The Agrobacterium-plant cell interaction. Taking biology lessons from a bug.

Agrobacterium elicits neoplastic growths (called crown gall tumors) that affect most dicotyledonous plants. Moreover, although plants represent the natural hosts for Agrobacterium, this microorganism can also genetically transform a wide range of other eukaryotic species, from yeast (Bundock et al., 1995; Piers et al., 1996; Sawasaki et al., 1998) to mushrooms (de Groot et al., 1998; Chen et al., 2000) and filamentous fungi (de Groot et al., 1998; Gouka et al., 1999) to phytopathogenic fungi (Rho et al., 2001; Rolland et al., 2003) to human cells (Kunik et al., 2001). Most functions for Agrobacterium-host cell DNA transfer are coded by a large (200-kb) tumor-inducing (Ti) plasmid that resides in the bacterial cell and carries two important genetic components: the vir (virulence) region and the T-DNA delimited by two 25-bp direct repeats at its ends, termed the T-DNA borders (for review, see Citovsky et al., 1992a; Zupan et al., 2000; Gelvin, 2003). The vir region comprises seven major loci, virA, virB, virC, virD, virE, virG, and virH, which encode most of the bacterial protein machinery (Vir proteins) of the DNA transport. After induction of vir gene expression by small phenolic signal molecules secreted from wounded susceptible plant cells (Stachel et al., 1985), the T-DNA borders are nicked by the bacterial VirD2 endonuclease (Wang et al., 1987), generating a transferable single-stranded (ss) copy of the bottom strand of the T-DNA region, designated the T strand (Stachel et al., 1986). Interestingly, the T strand does not travel alone but is thought to directly associate with two Agrobacterium proteins, VirD2 and VirE2, forming a transport (T) complex (Zupan and Zambryski, 1997) in which one molecule of VirD2 is covalently attached to the 5 -end of the T strand, whereas VirE2, an ssDNAbinding protein, is presumed to cooperatively coat the rest of the T strand molecule (for review, see Zupan and Zambryski, 1997; Tzfira et al., 2000; Zupan et al., 2000). Although only the wild-type T-DNA carries Ti genes, any DNA placed between the T-DNA borders will be transferred to the plant host (for review, see Zambryski, 1992; Sheng and Citovsky, 1996). This lack of sequence specificity implies that a T-DNA molecule itself does not encode protein machinery for its transport from the bacterial cell into the host cell, import into the host cell nucleus, and integration into the host cell genome. Instead, these functions are fulfilled by the bacterial Vir proteins and their host cell partners (for review, see Gelvin, 2000; Tzfira et al., 2000). In the last quarter of a century, since the discovery of the stable integration of the bacterial DNA in crown gall tumors (Chilton et al., 1977), Agrobacterium has served as a primary a tool for plant genetic engineering. Furthermore, the Agrobacterium-host cell interaction also represents a unique and powerful experimental system to study a wide spectrum of basic biological processes such as cell-cell recognition and cell-to-cell transport, nuclear import, assembly and disassembly of protein-DNA complexes, DNA recombination, and regulation of gene expression. This special Focus Issue reviews the use of Agrobacterium as a gene vector for plants and reports new insights into the processes of cell-cell recognition and attachment, intercellular transport, DNA integration, and transgene expression gained from the Agrobacterium research.