PubMed Health⌕ Search

Biomedical subjects

P B Green

Publications and source records attributed to P B Green.

14 recordsLinked to original sources
PubMed1999

Expression of pattern in plants: combining molecular and calculus-based biophysical paradigms.

Pattern formation in plant meristems occurs across a broad scale. At the topographical level (large scale), tissue folding in the meristem is responsible for the initiation of new organs in specific phyllotactic patterns and also determines organ shape. At the cellular level (small scale), oriented cell division and microtubule-based cellulose reinforcement control cell pattern and growth direction. I argue here that structural specification at each scale is highly efficient if the pertinent gene activity is manifested in two complementary biophysical categories. At large scale, one category is the tendency of the formative tissue to fold with a certain spatial periodicity determined by its material properties (e.g., bending stiffness from cellulose content). This latent tendency is formalized in a differential equation for physical buckling. The second category at this scale comprises boundary conditions that specify how the latent tendency is manifested as topography: whether tissue humps occur as whorls or Fibonacci spirals. This versatile combinatorial format accounts for the relative stability of alternative organ patterning as well as alternative organ shaping (e.g., stamens vs. carpels). It also accounts for the structural shifts seen in normal development and after mutation or chemical/physical intervention. At small scale, the latent differential activity is the tendency for groups of dividing cells to co-align their cytoskeletons. The curvature of the surface opposes this tendency. The least curved part of a new primordium is its quasicylindrical midportion. There, by aligning microtubules and cellulose coherently around the organ, a new growth direction is set. Thus large-scale buckling produces curvature variation, which, in turn, affects the localization and orientation of the cytoskeleton. This scheme for the coherent production of diverse geometrical features, involving calculus at two structural levels, is supported by complex organogenetic responses to simple physical intervention. Also, many morphological alternatives, wild type vs. mutant, reflect single changes in parameters in this differential-integral format.

Journal Article↗
PubMed1992

Biophysical mechanisms for morphogenetic progressions at the shoot apex.

Leaf primordia, first visible as small bumps, are produced in a cyclical pattern at the edges of the shoot apex, a smooth region at the top of the stem. Their formation is a biomechanical process. This review first considers hypothetical construction mechanisms and then summarizes research that provides information about how and where the primordia are made. Studies of growth at the primordium site indicate the importance of growth parallel to the surface in generating the forces for primordium emergence. The symmetry of the pattern of reinforcement by cellulose microfibrils correlates with the subsequent pattern of primordium production. Finite element models of the apex reveal that lateral bulging of the apex results in a gradient of shear stress, with high shear at the future primordium site. In contrast, tension parallel to the surface is lowest at the primordium site. Response of apical surface tissue to punctures indicates that an existing primordium can exert a pulling force tangential to its base and a compressive force perpendicular to its base. These observations lead to identification of a continuous biophysical cycle for apex morphogenesis, in which most of the steps are direct physical consequences of the previous step. Biophysical processes, subject to input from genetic, hormonal, and environmental sources, are thus involved in the construction and patterning of leaf primordia.

Computer Simulation↗
PubMed1986

Plasticity in shoot development: a biophysical view.

The construction and spacing of leaves can be analysed in terms of the direction of reinforcement in the walls of the organ surface. In general, growth is at right angles to the reinforcement. When, however, tissues are actively stretched by adjacent organs they apparently take on, by cell activity, reinforcement which lies in the direction of stretch. Thus reinforcement can dictate extension direction; extension direction, when imposed on a tissue, may dictate reinforcement direction. This proposed two-way relationship has been used to model the activity of shoot meristems. It produces biophysically plausible schemes for the progressive development of various leaf structures and for the cyclical revision of apical structure seen in various types of phyllotaxis.

Models, Biological↗
PubMed1985

Surface of the shoot apex: a reinforcement-field theory for phyllotaxis.

