72
EDWARD C. CANTINO AND JAMES S. LOVETT
All the evidence, then, points to the conclusion that plants derived
from RS thalli by removal of exogenous bicarbonate function like OC
cells, but that they are not entirely identical to them in total structure
and chemical composition (see Cantino and Goldstein, 1962). This
conclusion in no way vitiates the functional significance of morphological
reversibility in B. emersonii ; the influence of a previous environment on
the chemical composition of other micro-organisms has been amply
demonstrated (Herbert, 1961). Our results suggest that gross structural
de-differentiation is much less easily accomplished (if at all) than other
characteristics by removal of the morphogenetic inducer. Indeed, it
seems reasonable to assume that a structural or storage material would
be less directly and less rapidly subject to induction or repression than
an enzyme. Thus, while our knowledge is admittedly far from complete,
we can conclude : (a) that prior to formation of the cross-wall, no changes
occur in an RS cell which, upon removal of the inducer, prevent it from
regaining the function of an OC cell ; and (b) that at this stage in ontogeny
at least, the structural changes which occur in an RS cell have a secondary and dependent role, rather than a central and regulatory function,
in the morphogenetic events which follow.
We are confronted, then, with the need to interpret the functional
significance of the cellular parameters altered during morphogenetic
reversal. In short, we must decide whether or not these changes are, in
fact, responsible for the new developmental pathway. The various
correlations observed so far, though consistent with our hypothesis, do
not in themselves establish without question that they play a controlling
role. To do this, it will be necessary to establish the existence of an
obligatory connection between experimentally-altered enzyme activities,
for example, and the morphogenetic response.
The fragmentary evidence so far available implies that the conversion
of a 32-hour RS cell to an OC cell involves protein turnover ; presumably
this results from the release of inductive and/or repressive activities
mediated directly or indirectly by bicarbonate. That it is turnover,
rather than an overall transformation, is suggested by: (a) the lack of
net change in total protein; (b) the increase in some enzyme activities
and the simultaneous decrease in others; and (c) the fact that the
process occurs in the absence of exogenous substrates (i.e., it will occur
in water). Thus, if synthesis occurs it must do so at the expense of the
degradation of other, non-essential intracellular components; this would
be analogous to rapid turnover in bacteria under conditions which
prevent growth (Mandelstam, 1960). A similar situation was described
for sporulating bacteria in water and inorganic salts (Foster and Perry,
1954; Monro, 1961). Since there is also some evidence for RNA turnover
during the transformation of RS to OC cells (Cantino, 1961b), it should
EDWARD C. CANTINO AND JAMES S. LOVETT
All the evidence, then, points to the conclusion that plants derived
from RS thalli by removal of exogenous bicarbonate function like OC
cells, but that they are not entirely identical to them in total structure
and chemical composition (see Cantino and Goldstein, 1962). This
conclusion in no way vitiates the functional significance of morphological
reversibility in B. emersonii ; the influence of a previous environment on
the chemical composition of other micro-organisms has been amply
demonstrated (Herbert, 1961). Our results suggest that gross structural
de-differentiation is much less easily accomplished (if at all) than other
characteristics by removal of the morphogenetic inducer. Indeed, it
seems reasonable to assume that a structural or storage material would
be less directly and less rapidly subject to induction or repression than
an enzyme. Thus, while our knowledge is admittedly far from complete,
we can conclude : (a) that prior to formation of the cross-wall, no changes
occur in an RS cell which, upon removal of the inducer, prevent it from
regaining the function of an OC cell ; and (b) that at this stage in ontogeny
at least, the structural changes which occur in an RS cell have a secondary and dependent role, rather than a central and regulatory function,
in the morphogenetic events which follow.
We are confronted, then, with the need to interpret the functional
significance of the cellular parameters altered during morphogenetic
reversal. In short, we must decide whether or not these changes are, in
fact, responsible for the new developmental pathway. The various
correlations observed so far, though consistent with our hypothesis, do
not in themselves establish without question that they play a controlling
role. To do this, it will be necessary to establish the existence of an
obligatory connection between experimentally-altered enzyme activities,
for example, and the morphogenetic response.
The fragmentary evidence so far available implies that the conversion
of a 32-hour RS cell to an OC cell involves protein turnover ; presumably
this results from the release of inductive and/or repressive activities
mediated directly or indirectly by bicarbonate. That it is turnover,
rather than an overall transformation, is suggested by: (a) the lack of
net change in total protein; (b) the increase in some enzyme activities
and the simultaneous decrease in others; and (c) the fact that the
process occurs in the absence of exogenous substrates (i.e., it will occur
in water). Thus, if synthesis occurs it must do so at the expense of the
degradation of other, non-essential intracellular components; this would
be analogous to rapid turnover in bacteria under conditions which
prevent growth (Mandelstam, 1960). A similar situation was described
for sporulating bacteria in water and inorganic salts (Foster and Perry,
1954; Monro, 1961). Since there is also some evidence for RNA turnover
during the transformation of RS to OC cells (Cantino, 1961b), it should