Plant Immunity

the insusceptibility of plants to pests and causal agents of diseases and to their metabolic products. Manifestations of plant immunity are resistance and tolerance. Resistance is the ability of plants of a particular variety (sometimes species) to withstand injury by disease or pests, or it may denote a less severe injury than occurs in other varieties (or species). Tolerance is the ability of diseased or injured plants to maintain productivity (the size and quality of the yield). The use of resistant varieties is the most reliable method of combating many plant diseases, such as rust of grain, corn smut, and corn rust. The growing of sunflower varieties resistant to broomrape and moths has almost completely eliminated the harm done by these pests.

A theory of plant immunity was proposed by the Soviet biologist N.I. Vavilov, who was the first to study its genetic nature. He held that resistance to parasites developed, during plant evolution in the place of origin, after thousands of years of natural infection by the causal agents. If as a result of evolution plants acquired genes enabling them to resist pathogens, or causal agents of diseases, then the pathogens acquired the ability to injure resistant varieties through the appearance of new strains. Thus, more than 250 strains have evolved from the causal agent of stem rust, the fungus Puccinia graminis tritici. Every wheat variety is likely to be susceptible to some strains and immune to others. New strains of pathogenic microorganisms are the result of such processes as hybridization, mutation, and heterokaryo-sis. The number of strains in a population of microorganisms may also vary as a result of changes in the composition of plant varieties in a given region. The appearance of new strains of causal agents may be linked to the loss of resistance by a variety previously insusceptible to a particular causal agent.

Plant immunity to diseases is controlled by a comparatively small number of genes that can be estimated by hybrid analysis. For example, wheat species have about 20 genes of resistance to stem rust localized in nine chromosomes occurring in different chromosome sets (genomes). Resistance or susceptibility results from the interaction of two genomes (plant and parasite), which accounts for both the variety in the genes of plant resistance to the same species of causal agent and for the variety in the physiological strains of the parasite capable of overcoming the effect of these genes. This variety is the consequence of the parallel evolution of parasite and plant host (N. I. Vavilov, P.M. Zhukovskii). The American geneticist and plant pathologist H.H. Flor advanced the hypothesis of “gene for gene,” according to which all the genes of a resistant plant (R-genes) must sooner or later be overcome by the virulence genes of the parasite because its rate of reproduction is much higher than the plant’s. Nevertheless, in nature it is always possible to find plants that are resistant to all the known strains of parasites. One of the most important causes of such plant hardiness is “field resistance,” a type of resistance whereby the parasite may develop, but because of a food deficiency in the plant caused by such factors as the presence of a mechanical barrier or the unfavorable structure of the stomata, the parasite develops slowly and the loss of yield is therefore small. Field resistance is controlled by polymeric genes, which do not produce the visible effect of resistance, but different combinations of these genes determine the degree of resistance.

Because of the great variety in the types of causal agents and in the protective reactions of plants, there is no single theory of plant immunity. N.I. Vavilov subdivided plant immunity into structural (mechanical) and chemical immunity. Mechanical immunity is caused by the morphological characteristics of the host, particularly by the presence of certain protective adaptations, such as dense hairiness of shoots, preventing the pathogens from penetrating the plant’s body. Chemical immunity is caused by various chemical properties of plants. Sometimes a plant’s immunity depends on its deficiency in a substance required by a parasite; in other cases the plant secretes substances injurious to a parasite, for example, the phytoalexins isolated by the German biologist K. Müller or the phytoncides discovered by the Soviet biologist B.P. Tokin. The Soviet microbiologist T.D. Strakhov observed that regressive changes occur in pathogenic microorganisms found in the tissues of disease-resistant plants. These changes are induced by the action of the plant’s enzymes and its metabolic reactions. A number of Soviet biochemists, including B.A. Rubin, relate plant reactions aimed at neutralizing a pathogen and its toxins to the activity of the cell’s oxidative systems and energy metabolism. The various plant enzymes that regulate energy metabolism differ in the extent of their resistance to the metabolic products of pathogenic microorganisms. Enzymes resistant to the metabolites of pathogens play a more important role in immune forms of plants than they do in nonimmune forms. The oxidative systems (peroxidases and polyphenol oxidases) and some flavoproteins are highly resistant to the metabolites. In infected cells of immune plants, the activity of these enzymes, far from decreasing, may actually increase. This activation is caused by the biosynthesis of enzyme proteins, both those identical to the enzyme proteins present in noninfected tissues and those differing in several properties from such enzyme proteins (isozymes).

Plants, like invertebrate animals, have not been shown to possess the ability to form antibodies in response to antigens. Only vertebrates have special organs whose cells form antibodies. In infected tissues of immune plants there are formed functionally adequate protoplasm organelles—mitochrondria, plas-tids, ribosomes—that enable an immune form of a plant, when infected, not only to preserve but even to increase the energy efficiency of respiration. The respiratory disturbances caused by pathogenic agents are accompanied by the formation of various compounds that function as special chemical barriers against the spread of infection. Consequently, plant immunity reflects the characteristics of the protplast, the cell, the tissue, the organ, and the entire organism, which is a complex, qualitatively varied, and at the same time functionally single biological system. The plant responds to injury from pests and parasites by creating chemical, mechanical and growth barriers, by regenerating injured tissues, and by replacing lost organs—all of which play an important part in immunity to pests and parasites. In some cases the presence of certain chemical compounds in the tissues and the plant’s anatomical characteristics are also significant factors. This is particularly true of plant immunity to insect pests. For example, some products of secondary metabolism (alkaloids, glycosides, terpenes, saponins) have a toxic effect on the digestive apparatus and endocrine and neurohumoral systems of insects and other pests.

Hybridization between varieties, species, and genera is of the greatest importance in breeding plants for resistance to diseases and pests. Autopolyploids and amphipolyploids, from which hybrids between species with different chromosomes are obtained, serve as the basic material for breeding. Amphidiploids were produced by the Soviet breeder M.F. Ternovskii to obtain tobacco varieties resistant to powdery mildew. Artificial mutagenesis can be used to create resistant varieties, and selection among heterozygous populations is employed for cross-pollinated plants. The Soviet breeders L.A. Zhdanov and V.S. Pustovoit used these methods to obtain sunflower varieties resistant to broomrape. One method of obtaining varieties with longlasting resistance is the development of multistrain varieties by crossing economically valuable varieties with varieties possessing different resistance genes. As a result of the variety of resistance genes in the hybrids, new strains of parasites cannot accumulate in sufficient quantities. A second method consists in combining R-genes with field-resistance genes in a single variety. Periodic changes in the varieties found in a particular region or farm also help to increase resistance.