many thorough reviews on the mechanism ofAgrobacteriumT-DNA transfer (Binns and
Thomashow 1988; Zambryski 1992; Tzfira and Citovsky 2006), only the basic features
as relating to transformation are presented here (Fig. 10.4). To start, the plasmid that is
used as a vector forAgrobacterium-mediated transformation has been whittled down to
contain only the essential components. Agrobacterium vectors are called “binary
vectors” because they are the second of two plasmids that are involved in the overall
process. Many of the transfer functions are retained on a modified Ti plasmid with the
T-DNA removed. The second binary vector contains the T-DNA, but the hormone and
opine biosynthesis genes have all been deleted. What is left on the binary vector, aside
from the components that allow the plasmid to be retained in the bacterium, are the left
and right “borders” of the T-DNA region. Genes of interest are cloned between the
borders, which are recognition sequences for the T-DNA processing machinery. The
genes for the T-DNA processing machinery are still located primarily on the modified Ti
plasmid where they direct T-DNA processing on the binary plasmid. After
Agrobacteriumis inoculated on the appropriate plant tissue, the bacteria may recognize
the target tissue as a suitable host; remember that this bacterium is a pathogen that
infects plant tissue. Chemical signals are put out by both the plant tissue and the bacteria.
Wounded plant tissues from appropriate plant tissues produce acetosyringone, which acti-
vates the bacterialvirulence(vir) genes, which initiates the T-DNA transfer machinery. Not
all wounded plant tissues produce acetosyringone, and the lack or poor production of acet-
osyringone by monocot cells originally made it difficult to impossible to produce transgenic
monocots usingAgrobacterium. Addition of synthetic acetosyringone to the inoculated
plant tissues allowsAgrobacterium-mediated transformation of monocots to proceed and
tremendously enhances transformation of other moderately susceptible target plants.
Once the bacteria infect plant tissue, most plants will respond by trying to fight off the inva-
sion by either producing antipathogenic compounds or sacrificing cells adjacent to the
Figure 10.4.Schematic ofAgrobacterium-mediated transformation of a plant cell, showing pro-
duction of the T strand from the binary vector, transport through the bacterial pillus, and integration
into plant chromosomal DNA. See color insert.
252 TRANSGENIC PLANT PRODUCTION