Plant genetic engineering involves the intentional introduction of foreign DNA into the plant genome. The two most common transformation methods are Agrobacterium-mediated transformation and biolistic particle delivery.
Agrobacterium tumefaciens is a soil bacterium that naturally transfers a segment of its Ti plasmid (T-DNA) into plant cells. The T-DNA is flanked by left and right border sequences recognized by the bacterial Vir protein complex. In binary vector systems, the T-DNA carrying the gene of interest and a selectable marker is cloned in a small plasmid, while the Vir proteins are provided in trans by a helper plasmid in the same Agrobacterium strain.
Agrobacterium-mediated transformation is the method of choice for dicotyledonous plants and many monocots. Leaf disc, cocultivation, and floral dip methods expose plant tissues to Agrobacterium. The floral dip method,broadly used for Arabidopsis, involves dipping developing flowers into an Agrobacterium suspension, transforming the female gametophyte.
Biolistic transformation (gene gun) accelerates DNA-coated microprojectiles into plant cells using compressed helium. Particles penetrate the cell wall and membrane, delivering DNA directly into the nucleus or chloroplast. This method is effective for plants recalcitrant to Agrobacterium, including cereals like wheat and maize, and for chloroplast transformation.
Selection of transformed cells uses marker genes conferring resistance to antibiotics or herbicides. Common selectable markers include nptII (kanamycin resistance), hpt (hygromycin resistance), and bar (phosphinothricin resistance). Reporter genes such as gus (β-glucuronidase) and gfp (green fluorescent protein) enable visual identification of transformed tissues.
Selectable marker genes can be removed in subsequent generations using site-specific recombination systems such as Cre-lox or FLP-FRT. Marker-free transformation produces plants without antibiotic resistance genes, addressing regulatory and public acceptance concerns.
Applications include herbicide-resistant crops (Roundup Ready), insect-resistant Bt crops, improved nutritional content (Golden Rice), drought tolerance, and production of pharmaceutical proteins in plants. Genome editing using CRISPR-Cas9 is increasingly replacing traditional transgenic approaches.
