Chapter 20: Crop Improvement

 This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on February 18, 2012.

The generic alteration of plants to satisfy human needs is demarcated as crop improvement.  The common factors that are hampering crop production are abiotic, e.g. cold, salinity, soil aluminium toxicity and drought; as well as biotic, e.g. diseases and pests.  Modern Approaches like induced mutations and biotechnology offer new means and significant potential to breed desired varieties in a relatively short time.  Furthermore, these approaches facilitate the breeding of some vegetatively propagated crops which until now were improved mainly through selection of rare spontaneous mutants in natural or cultivated populations. Slide1

Induced Mutations (Successful nuclear techniques) designed to change the genetic makeup of a given plant variety without crossing with another variety.  With this approach, a variety retains all its original attributes but is upgraded in one or two changed characteristics.  This approach is based on radiation-induced genetic changes.  Scientific methods, mainly the use of radiation, can increase by a hundred thousand times the likelihood of beneficial changes in plants grown for man’s use, and provide a tool to break through present limitations in variability.  Already there are examples of better crops of wheat, barley, rice, oats, peanuts, soybean and other plants.

Here are the reasons to be seriously concerned about the current food situation generally and the food supply in particular:

  • Immoderate continuing human population increases, most pronounced in some poor developing countries;
  • The highly accelerated consumption of animal foods associated with increasing affluence in the richer countries of the world.  The production of such foods as meat demands great expenditures of grain, which is an inefficient mode of obtaining the required calories and protein for human consumption;
  • The over-exploitation of many of the world’s fishery resources resulting in reduced yields, perhaps irreversibly, of some fish species;
  • Recent price increases in petroleum and fertilizer products which have imposed a major obstacle to increasing crop production; and
  • The apparent alteration of climates in places like Africa, Asia and other parts of the Northern hemisphere which may put significant restrictions on crop production.

Green plants are the ultimate source of resources required for human life, food, clothing, and energy requirements. Prehistoric people, who depended on their skills as hunters, drew upon abundant natural vegetation to collect nutritious and non-poisonous fruits, seeds, tubers, and other foods.  As human populations increased, greater and safer supplies of food had to be found, and gradually production systems based on plant domestication were developed.

The domestication of crops historically has been influenced by ecological and agricultural conditions, as well as by food gathering preferences.  Genotypes that have adapted to a wide range of climatic and edaphic conditions typically have been selected for cultivation.  The achievement of higher yielding crops facilitated population growth, sedentary settlements, and further development.  Which crops were domesticated depended not only on the number of seeds or the size of fruits, but also on taste, palatability, and other factors.  This process of domestication involved the identification of certain useful wild species combined with a process of selection that brought about changes in appearance, quality, and productivity.  The exact details of the process that altered the major crops are not fully understood, but it is clear that the genetic changes were enormous in many cases. In fact some crop plants have been so changed that for many of them, maize, for example, their origins are obscure, with no extant close wild relatives.Slide2Only a small fraction of the world’s approximately 200 000 plant species have been found suitable for domestication; humans have used about 3000 of these for food, fiber, spices, etc., with 200 ultimately domesticated as crops. Today, only 15-20 of these are food crops of major importance.

The means of developing new plant varieties for cultivation and use by humans has come to be called plant breeding. Early on, it primarily involved selection, the choice between good and bad plants.  People learned not to eat all the “best fruit” but to plant the seed from some of them.  Genetics became a fundamental science of plant breeding after the Moravian monk J.G. Mendel discovered the laws of heredity in the mid-19th century.

Plant breeding further advanced when the methodology of hybridization was developed.  Its aim was to combine various desirable properties of many plants in one plant, instead of just choosing between good and bad plants.  This method, often supplemented by germplasm derived from induced mutation, has become the most common one for breeding plants through sexual reproduction.  However, some crops—including bananas, apples, cassava, and sugar cane—reproduce vegetatively, especially those that are fully sterile without seeds.  For this important group, alternative approaches had to be developed, namely techniques of manipulation with somatic tissue: Mutation Breeding and Biotechnology.Slide3

Plant breeding requires genetic variation of useful traits for crop improvement.  Often, however, desired variation is lacking.  Mutagenic agents, such as radiation and certain chemicals, then can be used to induce mutations and generate genetic variations from which desired mutants may be selected.  Mutation induction has become a proven way of creating variation within a crop variety.

Artificial induction of mutations by ionizing radiation dates back to the beginning of the 20th century.  But it took about 30 years to prove that such changes could be used in plant breeding.  Initial attempts to induce mutations in plants mostly used X-rays: later, at the dawn of the “Atomic Age”, gamma and neutron radiation were employed as these types of ionizing radiations became readily available from newly established nuclear research centers.

Major efforts were devoted during this initial phase of mutation induction to define optimal treatment conditions for reproducibility.  Research focused on changing “random” mutation induction into a more directed mutagenesis to obtain more desirable and economically useful mutations.  However, it did not lead to the desired alterations in the mutant spectrum.  Limitations were the concomitant increase of plant injury with increasing radiation dose and the low frequency of economically useful mutations.  This led scientists to search for potentially better mutagens.  As a result, new methods of radiation treatment, as well as chemical agents with mutagenic properties, were found.