Theory for leaf patterning, phyllotaxis, is usually expressed in terms of interactions in the surface of the apical dome of the shoot. Mechanisms for leaf formation, however, usually relate to phenomena in the longitudinal section, e.g. periclinal divisions. Studying epidermal cell file patterns and the directionality of cellulose in the outer walls of the dome we have found distinct patterns of cells and reinforcing cellulose on the surface. Changes in the epidermal pattern correlate with the phyllotactic sequence to suggest that: Recently established leaves are associated with fields of tangential cellulose reinforcement which extend toward the apical dome. Where such aligned fields come into contact so as to generate relatively abrupt angular changes in reinforcement pattern, a new leaf will appear. As this region of discontinuity develops into a hoop-reinforced structure, the visible primordium, a new single field of tangentially aligned reinforcement is generated. The new field interacts with other fields to continue the cycle. In whorled phyllotaxis two angular discontinuities appear to be involved with each new leaf; the pertinent older leaves are just one plastochron older than the leaf being initiated. In spiral phyllotaxis (3:2) a single angular discontinuity appears to be involved initially; the pertinent older leaves are three and five plastochrons older than the leaf being formed. There are two major differences from previous theories of phyllotaxis. First, the cyclic changes in leaf initiation are thought to be based on the constructive involvement of new leaves in modifying the dome's reinforcement pattern. Many theories assume that leaf formation is spontaneous and the role of nearby leaves is inhibitory. Second, the pattern sequence can explain the concurrent appearance of reinforced stem tissue along with leaves.

Cell Division↗
PubMed1984

Shifts in plant cell axiality: histogenetic influences on cellulose orientation in the succulent, Graptopetalum.

The elongation of typical plant meristems can be explained, biophysically, by the cellulosic hoop reinforcement in the longitudinal walls of most of the cells. A cortical microtubule array is believed to govern the reinforcement alignment. The orientation of the reinforcement is studied here with polarized light. Transverse orientation appears to be invariant, regardless of division direction, in interior cells of at least some established meristems. Exceptional longitudinal reinforcement (i.e., in the direction of growth) is occasionally seen, however, in the outer epidermal wall of established Graptopetalum roots, leaves, and stems. It is found in pairs of elongate cells that arise from a longitudinal division. This behavior is prominent when the epidermis is shifting reinforcement to initiate new organs. Usually the division direction and the new cell long axis coincide; reinforcement parallels both. When these two factors are in marked opposition, as in certain longitudinal divisions of very broad cells, the reinforcement can follow the new cell's long axis even though this orients alignment perpendicular to the new wall. The normal prevailing effect of the division direction may relate to the formation of the cortical microtubule array parallel to the orientation of the preprophase band. The normally synergistic effect of the new cell's long axis may stem from the fact that when the band-like array occupies the smaller pair of anticlinal faces of the cuboidal cell (and hence parallels the cell's long dimension in surface view), it will have the densest and presumably most stable structure. Reinforcement orientation is seen as a function of at least two factors which bear on the stability of the newly forming array.

Cell Division↗
PubMed1967

Turgor pressure: direct manometric measurement in single cells of Nitella.

A small capillary, fused at one end, serves as a micromanometer when the open end is inserted into a large Nitella cell. The cell's ability to compress the gas reveals its turgor pressure directly-save for a small correction due to capillarity. The method gives a lower limit to turgor pressure for the same cell in the normal state. The common method, incipient plasmolysis, gives an upper limit. On Nitella axillaris cells the two methods limit the turgor pressure at 5.1 to 5.7 atmospheres. The manometric method is also applicable to growing cells, where osmotic equilibrium is not present.

Eukaryota↗
PubMed1965

Pathways of cellular morphogenesis. A diversity in Nitella.

Evidence is presented to show that a given change in cell form or size may generally be brought about by a variety of patterns of local surface distortion and expansion. Structural and chemical features of the cell which are important in morphogenesis may thus be expected to relate not to form per se but to the kinetics of surface behavior which establish form. These kinetics evaluate both the rate at which local regions of cell surface expand and the directed character (anisotropy) of this expansion. These variables have been studied in model systems and, through marking experiments, in growing cells of various shapes in Phycomyces, Clypeaster, and particularly Nitella. In the latter plant, prominent "giant internodes" display a well defined longitudinal anisotropic expansion devoid of sizeable gradients in expansion rate. These cells have their origin, however, in apical cells which have a pronounced gradient in area expansion rate (maximal at the tip). The great part of the expansion in the apical cell is apparently isotropic (equal in all directions), but the basal region often shows predominant expansion laterally. This transverse stretching in the apical cell could align cell wall texture and possibly fibrous cytoplasmic constituents, such as microtubules, into configurations significant in later morphogenetic stages, including the elongation of the internodes.

Eukaryota↗