''Agrobacterium radiobacter'' (more commonly known as ''Agrobacterium tumefaciens'') is the causal agent of crown gall disease (the formation of

tumour A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

s) 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 dic ...

s. It is a rod-shaped,

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

soil

bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were a ...

. 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 f ...

, 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 In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...

. 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'' ...

'' for the delivery of sequences hosted in T-DNA binary vectors. ''Agrobacterium tumefaciens'' is an Alphaproteobacterium 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 Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. At ...

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 fo ...

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

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...

ic 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 A walnut is the edible seed of a drupe of any tree of the genus ''Juglans'' (family Juglandaceae), particularly the Persian or English walnut, '' Juglans regia''. Although culinarily considered a "nut" and used as such, it is not a true b ...

grape vine ''Vitis'' (grapevine) is a genus of 79 accepted species of vining plants in the flowering plant family Vitaceae. The genus is made up of species predominantly from the Northern Hemisphere. It is economically important as the source of grapes, b ...

stone fruit In botany, a drupe (or stone fruit) is an indehiscent fruit in which an outer fleshy part ( exocarp, or skin, and mesocarp, or flesh) surrounds a single shell (the ''pit'', ''stone'', or '' pyrena'') of hardened endocarp with a seed (''kernel ...

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 r ...

, 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. At temperatures above 30 °C, ''A. tumefaciens'' begins to experience heat shock which is likely to result in errors in cell division.

Conjugation

To be

virulent Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ...

, the bacterium contains tumour-inducing plasmid (Ti plasmid or pTi), of 200 kbp, which contains the T-DNA and all the

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s 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 The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microor ...

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 bacteria ...

- exchange of plasmids amongst bacteria. In the presence of

opines Opines are low molecular weight compounds found in plant crown gall tumors or hairy root tumors produced by pathogenic bacteria of the genus ''Agrobacterium'' and '' Rhizobium''. Opine biosynthesis is catalyzed by specific enzymes encoded by ge ...

, ''A. tumefaciens'' produces a diffusible conjugation signal called 30C8HSL or the ''Agrobacterium'' autoinducer . This activates the

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...

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 Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

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 Acetosyringone is a phenolic natural product and a chemical compound related to acetophenone and 2,6-dimethoxyphenol. It was first described in relation to lignan/phenylpropanoid-type phytochemicals, with isolation from a variety of plant sour ...

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 VirA is a protein histidine kinase which senses certain sugars and phenolic compounds. These compounds are typically found from wounded plants, and as a result VirA is used by '' Agrobacterium tumefaciens'' to locate potential host organisms for i ...

, 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. At least 25 ''vir'' genes on the Ti plasmid are necessary for tumor induction . In addition to their perception role, ''virA'' and ''chvE'' induce other ''vir'' genes. The VirA protein has auto

kinase In biochemistry, a kinase () is an enzyme that catalysis, catalyzes the transfer of phosphate groups from High-energy phosphate, high-energy, phosphate-donating molecules to specific Substrate (biochemistry), substrates. This process is known as ...

activity: it

phosphorylates In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, whi ...

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

, inducing the transcription of the ''vir''

operon In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo spli ...

s. 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 Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

fibril Fibrils (from the Latin ''fibra'') are structural biological materials found in nearly all living organisms. Not to be confused with fibers or filaments, fibrils tend to have diameters ranging from 10-100 nanometers (whereas fibers are micro t ...

s 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

microcolony {{Short pages monitor