Agrobacterium tumefaciens

Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Alphaproteobacteria
Order	Rhizobiales
Family	Rhizobiaceae
Genus	Rhizobium

Scientific Name

Rhizobium radiobacter

Scientific Name Synonyms

Agrobacterium radiobacter

Agrobacterium tumefaciens

Common Name

crown gall

Author: John Bonkowski, Bacheline Joseph, Deanna Bayo University of Florida

Reviewed by:Name, Organization

Pathogen

Rhizobium radiobacter (previously known as Agrobacterium tumefaciens) is an aerobic, gram-negative, motile, rod-shaped bacterium with one to six flagella. It is about 1.5-3.0 x 0.6-1.0 µm in size and causes the formation of galls on plant hosts (2). This bacterium is closely related to nitrogen-fixing bacteria that can be found on root nodules in legumes (1). Rhizobium radiobacter has also been used for the genetic modification of different organisms (1).

Symptoms and Signs

Symptoms of plants infected with Rhizobium radiobacter are dependent on the type of host infected. The first symptom plants commonly exhibit when infected by Rhizobium radiobacter is the presence of tumorous overgrowths (galls) on the roots, stem (trunk), and crown at or below the soil surface. Initially, these galls are round, white or flesh colored, and soft and spongy. In later stages, the emerging gall develops into a woody interior with an irregular, rough, corky surface. This outer tissue darkens (8). The gall color will range from light brown to black depending on age and host. Size also varies ranging from 5 to 16 cm diameter. In trees, galls can sometimes grow to be 30 cm diameter (2). As galls age, the outer surface may slough off and crack due to normal rotting, weathering, and secondary microorganism activity leading to further tissue breakdown (5,8). However, galls can reappear in the same places the following year along with the formation of new tissues. Some host plants develop secondary tumors at a few inches above or below the previous site of infection. Severely infected plants lack vigor, have stunted leaves, may turn yellow or red, and shoots often die back. Wilting and death are the primary symptoms of continued infection (8). Death is more common when galls occur at the plant's base and girdle the stem of the host, blocking the vascular system (5). Young plants are more susceptible to the disease and are also more likely to die from the disease (2).

Ecology and Spread

Rhizobium radiobacter is a soilborne pathogen that can survive in the soil or inside plants as a parasite. There are over 600 plant species and over 90 plant families that are susceptible to crown gall (8). Pathogen activity and disease development are favored at temperatures around 15°C to 32°C (2). Infection can only occur when a wound provides an entry way for the bacterium. Upon entry, a portion of the bacterium DNA is inserted into the DNA of the host plant. This results in over production of plant hormones, such as auxins and opines, which induces gall formation (6). These galls are nutrient rich and great for sustaining the pathogen (2, 6). Common causes of wounds include: 1) chewing insects; 2) bud scales and leaf scars; 3) mechanical injuries such as pruning, grafts and cultivation; and 4) frost damage. When temperatures are warm, new wounds may become infected for two to four days. Wounds can remain susceptible for weeks when temperatures are below 13°C (2). After infection, tumors are noticeable after two to four weeks (8). The plant's growth and vigor and environmental conditions at the time of infection will determine the rate of gall development (5).

Infection of Rhizobium radiobacter is most likely to occur under the following conditions: 1) temperatures above 20°C (2); 2) during times of high humidity; and 3) when the host surfaces are moist due to rainfall. The pathogen can be spread through rain, irrigation water, wind, insects, tools, and use of contaminated soil during propagation or planting. Detached galls that rest on the ground or in the soil are primary inoculum sources (7). As galls degrade and breakdown, infected tissue that sloughs off into the soil releases bacteria (5). Water can then facilitate subsequent infections of the host plant or other plants in that area (2, 5). Some rootstocks are more susceptible than others to the disease, so it is important to use healthy rootstock to prevent transmission. Although the bacterial population will decline in the absence of a host plant, the pathogen may survive in soil for up to 3 years (5).

