Plant genetic transformation and transgenic crops: Methods and applications

Abstract

The combined use of recombinant DNA technology, gene transfer methods, and tissue culture techniques has led to the efficient transformation and production of transgenics in a wide variety of crop plants. In fact, transgenesis has emerged as an additional tool to carry out single-gene breeding or transgenic breeding of crops. Unlike conventional breeding, only the cloned gene(s) of agronomic importance is/are being introduced without cotransfer of undesirable genes from the donor. The recipient genotype is least disturbed, which eliminates the need for repeated backcrosses. Above all, the transformation methods provide access to a large gene pool, as the gene(s) may come from viruses, bacteria, fungi, insects, animals, human beings, unrelated plants, and even from chemical synthesis in the laboratory. Various gene transfer methods such as Agrobacterium, physicochemical uptake of DNA, liposome encapsulation, electroporation of protoplasts, microinjection, DNA injection into intact plants, incubation of seeds with DNA, pollen tube pathway, use of laser microbeam, electroporation into tissues/embryos, silicon carbide fiber method, particle bombardment, and “in planta” transformation have been developed. Among these, Agrobacterium and “particle gun” methods are being widely used. Recently RNAi and CRISPR/Cas9 systems have further expanded the scope for genome engineering. Using different gene transfer methods and strategies, transgenics carrying useful agronomic traits have been developed and released. Attempts are being made to develop transgenic varieties resistant to abiotic stresses, such as drought, low and high temperature, salts, and heavy metals, and also to develop transgenic varieties possessing better nutrient-use efficiency and better keeping and nutritional and processing qualities. Genetically modified foods, such as tomato containing high lycopene, tomato with high flavonols as antioxidants, edible vaccines, are leading examples of genetically engineered crops. Several genes of agronomic importance have been isolated from various organisms; cloned and suitable constructs have been developed for plant transformation. Agrobacterium and “particle gun” methods have been refined and now being used for genetic transformation of a wide variety of field, fruit, vegetable, forest crops, and ornamental plant species. Transgenic crops such as cotton, maize, papaya, potato, rice, soybean, and tomato, carrying mainly insect resistance, herbicide resistance, or both, are now being grown over an area of 185 million hectares spread over 28 countries of the world.