Biophysics of Development of the Plant Shoot

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Multicellular development in plants involves a long causal chain between genome and macroscopic phenotype. Cell behavior is a key intermediate stage. One the one hand the genome provides the basis for the cell behavior. On the other, the cell behavior, when integrated through time and space, produces the successive developmental stages. Dr. Green specialize in these later aspects of the causal chain. Dr. Green studies the development of the shoot in terms of the directional reinforcement, by cellulose, in the walls of growing cells. Cells maintain, or reorient, the direction of cellulose synthesis at mitosis. A biophysical theory to explain the origin and spacing of leaves, phyllotaxis, is being developed. Its main tenet is that growth of recently formed leaves influences the apical dome by stretching its cells which then establish new reinforcement patterns on the dome. This can explain the placement of subsequent leaves. Now he proposes to test experimentally the apparent key relationship- between cell stretch and reinforcement behavior by altering organ stretch patterns and studying cell responses. He also proposes to find out if the present theory can be extended to additional phyllotactic patterns and if it can be applied to the generation of form in leaves. Sufficiency of the biophysical explanations will be tested by a finite element program which correlates physical forces and cell responses in model meristems. A general biophysical theory for shoot development should result. %%% Research in this project centers on the way that plants inherit their geometrical features. The best known examples of pattern in plant shoots are the spiral lines seen in the heads of sunflowers and the helical lines made by the scales on a pine cone. These spiral patterns are also found in the leaf patterns of most plants growing in temperate zones. Obviously, plants have a "developmental engine" that produces leaves in a pattern specific to the plant. The present research is concerned with the cellular basis of the formation of these patterns. The patterns are generated by a cycle of activity at the stem tip. At the stem tip, leaves or other appendages, arise at points of discontinuity in the reinforcements patterns around cell surfaces. These reinforcement patterns are a result of cellulose microfibrils. Once a leaf is establishes, the growth of its base act of the adjacent tissue, stretching it. This causes a response in neighboring cells which create new discontinuities. Thus surface irregularities lead to organs and the organs make irregularities. The cycle can go on to perpetuate a fixed pattern. This research is to expand this interpretation of all patterns and to examine the apparently key connection between physical stretch and the cell response which alters reinforcement direction.