It was concluded by scientists that there is no difference between artificially produced induced mutants and spontaneous mutants found in nature.  Compared to cross-breeding, special care is taken in selecting homozygous, non-chimerical mutant lines when working with artificially induced mutants.  To achieve this, induced mutants are passed through several generations of selfing (in order to achieve homozygosity); or clonal propagation, usually through in vitro techniques (in order to dissociate chimeras).  This is exactly what happens in nature (through evolution) and leads to the fixation of the mutation events.  All that plant breeders do is mimic nature in this regard.  It should also be noted that in most cases, the induced mutants are merely “raw materials” that in order for their potentials to be realized must be integrated into established breeding schemes through hybridizations and repeated backcrosses.  Like in all such processes that involve crossing, care should be taken to eliminate linkage drags of agronomically unfavorable alleles as such mediocre or undesirable alleles could show up in subsequent generations through segregation.

The use of induced mutations (through irradiation and chemical agents) has over the past 50 years played a major role in the development of superior crop varieties translating into a tremendous economic impact on agriculture and food production that is currently valued in billions of dollars and millions of cultivated hectares.  For the past 30 years, the International Atomic Energy Agency (IAEA) and the Food and Agriculture Organization (FAO) of the United Nations have through the Joint FAO/IAEA Division for Nuclear Techniques in Food and Agriculture, Vienna, sponsored extensive research and development activities in their Member States on mutation induction to enhance the genetic diversity in the germplasm of food and industrial crops and these efforts have resulted in the official release to farmers of over 2300 new crop varieties including rice, wheat, barley, apples, citrus, sugar cane, banana, among others.  These represent the information submitted to IAEA voluntarily by member states but they are aware that many more mutants are not registered with IAEA.Slide4

Mutation induction techniques are undergoing a renaissance in crop improvement because of advancements in modern efficiency enhancing biotechnologies – irreplaceable tools in the tool box of the breeder.  In the context of climate change and variability, mutation induction is a proven way to generate diversity in existing crop varieties, to widen the extent of adaptability and enhance productivity of crop biomass.  In many countries, a broad variety of plant species and target traits are addressed using mutation induction.

All major activities of the Plant Breeding and genetics Sub-programme in the Joint Programme are within the ‘public goods’ area both in developing and developed countries and addresses urgent needs and requirements from FAO and IAEA Member States.  In most cases regional collaboration is necessary and collaborating with several national and international institutions, IAEA is in the best position to coordinate these activities.  Here are some highlights the following achievements:

  • There have been more than 2700 (Figure 04) officially released mutant varieties from 170 different plant species in more than 60 countries throughout the world.  These thousands of plant mutants produced by Joint Programme not only increased biodiversity, but also provided breeding material for conventional plant breeding, thus directly contributing to the conservation and use of plant genetic resources;
  • Over 1,000 mutant varieties of major staple crops enhance rural income, improve human nutrition and contribute to environmentally sustainable food security in the world;
  • Worldwide, more than 60 percent of all mutant varieties were officially released after the year 1985, in the era of biotechnology in plant breeding.  The integration of mutation techniques and efficiency-enhancing bio-molecular techniques that permit rapid selection of the most beneficial mutants has pushed the use of mutation induction to new and higher levels of applicability;
  • In vegetatively propagated crops, where genetic variation is difficult to obtain due to limited sexual reproduction, sterility and polyploidy mutation induction appears as the tool of choice for rapidly obtaining tangible results. Mutation induction allows for escaping the deadlock of sterility and parthenocarpy by creating useful variants; and
  • Tens of millions of hectares of higher yielding or more disease-resistant crops developed through induced mutations and released to poor farmers. Slide5

Mutation breeding has its advantages and limitations.  The advantages include creation of new gene alleles that do not exist in germplasm pools and the induction of new gene alleles for a commercial variety so new varieties carrying desired mutation alleles can be directly used as a commercial variety.  The limited genetic changes of any single plant of a mutated population and the often recessive nature enable breeders to develop a new variety in a short breeding cycle.  The disadvantage of mutation breeding is its limited power in generating the dominant alleles that might be desired; it is also less effective than cross breeding for a trait needs for a combination of multiple alleles, such as tolerance to abiotic stresses.  The low mutation frequency requires growing and screening a large population for selection of desired mutants at a reasonable confidence.  This becomes very expensive for traits that have to be evaluated through laborious phenotypic analysis.

Resources:

  1. IAEA – Induced mutations in connection with biotechnology for crop improvement in Latin America;
  2. Advances in Breeding for Better Crops;
  3. Protein Improvement in Crop Plants;
  4. Plant Breeding – Induced Mutation Technology for Crop Improvement;
  5. Answers – Crop Improvement;
  6. IAEA – Nuclear Techniques in Food & Agriculture;
  7. IAEA – Facts about the Joint FAO/IAEA Programme;
  8. Plant Mutation Report – April 2011;
  9. IAEA – Joint FAO/IAEA Programme; and
  10. Turning Plant Breeding into a New Era: Molecular Mutation Breeding.

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