Plant Breeding

Plant breeding began when early humans saved seeds and planted them. The cultural change from living as nomadic hunter-gatherers, to living in more settled communities, depended on the ability to cultivate plants for food. Present knowledge indicates that this transition occurred in several different parts of the world, about 10,000 years ago.

Today, there are literally thousands of different cultivated varieties (cultivars) of individual species of crop plants. As examples, there are more than 4,000 different peas (Pisum sativum), and more than 5,000 grape cultivars, adapted to a wide variety of soils and climates.

The methods by which this diversity of crops was achieved were little changed for many centuries, basically requiring observation, selection, and cultivation. However, for the past three centuries most new varieties have been generated by deliberate cross-pollination, followed by observation and further selection. The science of genetics has provided a great deal of information to guide breeding possibilities and directions. Most recently, the potential for plant breeding has advanced significantly, with the advent of methods for the incorporation of genes from other organisms into plants via recombinant DNA-techniques. This capacity is broadly termed "genetic engineering." These new techniques and their implications have given rise to commercial and ethical controversies about "ownership," which have not yet been resolved.

Somaclonal variation

Replicate plant cells or protoplasts that are placed under identical conditions of tissue culture do not always grow and differentiate to produce identical progeny (clones). Frequently, the genetic material becomes destabilized and reorganized, so that previously-concealed characters are expressed. In this way, the tissue-culture process has been used to develop varieties of sugar cane, maize, rapeseed, alfalfa, and tomato that are resistant to the toxins produced by a range of parasitic fungi. This process can be used repeatedly to generate plants with multiple disease resistance, combined with other desirable characters.

Vectors for gene transfer

Agrobacterium tumefaciens and A. rhizogenes are soil bacteria that infect plant roots, causing crown gall or "hairy roots" diseases. Advantage has been taken of the natural ability of Agrobacterium to transfer plasmid DNA into the nuclei of susceptible plant cells. Agrobacterium cells with a genetically-modified plasmid, containing a gene for the desired trait and a marker gene, usually conferring antibiotic resistance, are incubated with protoplasts or small pieces of plant tissue. Plant cells that have been transformed by the plasmid can be selected on media containing the antibiotic, and then cultured to generate new, transgenic plants.

Many plant species have been transformed by this procedure, which is most useful for dicotyledonous plants. The gene encoding Bt, as well as genes conferring resistance to viral diseases, have been introduced into plants by this method.

Direct gene transfer

Two methods have been developed for direct gene transfer into plant cells—electroporation and biolistics. Electroporation involves the use of high-voltage electric pulses to induce pore formation in the membranes of plant protoplasts. Pieces of DNA may enter through these temporary pores, and sometimes protoplasts will be transformed as the new DNA is stably incorporated (i.e., able to be transmitted in mitotic cell divisions). New plants are then derived from cultured protoplasts. This method has proven valuable for maize, rice, and sugar cane, species that are outside the host range for vector transfer by Agrobacterium.

Biolistics refers to the bombardment of plant tissues with microprojectiles of tungsten coated with the DNA intended for transfer. Surprisingly, this works. The size of the particles and the entry velocity must be optimized for each tissue, but avoiding the need to isolate protoplasts increases the potential for regenerating transformed plants. Species that cannot yet be regenerated from protoplasts are clear candidates for transformation by this method.

Genetically-modified plants

In 1992, a tomato with delayed ripening became the first genetically-modified (GM) commercial food crop. More than 40 different GM crops are now being grown commercially. GM corn and cotton contain bacterial genes that kill insects and confer herbicide-resistance on the crops. GM squash contains viral genes that confer resistance to viruses. Potatoes carry the Bt gene to kill the Colorado potato beetle and a viral gene that protects the potato from a virus spread by aphids. Mauve-colored carnations carry a petunia gene required for making blue pigment. In many cases, GM crops result in increased yields and reduced use of pesticides. New research is focused on producing GM foods containing increased vitamins and human or animal vaccines.

GM crops are very controversial. There is concern that the widespread dissemination of the Bt gene will cause insects to become resistant. It has been reported that pollen from Bt corn is toxic to the caterpillars of monarch butterflies. It also is possible that GM crops will interbreed with wild plants, resulting in "superweeds" resistant to herbicides. There is also concern that the antibiotic-resistance genes, used as markers for gene transfer, may be passed from the plants to soil microorganisms or bacteria in humans who eat the food. Finally, the possibility of allergic reactions to the new compounds in food exists. Many countries have banned the production and importation of GM crops.