Geographic Distribution

A. tumefaciens is found on all continents, but the countries listed below have the most occurrences of crown gall

Europe

Austria, Azores, Belgium, Britain and Northern Ireland, Bulgaria, Cyprus, Czechoslovakia, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Romania, Spain, Sweden, Switzerland, USSR (Estonia) (Ukraine)Caucasus), Yugoslavia.

Asia

Afghanistan, China (S.) (Honan) (N.E.), India, Indonesia, Iran, Israel, Japan, Korea, Lebanon, Malaysia (Sarawak), Saudi Arabia, Sri Lanka, Syria, Turkey, Union of Soviet Socialist Republics (Central Asia, Western Siberia).

Africa

Algeria, Egypt, Ethiopia, Kenya, Libya, Malawi, Morocco, Mozambique, Rhodesia, Seychelles, Somalia, South Africa, Tanzania, Uganda, Zambia.

North America

Canada, Mexico, United States of America.

Central America and Caribbean

Bermuda, Cuba, French Antilles, Guadeloupe, Jamaica, Puerto Rico.

South America

Argentina, Bolivia, Brazil (Sao Paulo, Pernambuco), Colombia, Chile, Guyana, Peru, Uruguay, Venezuela.

Oceania

Australia (All territories), New Zealand (3).

Management

Management of any pathogen is often dependent upon both cultural and chemical options. Consult your local extension specialist or agent for recommendations relevant to your particular host and state. Remember: the label is the law.

Diagnostic procedures

Procedures/Media	Result of Rhizobium radiobacter
KOH test	Positive reaction; stringing is observed (Gram-negative)
Oxidase	Positive; color change occurs
Kings medium B	Negative; nonfluorescent
Nutrient agar	Cream-colored colonies after 48 hours
Yeast extract-dextrose CaCO3 medium	Cream-colored colonies colonies
Sucrose peptone agar	Positive; colonies are mucoid
Urease	Positive
Hypersensitive Response	Tomato: No response
	Tobacco: No response
Crystal violet pectate medium	Negative; no pitting observed
D1 medium	Positive; blue-green centered colonies with clear border
1A agar	Red to purple colonies

Isolation of the bacterium on selective media, specifically D1M and 1A, can be used in conjunction with molecular techniques such as polymerase chain reaction (PCR) to help identify the bacterium.

• PCR

Cubero, J., Martínez, M. C., Llop, P., López, M. M. (1999). A simple and efficient PCR method for the detection of Agrobacterium tumefaciens in plant tumours. Journal of Applied Microbiology. 86(4): 591-602.

DOI: 10.1046/j.1365-2672.1999.00700.x

Selected References

1. Aldemita, R. R., and Hodges, T. K. 1996. Agrobacterium tumefaciens-mediated transformation of japonica and indica rice varieties. Planta Vol 199 (4), pp 612-617.

2. Caruso, F. L., and Ramsdell, D. C. 1995. Compendium of Blueberry and Cranberry Diseases. American Phytopathological Society, St. Paul, MN.

3. Commonwealth Mycological Institute. Distribution Maps of Plant Diseases: Agrobacterium tumefaciens. Online: http://www.cabi.org/dmpd/abstract/20056500137.

4. Holt, J. G., Krieg, N. R.,Sneath, P. H. A.,Staley, J. T., and Williams, S. T. 1994. Bergley’s Manual of Determinative Bacteriology. 9th ed. Williams and Wilkins, Baltimore, MD. 74.

5. Horst, R. K., and Cloyd, R. A. 2007. Compendium of Rose Diseases and Pests. 2nd ed. American Phytopathological Society, St. Paul, MN.

6. Kado, C.I. 2002. Crown gall. The Plant Health Instructor. Online publication. doi:10.1094/PHI-I-2002-1118-01.

7. Paret, M. L., Momol, T., Ritchie, L., and Dankers, H. 2011. Florida Plant Disease Management Guide: Apple (Malus domestica). University of Florida. EDIS. #PDMG-V3-05. Online publication.

8. Sinclair, W. A. and Lyon, H. H. 2005. Diseases of Trees and Shrubs. 2nd ed. Cornell University Press, Ithaca, NY.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)