potential of agricultural crops can become one of the alternative ways of developing 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 structure of the 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 cultures. Recently, methods for creating transgenic plants have begun to be used to obtain plants that are resistant to viral, fungal and bacterial diseases, as well as to some pests (Colorado beetle, corn stalk moth, cotton moth and scoop, tobacco leafworm, etc.). In terms of its methods and objects, this direction sharply differs from traditional breeding for plant immunity, but it pursues the same goal - the creation of forms that are highly resistant to harmful organisms.
A brilliant substantiation of the role of resistant varieties in plant protection was given by N.I. Vavilov, who wrote that among the measures for protecting 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. In relation to cereals, which occupy three quarters of the entire sown area, replacing susceptible varieties with resistant forms is, in fact, the most affordable way to fight infections such as rust, powdery mildew, wheat dust smut, various fusariums, and spotting.
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 varieties of agricultural crops resistant to diseases and pests.
In the following chapters, we will consider the main regularities that determine the presence of resistance traits in plants, the ways of their effective use in the breeding process, and ways of imparting induced immunity to plants.
1. THE HISTORY OF THE ORIGIN AND DEVELOPMENT OF THE TEACHING ABOUT PLANT IMMUNITY.
The idea of 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 one who has once had one of these terrible diseases does not get sick with it again.
By the middle of the II century. BC e. the idea of the non-recurrence of human diseases such as plague and others is becoming generally accepted. At the same time, people who had recovered from it began to be widely used to care for patients with plague. It is logical to assume that it was at this stage in 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 strove to use the collected observations for the practical protection of the population from infectious diseases. For many centuries, to protect people from smallpox, in one way or another, deliberate infection with this disease was carried out, 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, which is that many of the vaccinated had smallpox in a severe form, often with a fatal outcome. In addition, the vaccinated 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 by transferring the disease in a mild form.
The work of the English physician Edward Jenner (1798), in which he summed up the results of 25 years of observations and showed the possibility of vaccinating cowpox in humans and acquiring immunity to a similar human disease, was of epochal importance in the development of immunity. These vaccinations are called vaccinations (from 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, allowed a new look at Jenner's results and appreciated them, which influenced both the subsequent development of immunology and the work of Pasteur himself, who proposed using 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 organism of animals and humans from pathogens, II 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 inside living tissues and concentrate in places where microbes enter. 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 theory of animal immunity, the concept of 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 attributed such features of a plant as a thickened cuticle, a peculiar structure of flowers, the ability to quickly form at the site of damage to the outer tissues of the wound peridermis, and others as mechanical protective devices. Subsequently, this method of protection was called passive immunity. However, the mechanical theory could not exhaustively explain such a complex and 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 content of sugars 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 diseases that affect it. Varieties with a high sugar content and relatively low acids and tannins are more susceptible to disease. So, 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, the theory of Comes is not universal and cannot explain all cases of manifestation of immunity. Thus, the study of many varieties of wheat and rye, which have unequal susceptibility to rust and smut, did not reveal a clear correlation between immunity and the content of acids in leaf tissues. Similar results were obtained for many other cultivated plants and their pathogens.
At the beginning of the XX century. new hypotheses appeared, the authors of which tried to explain the causes of plant immunity. Thus, the English researcher Massy proposed a chemotropic theory, according to which plants that lack the substances necessary to attract parasites are immune. Investigating the 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 come under serious criticism from a number of researchers. The most detailed criticism of this theory was given by N.I. Vavilov, who considered it unlikely that the cell sap contained in the vacuoles could remotely act on the fungal hyphae and that some substances secreted from the tissues to the outside cannot be identified with the cell sap obtained by pressing the substrates where the mushroom was grown.
The protection of 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 especially dangerous diseases (rust of cereals, potato late blight, oidium and mildew of grapes) stimulated the emergence of scientifically based plant breeding for immunity to diseases. In 1911, the 1st breeding congress was held, where A. A. Yachevsky (1863-1932) presented a summary report “On the importance of breeding in the fight against fungal diseases of cultivated plants”. The data presented in the report indicated that successful work on the development of disease-resistant varieties is impossible without the development of 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 substantiation of all the material accumulated by that time in the field of immunity studies. ... In the same years, the works of N.I. Litvinov (1912) on the assessment of the resistance of cereals to rust and E.N. Iretskaya (1912) on the methods of breeding 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 fight against rust of cereals in 1937 and at the Biological Department of the Academy of Sciences of the USSR 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 that determine varietal and species resistance. NI Vavilov substantiated the proposition that plant immunity is inextricably linked with their genetic characteristics. Therefore, the main task of breeding for resistance NI Vavilov considered the search for species differences in plants on the basis of immunity. The world collection of cultivated plant varieties collected by him and by the VIR staff still serves as a source for obtaining immune forms. Of great importance in the search for immune forms of plants is his concept of 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 of the plant and the pathogen, NI Vavilov attributed to the field of physiological immunity.
