Breeding Aims
Plant varieties must meet the requirements of plant production under special environmental and economical conditions. Moreover, the demands of food and feed industry as well as the consumers' preferences must be regarded.
The yield potential is often the most important aim. Today's varieties are elites with high-yielding potential due to countless rounds of recombination and selection. Yield potential is a typical quantitative character controlled by many genes. Thus high yielding varieties differ substantially from their wild relatives, landrace and any other non-adapted material. Therefore, transgenic material with
C.Jung
Plant Breeding Institute, Christian Albrechts University of Kiel, Olshausenstrasse 40, 24098 Kiel, Germany e-mail: c.jung@plantbreeding.uni-kiel.de
F. Kempken and C. Jung (eds.), Genetic Modification of Plants, 103
Biotechnology in Agriculture and Forestry 64,
DOI 10.1007/978-3-642-02391-0_6, © Springer-Verlag Berlin Heidelberg 2010
natural genetic variation ^^^^^^^^^^^ artificial crosses with elite material wide crosses crosses with landraces
cell fusion hi mutation transformation
Increasing genetic variation primary gene pool
secondary gene pool
Crossing and recombination
Selection
Variety
Fig. 6.1 Methods for increasing genetic variation in plant breeding poor-yielding capacities must be backcrossed several times with elite lines (see Sect. 6.3). The yield potential itself is not accessible to transgenic modification due to its polygenic nature.
Plants are attacked by numerous pathogens and pests and they suffer from different environmental constraints like drought, heat, frost, water, salt and low soil pH. Therefore resistances or tolerances to these stresses are needed. These measures increase the yield stability of a crop. They are often a prerequisite for crop cultivation mainly when technical measures like pesticides are unavailable. It happens quite often that modern high-yielding varieties lack resistance or tolerance genes. Transgenic technology has been tremendously successful in increasing the genetic variation in this field, mainly for virus and pest resistance (see Chap. 10). Excellent perspectives exist for drought tolerance and other environmental stresses (see Chap. 8). Transgenic technology is particularly successful in breeding varieties with tolerance or resistance to herbicides. Although non-transgenic herbicide tolerances have been used before, the availability of these genes identified from microorganisms tremendously broadens the genetic variation of crops (see Chap. 9). Moreover, this character is simply inherited and can be easily selected in segregating populations. Therefore, herbicide resistances can easily be combined with pathogen or pest resistances and a number of varieties with dual resistances are available today, e.g. corn and soybean.
Biotic stress resistance is frequently broken by the pathogen when resistance relies on single genes only (monogenic resistance). A simple mutation within the pathogen's genome can break the resistance and diminish the value of a variety. Gene stacking of resistance genes by transgenic technology offers a perspective for durable resistance breeding because double mutations on the pathogen's side are highly unlikely (see Chap. 3). The genes can be natural ones isolated from the given species or from related or non-related species or from artificial resistance genes not present in the primary to tertiary gene pools of the species. An example is gene stacking with synthetic crylC genes which gave multiple resistances to leaffolders and stemborers in rice. The transgenic line was used as a parent for hybrid rice production and the hybrids proved to be resistant as well (Tang et al. 2006).
Improving the quality of harvested parts of the plant is another major aim in many breeding programs. Often single genes have a major impact on the phenotype, thus quality improvement is readily accessible to genetic modification. Consequently, the quality of major storage components like fatty acids, protein or starch has been improved by genetic modification (see Chap. 11). Moreover, transgenic plants with higher vitamin and mineral content and better feeding or processing quality have become available.
Altering the phenological development has often been a major requirement for plant production. Transgenic technology offers new perspectives by modification of flowering time genes. The onset of flowering is of uppermost importance for plant production. Often early flowering is desired to avoid stress conditions. However flowering must be avoided when the vegetative parts of a plant are harvested, like beet roots, tubers or leaves.
The key regulators for flowering time control have been cloned from Arabidop-sis thaliana. High conservation of regulatory pathways was found among dicot species. Genes isolated from A. thaliana were often found to have the same function after transformation into crop species. Likewise flowering time genes were identified for monocot species, using rice as a model. These findings offer possibilities for shortening the generation cycle of crop species by using genetically modified early-flowering genotypes as crossing parents. This is of major interest for introducing genes from exotic material by repeated backcrossing (Fig. 6.2). After selfing plants from advanced backcross generations the transgene can easily be eliminated from the offspring, either by marker assisted selection, or by selecting for the phenotype itself. Many plants have long generation cycles which substantially delay the breeding progress. These are biennials with a long seed ripening phase like sugar beet or tree species which often need many years to flower. Breeding strategies employing short generation transgenics are presently discussed for forest tree and fruit tree breeding (Flachowsky et al. 2007). Manipulation of flowering time regulators also offers a possibility to produce plants with completely altered bolting or flowering behavior (see Sect. 6.6).
