ir:;.;i;iiL
i r.1
.j An
Agrobacteium
carrying a
Ti
plasmid
of
the nopatine
type
induces
a teratoma,
jn
whjch
djfferen-
t'iated structures
devetop. Photo
courtesy of the estate of
Jeff Schett.
Used with
oermission of the
Max P[anck Insti-
tute for Ptant
Breeding Research,
Cologne.
i-ii:l-:{i
r i=-.:.i;
Ti
ptasmids
carry
genes
involved in
both
p[ant
and
bacterial
functions.
Ti
plasmids (and
thus tlire Agrobacteria
in
which
they reside)
can be
divided into four
groups,
according
to the types
of opine that are
made:
.
Nopaline
plasmids
carry
genes
for
syn-
thesizing nopaline
in
tumors and for
utilizing it in
bacteria.
Nopaline tumors
can
differentiate into
shoots
with abnor-
mal
structures. They
have
been called
teratomas
by analogy
with certain
mammalian
tumors
that retain the
abil-
ity
to differentiate
into
early embryonic
structures.
.
Octopine
plasmids
are similar
to
nopa-
line
plasmids,
but the relevant
opine is
different.
Octopine
tumors are
usually
CHAPTER
1.6 Extrachromosomal
Reoticons
undifferentiated, however,
and do
not
form
teratoma shoots.
.
Agropine
plasmids
carry
genes
for
agropine metabolism; the tumors
do not
differentiate, and they develop
poorly
and die early.
.
Ri
plasmids
can
induce
hairy root dis-
ease on some
plants
and crown
gall
on
others. They have agropine
type
genes,
and may
have
segments derived from
both nopaline and octopine
plasmids.
The
types of
genes
carried by a Ti
plasmid
are summarized in
fl'i*lifit t*.i'i.
Genes utilized
in
the bacterium code
for
plasmid
replication
and incompatibility, transfer
between bacteria,
sensitivity to
phages,
and synthesis
of other
compounds, some of which are
toxic to other
soil bacteria. Genes used
in
the
plant
cell code
for
transfer of
DNA into
the
plant,
induction
of
the transformed state, and shoot
and root
induction.
The specificity of the opine
genes
depends
on the type of
plasmid.
Genes needed for
opine
synthesis are linked to
genes
whose
products
catabolize the same opine; thus each strain of
Agrobacterium
causes crown
gall
tumor
cells to
synthesize opines that are useful for
survival of
the
parasite.
The
opines can be used
as the sole
carbon and/or nitrogen
source for the induc-
ing Agrobacterium strain The
principle
is that
the transformed
plant
cell synthesizes those
opines that the bacterium can use.
T-DNA
Carries Genes
Required
for Infection
.
Part
of the
DNA
of the
Ti
plasmid
is
transferred to
the olant ce[[ nucleus.
r
The vrr
genes
of the
Ti
ptasmid
are
located outside
the
transferred
region
and are
required
for
the
transfer
process.
o
The vir
genes
are induced by
phenotic
compounds
reteased
by
plants
in response
to
wounding.
o
The membrane
protein
VirA is autophosphorytated
on
histidine
when it binds an inducer.
r
VirA activates VirG
by transferring the
phosphate
group
to it.
r
The VirA-VirG is one of
several bacterial two
component systems that
use a
phosphohistidine
reray.
The interaction
between Agrobacterium
and
a
plant
cell is illustrated
in
E{#liSf;
i*"1h.
The
bac-
terium does not
enter the
plant
cell.
but rather
transfers
part
of the Ti
plasmid
to the
plant
Locus Function
Ti
Plasmid
vtr
snt
rot
nos
noc
ocs
occ
DNA
transfer into
plant
all
shoot
induction
all
root
induction
all
nopaline
synthesis
nopaline
nopaline
catabolism
nopaline
octopine
synthesis
octopine
octopine
catabolism
octopine
tra
bacterial transfer
genes
all
lnc incompatibility
genes
all
oriV
origin for replication
all
402