The development of theoretical questions of the doctrine of plant immunity, begun by NI Vavilov, was continued in our country in subsequent years. Research was carried out in different directions, which was reflected in different explanations of the nature of plant immunity. Thus, BA Rubin's hypothesis, based on the teachings of AN Bach, links plant resistance to infectious diseases with the activity of plant oxidative systems, mainly peroxidases, as well as a number of flavone enzymes. The activation of the oxidative systems of plants leads, on the one hand, to an increase in the energy efficiency of respiration, and on the other, to a 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.Aksyonova, and others also took part in the development of this hypothesis.
The phytoncidal theory, developed in 1928 by B.P. Tokin on the basis of the discovery of bactericidal substances in plants - phytoncides, was developed by D.D.Verderevsky (1904-1974), as well as 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, arising in response to their infection with incompatible species or races of pathogens. They discovered a new phytoalexin of potatoes - Lyubimin.
A number of interesting provisions of the theory of immunity were developed by K. T. Sukhorukoye, who worked in the Main Botanical Garden of the USSR Academy of Sciences, and also by a group of collaborators led by L. N. Andreev, engaged in the development of various aspects of the theory of plant immunity to rust diseases, peronosporosis and verticillary wilt.
In 1935. TI Fedotova (VIZR) was the first to discover the affinity of host and pathogen proteins. All of 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, even NI Vavilov emphasized that the nature of immunity is complex and cannot be associated with any one group of factors, because the nature of the relationship of plants with different categories of pathogens is too diverse.
In the first half of the XX century. in our country, only an assessment was made of the resistance of varieties and species of plants to diseases and parasites (grain crops to rust and smut, sunflower to broomrape, etc.). Later, breeding for immunity began. This is how the sunflower varieties bred by E.M. Pluchek (Saratovsky 169 and others), resistant to broomrape (Orobanche sitapa) of race A and sunflower moth, appeared. The problem of combating the broomrape of the race B "Zloy" was removed for many years thanks to the works of V. S. Pustovoit, who created a series of varieties resistant to broomrape and moths. V.S. Pustovoit has developed a seed production system that allows for a long time to maintain the stability of sunflower at the proper level. In the same period, oat varieties were created that are resistant to crown rust (Verkhnyachsky 339, Lgovsky, etc.), which have retained resistance to this disease to the present day. Since the mid-1930s, P.P. Lukyanenko and others began breeding for wheat resistance to brown rust, 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 peronosporosis. Breeding for the immunity of sugar beets to a number of diseases was successfully carried out.
The cultivars obtained are resistant to powdery mildew (Hybrid 18, Kyrgyz odnosemyanka, etc.), cercosporosis (Pervomaisky polyhybrid, Kuban polyhybrid 9), peronospora (MO 80, MO 70), root rot and kagatnaya rot (Verkhneyachskaya 031, Belotserkovskaya TsG 19).
AR 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-1930s, KN Yatsynina obtained tomato varieties resistant to bacterial cancer.
A number of interesting and important works on the creation of varieties of vegetable crops resistant to keel and vascular bacteriosis were carried out under the leadership of B.V. Kvasnikov and N.I. Karganova.
The selection of cotton for immunity to verticillary wilt was carried out with varying success. Bred in the mid-30s of the last century, the 108 f variety remained stable for about 30 years, but then lost it. The varieties of the Tashkent series that replaced it also began to lose their resistance to wilt due to the emergence of new races 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 Industry named after N.I. Vavilov. The world collections of samples of cultivated plants, collected at this institute, still serve as a fund for donors of resistance of various crops necessary for breeding for immunity.
