MORPHOGENESIS OF CROWN GALL
75
VII. Summary
Crown gall can be induced on a great number of dicotyledonous plants
by inoculating a wound with Agrobacterium
tumefaciens
under appropriate conditions. The type of gall formed—from fast growing,
malignant outgrowths to slow growing tumors—is determined by the
host species, the bacterial strain, the inoculation site, the timing of the
conditioning, and the length of the transformation period. (Wounding
conditions the host cells in some way so that the bacteria are able to
transform them into tumor cells.) Not only is the transformation strictly
dependent on the temperature, but recent observations have shown that
the wound healing, and thus the conditioning, is greatly influenced by it.
The earlier observation that the conditioning is related to the first cell
divisions has been confirmed, and it seems probable that the critical stage
is the first DNA synthesis.
The present review on the crown gall problem deals particularly with
its anatomical and cytological aspects. Special attention has been paid to
normal wound healing as compared to the development of crown gall.
Anatomically the tumors on the stem—the most widely used organ—
can be divided into two classes: the teratoma type (pea, tomato, broad
bean, tobacco) and the unorganized type (sunflower). Whether the difference between these is caused by the anatomy and/or cytology of the
host is not clear. At any rate, the differentiated cells of the sunflower are
all diploid, whereas the other hosts show in addition to diploidy various
degrees of somatic polyploidy.
During the development of the crown gall, the diploid and tetraploid
cells continue to divide, whereas most of the more highly polyploid cells
merely undergo endoreduplication to become even more polyploid. The
cells of the sunflower tumor remain diploid. The often highly disorderly
structure of galls can be explained by an increase in the growth substance production of the transformed cells and the resulting lack of any
orderly gradients.
The various hypotheses on the origin of crown gall have been discussed.
A gene or chromosome mutation seems to be excluded, but both the
plasmagene and dedifferentiation hypotheses fit the known facts. The
present authors slightly favor the latter. A number of experimental approaches to decide between these hypotheses are suggested.
Acknowledgments
This work has been supported by a grant from the Finnish National Research
Council for Sciences and grant No. FG-Fi-142 from the United States Department
of Agriculture, Agricultural Research Service, to Dr. Kupila-Ahvenniemi, and in
75
VII. Summary
Crown gall can be induced on a great number of dicotyledonous plants
by inoculating a wound with Agrobacterium
tumefaciens
under appropriate conditions. The type of gall formed—from fast growing,
malignant outgrowths to slow growing tumors—is determined by the
host species, the bacterial strain, the inoculation site, the timing of the
conditioning, and the length of the transformation period. (Wounding
conditions the host cells in some way so that the bacteria are able to
transform them into tumor cells.) Not only is the transformation strictly
dependent on the temperature, but recent observations have shown that
the wound healing, and thus the conditioning, is greatly influenced by it.
The earlier observation that the conditioning is related to the first cell
divisions has been confirmed, and it seems probable that the critical stage
is the first DNA synthesis.
The present review on the crown gall problem deals particularly with
its anatomical and cytological aspects. Special attention has been paid to
normal wound healing as compared to the development of crown gall.
Anatomically the tumors on the stem—the most widely used organ—
can be divided into two classes: the teratoma type (pea, tomato, broad
bean, tobacco) and the unorganized type (sunflower). Whether the difference between these is caused by the anatomy and/or cytology of the
host is not clear. At any rate, the differentiated cells of the sunflower are
all diploid, whereas the other hosts show in addition to diploidy various
degrees of somatic polyploidy.
During the development of the crown gall, the diploid and tetraploid
cells continue to divide, whereas most of the more highly polyploid cells
merely undergo endoreduplication to become even more polyploid. The
cells of the sunflower tumor remain diploid. The often highly disorderly
structure of galls can be explained by an increase in the growth substance production of the transformed cells and the resulting lack of any
orderly gradients.
The various hypotheses on the origin of crown gall have been discussed.
A gene or chromosome mutation seems to be excluded, but both the
plasmagene and dedifferentiation hypotheses fit the known facts. The
present authors slightly favor the latter. A number of experimental approaches to decide between these hypotheses are suggested.
Acknowledgments
This work has been supported by a grant from the Finnish National Research
Council for Sciences and grant No. FG-Fi-142 from the United States Department
of Agriculture, Agricultural Research Service, to Dr. Kupila-Ahvenniemi, and in