Plant Breeding: Induced Mutation Technology for Crop Improvement
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FEATURES Plant breeding: Induced mutation technology for crop improvement Scientists at the IAEA's Seibersdorf Laboratories are helping breeders to develop crops having more desirable traits present forms of life are the product of found suitable for domestication; humans have by F.J. Novak three factors: used about 3000 of these for food, fibre, spices, and • mutation, the fundamental source of heritable etc., with 200 ultimately domesticated as crops. H. Brunner variation, Today, only 15-20 of these are food crops of • environmental factors, which influence the major importance. selection of those mutations that survive and The means of developing new plant varieties reproduce, and for cultivation and use by humans has come to be • time, during which the genotype and environ- called plant breeding. Early on, it primarily in- ment constantly interact and evolutionary volved selection, the choice between good and change is realized. bad plants. People learned not to eat all the "best Genetic variation found in nature does not fruit" but to plant the seed from some of them. represent the original spectrum of spontaneous Genetics became a fundamental science of mutations. Rather, this is the result of genotypes plant breeding after the Moravian monk J.G. recombining in populations and continuously in- Mendel discovered the laws of heredity in the teracting with environmental forces. mid-19th century. Plant breeding further ad- Green plants are the ultimate source of vanced when the methodology of hybridization resources required for human life, food, clothing, was developed. Its aim was to combine various and energy requirements. Prehistoric people, desirable properties of many plants in one plant, who depended on their skills as hunters, drew instead of just choosing between good and bad upon abundant natural vegetation to collect plants. This method, often supplemented by nutritious and nonpoisonous fruits, seeds, tubers, germplasm derived from induced mutation, has and other foods. As human populations in- become the most common one for breeding creased, greater and safer supplies of food had to plants through sexual reproduction. be found, and gradually production systems However, some crops—including bananas, based on plant domestication were developed. apples, cassava, and sugar cane—reproduce The domestication of crops historically has vegetatively, especially those that are fully been influenced by ecological and agricultural sterile without seeds. For this important group, conditions, as well as by food gathering alternative approaches had to be developed, preferences. Genotypes that have adapted to a namely techniques of manipulation with somatic wide range of climatic and edaphic conditions tissue: mutation breeding and biotechnology. typically have been selected for cultivation. The achievement of higher yielding crops facilitated population growth, sedentary settlements, and Mutation breeding further development. Which crops were domes- ticated depended not only on the number of Plant breeding requires genetic variation of seeds or the size of fruits, but also on taste, useful traits for crop improvement. Often, how- palatability, and other factors. ever, desired variation is lacking. Mutagenic Only a small fraction of the world's ap- agents, such as radiation and certain chemicals, proximately 200 000 plant species have been then can be used to induce mutations and generate genetic variations from which desired mutants may be selected. Dr Novak is Head of the Plant Breeding Unit at the IAEA's Seibersdorf Laboratories, and Dr Brunner is a senior scientist Mutation induction has become a proven in the Unit. way of creating variation within a crop variety. IAEA BULLETIN, 4/1992 25 FEATURES One natural evolutionary product of genetic variation: a mutant of dwarf coconut palm. It offers the possibility of inducing desired at- Major efforts were devoted during this initial tributes that either cannot be found in nature or phase of mutation induction to define optimal have been lost during evolution. When no gene, treatment conditions for reproducibility. Re- or genes, for resistance to a particular disease, or search focused on changing "random" mutation for tolerance to stress, can be found in the avail- induction into a more directed mutagenesis to able gene pool, plant breeders have no obvious obtain more desirable and economically useful alternative but to attempt mutation induction. mutations. However, it did not lead to the desired Treatment with mutagens alters genes or alterations in the mutant spectrum. Limitations breaks chromosomes. Gene mutations occur were the concomitant increase of plant injury naturally as errors in deoxyribonucleic acid with increasing radiation dose and the low fre- (DNA) replication. Most of these errors are quency of economically useful mutations. This repaired, but some may pass the next cell led scientists to search for potentially better division to become established in the plant off- mutagens. As a result, new methods of radiation spring as spontaneous mutations. treatment, as well as chemical agents with Although mutations observed in a particular mutagenic properties, were found. gene are rare, there are probably 100 000 genes in a cell of a higher plant. This means that every plant may carry one or more spontaneous muta- Plant biotechnology tions into the next generation. Gene mutations without phenotypic (visible) expressions are Breeding for improved plant cultivars is usually not recognized. Consequently, genetic based on two principles: genetic variation and variation appears rather limited, and scientists selection. The process is extremely labourious have to resort to mutation induction. There are and time consuming with high inputs of intellec- no other economic ways of altering genes, ex- tual and manual work. (See box.) However, the cept to wait a long time for spontaneous muta- development of plant cell and tissue culture over tions to occur. the last 20 years has made it possible to transfer Artificial induction of mutations by ionizing part of the breeding work from field to laboratory- radiation dates back to the beginning of the 20th conditions. century. But it took about 30 years to prove that Extensive research has resulted in new areas such changes could be used in plant breeding. of plant breeding, namely "plant biotechnology" Initial attempts to induce mutations in plants and "genetic engineering". They are based on mostly used X-rays: later, at the dawn of the cellular totipotency. or the ability to regenerate "Atomic Age", gamma and neutron radiation whole, flowering plants from isolated organs were employed as these types of ionizing radia- (meristems). pieces of tissue, individual cells. tions became readily available from newly estab- and protoplasts. The isolated plant parts are lished nuclear research centres. aseptically grown in test tubes on artificial media 26 IAEA BULLETIN, 4/1992 FEATURES Some tools and products of plant breeding (clockwise from top left): a mutant of paddy rice induced by ionizing radiation; yams and other root and tuber crops can be genetically improved by mutation breeding; tissue culture and in vitro mutagenesis are basic methods of biotechnology for improving crops; "Golden Maidegg", an apple mutant with improved market value, was induced at the Seibersdorf Laboratories by irradiation of cuttings from "Golden Delicious" apples; mutation breeding has improved the tolerance to environmental stress of Azolla, a water fern used as biofertilizer in rice paddies. IAEA BULLETIN. 4/1992 27 FEATURES General scheme of mutation breeding Breeding a new variety of crop takes anywhere from 12 to 15 years of intensive effort The steps in- clude: Generation Characterization Seeds, pollen, vegetative parts, or tissue cultures treated by physical (radiation) or chemical mutagens. Mi(MiVi) Plants grown from treated seeds (Mi) or vegetative propagula (MiVi). M2(MiV2) Population of plants grown from seeds (M2) or vegetative parts (MiV2> harvested from Mi or MiVi respectively. Selection of desired mutants may start in this generation or later. MS - MS Continuing selection, genetic confirmation, mulitphcation (MiV3 - MiV8) and stabilization of field performance of mutant lines. Next 2 - 3 generations Comparative analyses of mutant lines during different years and in different locations. Next 2 - 3 generations Official testing before release as a new variety. Applications of nuclear techniques in plant breeding Cross Breeding Mutation Induction (using mutants) Genetic Engineering | | Mutation breeding Tracer techniques Both Biochemical-and DNA Markers Crop improvement is based on two basic principles: genetic variation and selection. 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