11. Growth and Morphogenesis of a Plant Cell: Nitella axillaris

11. Growth and Morphogenesis of a Plant Cell: Nitella axillaris

The preservation of the Developmental Biology Film Series was made possible by generous contributions from Publisher of “The Biggest Picture” Producer of the documentary, “Symbiotic Earth” Growth is a developmental activity of all organisms. In plants, one major aspect of growth is elongation. This is easily seen in stems, roots, and certain leaves such as those of grass. Elongation of a structure such as a stem or root is mainly the result of the extension of its individual cells. In the formative part or meristem of the elongating organ, cell enlargement and division are in balance so that a small average cell size is maintained. The cells that are to extend the axis of the growing structure soon stop dividing. The elongation process leads to the formation of very long cells. This sequence of events occurs in the shoot development of the pond alga, nitella. At the growing tip of this plant a single apical cell alternately enlarges and divides, but behind the dividing tip, cell division soon stops and the process of elongation produces a cell of gigantic size. The immediate cause of growth of these giant internode cells is the yielding of the cell wall to the pressure of the central vacuole. In a plant cell which has a high concentration of solutes in it, there is normally considerable turgor pressure. Living nitella cells in pond water are quite rigid. The osmotic origin of the internal pressure is revealed when the cell is transferred to a concentrated sugar solution. The high solute concentration of the cell is now matched by that of the outside medium. Water escapes from the cell and the cell collapses. Returning the plant to the pond water restores the gradient, the pressure, and the rigidity. From the fact that a one-fifth molar sucrose solution will just suffice to collapse the cell, it can be calculated that the turgor pressure of the cell in pond water should reach about five atmospheres, or 75 pounds per square inch. This is remarkably high considering that a pressure cooker or autoclave develops a pressure of only one atmosphere. To measure this pressure, one can observe the compression of a volume of air trapped in a u-Shaped glass capillary which is closed at one end. When the open point is inserted into a living nitella cell the internal pressure of the cell compresses the trapped air initially at atmospheric pressure to approximately one-sixth of its original volume. This shows that the pressure within the cell is 5 atmospheres. When sucrose is added to the water the turgor pressure drops and the trapped air bubble expands. As the sugar diffuses away the pressure builds up again. This capillary method for the direct measurement of turgor pressure has the advantage that it can be applied to the growing cell, and the relation between growth rate and turgor pressure can be observed. Here seen in time-lapse, the growing cell has a constant high turgor pressure. When the medium is changed for a more concentrated one the pressure falls and suddenly elongation stops. With a return to normal conditions the pressure is restored and growth resumes. The normal extension of a stem or root is the result of hundreds of microscopic cells all extending in the same direction. While elongation is widespread among plants, how a cell grows as a cylinder rather than as some other shape is not evident. One would expect cells with high internal pressure to become round, in fact Nitella cells do become round when their normal morphogenesis is impaired by chemical treatment. There are two general ways by which a cell may depart from the spherical form and grow as a cylinder. First it may elongate by localized non-directional expansion at the tip like this pollen tube. The second alternative is that the cell may in effect stretch itself throughout its whole length. This is what happens in the tissue cells of higher plants and in the internodes of Nitella. To prove this for Nitella the cell is marked and the behavior of the marks followed during subsequent growth. A convenient way to mark the cell is with resin beads which carry a charge enabling them to cling to the surface of the cell. These are placed in position and spaced regularly with a human hair. Extension of the cell wall can be seen between each pair of beads showing that every part of the cell is displaying directed growth. This is what happens in tissue cells. The twisting motion which shows up is characteristic of Nitella, but it is not essential to the growth process in general. The longitudinal stretching of a cell with high internal pressure can take place only if there is some reinforcement, resisting increase in girth. The ordinary light microscope reveals no structure that could play this reinforcing role. In polarized light however the cell appears bright at the oblique positions, so one can conclude that ordered crystallites, in this case cellulose microfibrils, are present. Does their orientation give the necessary reinforcement? Optical analysis is easier when the cell wall is isolated and flattened. To do this a piece is cut from a Nitella internode cell and the cytoplasm and chloroplasts are removed by stroking it with a hair loop. The transparent cell wall which remains is then examined in polarized light. It is seen to behave like a simple crystal, allowing light to pass through when in the oblique positions. This shows that the cellulose microfibrils are oriented either parallel to or perpendicular to the axis of the cell. The correct orientation is found by comparing the behavior of the wall with that of two test strips of stretched cellulose in which the orientation of the cellulose is known. A thin crystal gives a lavender tint to the field. When the test strips are rotated one turns red and the other turns blue according to their orientation. The piece of cell wall is now introduced. It is seen to have a transverse arrangement of its constituent cellulose microfibrils. The cell then is reinforced by the synthesis of strands or hoops of cellulose, which strongly resist expansion of the circumference under the influence of turgor pressure. It follows that most of the growth must of necessity take place along the long axis of the cell. In the living cell the curvature of the wall causes a striped appearance under polarized light. The presence of a central bright band flanked by a dark and light band on either side is characteristic for a predominantly transverse arrangement of cellulose. This arrangement is present during growth. If a drug such as colchicine is added the cell is unable to make an organized wall. The cellulose strands are scattered at random in the surface of the cell. Lacking the reinforcements of regular hoops of cellulose the cylinder swells into a sphere. The random arrangement of cellulose in the curved wall brings on a black cross image when the cell is seen in polarized light. The same transition in cell form from the cylindrical to the spherical occurs naturally in the alga Hydrodictyon africanum. At higher power in polarized light the corresponding shift in wall structure is seen. Cylindrical cells have the striped pattern, round cells show the black cross. To sum up, cell elongation is an expansion process where high pressure serves as the driving force of growth and at the same time, contributes to the rigidity of the plant. The cells of Nitella as well as those of elongating plant organs such as stems and roots, grow as cylinders because their cells undergo directed expansion along the cell axis. This is achieved by circumferential reinforcement with cellulose microfibrils. Cell elongation is thus seen to be based on the interaction between turgor pressure and the special yielding qualities of the cell wall. [Music]


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