70
EDWARD C. CANTINO AND JAMES S. LOVETT
2. Reversibility
of RS Development Before the Point of No Return
Early in the work with synchronous cultures, it became obvious that
many changes (in addition to those emphasized in Section III, D)
associated with RS morphogenesis occur several hours before the point
of no return. Of particular importance is the appearance of melanin in
the cell wall and carotene in the cytoplasm. Melanin (and by inference,
the polyphenoloxidase associated with its synthesis; Cantino and
Horenstein, 1955) is first detectable in a young RS cell at approximately
24 hours ; the exact point at which carotenoid first appears has not been
determined. Since both of these materials are characteristically present
in a mature RS cell but absent from an OC cell, their presence at an early
stage in ontogeny indicates an expeditious synthesis of at least two new
pathways as a result of bicarbonate induction. All other chemical and
physiological differences between OC cells and RS cells (which have not
reached the point of no return) are quantitative rather than qualitative;
e.g., levels of enzyme activity (Lovett and Cantino, 1961), relationships
among soluble protein fractions (Cantino and Goldstein, 1962), and
rates of both growth and respiration (Cantino and Lovett, 1960).
Finally, the first morphological expression of RS formation, which occurs
at approximately 30 hours, is the obvious migration of cytoplasmic material towards the apical region of the thallus where the RS cell is destined
to be formed. In OC cells, this occurs only under certain conditions.*)*
These changes clearly show that early in ontogeny, bicarbonate exerts
a profound effect upon the growing thallus of Blastocladiella.
Nevertheless, up to the point of no return at 36 hours, the organism retains an
inherent plasticity ; it is able to revert to an OC-like cell upon removal of
the morphogenetic inducer. These observations, therefore, raise some
fundamental questions: (1) are these changes, associated with RS
formation but occurring before 36 hours, obligatory for completion of
morphogenesis; (2) when the developmental pathway is reversed by
t Under ideal conditions in synchronized, single-generation cultures, an OC plant is
essentially a single cell; the empty, basal rhizoidal cell so typical of an RS plant is
virtually non-existent (Goldstein and Cantino, 1962). The formation of OC plants
consisting of two well-defined cells separated by a cross-wall (as found in OC plants
resulting from morphogenetic reversal) does occur automatically, however, when such
plants are crowded together in clones ; these are the same conditions that usually lead to
formation of RS cells without addition of exogenous bicarbonate (Cantino, 1951, 1956).
Acropetal migration of the cytoplasm and concomitant formation of a large, basal cell are,
we think, expressions essentially characteristic of RS plants. The basipetal formation of
rhizoids, on the one hand, and the direction of cytoplasmic migration towards the future
site of the resistant sporangium, on the other, are clearly of opposite polarity. This
phenomenon has been strikingly demonstrated on several occasions by abnormal plants
which produce two, almost diametrically-opposite sets of rhizoids. When cytoplasmic
migration finally occurs in such thalli, it is towards a locus maximally distant from both
rhizoidal centres of attachment.
EDWARD C. CANTINO AND JAMES S. LOVETT
2. Reversibility
of RS Development Before the Point of No Return
Early in the work with synchronous cultures, it became obvious that
many changes (in addition to those emphasized in Section III, D)
associated with RS morphogenesis occur several hours before the point
of no return. Of particular importance is the appearance of melanin in
the cell wall and carotene in the cytoplasm. Melanin (and by inference,
the polyphenoloxidase associated with its synthesis; Cantino and
Horenstein, 1955) is first detectable in a young RS cell at approximately
24 hours ; the exact point at which carotenoid first appears has not been
determined. Since both of these materials are characteristically present
in a mature RS cell but absent from an OC cell, their presence at an early
stage in ontogeny indicates an expeditious synthesis of at least two new
pathways as a result of bicarbonate induction. All other chemical and
physiological differences between OC cells and RS cells (which have not
reached the point of no return) are quantitative rather than qualitative;
e.g., levels of enzyme activity (Lovett and Cantino, 1961), relationships
among soluble protein fractions (Cantino and Goldstein, 1962), and
rates of both growth and respiration (Cantino and Lovett, 1960).
Finally, the first morphological expression of RS formation, which occurs
at approximately 30 hours, is the obvious migration of cytoplasmic material towards the apical region of the thallus where the RS cell is destined
to be formed. In OC cells, this occurs only under certain conditions.*)*
These changes clearly show that early in ontogeny, bicarbonate exerts
a profound effect upon the growing thallus of Blastocladiella.
Nevertheless, up to the point of no return at 36 hours, the organism retains an
inherent plasticity ; it is able to revert to an OC-like cell upon removal of
the morphogenetic inducer. These observations, therefore, raise some
fundamental questions: (1) are these changes, associated with RS
formation but occurring before 36 hours, obligatory for completion of
morphogenesis; (2) when the developmental pathway is reversed by
t Under ideal conditions in synchronized, single-generation cultures, an OC plant is
essentially a single cell; the empty, basal rhizoidal cell so typical of an RS plant is
virtually non-existent (Goldstein and Cantino, 1962). The formation of OC plants
consisting of two well-defined cells separated by a cross-wall (as found in OC plants
resulting from morphogenetic reversal) does occur automatically, however, when such
plants are crowded together in clones ; these are the same conditions that usually lead to
formation of RS cells without addition of exogenous bicarbonate (Cantino, 1951, 1956).
Acropetal migration of the cytoplasm and concomitant formation of a large, basal cell are,
we think, expressions essentially characteristic of RS plants. The basipetal formation of
rhizoids, on the one hand, and the direction of cytoplasmic migration towards the future
site of the resistant sporangium, on the other, are clearly of opposite polarity. This
phenomenon has been strikingly demonstrated on several occasions by abnormal plants
which produce two, almost diametrically-opposite sets of rhizoids. When cytoplasmic
migration finally occurs in such thalli, it is towards a locus maximally distant from both
rhizoidal centres of attachment.