Somaclonal variation

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Callus culture of Nicotiana tabacum . Callus cells are considered to be particularly susceptible to the occurrence of somaclonal variants

Under somaclonal variation (also: Somaclonal variability ) is meant primarily occurring every deviation from the genetic initial state of an individual. In in vitro culture, this relates to the genotypic and often also phenotypic changes that occur during the cultivation of plant cells or plant tissue compared to the original plant. These often occur, for example, after the differentiation of plant tissue from callus cells . If, in the context of maintenance or mass propagation culture in in vitro culture, identical propagation of the plant, i.e. the generation of clones, is desired, somaclonal variants are therefore undesirable.

Initial investigation and definition of terms

In the course of the general and increasingly commercial establishment of the plant tissue culture in the 1970s and 80s, undesirable, apparently genetically determined changes in plant material occurred more frequently. In 1981 it was proposed for the first time that all genetic changes that occurred in the context of in vitro cultivation should be referred to as somaclonal variation.

In the meantime, there is a definition in the relevant specialist dictionaries that somaclonal variation is the variation of properties that occurs in in vitro cultures of plant cells, calluses or organs . Their causes are often unclear and in some cases can be found on the genetic or epigenetic level. Known causes for this effect are, for example, gene loss or numerical changes in the karyotype . It is also possible to change traits in which several genes are involved ( polygeny ).

in vitro methods and occurrence of somaclonal variation in plants

Plants often have genetic differences that already exist before an in vitro culture. The development and manifestation of the genetic deviation occurs less often in the run-up to in vitro cultivation, but more often during the implementation of certain in vitro culture processes. Using some in vitro cultivation methods and their framework conditions or external influencing factors such as phytohormones , these differences can be selected and later also appear phenotypically. New genetic deviations that only appear during in vitro cultivation are also possible.

Above all, the high rate of cell division in tumor cell-like plant callus tissue leads to more errors when reading the genetic information. These mostly lead to aneuploidy and ploidy mutation. If plant organs or entire plants are later regenerated from this callus tissue via organogenesis , this genetic misinformation, which has meanwhile been copied many times, is responsible for the occurrence of typical characteristics of somaclonal variation.

In addition, there is the relationship between genetic stability and the starting tissue of regeneration during in vitro culture. A direct regeneration from already existing meristem tissue is considered to be genetically stable. If secondary meristem tissues form, the first genetic changes already occur. The type and quantity of these changes increase the more the regeneration of the tissue during the in vitro cultivation moves from the primary regeneration to secondary tissue regeneration processes.

In the case of plants, clonal propagation takes place via in vitro culture methods such as shoot tip or meristem culture or the cultivation of other suitable explant forms such as leaf or shoot segments. The aim of clonal propagation is the generation of genetically stable and identical offspring. As a rule, the intention is not so much a quantitative mass reproduction of the clones, but rather - in commercial horticulture or agriculture - the elimination of pathogens while at the same time maintaining the species- or variety-specific genetic status. The production of so-called elite material for further conventional propagation ( cuttings ) is in the foreground , especially in horticulture with crops such as Pelargonium , New Guinea Impatiens or Chrysanthemum .

In plants, the risk of somaclonal variation occurring in callus, suspension, single-cell and protoplast culture is high, as is the formation of indirect embryoid formation in the context of somatic embryogenesis . In general, the somaclonal variation is considered undesirable for maintenance breeding and in vitro cloning since genetically identical individuals are required here.

Further possibilities for the development of somaclonal variation

In addition to the causes mentioned, which can be found directly in changes in the genome , other complexes of causes can also be considered. The somaclonal variation can also be epigenetic. This means that if the genome remains the same, variations can still occur which have causes other than genetic changes. Changes in the metabolic balance should be mentioned here as examples, above all in the area of ​​phytohormone quality and quantity. In the case of ex vitro plants, there is also the possibility of an infectious agent, as in the case of dwarf fruit bases or heavily branched poinsettias . In addition, in relation to undesired changes there are also special, rather rare causes such as "jumping genes" ( transposons ).

Thus, the cause for the development of somaclonal variants are both genetic and non-genetic factors, as well as a multitude of smooth transitions between them.

