Physical map of chloroplast DNA of the onion Allium cepa L., showing the location of photosynthesis-related genes.

To compare the gene order of chloroplast genome among monocotyledonous plants, we constructed a physical map of chloroplast DNA (cpDNA) of onion (Allium cepa L. cv. Shounan red, 2n=16) with four restriction endonucleases, PstI, SalI, XhoI, and SmaI. The cpDNA of Allium cepa was found to be a circular molecule with a total size of ca. 155 kb and containing two inverted repeats of 26 kb each that disrupt the rest of the molecule into small (ca. 16 kb) and large (ca. 86 kb) single copy regions. The endonuclease recognition sites of the physical map were confirmed by Southern hybridizations of total onion cpDNA with homologous and heterologous probes. 16 genes were localized on the physical map of the onion cpDNA. Genome structure of onion cpDNA was similar in terms of genome size and gene order of cpDNA to that of tobacco cpDNA, differing from the cpDNA structure of gramineous plants.