crown gall

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crown gall:

see gallgall,
abnormal growth, or hypertrophy, of plant tissue produced by chemical or mechanical (e.g., the rubbing together of two branches) irritants or hormones. Chemical irritants are released by parasitic fungi, bacteria, nematode worms, gall insects, and mites.
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Crown gall

A neoplastic disease of primarily woody plants, although the disease can be reproduced in species representing more than 90 plant families. The disease results from infection of wounds by the free-living soil bacterium Agrobacterium tumefaciens which is commonly associated with the roots of plants.

The first step in the infection process is the site-specific attachment of the bacteria to the plant host. Up to half of the bacteria become attached to host cells after 2 h. At 1 or 2 weeks after infection, swellings and overgrowths take place in tissue surrounding the site of infection, and with time these tissues proliferate into large tumors (see illustration). If infection takes place around the main stem or trunk of woody hosts, continued tumor proliferation will cause girdling and may eventually kill the host. Crown gall is therefore economically important, particularly in nurseries where plant material for commercial use is propagated and disseminated.

Crown gall on peachenlarge picture
Crown gall on peach

Unlike healthy normal cells, crown gall tumor cells do not require an exogenous source of phytohormones (auxins and cytokinin) for growth in culture because they readily synthesize more than sufficient quantities for their own growth. They also synthesize basic amino acids, each conjugated with an organic acid, called opines. The tumor cells also grow about four times faster and are more permeable to metabolities than normal cells.

These cellular alterations, such as the synthesis of opines and phytohormone regulation, result from bacterial genes introduced into host plant cells by A. tumefaciens during infection. Although it is not understood how these genes are introduced into the plant cell, the genes for the utilization of these opines and for regulating phytohormone production have been found to be situated on an extrachromosomal element called the pTi plasmid. This plasmid, harbored in all tumor-causing Agrobacterium species, also carries the necessary genetic information for conferring the tumor-inducing and host-recognition properties of the bacterium.

Crown gall is consequently a result of this unique bacteria-plant interaction, whereby A. tumefaciens genetically engineers its host to produce undifferentiated growth in the form of a large tumor, in which there is the synthesis of a unique food source in the form of an opine for specific use by the bacterial pathogen. See Bacterial genetics, Genetic engineering, Plant hormones, Plant pathology

crown gall

[′krau̇n ‚gȯl]
(plant pathology)
A bacterial disease of many plants induced by Bacterium tumefaciens and marked by abnormal enlargement of the stem near the root crown.
References in periodicals archive ?
Smith and Townsend (1907) were among the earlier major investigators of crown gall disease (Braun, 1954, 1982).
Nucleic acid hybridization studies to determine the genetic involvement in crown gall tumors and the nature of TIP were made, but no concrete evidence of bacterial DNA sequences in sterile crown gall tissue was found (Butcher, 1977).
Whereas the initial phase of knowledge about crown gall tumors was concerned with the general external description and cell culture without exogenous hormones, the second major phase, in the 1970s and into the 1980s, described a unique process in which a DNA component of a bacterial plasmid was transferred into the nucleus of an eukaryote organism to produce the hormones for plant-cell growth and opines for bacterial consumption.
Experimental evidence of the essential role of T-DNA in the transformation process in crown gall disease was published in 1980 by several scientific groups (Braun, 1982; Yadav et al.
Research on the main details of crown gall disease may be completed in the 2000s, but major information is still needed regarding the incorporation of the T-strand into the plant nucleus and the subsequent steps producing plant hormones and opines in the plant cytoplasm.
Much is now known about crown gall disease in many dicotyledonous plants, but many aspects of the infection process are still unknown.
Figure 2 gives a general scheme of the sequence of reactions in crown gall disease leading up to the formation of the nodule on the external surface of the stem.
Ethylene has also recently been shown to play a critical role in crown gall morphogenesis (Aloni et al.