Somaclonal Variation in New Breeds

In the 1980s and 1990s, the phenomenon of the somaclonal variation of plant cells via in vitro culture was seen as an interesting opportunity to obtain new genetic types (new breeds) of the parent plants. It was hoped to be able to create interesting new genotypes in a targeted manner and with little expenditure of time. The selection of these genetic variants should then lead to the direct isolation of new genotypes from cell cultures.

However, based on practical experience, this hope could not be maintained. The generation of interesting genotypes is considered to be too random and difficult to regulate, and undesirable negative mutations also occur. Such genetic instability is undesirable, particularly in connection with transformed plant cells. However, there are varieties that have emerged from somaclonal variants in tomatoes , potatoes or sugar cane . For the targeted mutagenesis of plant cells, options such as colchicine treatment or radiation with X-rays , which are much more efficient, are primarily used today . Targeted interventions in the genome are now often carried out using molecular biological methods, such as biolistic transformation or transformation with agrobacteria .

Individual evidence

  1. ↑ PJ Larkin, WR Scowcroft: Somaclonal variation - a novel source of variability from cell cultures for plant improvement. Theoretical and Applied Genetics, Vol. 60 No. 4, pp. 197-214. 1981
  2. ↑ after Rudolf Schubert, Günther Wagner: Botanical Dictionary. 11th edition, Eugen Ulmer Verlag Stuttgart 1993. ISBN 3-8252-1476-1
  3. ↑ Klaus Olbricht: Investigations on the genetic and histogenetic variability in transgenic Petunia hybrida Hort. (Vilm.). Dissertation at the Agricultural and Horticultural Faculty of the Humboldt University, Berlin 1998, p. 57
  4. ↑ DA Evans: Somaclonal variation - genetic basis and breeding applications.
  5. ↑ Paul Präve, Uwe Faust, Wolfgang Sittig, DA Sukatsch (ed.): Handbuch der Biotechnologie. P. 240
  6. ↑ pflanzenforschung.de /biosicherheit/ - encyclopedia entry Somaklonale Variation
  7. ^ Frame, BR, HY Zhang, et al. ( 2000 ). "Production of transgenic maize from bombarded type II callus: Effect of gold particle size and callus morphology on transformation efficiency." In Vitro Cellular & Developmental Biology-Plant 36 (1): 21-29.
  8. ↑ Brettschneider, R., D. Becker, et al. ( 1997 ). "Efficient transformation of scutellar tissue of immature maize embryos." Theoretical and Applied Genetics 94 (6-7): 737-748.
  9. ^ Frame, BR, HX Shou, et al. ( 2002 ). "Agrobacterium tumefaciens-mediated transformation of maize embryos using a standard binary vector system." Plant Physiology 129 (1): 13-22.
  10. ↑ Sidorov, V., L. Gilbertson, et al. ( 2006 ). "Agrobacterium-mediated transformation of seedling-derived maize callus." Plant Cell Reports 25 (4): 320-328.

literature

  • PJ Larkin, WR Scowcroft: Somaclonal variation - a novel source of variability from cell cultures for plant improvement. Theoretical and Applied Genetics, Vol. 60 No. 4, pp. 197-214. 1981
  • Dieter Hess: Plant biotechnology . Verlag Eugen Ulmer, Stuttgart, 1992. ISBN 3-8252-8060-8
  • DA Evans: Somaclonal variation - genetic basis and breeding applications. Trends in Genetics , 5, 46, 1989.
  • Paul Präve, Uwe Faust, Paul Praeve, Wolfgang Sittig, DA Sukatsch (Hrsg.): Handbuch der Biotechnologie. Oldenbourg Industrieverlag, Essen, 4th edition 1994. ISBN 3-83566-223-6
  • MK Razdan: Introduction to Plant Tissue Culture . Second edition. Science Publishers Inc., Enfield (NH), 2003. ISBN 1-57808-237-4
  • PC Debergh, RH Zimmerman: Micropropagation - Technology and Application . Kluwer Academic Publishers, Dordrecht, 1991. ISBN 0-7923-0818-2

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