Crop improvement depends on genetic variation, which may arise through (1) sexual crosses between closely related plants of the same species and the subsequent segregation and recombination among their progenies, which provide the raw material for artificial selection; (2) chemical and physical mutagenesis that randomly induces hereditary changes, desirable and undesirable, at a low frequency; and (3) wide crosses between the crop species and its distantly related relatives or an unrelated organism.

Wide crosses can be accomplished by (1) sexual means or direct crossing where fresh pollen grains from one species are placed on the stigmatic surface of the other, often ollowed by application of plant growth regulators, such as gibberellic acid, naphthalene acetic acid, 2,4-dichlorophenoxy acetic acid, or others, to stimulate and induce the pollen to germinate and grow down the style; (2) parasexual means or fusion of the protoplasts of the species involved followed by dedifferentiation for callus formation and redifferentiation for shoot and root development; and (3) asexual means or incorporating genes encoding proteins using microprojectile bombardment and the soil bacterium. Agrobacterium, by which alien genes from any organism can be incorporated into the genomes of plants from a related or unrelated species.

Production of transgenie plants using microprojectile bombardment and Agrobacterium is a powerful means of providing new and novel genes to crop species with minimal disturbance of the genetic background of the host plants. Commercially available Bt-maize. Bt-cotton, and Bt-potato are just a few of the examples of crops in which a gene encoding a toxin from the bacterium Bacillus thuringiensis (Bt) has been incorporated to defend them against pests, with no effect on human and livestock. Herbicide-resistant soybean is another example of a genetically engineered crop currently on the market.

Indeed, the advent of molecular genetics and the development of novel technologies have been revolutionizing the crop improvement system and have produced fruitful results, providing benefits to seed industries, fanners, and all humankind. However,  novel techniques must be used with caution, because unlimited and unregulated gene transfer between species may lead to unknown but profound ecological impacts; concerns for food safety (such as allergenic substances in flour); and unintended spread of resistance to herbicides and insecticides among pests (weeds, insects, microbials, or viruses).