Genetics of resistance. Theory gene versus gene
The study of the genes that control this or that property of the organism was carried out by crossing two individuals, differing in the expression of this property (plants with red and white flowers, long and short shoots, short or long maturation of seeds, etc., until the recent epoch of molecular genetics) P.). This study of genes that control disease resistance is extremely difficult, because it (stability), as seen from the previous paragraph, is governed by a great variety of genetic systems. Indeed, the resistance of wheat to rust may be due to the downward folded leaves or the presence of a R-protein recognizing the structure of the parasite effector. The genes that control these properties have nothing in common. There was a need for the existence of a system that would classify the genetic nature of different types of resistance. Such a system was proposed by the South African phytopathologist J. van der Planck, who combined the different types of plant resistance into two groups, named him vertical and horizontal stability. To understand the course of Van der Planck's reasoning consider an example of the stability analysis of potato to late blight.
The degree of affection of potato bushes with late blight can be taken into account with the help of a five-point scale, in which 0 means plants without phytofluorous spots, point 1 - plants that have up to 1/4 of all leaves, with a score of 2 - up to 1/2 leaves, with a score of 3 - from 1/2 to 3/4, with a score of 4 - all leaves of the bush are affected. Assume that the potato grade A is hit by 0 points, and the grade B under the same conditions - by 3-4 points. Hybrids of the first generation (F1) between these varieties will be struck, as well as grade A, by 0 points. Consequently, the resistance of the variety A is dominant, and the susceptibility of the B type is recessive. In the second generation, instead of the monotony of the first generation, splitting into two classes will be observed: n% the plants will be stable as a parent A (0 points) and (100 - n)% will be susceptible as a variety B (3-4 points). But the ratio of these classes can be determined by how many genes the resistance of the variety A is controlled (in the presence of one gene the splitting will be close to 3: 1, two genes - 15: 1, etc.). Hence the first conclusion follows: the stability of the variety A is controlled by oligogen or by large genes (highly expressive). Each of these genes causes significant changes in the phenotype of the plant, which makes it possible to establish its presence in the genome, therefore, the number of genes can be determined from the second generation (F2)
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Now we collect the controversies of the causative agent from different fields and regions and infect the leaves of the variety A with samples from different populations. We can find strains of the parasite that will infect grade A as strongly as grade B. Therefore, the second conclusion is that the effect of the resistance gene present in strain A , manifests itself not to the whole species Phytophihara infestans, but only to certain strains (they are called physiological races or races but to the host), while other races can hit this variety. Finally, we will tear off the leaves A from the plant, place them in a moist chamber (a Petri dish with moistened filter paper) and infect millions of spores, i. E. create optimal conditions for the parasite. Anyway, the leaves will remain stable. Hence the third conclusion: the stability of the variety A is not modified by external conditions, is preserved in a wide range.
The stability inherent in the A, variety Van der Planck called the vertical (VU).
Now with the grade B we will compare another stable variety, call it C, which is hit by 1 point. In the first hybrid generation, unlike the previously described situation, there will be no monotony. There are plants that are both highly stable and highly susceptible. Even greater dispersion will be observed in the second generation. C) the number of genes judged by such splitting of offspring is extremely difficult, therefore statistical methods are used for the analysis. We plot the graph by plotting the damage points on the abscissa axis, and on the ordinate axis - the number of plants in the fissionable population affected by this score. The result is a bell-shaped curve that is close to the normal distribution curve, according to which the largest number of plants will have an average lesion score between parents (Figure 2.6).
Fig. 2.6. Splitting by the degree of affection by late blight of hybrids of the first generation F1 from crossing varieties of potatoes "Voltman" and Sickingen & quot ;, differing in the degree of horizontal stability
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When analyzing a sufficiently large population, one can find plants more stable than a stable parent, and more susceptible than a receptive parent. This phenomenon was called transgression.
