AGROBATERIUM TUMEFACIENS
It is Rod-shaped Gram-negative bacterium
that belongs to the family rhizobiaceae which transforms normal plant cells
into tumor cells. It causes disease in dicots. Agrobactrium tumefaciens
contains Ti (tumor-inducing) plasmid: 200kb; contains T-DNA. It is inter-kingdom
genetic transfer, Ti plasmid is essential to cause disease. During infection,
Ti plasmid is transferred to host plant cells. A small segment of Ti plasmid
called T-DNA gets integrated into the host chromosomal DNA. VIR genes on Ti
plasmid are necessary for tumor formation.
The plant cells that are infected by
Agrobacterium tumefaciens and have T-DNA incorporated into their genome
are referred to as transformed. Transformed cells are able to produce opines
(unusual amino-acid derivatives such as octopine and nopaline) that are source
of carbon and nitrogen for infecting bacteria. Tumors induced by A.
tumefaciens contain high concentrations of auxin and cytokinin. The T-DNA
is the transferred portion of Ti plasmid. This DNA transferred into the host
plant’s nuclear DNA genome. T-DNA about 24000 bp long. T-DNA bordered by 25bp
repeats: Left border, right border. It contains genes that code for opine synthesis
and phytohormone synthesis.
Two genes auxA and auxB encode
proteins involved in the production of IAA. The transfer of T-DNA is by :
Signal
recognition by Agrobacterium:
The bacterium perceives signals, such as phenolics and sugars, which are
released from wounded plant cells. These substances are part of plant defence
mechanism. The substances released from wounded cells signal the presence of
plant cells that are competent for transformation.
Attachment
to plant cells:
It is 2-step process: It involves initial attachment via a polysaccharide.
Subsequently, a mesh of cellulose fibres is produced by the bacterium. chvA and
chvB genes are involved in attachment of bacterium to the plant cell. These
genes are involved in the production and secretion of cyclic beta-1,2-glycans.
Induction
of vir genes:
VirA, a membrane-linked sensor kinase, senses phenolics and authphosphorylates,
then phophorylates VirG. VirG induces expression of all the vir genes. Many
sugars enhance vir gene induction.
T-strand production:
VirD1-VirD2 complex and VirC1 are integral components of the process.
VirD1-VirD2 complex recognizes the left and right borders. VirD2 produces
single-stranded nicks in the DNA.
Transfer
of T-DNA out of the bacterial cell:
T-DNA-VirD2 complex is exported from the
bacterial cell by a T-pilus (membrane-channel secretory system) composed of vir
genes. VirD4 is important for the process.
Transfer
of T-DNA and Vir proteins into the plant cell and nuclear localization:
T-DNA-VirD2 complex and other Vir
proteins cross the plant plasma membrane. Inside the plant cell cytoplasm, the
T-DNA strand becomes covered with VirE2 protein, and is now known as mature
T-complex. VirD2 and VirE2 interact with variety of plant proteins that ensure
nuclear localization of T-DNA. VirE2 protects the T-DNA from plant nucleases.
T-DNA is integrated into the host plant genome by illegitimate recombination.
This process, unlike homologous recombination, does not depend on extensive
regions of sequence similarity between plant and Agrobacterium T-DNA.
EFFECTS OF PATHOGEN ON PLANTS
- Galls (tumors composed of undifferentiated cells)
- Wilts (loss of turgor)
- Cankers (localized wounds or lesions resulting from necrosis of stems or bark)
- Rots
- Deformed fruits
- Leaf spots
- Dwarfing
- Delayed fruit ripening
CROWN GALL DISEASE IN PLANTS
- Disorganized masses of plant cells
- Occur on fruit trees, sugar beets, other broad-leafed plants, walnuts, grape vines
- Grow where the stem comes out of the ground
- Cut off the flow of water and nutrients in the plant
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