After E. Stekman discovered physiological races in the causative agent of stem rust of cereals, similar work was launched in our country. Since 1930, the VIZR (V.F.Rashevskaya and others), the Moscow Agricultural Experimental Station (A.N.Bukhgeim and others), the All-Union Institute of Breeding and Genetics (E.E. Geshele) began to study physiological races brown and stem rust, smut. In the postwar 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 the races of rust fungi. In subsequent, 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. So, it was revealed that race 77 of the causative agent of wheat leaf rust, prevailing in the 70s of the XX century. in the North Caucasus and southern Ukraine, consists of a series of biotypes differing in virulence, which are formed not on wheat, but on susceptible cereals. Studies of the races of smut fungi, begun at the VIZR by S.P. Zybina and L.S.Gutner, as well as by K.E. Murashkinsky in Omsk, were continued at the VIR by V.I. Tymchenko at the Institute of Agriculture of the Non-Black Earth Zone.
N. A. Dorozhkin, Z. I. Remneva, Yu. V. Vorob'eva, K. V. Popkova were very productive in the study of the races 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, allowing to some extent explain the mechanism of variability of this fungus.
In 1962 P. Akhizhnyak and V.I. Yakovlev discovered an aggressive race of the causative agent of potato cancer Synchythrium endobioticum. It was found that at least three races of S. endobioticum are widespread on the territory of our country, affecting potato varieties resistant to the common race.
In the late 70s - early 80s of the last century, the study of the physiological races of the fungus Verticillium dahliae was studied by A.G. Kasyanenko, Cladosporium fulvum - L.M. Levkina, the causative agent of wheat powdery mildew - M.N. Rodigin et al., Peronosporosis tobacco - A. A. Babayan.
Thus, the study of plant immunity to infectious diseases was carried out in our country in three main directions:
Study of race formation of pathogens and analysis of the structure of populations. 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: accounting for races, forecasting and regularities of the appearance of some races and (or) the disappearance of others;
assessment of disease resistance of available varieties, search for resistance donors and, finally, creation of resistant varieties.
Congenital, or natural, immunity is the property of plants not to be affected (not damaged) by one or another disease (pest). Congenital immunity is inherited from generation to generation.
Within the innate, 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, proceeding without changing the genome, is called acquired or induced resistance. The factors, the impact of which on seeds or plants leads to an increase in plant resistance, are called inducers.
Acquired immunity is the property of plants not to be affected by one or another pathogen of the disease that arose in plants after transferring a disease or under the influence of external influences, especially the conditions of plant cultivation.
The resistance of plants can be increased by various methods: the introduction of micronutrient fertilizers, changing the dates of planting (sowing), the depth of planting seeds, etc. Methods of gaining resistance depend on the type of inductors, which can be of 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 crops at optimum early times, and winter crops at optimum late periods; wheat resistance to hard smut, which affects 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 infect plants of a given species. Thus, grain crops are not affected by late blight and scab of potatoes, cabbage - by smut diseases, potatoes - by rust diseases of grain crops, etc. In this case, immunity is manifested by the species of plants as a whole. Immunity based on the inability of pathogens to infect plants of a particular species is called non-specific.
In some cases, immunity may not be manifested by the plant species as a whole, but only by a separate variety within this species. In this case, some varieties are immune and are not affected by the disease - ^ / new, others are susceptible and are strongly affected by it. So, the causative agent of potato cancer Synchytrium endobioticum affects the Solanum species, but inside it there are varieties (Kameraz, Stoilovy 19, etc.) that are not affected by this disease. This type of immunity is called varietal specific. It is of great importance for the breeding of 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 variety or type of plant 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, powdery mildew. There are known varieties of tobacco that are resistant to the tobacco mosaic virus and the causative agent of downy mildew. By zoning such varieties in production, it is possible to solve the problem of protecting a particular crop from major diseases.
Unlike medicine and veterinary medicine, where acquired immunity is of decisive importance in protecting humans and animals, acquired immunity has been used very little in practical phytopathology until recently.
There is a significant circulation of sap in plants, although not in closed vessels. When solutions of mineral salts or other substances are applied to parts of a plant, after a while, these substances can be found in other places of the same plant. On this principle, Russian scientists I. Ya. Shevyrev and S. A. Mokrzhetsky developed a method of foliar nutrition of a plant (1903), which is widely used in agricultural production. The presence of sap circulation in plants can explain the appearance of tumors of root cancer far from the site of introduction of the causative agent of this disease, Pseudomonas tumefaciens Stevens. This fact also indicates that the formation of tumors is not only a local disease, but the whole plant as a whole reacts to the disease.