Such a curve indicates that, firstly, one can not judge the dominance or recessivity of the genes of resistance of grade C, and secondly, one can not judge the number of genes that control the resistance of this variety. Such genes are called noninteracting (there is no interaction between the dominant and recessive alleles of one gene) or additive. Each of these genes has a weak phenotypic effect compared to its allele, controlling the stability, the distribution of the trait in the offspring from crosses is not as easy to determine as in the first example. This will require the implementation of specially planned experiments, during which it is possible to obtain only a statistically probabilistic answer. The overall degree of resistance will be determined by the number of genes in a given variety. For example, if each gene increases resistance by 0.5 points, then in the absence of genes, the infectivity will be 4 points, with one gene - 3.5 points, two genes - 3 points, three genes - 2.5 points, four genes - 2 a score, five genes - 1.5 points, six genes - 1 point, seven genes - 0.5 points and eight genes - 0 points.
In this example, the variety C has six resistance genes, and grade B two. The greatest number of hybrids will have an average number of genes between the parents (6 + 2): 2, i.e. four, and score at 2 points, although rare combinations can have all eight genes (resistance transgression) and 0 genes (transgression by susceptibility). Such genes are called polygens or small genes, and resistance to diseases controlled by them, van der Planck called horizontal (PG).
If, as in the first experiment, the infection of the variety C is carried out by different races, then, unlike the varieties from the VU, they will not have a significant difference in the damage, therefore, the GD is manifested in relation to all races of this parasite, and its destruction by individual races will not happen. On the other hand, in conditions that are very favorable for the development of the disease, varieties that have a high level of GU may be affected, although not as much as varieties that do not have it. Consequently, the stability of this type operates in a narrower range of external conditions than the vertical one.
So, the two types of stability, postulated by Van der Planck, have a number of features that distinguish them from each other. Among them:
- genetic control, which in the case of VU is carried out by oligogenes with a dominant effect, in the case of PG - polygens with an additive effect;
is a phenotypic expression: when BY is an alternative, while for PI, it is not an alternative. This means that with the VU of one dominant gene, it is necessary and sufficient that the plant be completely resistant, and additional genes do not add anything to this, i.e. only two alternatives are possible: in the presence of a dominant allele, the plant is completely stable, in the absence of it it is completely susceptible ("all or nothing").
With PG, the more genes are present in a variety, the higher is its resistance, i.e. relationship is not "all or nothing", and "more or less". The most frequent manifestation of DU is a qualitative (supersensitivity reaction), and PI is quantitative (the number of spots on the sheet, the number of spores in one spot, etc.);
- relationship with the parasite. In the case of VU, they are rasospecific: in populations of parasites, races capable of eroding erosion and infecting a previously resistant variety can arise and accumulate. In the presence of GU, the situation of loss of the variety of its resistance is unlikely (Figure 2.7):
Fig. 2.7. Schematic representation of the action of vertical and horizontal stability:
1, 2 - the varieties possessing different genes of VU; 3 - a variety with a high degree of PG; 4 - a variety that does not have any ВУ, ГУ
- modifications by external conditions: the VU protects the variety from damage in almost all weather conditions, while PG can be significantly weakened in weather conditions favorable for the development of the disease;
The stability mechanism. Comparing the given data on the properties of the VU and PG with the stability mechanisms described in paragraph 2.1, we can make an unambiguous conclusion that the mechanism of the DU is the synthesis of the R proteins, and the mechanism of its destruction by the parasite races is the synthesis of the effectors, not recognizable R - proteins. All other mechanisms of immunity are within the purview of the PG.The arms race, caused by changes in the structure of parasite effectors and recognizing their plant I-proteins, proceeds according to the concept of the gene against the gene. This concept was proposed in the middle of the last century by the American phytopathologist H. G. Flor as a result of experiments on the inheritance of flax resistance to rust rivals and the virulence of the rust agent to flax varieties. According to the theory gene versus gene a parasite can infect a plant variety that has a dominant resistance gene, only if there is an appropriate recessive virulence gene (more precisely, a recessive allele of the dominant avirulence gene). Similar diallelic relationships are presented in Table. 2.3.
