The identification of numerous mutations affecting plant morphology has allowed the construction of genetic linkage maps for all major cultivated species. These maps are constantly being refined. They serve as a guide to the physical location of individual genes on chromosomes.
DNA sequencing of plant genomes has shown that gene expression is controlled by distinct "promoter" regions of DNA. It is now possible to position genes under the control of a desired promoter, to ensure that the genes are expressed in the appropriate tissues. For example, the gene for a bacterial toxin (Bt) (from Bacillus thuringiensis) that kills insect larvae might be placed next to a leaf-development promoter sequence, so that the toxin will be synthesized in any developing leaf. Although the toxin might account for only a small proportion of the total protein produced in a leaf, it is capable of killing larvae that eat the genetically-modified leaves.
Science EncyclopediaScience & Philosophy: Planck mass to PositPlant Breeding - Early Selection, Seed Dormancy, Quality, Climatic Adaptation, Pollination And Hybridization, The Impact Of Hybridization On Plant Breeding In The United States