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GENE TRANSFER METHODS IN 1. A promoter which determines the PLANTS site and timing of expression of the introduced gene Gene transfer methods differ for 2. A terminator to identify the end of both animals and plants. In plants, transcription and the uptake of genes by cells is often 3. A marker gene, which allows described as transformation, while selection of plants having the in animals this term is replaced by introduced gene. the term transfection, leading to the Various desirable traits have been production of transgenic plants and efficiently introduced and stably animals. Plant transformation thus expressed in almost 150 plant depends on two events: successful species. Different methods are introduction of foreign DNA into available to achieve genetic target plant cells, and subsequent transformation of plants i.e. the development of a complete plant delivery of the foreign DNA into the derived from the transformed cells. host plant (Figure 1). These are Plant transformation methods divided into two main groups therefore require an efficient way to 1. Indirect methods: In this case introduce DNA into cell and the vector is needed for insertion of the regeneration of the transformed foreign DNA into the host genome. cells or tissues into whole plants. 2. Direct methods: This method is The DNA segment which is vector independent. The DNA is introduced in this process contains directly inserted into the host the gene of interest and a cassette genome. containing additional genetic material. Additional genetic material includes INDIRECT METHODS: Agrobacterium can only infect plants Agrobacterium-mediated plant through wounds. When a plant root transformation or stem is wounded it gives off (produce) certain chemical signals. A. tumefaciens are soil bacteria that In response to these signals, have the ability to infect plant cells agrobacterial vir genes become and transfer a defined sequence of activated and direct a series of their DNA to the plant cell in the events necessary for the transfer of infection process. Upon integration the T-DNA from the Ti plasmid to the of the bacterial DNA into a plant plant cell through the wound. chromosome, it directs the synthesis To harness A. tumefaciens and the Ti- of several proteins, using the plant plasmid as a transgene vector, the cellular machinery, that ensure the tumor inducing section of T-DNA is proliferation of the bacterial removed, while the T-DNA border population within the infected plant, regions and the vir genes are Agrobacterium infections result in retained. The desired transgene is crown gall disease (Figure 2). inserted between the T-DNA border In addition to its chromosomal regions, applying recombinant DNA genomic DNA, an A. tumefaciens cell technology. Thus, in the infection contains a plasmid known as the Ti process, the transgene DNA is (tumour-inducing) plasmid. The Ti transferred to the plant cell and plasmid contains a series of vir integrated into the plant’s (virulence) genes that direct the chromosomes. To achieve infection process, and a stretch of transformation, Agrobacterium cells DNA termed T-DNA (transfer DNA), carrying an appropriately approximately 20 kb in length, that constituted Ti plasmid vector is transferred to the plant cell in the containing the desired transgene can infection process. The T-DNA be inoculated into plant stems, leaf encodes proteins required for the disks etc., to allow infection and T- maintenance of infection. These DNA transfer to the plant cells. The proteins include certain plant explants that have been co- hormones that stimulate cell growth, cultivated with Agrobacterium are resulting in the formation of galls, subsequently processed through and proteins required for a certain various tissue culture steps resulting metabolic pathway that secures the in the selection and production of availability of nutrients for the transformed cells and plants. bacteria (Figure 3). Figure 1| Depending on species, plant tissues are extremely pliable and can be easily interconverted and regenerated in culture. This diagram shows some of the ways in which this flexibility can be exploited to facilitate gene transfer and the creation of transgenic plants. Figure 2| Steps involved in the generation of genetically transformed plants using either the Agrobacterium tumefaciens or micro-projectile bombardment approaches (Following introduction of foreign DNA into the plant cell, successfully transformed cells are selected and used to regenerate a transgenic plant.)
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