Agrobacterium tumefaciens Missouri State

Agrobacterium tumefaciens,

The Causal Agent of Crown Gall Disease

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19 November 2001, AGH 483 Diseases of Economic Plants

Laszlo Kovacs, State Fruit Experiment Station, Missouri State

I. Disease Mechanism

Most plant pathogenic bacteria cause disease by the secretion of enzymes, polysaccharides, proteins, or hormones.

Agorbacterium tumefaciens:

· A unique, highly sophisticated plant-pathogenic bacterium

· Causes disease by the transfer of its own DNA into plant cells

· The transferred DNA (T-DNA) is carried by a Ti (tumor-inducing plasmid)

· T-DNA has genes to direct the plant cells to synthesize:

1. Opines

Low molecular weight, unusual compounds

Secreted by the plants cells

Uniquely taken up and metabolized by Agrobacterium

Provide competitive advantage for the bacterium

2. Phytohormones

Auxin and cytokinine

Induce cell proliferation that is analogous to animal cancer

Increase the number of cells that synthesize opines

· Genetic colonization: Agrobacterium makes the plant produce specific metabolites, create a favorable environment for the bacterium

II Economic Impact

Mostly woody perennials and herbaceous ornamentals are affected

· Fruit crops: Grapes, apple, walnut, cherry, brambles

· Landscape plants: Euonymus

· Ornamentals: Chrysanthemum

Only wounded cells can be transformed.

Wounding is inflicted by freeze damage, grafting, or mechanical injury.

Disease can be very severe following cold winters and in nurseries where grafted plants are used.

Transformed cells are liberated from constrains of the normal hormonal regulation by the plant.

Cells containing the T-DNA do not differentiate into functional tissues, but form a tumorous, non-differentiated callus, called crown gall.

Most frequently, the cambium is affected – transformed cambium cells form tumors (galls) instead of xylem and the phloem tissues.

Phloem and xylem transports are disrupted - if affected tissues girdle the stem, the plant will starve to death.

Galls become infected by other pathogens and they decay at the end of season – provides access to the healthy parts of the plant for many other microorganiMissouri State.

Cambium dies of - grapevines are never able to from new cambium.

Agronomic Impact:

	Severely affected plants are killed by the disease
	Other affected plants are poor producers – cultivation of vineyards/orchards are poorly profitable
	Affected plants have shorter lifespan - replanting is expensive
	Nursery plants are not marketable (fruit/landscape propagating material, ornamentals)

III DISEASE control:

Curative measures: Not available: once the plant gets infected, it is not possible to cure it of Agrobacterium

Preventive measures:

Biological control: Non-pathogenic strains of Agrobacterium radiobacter – strain K84 produces antibiotic agrocin 84

Effective and widely used in fruit trees, but ineffective in grapes

Shoot tip culture: bacteria are unable to invade shoot apex.
Sanitary selection: detect and discard infected plants. Detection techniques include i) microbiological detection ii) polymerase chain reaction (PCR), iii) immunological detection, and iv) immunocapture PCR,
Hot water treatment: Reduces endophytic bacterial populations, but does not eliminate them

	Clean propagating material:
	Most promising technique: planting shoot tip culture-derived plants in non-horticultural soil.

Resistance breeding:

Suppress the expression of crown gall oncogenes
Prevent the integration of the T-DNA into the plant nucleus

	Traditional plant breeding: Certain wild plants are resistant/tolerant to crown gall (for example, wild East-Asian and North American grape species).
	Molecular breeding: Creation of transgenic plants by pathogen-derived resistance.

**IV Agrobacterium and Plant Genetic Engineering.**

	Crown gall is a natural form of genetic engineering.
	T-DNA oncogenes can be replaced with genes of desired phenotype.
	Plant genomes can be improved with useful genes not only from related plants (traditional breeding), but with genes from any other life form, including animals, fungi, bacteria, or viruses.
	Agrobacterium-mediated plant transformation revolutionized plant biotechnology during the 1980s and 1990s.

Selected Internet Resources:

Agrobacterium biology:

http://helios.bto.ed.ac.uk/bto/microbes/crown.htm

Agrobacterium-mediated plant transformation:

http://www4.nas.edu/beyond/beyonddiscovery.nsf/web/seeds4?OpenDocument

Engineering of crops using Agrobacterium:

http://opbs.okstate.edu/~melcher/MG/MGW4/MG4373.html

Genetically Modified Corps:

http://www.sdcma.org/GMFoodsBrochure.pdf

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