Ti-plasmid

Fig. 2. Schematic representation of the Ti-plasmid of Agrobacterium, showing the genes involved in transformation of host plants. Note that the genetic map is not to scale in reality, the T-region makes up about 10% of the total plasmid genome (from Bryant, 1988).

Fig. 3. One of the experimental systems in common use for genetic engineering of plants with Agrobacterium containing a recombinant Ti-plasmid (from Bryant, 1988).

Leaf discs are floated on a suspension of Agrobacterium ceWs that contain genetically engineered Ti-plasmids. The bacterium infects cells around the edges of the discs, and the T-DNA is transferred to the chromosomes of the infected cells. 

Under natural conditions. Agrobacterium does not appear to infect monocotyledonous plants. Genetic manipulation of cereal crops using a Ti-plasmid delivered by Agrobacterium therefore seems out of the question. However, there are some data to suggest that Agrobacterium can, under certain circumstances, infect monocots, but that it does not cause... 

Hooykaas-van Slogteren, G.M.S., Hooykaas, P.J.J. Schilperoort, R.A. (1984). Expression of Ti plasmid genes in monocotyledonous plants infected with Agrobacterium tumefaciens. Nature, 311, 763-4. 

Herrera-Estrella L, Depicker M, Van Montagu M, Schell J (1983) Expression of chimeric genes transferred into cells using a Ti-plasmid-derived vector. Nature 303 209-213... 

A. M. Ashby, M. D. Watson, and C. H. Shaw, A Ti-plasmid determined function is responsible for cheinotaxis of Agrobacterium tumefaciens towards the plant wound product acetosyringone, FEMS Microbiology Letters 47 189 (1987). 

Plasmid A relatively (cf. the chromosome) small (>20 Kb), usually circular, double-stranded DNA molecule found in prokaryotes capable of replicating independently of the chromosome. Plasmids carry genes which are usually not essential for the growth of the organism except under special conditions. Some plasmids carry genes for antibiotic resistance. See also Ti-plasmid. Some plasmids however can be very large, e.g. the plasmids in Rhizobium species. 

Morris, P.F., Bone, E., and Tyler, B.M., Chemotropic and contact responses of Phytophthora sojae hyphae to soybean isoflavonoids and artificial substrates. Plant Physiol, 117, 1171, 1998. Zerback, R., Dressier, K., and Hess, D., Flavonoid compounds from pollen and stigma of Petunia hybrida inducers of the vir region of the Agrobacterium tumefaciens Ti plasmid. Plant Scl, 62, 83, 1989. 

Cloning in plants, which makes use of the Ti plasmid vector from Agrobacterium, allows the introduction of new plant traits. 

Physical methods of gene transfer. Genes can often be transferred without the use of a cloning vehicle. This is especially important for certain plant cells, such as those of cereal grains, for which transfer of genes via the Ti plasmid has been difficult.167 If DNA, which may be in a plasmid, is coprecipitated with calcium phosphate, it can often be taken up directly either by animal cells or by plant protoplasts.168 169 Polycations also facilitate DNA uptake cationic liposomes seem to be especially effective.170 In the widely used electroporation technique a short electrical pulse of a few hundred volts / cm is applied to create transient pores in the plasma membrane through which the DNA can enter a cell.111 8,171 175 Chromosomes can be transferred by cell fusion and either... 

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