USE OF BIOTECHNOLOGY IN PLANT BREEDING
Crop improvement is the exploitation of genetic variability, followed by several generations of selection. All these conventional methods and process are time consuming and slow process. Breeders have always used the most modern technologies available to them. This has permitted them to make considerable progress during the last twenty years, the development of biotechnology helps in ease and fasten the plant breeding programme and leads to crop improvement. These biotechnological tools permit:
• an acceleration of the selection process,
• new genetic combinations that are not possible through conventional breeding, and
• greater precision in the desired modifications of the genome.
Using in vitro techniques, it is possible to regenerate plants from pollen or ovules. These plants, which contain only one copy of each chromosome, are called haploids. They are not viable. After appropriate chemical treatment, it is possible to restore the normal number of chromosomes and to regenerate viable plants. These plants, called double-haploids, are homozygous for all their genes. Such plants are of tremendous interest to plant breeders, since they allow development of pure line varieties or inbred parental lines much more quickly than through conventional breeding.
Androgenesis (regeneration from pollen) has been successfully used for crops such as eggplant, pepper and wheat. Gynogenesis (regeneration from ovules) is used on barley. However, the bulbosum method used with barley does not require in vitro cultivation of ovules; development of haploids is obtained in vivo through interspecific crosses between barley and Hordeum bulbosum, a wild relative.
Markers may be either phenotypic or genotypic, and marker-assisted breeding developed in the 1980s with the evolution of DNA marker technologies. Today, the main DNA markers used in breeding programmes are Random Amplified Polymorphic DNA (RAPD), Amplified Fragment Length Polymorphism (AFLP), microsatellites, and Expressed Sequence Tags (ESTs). Each of these markers has a different set of advantages and limits. Cost and possible automation of the techniques are of particular importance for their adoption.
Use of molecular markers, in association with linkage maps and genomics, offers plant breeders the potential to make genetic progress much more precisely and rapidly than through phenotypic selection. It also offers the possibility of addressing previously unattainable goals.
There are many applications for the use of DNA markers in breeding programmes, which fall into four broad groups, based on the purpose of the intervention:
• enhancing knowledge of breeding material and systems, such as better understanding and more effective breeding of Quantitative Trait Loci (QTL);
• rapid introgression or backcross breeding of simple characters, as the number of back-crosses required can be reduced drastically if there are markers for the character to be introduced and for the genetic background of the recurrent parent;
• early or easy indirect character selection, which is important for genes that cannot be detected at an early development stage, such as high lysine and tryptophan genes in maize; and
• new goals not possible through traditional breeding, including pyramiding of disease resistance genes with indistinguishable phenotypes.
Breeders need access to the largest possible genetic variability. In some cases, variability available within a given species is not sufficient to answer a specific problem (e.g. resistance to some new disease). A solution available to breeders is inter-specific hybridization (crossing plants from separate but related species). However, embryos resulting from such hybridization rarely survive, due to incompatibilities between the embryo and the mother plant. Saving embryos is sometimes possible through their in vitro cultivation, which make it possible to isolate the inter-specific embryo from the hostile mother plant environment.
This technique has been used for the introduction of disease resistance into squash, lettuce, tomato, etc. This technique will be replaced by transgenesis, which will provide the same result, but much faster and with much more accuracy. Indeed, contrary to inter-specific hybridization, transgenesis leads to the introduction of solely the target gene, and eliminates the need for several generations of backcrossing.
Fusion of protoplasts is another technique to allow inter-specific hybridization between species that cannot be crossed through conventional breeding, even using in vitro embryo rescue. Protoplasts are plant cells that have had their outer walls removed through chemical treatment. While it is difficult or impossible to fuse plant cells, it is possible through various techniques (using either chemical or physical treatments) to merge protoplasts from different crop species or genera, and then to regenerate a whole plant resulting from the fusion process.
This technique has been used to introduce traits such as male sterility into rapeseed, or disease resistances in potato. Similarly to embryo rescue, this technique will most probably be replaced by transgenesis, which is a faster, more effective and more precise technique. Moreover, protoplast fusion is generally not effective beyond the family level due to incompatibilities between two too-distant genomes making it impossible to regenerate plants.
Transgenesis (also called genetic transformation or genetic engineering) is the most recent step to increase genetic variability available within a crop. Transgenesis aims to introduce, through different techniques, a specific gene (included in a gene construct) from a donor species into the genome of a host species. Organisms resulting from transgenesis are commonly called Genetically Modified Organisms (GMOs).
Transgenesis is being used to introduce a broad range of new agronomic, processing and nutritional traits into the main agricultural and vegetable crops (see section 4, below). In comparison to embryo rescue and protoplast fusion, transgenesis is not constrained by reproduction barriers. Genes can be transferred from one realm into another. Moreover, only the specific gene construct is introduced in the host organism. This provides great precision and rapidity to the enhancement process. Transgenesis is a very promising tool for the development of new varieties with specific traits that are not present within the crop genepool.
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