& quot; Care & quot; from the parasite - the third line of defense, Vegetative integuments - the fourth line of defense - General phytopathology

& quot; Care & quot; from the parasite - the third line of defense

Unlike animals, plants can not walk, so "care" It should not be understood literally, but as a change in the rate of growth and development in a direction that reduces the likelihood of infection. As we know, human diseases can be divided into two groups: diseases that affect mainly people at a young age (measles, chickenpox, whooping cough, finally, tuberculosis), and diseases that are more likely to affect older people (cancer, heart attacks, stroke, Parkinson's disease and Alzheimer's). The same division exists in relation to certain plant diseases. Sprouts of the causative agent of the hard wheat mallet niletia tritici infect shoots at the earliest age, until the cell walls of the covering tissues have hardened (see paragraph 2.1.4). Also, the shoots of various plants suffer from numerous necrotrophic fungi affecting the young rootlets and the underground part of the stem and causing root rot. Such diseases as potato blight and tomato, cotton verticillosis, numerous leafy spotting, affect aging plants and appear in mass after flowering, when the processes of outflow of photosynthetic products from the leaves begin to predominate over the accumulation processes. Consequently, species and varieties of plants that quickly develop the first stages of ontogeny will show resistance to diseases of the first group, and species and varieties that delay the onset of aging stages or, conversely, these stages quickly end, will be weakerly affected by diseases of the second group. Russian phytopathologist MS Dunin called changes in plant resistance in the process of ontogeny by immunogenesis and proposed agrotechnical methods for regulating the speed of the course of sensitive stages of ontogenesis (by choosing optimal seeds for sowing seeds, applying fertilizers, etc.). For example, when growing cotton , the seeds are treated with a mixture of fungicides for disinfection from parasites on their surface before seeding and creating a sterile zone around the germinating seed and nitrogen fertilizers (ammonium nitrate) for the accelerated passage of the first stages of ontogeny.

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Vegetative covers - the fourth line of defense

1. Some organs of plants are covered with dead cells, in which toxic products for mushrooms and bacteria are deposited. For example, scales onions consists of dead cells, in which simple phenols are toxic for mushrooms - protocatechic acid and orthocatechol. They are soluble in water, diffuse into infectious drops and kill spores and sprouts of fungi. Also from the dead cells is a bark covering the trunks of trees. In the bark of conifers, toxic for fungi, diphenol pinesilvin accumulates.

2. The surface of the body of plants, like animals, has a cuticular cover. Vegetative cuticle is a hydrophobic (repelling water) layer - kutin, consisting of a mixture of fatty acids connected to each other by ether bonds, and immersed in wax. Due to the hydrophobic properties of the cuticle, water drops from it. The following observations indicate the protective role of the cuticle. In areas where sandstorms are frequent, plants are strongly affected by fungi that cause spotting. The reason for this is that small particles of sand locally break the cuticular layer and create holes through which fungi penetrate, unable to penetrate the cuticle themselves. Phyto-itogeous organisms overcome cuticular protection in three ways.

The first way. Some types of fungi - fusariums, piricularia and others - form an extension at the end of the germ tube - apressorium, which, with the help of hydrophobic hydrophobin proteins, adheres tightly to the surface of the cuticle and due to strong turgor pressure, the penetration of the cuticle by the growing infectious hyphae. The mechanical pressure on the cuticle is helped by chemical compounds: the fungi synthesize the enzyme kutinase, which ruptures the ethereal bonds and the loosening cuticle layer. This enzyme is formed very sparingly - only if there are fungi fragments in the growth medium of the fungus on.

By penetrating the cuticle, the leaves of the spring host barberry infect the germinal tubules of the mononuclear basidiospore of the causative agent of the stem rust of the cereals Risinia graminis. In this connection, barberry species having a thick cuticle and a thickened cuticle the top wall of the epidermis, are resistant to rust (Table 2.1).

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Table 2.1

The thickness of the cuticle and the outer wall of the epidermis in species of barberry different in rust resistance (according to E. Gojman)

Barberry type

Average cuticle thickness, μm

Highly responsive (three kinds)

0.93

Weak-responsive (four kinds)

1.28

Insensitive (two kinds)

1.66

This leads to a simple conclusion - by selecting the thickness of the cuticle and epidermal wall, you can get resistant rust varieties barberry.

The second way. Many species of fungi and bacteria are able to penetrate plants only through wounds on their surfaces. They are called wound parasites. Since the wounds are always present on the roots, rubbing during the growth process on the solid particles of the soil, and on the leaves - from insect damage and other causes, wound parasites such as pathogens of root rotten of the genus Pythium, leaf patches of the genus Septoria or the rotting of fruits from the genus Mycosphaerella , often cause massive diseases.

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In order to protect against these diseases, the speed of sealing the wound surfaces (suberinization) is of great importance. Suberinization - is an energy-consuming process that requires the passage of chemical reactions involving oxygen in cells. Therefore, in order to accelerate the suberinization of tissues in fruits and vegetables intended for long-term storage, engineering devices have been developed in potato and vegetable stores to ensure air purging with a certain temperature and humidity through the thickness of the product (active ventilation).

The third way. A large number of species of fungi and bacteria are introduced through natural openings on the surface of plants - stomata, pores, hydratodes, lenticles (see Figure 1.16, Figure 2.1).

Sprouts of rust fungi grow in the direction of the stomata, being guided by the structure of the wax crystals on the surface of the leaf. Therefore, the structure and location of stomata can play an important role in protecting against fungi and bacteria. For example, mandarin is resistant to bacterial citrus cancer due to the presence of protrusions on the outer walls of stomata that prevent ingestion of bacterial cells.

Fig. 2.1. Introduction of the infectious hyphae of the rust fungus in the stomata

3. Unlike animals, plants, in addition to the common cuticular cover, have a second type of protective surface, a polysaccharide cell wall. It consists of fibers of cellulose (long linear polymers of glucose of p-glucans bound to each other by hydrogen bonds) immersed in the matrix from other polysaccharides called and which are related to each other with a friend of the third group of polysaccharides - polymers of methylated galacturonic acid, which contain sugar molecules rhamnose (pectins). The destruction of cellulose requires a large expenditure of energy and is not available to all microorganisms. Cellulose is actively destroyed by wood-breaking basidiomycetes and some bacteria - cellulolytic. Most phytopathogenic fungi and bacteria have enzymes that destroy pectin-methyl pecti nesterase, polygalacturonase, and pectatlase. Like kutinases, these enzymes are formed only in the presence of pectin in a growth medium. However, in the course of evolution, plants created another protective barrier: a three-dimensional polymer of aromatic alcohols and acids, lignin, is deposited in the secondary cell wall (in ceasing growing cells) It makes the wall more rigid, shields polysaccharides from attack by enzymes of pathogens and has a direct toxic effect on them. Lignin is an extremely stable polymer, it can be destroyed only by certain fungi, mainly woody basidiomycetes.

The above-mentioned factors of stability Gojman called aksenie or passive stability. According to Gojman, aksenia, unlike true stability, is the inaccessibility of plant tissues or cells susceptible to disease, property inherent in the plant before infection.

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