74
SIRKKA KUPILA-AHVENNIEMI AND EEVA THERMAN
fast growing tumor is not only autonomous for all its growth substances,
but the auxin-type hormones are produced out of proportion with the
other systems (cf. Braun and Stonier, 1958). This would explain the
absence of any orderly gradients in tumor tissue (cf. Kupila and
Therman, 1962), which, in turn, causes the disorganized structure
characteristic of a neoplastic gall. Indeed, it has been shown by Braun
(1957) that normal tobacco pith tissue can be made histologically to
resemble crown gall by adding both auxin and kinetin in suitable
amounts.
All the observations on crown gall, so far recapitulated, would be
equally compatible with the plasmagene as with the dedifferentiation
hypothesis. The one finding which—at least to the present authors—
would seem to agree better with the latter idea, is that crown gall inception apparently is connected with a specific nuclear phase. As discussed
earlier, the absence of any somatic polyploidy in the sunflower crown
gall may also be pertinent in this connection. This would fit in with the
idea that only normal genes are derepressed during tumorigenesis: since
the sunflower does not possess the inherent tendency to somatic polyploidy, this is not induced in the crown gall either.
During recent years, histones have emerged as the most probable
regulators of gene action (cf. Ts'o and Bonner, 1964; Huang and Bonner,
1965; Littau et al.,. 1965). A review of the latest results in this field is
to be found in the book edited by de Reuck and Knight (1966). Dahmus
and Bonner (1965) have shown that a hormone, hydrocortisone, has the
ability to regulate DNA-dependent RNA synthesis in rat liver chromosomes. Whether histones play a role in crown gall induction might
possibly be tested using the methods developed by Bonner and his
associates.
To sum up, most of the known facts about the inception and behavior
of crown gall, although incompatible with gene or chromosome mutation,
agree both with the plasmagene and the dedifferentiation hypothesis. The
specific sensitivity of one cell stage, the S period, seems to fit better the
latter idea. Several experimental approaches which might help to decide
between these two possibilities have been proposed above. However, as
pointed out by Jinks (1964, pp. 146-161), differentiation to a certain
extent overlaps with phenomena that can be included under the term,
extranuclear inheritance. It does not seem completely impossible that
further research might establish a bridge between the two now seemingly
opposing hypotheses in the same way that the finding that both plastids
and mitochondria contain DNA is bridging the gap between nuclear
genes and plasmagenes.
SIRKKA KUPILA-AHVENNIEMI AND EEVA THERMAN
fast growing tumor is not only autonomous for all its growth substances,
but the auxin-type hormones are produced out of proportion with the
other systems (cf. Braun and Stonier, 1958). This would explain the
absence of any orderly gradients in tumor tissue (cf. Kupila and
Therman, 1962), which, in turn, causes the disorganized structure
characteristic of a neoplastic gall. Indeed, it has been shown by Braun
(1957) that normal tobacco pith tissue can be made histologically to
resemble crown gall by adding both auxin and kinetin in suitable
amounts.
All the observations on crown gall, so far recapitulated, would be
equally compatible with the plasmagene as with the dedifferentiation
hypothesis. The one finding which—at least to the present authors—
would seem to agree better with the latter idea, is that crown gall inception apparently is connected with a specific nuclear phase. As discussed
earlier, the absence of any somatic polyploidy in the sunflower crown
gall may also be pertinent in this connection. This would fit in with the
idea that only normal genes are derepressed during tumorigenesis: since
the sunflower does not possess the inherent tendency to somatic polyploidy, this is not induced in the crown gall either.
During recent years, histones have emerged as the most probable
regulators of gene action (cf. Ts'o and Bonner, 1964; Huang and Bonner,
1965; Littau et al.,. 1965). A review of the latest results in this field is
to be found in the book edited by de Reuck and Knight (1966). Dahmus
and Bonner (1965) have shown that a hormone, hydrocortisone, has the
ability to regulate DNA-dependent RNA synthesis in rat liver chromosomes. Whether histones play a role in crown gall induction might
possibly be tested using the methods developed by Bonner and his
associates.
To sum up, most of the known facts about the inception and behavior
of crown gall, although incompatible with gene or chromosome mutation,
agree both with the plasmagene and the dedifferentiation hypothesis. The
specific sensitivity of one cell stage, the S period, seems to fit better the
latter idea. Several experimental approaches which might help to decide
between these two possibilities have been proposed above. However, as
pointed out by Jinks (1964, pp. 146-161), differentiation to a certain
extent overlaps with phenomena that can be included under the term,
extranuclear inheritance. It does not seem completely impossible that
further research might establish a bridge between the two now seemingly
opposing hypotheses in the same way that the finding that both plastids
and mitochondria contain DNA is bridging the gap between nuclear
genes and plasmagenes.