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For more than two decades, scientists have used Agrobacterium-mediated genetic transformation to generate transgenic plants. Initial technologies to introduce genes of interest (goi) into Agrobacterium involved complex microbial genetic methodologies that inserted these goi into the transfer DNA (T-DNA) region of large tumor-inducing plasmids (Ti-plasmids). However, scientists eventually learned that T-DNA transfer could still be effected if the T-DNA region and the virulence (vir) genes required for T-DNA processing and transfer were split into two replicons. This binary system permitted facile manipulation of Agrobacterium and opened up the field of plant genetic engineering to numerous laboratories. In this review, we recount the history of development of T-DNA binary vector systems, and we describe important components of these systems. Some of these considerations were previously described in a review by Hellens et al. (2000b).
Agrobacterium transfers T-DNA, which makes up a small (approximately 5%-10%) region of a resident Ti-plasmid or root-inducing plasmid (Ri-plasmid), to numerous species of plants (DeCleene and DeLey, 1976; Anderson and Moore, 1979), although the bacterium can be manipulated in the laboratory to transfer T-DNA to fungal (Bundock et al., 1995; Piers et al., 1996; de Groot et al., 1998; Abuodeh et al., 2000; Kelly and Kado, 2002; Li et al., 2007) and even animal cells (Kunik et al., 2001; Bulgakov et al., 2006). Transfer requires three major elements: (1) T-DNA border repeat sequences (25 bp) that flank the T-DNA in direct orientation and delineate the region that will be processed from the Ti/Ri-plasmid (Yadav et al., 1982); (2) vir genes located on the Ti/Ri-plasmid; and (3) various genes (chromosomal virulence [chv] and other genes) located on the bacterial chromosomes. These chromosomal genes generally are involved in bacterial exopolysaccharide synthesis, maturation, and secretion (e.g. Douglas et al., 1985; Cangelosi et al., 1987, 1989; Robertson et al., 1988; Matthysse, 1995; O'Connell and Handelsman, 1999). However, some chromosomal genes important for virulence likely mediate the bacterial response to the environment (Xu and Pan, 2000; Saenkham et al., 2007). Several recent reviews enumerate factors involved in and influencing Agrobacteriummediated transformation (Gelvin, 2003; McCullen and Binns, 2006).
The vir region consists of approximately 10 operons (depending upon the Ti- or Ri-plasmid) that serve four major functions.
(1) Sensing plant phenolic compounds and transducing this signal to...