Plant Genetics

a branch of genetics that studies heredity and variation in the higher plants. (Such a study of fungi and algae is usually included in microorganism genetics.)

The following methods are used to study plant genetics, in addition to those used in other branches of genetics (hybridological analysis in particular). First, the role of each chromosome in the inheritance and development of different plant characters is determined by monosomic analysis. This method, worked out on the thorn apple, is used to study several allopolyploids (such as certain wheats and cotton) as well as diploids (barley). Experimental mutagenesis is very valuable in plant genetics because it produces a large variety of new forms used in breeding, as well as material useful for studying the genetics of individual plant species. Genetic chromosome maps are constructed with the help of mutants. With these maps one can investigate the effect of an altered gene (in a homozygous or heterozygous state) on the development of individual characters under various environmental conditions and on the physiological and biochemical properties of plants. The study of mutants helps one to clarify the evolution of a particular species.

Two methods of investigating the evolution of piants are hybridization and analysis of chromosome conjugation in hybrids during meiosis. (Unrelated chromosomes do not conjugate.) An important method is the artificial resynthesis of existing species by hybridization and the subsequent doubling of the number of chromosomes. Allopolyploidy plays an important role in the evolution of plants, including many cultivated ones such as wheat, oats, cotton, potatoes, and fruits. After it was discovered that the alkaloid colchicine can prevent doubled chromosomes from going to different poles of the cell, autopolyploidy was widely used to obtain new and sometimes very valuable forms. Distant hybridization combined with cytogenetics is used in the study of the role of individual chromosomes and segments of chromosomes in the inheritance of characters. It is also used to devise techniques for inserting the segments of chromosomes of wild plants responsible for the development of valuable characters, such as resistance to rust, into the chromosomes of cultivated plants. The role of the nucleus and the cytoplasm in the inheritance and development of characters is studied by means of distant hybridization and an analysis of the nature of male cytoplasmic sterility used to obtain heterotic forms. Plant geneticists investigate apomixis and self-incompatibility, that is, the incapacity of plants for self-fertilization, as well as the genetic peculiarities of self- and cross-pollinating plants and vegetatively and apomictically reproducing forms.

Plant genetics is being increasingly influenced by the ideas and methods of molecular biology (such as the hybridization of DNA and DNA—RNA and the study of isoenzymes). The methods of population genetics and biometrics are used in plant genetics to differentiate the genotypic and paratypic elements in the general phenotypic variation of characters, thus increasing the effectiveness of artificial selection. All these methods are used to improve the economically valuable properties of crops: yielding capacity, resistance to unfavorable environmental conditions, some biochemical and technological characteristics of a plant or its grain, and developmental characteristics (its condition in winter and in spring, early ripening, and so forth). Among the higher plants that are studied most from the genetic standpoint are corn, mouse-ear cress (a plant of the family Cruciferae known as the plant drosophila, a model object of genetic research), peas, tomatoes, and barley. Hybridization has been used in these plants to determine the location of genes and to compile chromosome maps. Under intensive study is the cytogenetics of bread wheat, a complex 42-chromosome allopolyploid that evolved in the course of the natural hybridization of three different cereal grasses, with subsequent doubling of the chromosome number in hybrids. Plant genetics has contributed greatly to breeding. For example, it includes the use of heterosis in the breeding of corn based on male sterility, the introduction of genes responsible for the high lysine content of the grain into high-yielding hybrids and fodder barley varieties, the creation of low, nonlodging, high-yielding wheat varieties by means of dwarfism genes (the “green revolution” in India and other countries), and the breeding of productive and sacchariferous triploid sugar beet hybrids.