The Ti plasmid

gene can be spliced to the virus vector. Also, the added gene affects the cell-to-cell movement (invasiveness) of the virus, since the virus can no longer mature. Moreover, the virus is mainly limited to Cruciferae as host plants and therefore the prospects of infecting other major crops are narrow. Nevertheless, in-depth studies on this virus will help provide important information on means to overcome these natural obstacles. Another promising gene vector is the Ti plasmid harbored in the crown gall bacterium Agrobacterium tumefaciens. Ti plasmids are very large, autonomously replicating, circular DNA molecules about 30 times larger than the DNA of cauliflower mosaic virus. This plasmid carries genes that cause crown gall tumors in many plants, representing more than 90 plant families. Although the mechanism by which these tumors occur is not well understood, A . tumefaciens naturally introduces the Ti plasmid DNA into plant cells during infection. A portion of this plasmid, known as the T-DNA, which carries genes for synthesis of unusual amino acids (opines) and phosphorylated sugars, is incorporated into the nuclear DNA of the plant cell. Thus, during infection of a wound, the crown gall bacterium “genetically colonizes” the plant by converting normal cells into tumor cells that are directed to produce the opines and phosphorylated sugars, which are assimilated by the infecting bacterium. With the knowledge that A . tumefaciens inserts a piece of its genetic material into plants, Davis plant pathologists and other workers have isolated the portion of the Ti plasmid that is transferred into plant cells. Foreign genes that confer resistance to certain antibiotics, such as methotrixate and chloramphenicol, and that direct the synthesis of seed proteins have now been inserted in the T-DNA on the Ti plasmid. A . tumefaciens cells carrying this hybrid plasmid are allowed to insert the chimeric T-DNA into tobacco, petunia, cowpea, and sunflower cells by natural infection. Although such genes have been transferred to plants, these genes are not stably maintained, particularly during meiosis (chromosome reduction, division, and segregation); however, the genes seem to be more stable in crop and ornamental plants that are vegetatively propagated. As with cauliflower mosaic virus, there are several limiting factors. The Ti plasmid carries genes that cause tumors in plants, and it will need to be “disarmed.” Once disarmed, an efficient means of selecting transformed cells will have to be developed. Also, most plants susceptible to crown gall have not been regenerated successfully from cell culture, an essential step in the development of useful plants. Finally, stability of the T-DNA in transformed plants needs to be enhanced. Other potential gene vectors being studied are transposition elements and organelle DNA. Well-defined segments of DNA that can insert themselves in random locations along a chromosome are known as transposition elements (or “transposons”). Organelles such as chloroplasts and mitochondria carry DNA with characteristics much like plasmids. The most promising aspect of these studies is that of harnessing existing plant DNAs as vehicles of genes that can be attached to them. In all of these examples of gene vectors, the pressing problem has been to find a suitable gene capable of confemng the desirable trait(s) on a plant. To solve this problem further studies are required on gene maintenance in plant cells. Two of the promising gene vectors are reported in the articles that follow.