From Wikipedia,
the free encyclopedia.
Agrobacterium is a
genus of
bacteria that cause
tumors in plants.
Agrobacterium tumefaciens
is the most commonly studied
species in this genus.
Agrobacterium is well known
for its ability to transfer
DNA between itself and plants,
and for this reason it has become
an important tool for
plant improvement by
genetic engineering.
Taxonomic note: The
Agrobacterium genus is quite
heterogeneous, recent
taxonomic studies have
reclassified all of the
Agrobacterium species in to
new genera, such as
Ruegeria,
Pseudorhodobacter and
Stappia. But most species
have been reclassified as
Rhizobium species.
Agrobacterium as a plant
pathogen
The large growths on
these roots are galls are
induced by Agrobacterium
sp.
A. tumefaciens causes
crown-gall disease in plants, the
disease is characterised by a
tumour like growth on the
infected plant at the junction
between the root and the shoot.
Tumors are induced by the transfer
of a transferred DNA (T-DNA)
segment form the bacterial tumour-inducing
(Ti)
plasmid A. rhizogenes
induces root tumors, its plasmid
is call Ri (root-inducing).
The plasmid T-DNA is
incorporated into the
genome of the host cell by
homologous
recombination, and the
virulence (vir) genes on the T-DNA
are expressed, causing the gall to
form. The T-DNA carries genes for
the biosynthetic enzymes for the
production of unusual
amino acids octapine and
nopaline. It also carries genes
for the biosyntheis of
plant hormones
auxin and
cytokinins. By altering the
hormone balance in the plant cell,
the division of those cells cannot
be controlled by the plant, and
tumors form.
Agrobacterium in biotechnology
The ability of Agrobacterium
to transfer
genes to
plants has been exploited for
genetic engineering for
plant improvement. A modified
Ti or Ri plasmid can be used. The
plasmid is 'disarmed' by deletion
of the tumor inducing genes, the
only essential parts of the T-DNA
are its two small (25 base pair)
border repeats, at least one of
which is needed for plant
transformation. A team of
researchers led by Dr
Mary-Dell Chilton were the
first to demonstrate that the
virulence genes could be removed
without adversely affecting the
ability of Agrobacterium to
insert its own DNA into the plant
genome.
The genes to be introduced into
the plant are cloned into the
plasmid, the plasmid will also
contain a selectable marker, like
an antibiotic resistance to enable
selection for plants that have
been successfully transformed.
Plants are grown on media
containing antibiotic following
transformation, those that have
not had the plasmid DNA integrated
into their genome will die.
Transformation with
Agrobacterium can be achieved
in two ways.
Protoplasts, or leaf-discs can
be incubated with the
Agrobacterium and whole plants
regenerated using
plant tissue culture. A common
transformation protocol for
Arabidopsis is the
floral-dip method, the flowers are
dipped in a culture of a
Agrobacterium, the bacterium
transforms the
germline cells that make the
female
gametes. The
seeds can then be screened for
antibiotic resistance, plants that
have not integrated the plasmid
DNA will die.
Agrobacterium cannot
infect all types of plant, there
are many other techniques for
transformation including the
gene gun.