6.3 Methods for Introducing Transgenes into Elite Plant Material
Often favorable alleles are absent from the primary gene pool of a plant, therefore exotic lines are used as parents and their offspring are backcrossed several times with the elite recipient line to produce an elite line with a small
marker assisted background selection
Phenotypic or , marker assisted bc X E '
„..transgeseselection.
Year 1
BC2 X E
D: transgenic line E: non transgenic elite line BC: backcross generation E': genetically improved elite line (S>: selfing
BC3 ® selection
Fig. 6.2 Introgression of a transgene from a transgenic donor (D) into an elite recipient plant line (E): backcross breeding in combination with phenotypic or marker-assisted selection. The share of the elite genome is shown in black
BC2 ® selection
introgression from the donor line. In most cases these are major genes with clear phenotypes.
Genetically modified plants directly resulting from a transformation process are often not adapted to local environmental conditions because standard genotypes with inferior yielding performance but good regeneration capacity have to be used. Thus, they must be backcrossed with recipient lines to create elites carrying the genetic modification (Fig. 6.2).
Molecular markers derived from the transgene itself turned out to be helpful for backcross breeding (Bernardo 2008). They are used to select for recombinant plants in offspring generations without phenotypic analysis (marker-assisted foreground selection). In addition markers covering the rest of the genome can be used to select plants with a high proportion of the recipient (elite) genome, even in early back-cross generations (marker-assisted background selection). This saves time because several generations of backcrossing can be avoided (Fig. 6.2).
Only single-copy transgenes are desired for breeding, otherwise selection will be complicated by complex segregation patterns. There are numerous examples for successful introduction of transgenes into elite material or existing varieties, e.g. resistances to insects, virus and leaf blight in rice. Existing rice varieties were also improved for quality characters like provitamine A and ferritin content (Kang and Priyadarshan 2007). When the transformation procedure is genotype-dependent and elite genotypes are non-accessible for transformation, the backcrossing procedure is the method of choice for introducing transgenes into a desired elite genotype. This could be an inbred line in the case of line or hybrid breeding or an inbred cultivar.
When an already existing variety has been used for transformation, the new variety is called an essentially derived variety (EDV). To protect the breeder's rights the GMO breeder needs approval from the variety's owner to commercialize the EDV. In the EC a 95% identity threshold has been established for defining an EDV.
For hybrid breeding the genetic modification can be introduced either into the male sterile parent or into the restorer parent. When however the degree of dominance is not complete, i.e. the performance of heterozygous is inferior to that of homozygous plants, the genetic modification must be introduced into both parents. In corn breeding, transformation is either done on a hybrid-by-hybrid basis or only one parent is transformed (Kang and Priyadarshan 2007).
When plants are clonally propagated elite material should be transformed. Due to the absence of recombination further improvement can only rely on extensive selection. When however the transgene cannot be incorporated into an already existing variety further crosses are needed and huge clone populations have to be created. Breeding clonally propagated transgenic varieties is further hampered by the fact that many clonally propagated plants are polyploid, which complicates selection due to their complex segregation pattern.
Today, the perspectives for transgenic breeding are rather limited because only single genes or a low number of genes can be transformed at one time. Quantitative characters are not accessible to genetic modification. Gene stacking is an interesting option to further increase genetic variability by transformation to accumulate a number of genes in an elite plant. Transgenic plants with different transgenes are crossed to each other. In the F2 generation recombinant genotypes with both transgenes can be selected. In the case of single genes, double homozygous plants are expected with a frequency of one in 16, provided that the transgenes are not genetically linked. Using molecular markers homozygous plants can be easily distinguished from homozygous ones. After testing for homozygosity, these lines can be used as parents for hybrid breeding. A transgenic rice restorer line has been bred in this way, combining multiple resistances against bacterial blight and striped stem borer together with a herbicide tolerance by repeated backcrossing and hybridization of transgenic parents (Wei et al. 2008).
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