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
       Dr Novak is Head of the Plant Breeding Unit at the IAEA's      mutants may be selected.
       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).
                                      M(MiV )                                      Population of plants grown from seeds (M ) or vegetative
                                        2     2                                                                           2
                                                                                   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
                                      (MiV  - MiV )                                and stabilization of field performance of mutant lines.
                                           3      8
                                      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.             Disease and Pest
          Serving as invaluable                   Resistance
            tools are mutagenic
         irradiation and isotope
       tracer techniques, which
       are incorporated into the                               Yield (Photo-
              various breeding                               synthesis Studies)
                     methods.
      28                          IAEA BULLETIN, 4/1992