ABSTRACT

The nuclear genome of a plant includes all the deoxyribonucleic acid (DNA) found in the nucleus of each cell and all the genes encoded by this DNA. The genome also comprises the cytological and biochemical structures that regulate the expression of these genes and ensure the stable inheritance of the DNA, in the form of chromosomes, from generation to generation. Plants also have two organeller genomes, one in the mitochondrion and one in the chloroplast, which are believed to be the degenerate genomes of ancient prokaryotic symbiotes. It appears that genes have migrated from the organellar genomes into the nuclear genome during millions of years of evolution, leaving the organellar genomes completely incapable of supporting a free living prokaryotic organism. Much of the DNA of all three genomes is organized into genes that encode either proteins or biochemically active ribonucleic acid (RNA) molecules such as transfer RNA and ribosomal RNA. The protein encoding regions of genes are flanked by regulatory elements that control the temporal and spatial expression of these genes. The programming that defines the architecture and the biochemical and physiological processes of a plant is contained within these genes and their associated regulatory elements, but the expression of this programming is subject to modification as a response to environmental factors.