''Agrobacterium radiobacter'' (more commonly known as ''Agrobacterium tumefaciens'')
is the causal agent of crown gall disease (the formation of
tumours) in over 140 species of
eudicot
The eudicots, Eudicotidae, or eudicotyledons are a clade of flowering plants mainly characterized by having two seed leaves upon germination. The term derives from Dicotyledons.
Traditionally they were called tricolpates or non-magnoliid dicot ...
s. It is a rod-shaped,
Gram-negative soil
bacterium.
[ Symptoms are caused by the insertion of a small segment of DNA (known as the ]T-DNA
The transfer DNA (abbreviated T-DNA) is the transferred DNA of the tumor-inducing (Ti) plasmid of some species of bacteria such as ''Agrobacterium tumefaciens'' and ''Agrobacterium rhizogenes(actually an Ri plasmid)''. The T-DNA is transferred fr ...
, for 'transfer DNA', not to be confused with tRNA that transfers amino acids during protein synthesis), from a plasmid into the plant cell, which is incorporated at a semi-random location into the plant genome. Plant genomes can be engineered by use of ''Agrobacterium
''Agrobacterium'' is a genus of Gram-negative bacteria established by H. J. Conn that uses horizontal gene transfer to cause tumors in plants. ''Agrobacterium tumefaciens'' is the most commonly studied species in this genus. ''Agrobacterium'' is ...
'' for the delivery of sequences hosted in T-DNA binary vectors.
''Agrobacterium tumefaciens'' is an Alphaproteobacterium
Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and p ...
of the family Rhizobiaceae
The Rhizobiaceae is a family of Pseudomonadota comprising multiple subgroups that enhance and hinder plant development. Some bacteria found in the family are used for plant nutrition and collectively make up the rhizobia. Other bacteria such as ...
, which includes the nitrogen-fixing legume
A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock f ...
symbionts
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
. Unlike the nitrogen-fixing symbionts, tumor-producing ''Agrobacterium'' species are pathogenic and do not benefit the plant. The wide variety of plants affected by ''Agrobacterium'' makes it of great concern to the agriculture industry.
Economically, ''A. tumefaciens'' is a serious pathogen of walnuts, grape vines, stone fruits, nut
Nut often refers to:
* Nut (fruit), fruit composed of a hard shell and a seed, or a collective noun for dry and edible fruits or seeds
* Nut (hardware), fastener used with a bolt
Nut or Nuts may also refer to:
Arts, entertainment, and media Com ...
trees, sugar beets
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double s ...
, horse radish, and rhubarb
Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of ''Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick rhizo ...
, and the persistent nature of the tumors or galls caused by the disease make it particularly harmful for perennial crops.
''Agrobacterium tumefaciens'' grows optimally at 28 °C. The doubling time can range from 2.5–4h depending on the media, culture format, and level of aeration.[
]
Conjugation
To be virulent, the bacterium contains tumour-inducing plasmid (Ti plasmid or pTi), of 200 kbp, which contains the T-DNA and all the genes necessary to transfer it to the plant cell. Many strains of ''A. tumefaciens'' do not contain a pTi.
Since the Ti plasmid is essential to cause disease, prepenetration events in the rhizosphere occur to promote bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. This takes place through a pilus. It is a parasexual mode of reproduction in bacteri ...
- exchange of plasmids amongst bacteria. In the presence of opines, ''A. tumefaciens'' produces a diffusible conjugation signal called 30C8HSL or the ''Agrobacterium'' autoinducer . This activates the transcription factor TraR, positively regulating the transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
of genes required for conjugation .
Infection methods
''Agrobacterium tumefaciens'' infects the plant through its Ti plasmid. The Ti plasmid integrates a segment of its DNA, known as T-DNA, into the chromosomal DNA of its host plant cells. ''A. tumefaciens'' has flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
that allow it to swim through the soil towards photoassimilate In botany, a photoassimilate is one of a number of biological compounds formed by assimilation using light-dependent reactions. This term is most commonly used to refer to the energy-storing monosaccharides produced by photosynthesis in the leaves ...
s that accumulate in the rhizosphere around roots. Some strains may chemotactically move towards chemical exudates from plants, such as acetosyringone and sugars, which indicate the presence of a wound in the plant through which the bacteria may enter. Phenolic compounds are recognised by the VirA protein, a transmembrane protein encoded in the virA gene on the Ti plasmid. Sugars are recognised by the chvE protein, a chromosomal gene-encoded protein located in the periplasmic space.kinase
In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
activity: it phosphorylates itself on a histidine residue. Then the VirA protein phosphorylates the VirG protein on its aspartate residue. The virG protein is a cytoplasmic protein produced from the ''virG'' Ti plasmid gene. It is a transcription factor, inducing the transcription of the ''vir'' operons. The ChvE protein regulates the second mechanism of the ''vir'' genes' activation. It increases VirA protein sensitivity to phenolic compounds.[
Attachment is a two-step process. Following an initial weak and reversible attachment, the bacteria synthesize cellulose fibrils that anchor them to the wounded plant cell to which they were attracted. Four main genes are involved in this process: ''chvA'', ''chvB'', ''pscA'', and ''att''. The products of the first three genes apparently are involved in the actual synthesis of the cellulose fibrils. These fibrils also anchor the bacteria to each other, helping to form a ]