Acquired immunity can be created in a variety of ways. In particular, it can be created by vaccination and chemical immunization of plants, their treatment with antibiotics, as well as some agricultural techniques.
In animals and humans, the phenomena of acquired immunity arising from the transferred disease and vaccination with weakened cultures of the causative agent of the disease are well known and studied in detail.
The great successes achieved in this area have served as a stimulus for the search for similar phenomena in the field of phytoimmunology. However, the very possibility of the existence of acquired immunity in plants was at one time questioned on the grounds that plants do not have a circulatory system, and this excludes the possibility of immunization of the whole organism. The acquired immunity of plants was considered as an intracellular phenomenon, which excluded the possibility of diffusion of substances formed in the affected cells into neighboring tissues.
It can be considered established that in some cases the resistance of plants to infection increases both after the disease, and as a result of vaccination. The waste products of pathogens (culture medium), weakened cultures and preparations from microorganisms killed by anesthesia or heating can be used as a vaccine. In addition, bacteriophage prepared in the usual way, as well as serum from animals immunized with a microorganism pathogenic for a plant, can be used for immunization. Immunizing substances are administered primarily through the root system. It is also possible to spray into the stems, apply in the form of lotions, spray on leaves, etc.
Methods of artificial immunization, widely used in medicine and veterinary medicine, are not very promising for the practice of plant growing, since both the preparation of immunizing agents and their use are very laborious and expensive. If we take into account that immunization is not always effective enough and its action is very short-lived, as well as the fact that the process of immunization, as a rule, inhibits the plant, it becomes clear why the results of work in the field of acquired immunity are not yet used in agricultural practice.
There are isolated cases of plant immunization as a result of a viral infection. Trust in 1952 the Canadian scientists Gilpatrick and Weintraub showed that if the leaves of Dianthus borbatus are infected with the necrosis virus, the uninfected leaves become resistant. Subsequently, similar observations were carried out by other researchers on many plants infected with various viruses. At present, facts of this kind are considered as phenomena of immunity acquired as a result of the transferred disease.
In search of a protective factor arising in the tissues of plant forms resistant to the virus, the researchers first of all turned to the hypersensitivity reaction, attributing a protective role to the polyphenols-polyphenol oxidase system. However, experimental data on this issue have not yielded definite results.
In some works, it is noted that the juice from the cells of the immune zone formed around the necrosis, as well as from tissues that have acquired immunity, has the ability to inactivate the virus. Isolation and study of this antiviral factor has shown that it has a number of properties similar to animal interferon. Interferon-like protein, like animal interferon, is found only in virus-infected resistant tissues, easily diffuses from infected cells into uninfected ones, and does not have antiviral specificity. It suppresses the infectivity of various viruses specific to plants from different families. The antiviral factor is active against viruses both in vitro, that is, when mixed with an extract from virus-infected leaves, and in vivo, that is, when it is introduced into plant leaves. The opinion is expressed that it can act either directly on the particles of the virus, or on the process of its reproduction, suppressing metabolic processes, as a result of which new viral particles are synthesized.
The phenomena of acquired immunity can be attributed to an increase in resistance to diseases caused by chemicals. Soaking seeds in solutions of various chemical compounds increases plant resistance to diseases. The properties of immunizers are possessed by macro- and microelements, insecticides and fungicides, growth substances and antibiotics. Pre-sowing soaking of seeds in solutions of microelements also increases plant resistance to diseases. The healing effect on the plant of trace elements persisted in some cases for the next year.
Phenolic compounds are effective as chemical plant immunizers. Soaking seeds in solutions of hydroquinone, paranitrophenol, ortonitrophenol, etc. can significantly reduce the susceptibility of millet to smut, watermelon, eggplant and pepper - by wilting, oats - by crown rust, etc.
Resistance caused by various chemical compounds, as well as natural, genetically determined, can be active and passive. For example, chemical treatment of seeds and plants can increase their mechanical resistance (increase the thickness of the cuticle or epidermis, affect the number of stomata, lead to the formation of internal mechanical barriers to the pathogen, etc.). In addition, most of the chemical immunizers of plants are substances of intra-plant action, i.e., penetrating into the plant, they affect its metabolism, thereby creating unfavorable conditions for the nutrition of the parasite. Finally, some chemical immunizers can act as neutralizing agents for the toxins of pathogens. In particular, ferulic acid, being an antimetabolite of piricularin, the toxin of Piricularia oryzae, increases the resistance of rice to this pathogen.
