INTERACTION OF AGROBACTERIUM AND ITS GENE PRODUCTS WITH ITS HOST PLANTS
E NESTER, W DENG, S SEKIGUCHI, L CHEN, W-T PENG, D WOOD and MP GORDON
Department of Microbiology, University of Washington, Seattle, WA 98195, USA
Agrobacterium induces crown gall tumours and transforms plants by transferring a fragment (T-DNA) of its tumour-inducing (Ti) plasmid into plant cells. The process is initiated when plant signal molecules, which include phenolic compounds and monosaccharides in an acidic environment, are synthesized by a wounded plant. Agrobacterium cells in the environment of the wound site chemotax towards the monosaccharides and appear to attach to plant cells by means of an acetylated polysaccharide on the bacterial cell surface. The plant signal molecules are then recognized by a sensor protein on the inner membrane of the bacteria, a member of a two-component regulatory system. This signal is then transduced to a second member of the two-component system, which then activates a set of genes (vir) which are required for the processing and transfer of the T-DNA.
A number of the vir gene products have now been identified. These include an endonuclease which is responsible for the formation of the single-stranded T-DNA. Once the endonuclease has cleaved the DNA, it becomes covalently bound to the 5' end of the T strand, thereby forming a nucleoprotein T complex, the molecule that is apparently transferred into the plant cell. Other vir gene products include a single-stranded DNA binding protein, the product of virE2, which interacts with the T complex; a chaperone-like molecule, the product of vire1, which interacts with VirE2 and is required for the transfer VirE2 out of Agrobacterium; and a transport pore which includes vir-specific pili, which are required for the transfer of the T complex into the plant cell. Once it enters the plant cell, the T complex interacts with a number of different plant proteins. These include a plant-encoded cylophilin, as well as other molecules involved in the transport of the T complex into the plant cell nucleus and its subsequent integration into the plant cell chromosome.