An Interpretation of Dose-Response Curves for Light-Induced Cell Elongation in Fern Protonemata

Current descriptions of the photocontrol of the filamentous growth of fern gametophytes contain several apparent photobiological anomalies. These anomalies are the result of the fundamental assumption that the rate of filament elongation is the growth parameter under direct photocontrol. This paper constructs and tests a model for the photocontrol of filamentous growth: k = A(dl/dt), where k, A, and (dl/dt) represent a constant rate of volume increase, the cross‐sectional area of the filament tip, and the rate of filament elongation, respectively. This model predicts that the photocontrol of filamentous growth could involve the direct regulation of the cross‐sectional area. Because this alternative perspective resolves the apparent photobiological anomalies, the photoresponse of the cross‐sectional area should represent the correct measure of the photocontrol over filamentous growth. It follows from the experimental results that the photocontrol of filamentous growth does not antagonize but rather preconditions the transition to two‐dimensional growth. Therefore, the original grounds for postulating that the transition in growth form involves the photoinduction of specific genes for two‐dimensional growth are no longer valid.

The Photobiology of Fern Gametophytes. Ii. The Photocontrol of Filamentous Growth and Its Implications for the Photocontrol of the Transition to Two‐dimensional Growth

Cooke¹,

Paolillo²

1979

Orientation of the Plane of Cell Division in Fern Gametophytes: The Roles of Cell Shape and Stress

Miller

1980

Self Cite

Apical cells of Onoclea sensibilis L. protonemata were measured to determine areas of new walls which were formed during both transverse and longitudinal cell division. Actual wall areas were compared with calculated areas of hypothetical walls oriented in the opposite sense (i.e., an actual transverse wall compared with a hypothetical longitudinal wall, and the reverse). Among 87 out of 90 cells which were analyzed the actual walls had the least area. Thus, the minimal area hypothesis of cell partitioning accurately predicts wall orientation in this instance, although it appears, on other grounds, that the hypothesis does not furnish a plausible mechanism for wall orientation. The application of Lintilhac's concept of the orientation of cell walls in response to anisotropic stresses in the cell was explored. Photographs of apical cells during deplasmolysis indicated that unequal stresses might be generated in apical cells as a result of the osmotic distension of the elastic protoplast. It is concluded that the primary factor which determines the plane of cell division in the apical cell, and the transition from one‐ to two‐dimensional growth, is the local pattern of stress which exists at the position of the nucleus at the time of onset of cell division and wall formation. Calculations of some geometrical properties of idealized model cells are interpreted to mean that the accuracy of the minimal area hypothesis results from a coincidence of its predictions with predictions of Lintilhac's hypothesis, and no causal significance is attributed to wall areas.

The Control of the Orientation of Cell Divisions in Fern Gametophytes

Cooke

Paolillo

1980

Time‐lapse observations of filamentous fern gametophytes were used to evaluate whether the plane of cell division is referable to the plane of minimal surface area before and during the transition to two‐dimensional growth. Cell dimensions of the apical cell were related to the length/width ratios associated with minimal area in the transverse plane vs. longitudinal plane, by modeling the apical cell as a hemisphere subtended by a cylinder. Our working hypothesis predicts that filamentous growth is perpetuated by an apical cell geometry that makes the transverse division plane the orientation of minimal surface area, whereas the transition to two‐dimensional growth (longitudinal division of the apical cell) occurs once the longitudinal plane becomes the position of minimal surface area. The predictions of this hypothesis are fulfilled regardless of variations in light intensity and light quality, the presence of regulators of metabolism, or whether the experimental perturbation causes a corresponding selective inhibition of the transition to two‐dimensional growth. Thus, the control of the plane of cell division in this system seems to depend on thermodynamic considerations of surface area. Furthermore, we favor the conclusion that the role of the genome in the transition to two‐dimensional growth involves its influence on apical cell dimensions rather than the induction of specific genes for specific morphogenetic mRNAs.