Diallel relationships of the host and parasite in the gene-versus-gene system
The gene of the host plant
RR (or Rr)
A A (or Aa)
From Table. 2.3 it follows that the plant can have vertical resistance to the parasite only if its dominant resistance gene is countered by the dominant parasite avirulence gene. Since most of the plant species studied, like most of their parasites, have many resistance and avirulence genes, a stable state occurs only when at least one pair of interacting host and parasite genes is dominant (if the parasite has a gene A1 < sub>, then a stable plant must have a gene R1 , a gene R2 must be present against the gene A2 , etc., etc.).
The concept of the "gene against the gene" played an important role both in theory (in studies of the molecular mechanisms of host-parasite interactions) and in practice - for the differentiation of parasitic races (see Chapter 3) and selection of resistant varieties (see Chapter 4).> The above-described relationships of potato varieties with the races of the causative agent of the late blight most clearly express the differences between the GU and the VU, therefore Van der Planck used the analysis of these differences as an analysis of these differences (however, in many plant diseases deviations occur from a similar picture).
Symptoms of the disease. With potato late blight , regardless of the potato genes and parasitic races, only two alternatives are always observed: point necrosis (hypersensitivity reaction) or spot with sporulation. With rust diseases of cereals, different VU genes have different expressiveness and include protective responses of cells at different rates. Therefore, along with point necrosis or chlorosis, a small pustule may be formed in the VU, containing spores of the parasite and surrounded by a chlorotic or necrotic halo. The American phytopathologist E. Stekman has created a scale of reaction types wheat , having different genes of VU, for infection with the causative agent of rust of wheat Puccinia graminis (Figure 2.8).
Modification by external conditions. Not all B V genes are phenotypically manifested in a wide range of external conditions. Some plants have temperature-sensitive resistance genes, the protective effect of which is lost in conditions of elevated temperatures. Some of them are of great practical importance (they are used in breeding for immunity). Thus, the wild American glue Nicotiana glutinosa has a resistance gene to TMV (L-gene), which is transferred to many varieties of cultured tobacco. If the leaves of adhesive tobacco growing at 24-25 ° C, grate tobacco juice containing viral
Fig. 2.8. Types of wheat resistance, controlled by different ВУ genes :
X - the designation of a heterogeneous reaction (the upper part of the leaf is susceptible, the lower part is stable)
particles, instead of spreading throughout the plant and the formation of mosaic symptoms in places where the virus penetrates, point necroses (a hypersensitivity reaction) are formed, beyond which the virus will not spread. But if the infected plants are transferred to high temperature conditions (35-37 ° C), then the virus will be blocked in the necrosis zone, and virus particles will spread throughout the plant. If we return the pots with sticky tobacco to a hothouse box with a lowered temperature, all areas of the leaves into which the virus particles have spread will be necrotic, but the virus will not fall outside it. Therefore, the gene N - is temperature sensitive: it protects plants against virus at optimal temperature for growth, but does not protect at a growth temperature close to maximum.
Some genes of VU cereals behave in a similar way to rust fungi. This was found in experiments on infection of wheat varieties with different rust rust in greenhouses. It turned out that some varieties show themselves in winter as stable, and in summer, when the temperature in the greenhouse becomes high as a result of the greenhouse effect, they are susceptible. This is the gene of resistance wheat to stem rust Sr6, which is contained in the genome of the popular Canadian variety "Selkirk". Other factors can influence the expression of the VU genes. For example, the reaction of the flax gene N2 to rust races varies depending on the illumination.
Inheritance of VU and PG. There are cases when polygenic resistance is rasospecific, and oligogenic - non-specific. For example, the polygenic, quantitative, but rasospecific resistance of wheat to brown rust is described, and the monogenic, qualitative, but non-specific resistance of corn to helminthosporium (causative agent of the fungus Cochliobolus carbonum).
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