50
SIRKKA KUPILA-AHVENNIEMI AND EEVA THERMAN
temperature in the range 21°-36°C. Plants maintained at 36°C were
18 hr ahead in the wound-healing process and plants maintained at
32°C, 12 hr ahead of those kept at 25°C. A comparison between the
time needed for reaching a certain level in conditioning and the time
required for initiation of the first oriented cell divisions indicated that
the minimum conditioning was completed before the first wound-induced
cell division occurred. The sensitivity of the cells to the tumorigenic
effect diminished at the time when wound cambium was completely
formed.
The suggestion of Lipetz (1966) that the cells are susceptible to the
tumorigenic effect only before the first cell division is in accordance with
the earlier observation of Braun and Mandle (1948) that it is before
or during the early stages of active wound healing, e.g., rapid cell division, that normal cells are most readily converted into tumor cells. As
Lipetz (1966) points out there are three possible sensitive stages, G x , S,
and G 2 , before the first mitosis; however, he does not make a choice
between them. Strong support in favor of the S period is provided by
Bopp's (1964a) experiments on the effect of fluorodeoxyuridine on
tumorigenesis. Fluorodeoxyuridine, which is known to inhibit specifically
DNA synthesis (cf. Kihlman, 1966), has been shown by Bopp virtually
to prevent tumor formation. This inhibition can be completely reversed
by high concentrations of thymidine. The virulence of the bacteria, the
wound reaction of the host, and the later stages of the tumor development are only mildly affected by the treatment. The exact relationship
of the DNA synthesis and the conditioning ought to be studied further,
for instance, by means of radioactive tracers. That the DNA synthesis
is only one factor in the complicated process of tumor induction is
demonstrated by the unsuccessful efforts to transform rapidly dividing
meristematic or callus cells (cf. Riker and Banfield, 1932; Braun, 1954b,
pp. 140-141). These relationships might possibly be elucidated by
applying various growth substances to increase the frequency of mitoses
during the early stages of tumor inception.
B. Tumor Induction
The two main questions in crown gall research are: "What is the
nature of the factor, produced by the bacteria, which causes the transformation of normal cells to tumor cells?" and, "What is the nature of the
events taking place in the cells during the transformation?" The answers
to these two questions are largely unknown. The hypotheses concerning
SIRKKA KUPILA-AHVENNIEMI AND EEVA THERMAN
temperature in the range 21°-36°C. Plants maintained at 36°C were
18 hr ahead in the wound-healing process and plants maintained at
32°C, 12 hr ahead of those kept at 25°C. A comparison between the
time needed for reaching a certain level in conditioning and the time
required for initiation of the first oriented cell divisions indicated that
the minimum conditioning was completed before the first wound-induced
cell division occurred. The sensitivity of the cells to the tumorigenic
effect diminished at the time when wound cambium was completely
formed.
The suggestion of Lipetz (1966) that the cells are susceptible to the
tumorigenic effect only before the first cell division is in accordance with
the earlier observation of Braun and Mandle (1948) that it is before
or during the early stages of active wound healing, e.g., rapid cell division, that normal cells are most readily converted into tumor cells. As
Lipetz (1966) points out there are three possible sensitive stages, G x , S,
and G 2 , before the first mitosis; however, he does not make a choice
between them. Strong support in favor of the S period is provided by
Bopp's (1964a) experiments on the effect of fluorodeoxyuridine on
tumorigenesis. Fluorodeoxyuridine, which is known to inhibit specifically
DNA synthesis (cf. Kihlman, 1966), has been shown by Bopp virtually
to prevent tumor formation. This inhibition can be completely reversed
by high concentrations of thymidine. The virulence of the bacteria, the
wound reaction of the host, and the later stages of the tumor development are only mildly affected by the treatment. The exact relationship
of the DNA synthesis and the conditioning ought to be studied further,
for instance, by means of radioactive tracers. That the DNA synthesis
is only one factor in the complicated process of tumor induction is
demonstrated by the unsuccessful efforts to transform rapidly dividing
meristematic or callus cells (cf. Riker and Banfield, 1932; Braun, 1954b,
pp. 140-141). These relationships might possibly be elucidated by
applying various growth substances to increase the frequency of mitoses
during the early stages of tumor inception.
B. Tumor Induction
The two main questions in crown gall research are: "What is the
nature of the factor, produced by the bacteria, which causes the transformation of normal cells to tumor cells?" and, "What is the nature of the
events taking place in the cells during the transformation?" The answers
to these two questions are largely unknown. The hypotheses concerning