gene manipulation techniques in plants
Working with plants is an obvious application of gene manipulation. Genetic modification was first attempted in plant species, and as of now it is still only plant based food which is commercially available in genetically modified form. Plants are easy to modify in either way, as you can find the strongest strains and repeatedly breed from them, while at the same time introducing genes from other plants to create completely new strains. Everything depends on the objective being sought, which is usually either improved durability and resistance of a greater food yield. The more complicated the genetic modification, the harder it is to predict the exact consequences. Producing sufficient food for an ever growing world population continues to be an ongoing struggle. Using gene transfer techniques it is possible to create new varieties of plants to help meet this challenge. There is much research into developing new plants that are easier to grow for a farmer, particularly those in poor countries where famine is a real problem and modern farming practices are rare. Rather than shipping grain to a nation facing drought, a better idea would be to provide seeds of plants that can grow in harsh conditions. The process for inserting desired genes into plants involves using a bacterium ready made for such a task. We can again turn to nature to find a vector delivery system for plants. There is a bacterium called Agrobacterium tumefaciens has a plasmid that naturally inserts DNA into a plant cell. This organism causes plant tumors and is considered a big problem in agriculture. The plasmid can be harnessed to deliver a target gene to plant tissues. In this case we build a vector using this natural bacterial system. All we have to do is insert a gene of interest into the plasmid using the exact procedure as outlined above. Then it's just a matter of growing the plant cells into a mature plant that will now contain our gene of interest. Lets take up an example of golden rice production. The development of golden rice involved the introduction of several genes into a plant to provide a multi step biochemical pathway. Rice grain, which serves as a food staple for much of the world, lacks vitamin A. An estimated 100 million to 200 million children worldwide have vitamin A deficiency, a condition that causes blindness; and increases susceptibility to diarrhea, respiratory infection, and childhood diseases, such as measles. Beta-carotene and other carotenes (the red, yellow, and orange pigments found in carrots and other vegetables) are the precursors of vitamin A. Rice synthesizes beta-carotene in its chloroplasts but not the edible seed tissue. The following explains some of the techniques used to genetically modify plants.
A package of DNA is usually transferred in the process which includes:
1. The 'gene of interest' (GOI) which gives the desired trait. The gene may come from any other organism but in commercial crops, bacteria are the commonest source of these genes.
2. Genes to switch the GOI on and off. These promoter and terminator genes, may also come from a range of organisms but viral, bacterial and plant promoters are the most commonly used in commercialised GM crops.
3. A marker gene which helps scientists identify when a plant cell has been successfully genetically modified or 'transformed'. This is necessary because the GM technique is inefficient, with only 1-2% of cells are successfully transformed. To identify these cells and use them to grow into plants, at the end of the modification process the cells are cultured in a medium where the presence of the marker genes means they can survive or produce a colour change, while other non-transformed cell will die or not have the colour mark. Marker genes include antibiotic resistance, herbicide tolerance and fluorescence.
4. There may also be remnants of the vector DNA (see below) also inserted.
There are two main methods that are used to genetically modify plants:
- Using the bacterium, Agrobacterium tumifaciens, as a vector for the DNA. Agrobacterium has the ability to infect plants and insert DNA into a plant's genome. It causes crown gall tumours in natural infections. This method has mainly been used to modify broad leaved plants, such as sugar beet and oilseed rape, but is now also being applied to monocot species, such as maize and rice.
- Particle bombardment or biolistics where the DNA to be inserted is coated on minute gold particles and fired into plants cells. This approach is used for monocot plants such as maize and rice. References: http://www.learner.org/courses/biology/textbook/gmo/gmo_5.html http://www.geneticengineeringinhumans.com/ar/articles-on-genetic-engineering.php
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