Plant Tumors Crown Galls

Within the intact plant, considered as a unit, active cell division is limited to certain zones around the meristems of the tips of shoots and roots. If one goes downward from the shoot tips and upward from the root tips, cell division becomes progressively less active and finally ceases altogether. After they have ceased to divide, the cells become visibly different from each other they begin to differentiate. 

The manner in which this transition from cell division to differentiation is regulated is largely unknown. We shall discuss a few relevant findings later (page 231). If this regulatory mechanism is disturbed, then cell division can occur without subsequent differentiation. The disturbances can be of quite different kinds. For example, one can take tissue from the whole plant and grow it in a defined culture medium. Under certain conditions, such tissue cultures can maintain their ability to divide almost indefinitely (page 246). 

Other examples of faulty regulation are tumors. Complexes of uninhibited cell division such as these can be due to different causes. For example, genetic tumors are well-known, particularly in hybrid species of the genus Nicotiana. They occur particularly frequently when one of the 

Crown galls are plant tumors that are induced by Agrobacterium tumefaciens. Their occurrence is in no way limited to roots they can occur on any part of the plant (Fig. 139). A precondition is a wound. In the first place it is necessary for the bacteria to penetrate into the plant, but, in addition, the wound leads to a conditioning of the plant cells. For if the bacteria are introduced without injury into the plant no tumor is formed. What this conditioning is, is unknown. At the site of the wound, a wound callus is first formed, as is usual after an injury. Later the tumor grows out of this callus. Wound callus and tumor can be readily distinguished from each other. This is because call divisions in cellus tissue are oriented periclinally, whereas in tumor tissue no orientation at all is to be found. 

DNA is thus implicated, but at first it was not known whether it was the DNA of the plant or of the bacterium. Many experiments strongly suggested the existence of a tumor-inducing principle (TIP) which was supposed to be released by the bacterial cells and initiate the change (transformation) in the plant cells. It now seems likely that the principle is indeed bacterial DNA. As has been shown, DNA migrates from the bacteria into the plant cells where it replicates itself and induces the formation of bacterial-specific proteins. It seems likely that this DNA is ultimately responsible for causing the tumors. 

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In our continuing interests in natural product discovery,40,41 ellagic acid and an extract of fruit from Melia volkensii (MP-extract) were screened for inhibition of Agrobacterium tume-faciens-md uced tumors using the potato disk assay.42 bioassay s usefu] for the examination of plant extracts and purified compounds which inhibit crown gall tumors (a plant neoplastic disease) that may have potential human anticancer activity.43,44... 

Plant tumors result from uncontrolled hormone production. Crown gall tumors, for example, are due to the infection of plant wounds by certain strains of Agrobacterium. These bacteria carry a large plasmid, the tumor-inducing, or T, plasmid, part of which is incorporated into the plant genome. This DNA encodes several genes that stimu-... 

The vectors used routinely to produce transgenic plants are derived from the soil bacterium Agrobacterium tumefaciens. In its natural form, this bacterium causes the crown gall disease in which the infected plant produces tumors ( galls ) usually at the base ( crown ) of the plant. 

Figure 13. Agrobacterium tumefaciens, the natural transformation system for plants. The bacterium contains a circular piece of DNA, the Ti-plasmid. The bacterium infects a dicotyledonous plant and transfers the T-DNA from the Ti-plasmid to the plant chromosomal DNA causing a tumor to form. The tumor is called a "crown gall". The gall, or teratoma, can be removed from the plant and placed into culture on medium without exogenous hormones.

Figure 6.33. Tumors in Plants. Crown gall, a plant tumor, is caused by a bacterium Agrobacterium tumefaciens) that carries a tumor-inducing plasmid (Ti plasmid). 

From the results so far obtained, certain crown gall cells and normal cells from C. roseus, tobacco and rice were found to produce brassinosteroid-like active substances, and their amounts were greater than that of their mother plant tissues. Since normal cells produced the active substances and the production by crown gall cells did not depend on their tumor types, T-DNA transformed to the cell genome is not directly involved in the production. Therefore, the production of brassinosteroid-like active substances was found to depend on neither the original plant nor the Ti-plasmid of crown gall cells. 

Srivastava, B.I.S. DNA polymerases of normal and crown gall tumor tissue cultures of tobacco Plant Physiol. (1972) Suppl. 74. 

Crown gall tumors, for instance, involve the transfer of genetic material coding for specific secondary products from a bacterium to cells of a higher plant (Krauspe 1981). 

Krauspe, R. Cell differentiation and tumor state - plant crown gall tumors. In Cell Differentiation (L. Nover, M. Luckner, B. Parthier, eds.), pp. 569-596. Fischer/Springer, Jena/Berlin-Heidelberg-New York 1982... 

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