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Biomedical subjects

P B Green

Publications and source records attributed to P B Green.

At least 19 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↗
PubMed1997

Transductions to generate plant form and pattern: an essay on cause and effect.

Many complex processes can be broken into transduction steps where one state is converted to another by a well defined activity. One difficulty for analysis is that transductions occur in chains or networks. Another, of primary concern here, is that a single transduction can be complex. Some such transductions can efficiently explain phenomena often thought to be summations or orchestrations of many simple transductions. Pattern formation is in this category. For a wide range of transductions one can define cause and effect in a differential equation. In its integral one can define the before and after states. The main experimental tactic to characterize unknown transductions is co-variation. The before state (input) is altered, change in the after state (output) is assayed. Thus an unknown transduction, with cause and effect embodied in the differential, is investigated through long-term changes in its integral. This is fully practical when all of the integral is known or readily surmised, as in simple discrete biochemical transductions. As causal differential expressions become complex, their integrals become more versatile in generating output because this changes not only with variation in the expression itself but also with boundary conditions and limits. These very features, however, make such a function increasingly intractable to discovery by co-variation. Only a small part of the integral is embodied in the before and after states; the remainder is not readily surmised. Accordingly, in contrast to reliance on the role of controls to deduce unknown simple transductions, the complex ones are generally established through formalization of the differential nature of the process itself.

Biophysical Phenomena↗
PubMed1996

Transductions to generate plant form and pattern: an essay on cause and effect.

Many complex processes can be broken into transduction steps where one state is converted to another by a well defined activity. One difficulty for analysis is that transductions occur in chains or networks. Another, of primary concern here, is that a single transduction can be complex. Some such transductions can efficiently explain phenomena often thought to be summations or orchestrations of many simple transductions. Pattern formation is in this category. For a wide range of transductions one can define cause and effect in a differential equation. In its integral one can define the before and after states. The main experimental tactic to characterize unknown transductions is co-variation. The before state (input) is altered, change in the after state (output) is assayed. Thus an unknown transduction, with cause and effect embodied in the differential, is investigated through long-term changes in its integral. This is fully practical when all of the integral is known or readily surmised, as in simple discrete biochemical transductions. As causal differential expressions become complex, their integrals become more versatile in generating output because this changes not only with variation in the expression itself but also with boundary conditions and limits. These very features, however, make such a function increasingly intractable to discovery by co-variation. Only a small part of the integral is embodied in the before and after states; the remainder is not readily surmised. Accordingly, in contrast to reliance on the role of controls to deduce unknown simple transductions, the complex ones are generally established through formalization of the differential nature of the process itself.

Biophysical Phenomena↗
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↗
PubMed1982

Bud Induction with Cytokinin : A LOCAL RESPONSE TO LOCAL APPLICATION.

A portion of the surface of detached Graptopetalum paraquayense E. Walther leaves can be used to assay small amounts of reagents in lanolin for their ability to induce shoots only at the site of application. The cytokinins benzyladenine, kinetin, and 6-(gamma,gamma-dimethylallylamino)purine (DMAAP) were tested, and DMAAP was most effective in bud induction at concentrations below 1%. The higher the hormone concentration, the sooner the appearance of leaf primordia and the higher the ultimate yield of buds. Leaves treated with DMAAP for 2 days developed buds as rapidly as those with longer treatments.

Journal Article↗
PubMed1981

Rapid Suppression of Growth by Blue Light : BIOPHYSICAL MECHANISM OF ACTION.

The mechanism of the rapid inhibition of hypocotyl elongation by blue light was investigated in cucumber (Cucumis sativus L.) and sunflower (Helianthus annuus L.) seedlings by measuring the changes in turgor during the response. A special device, based on the resonance frequency principle, was built which permitted simultaneous and continuous measurements of both tissue rigidity (turgor) and growth rate on a single intact hypocotyl. The large decrease in growth rate following blue irradiation was consistently accompanied by a small increase in resonance frequency. This result indicates that blue light inhibits growth by decreasing the yielding properties of the cell walls, resulting in a slight rise in turgor because of the coupling between growth rate and turgor.The nature of the blue-light inhibition was further studied by measuring the influence of light dose and temperature on the time course of inhibition (lag-time, half-time of inhibition, and amount of inhibition) with the aid of a microcomputer-based system for measuring growth rate and for controlling light duration and energy. The light dose has no influence on either the lag-time or the half-time of inhibition, but strongly affects the amount of inhibition. In contrast, a 10 degrees C drop in temperature (from 30 to 20 degrees C) lengthened the lag-time of the blue-light response, but did not significantly affect the half-time or the per cent inhibition by blue light. The half-time for changes in hypocotyl length (induced by applying a hydrostatic pressure to the roots or to the cut end of seedlings with roots excised) was found to be the same as the half-time of the blue-light inhibition (15 to 25 seconds in cucumber; 90 to 150 seconds in sunflower). These results support the idea that blue light, after a fixed lag period, induces an immediate decrease in the yielding properties of the cell walls. The growth rate subsequently decreases with a half-time that depends on the time required for cell turgor pressures to reach their new steady-state values.

Journal Article↗
PubMed1977

Cell growth pattern and wall microfibrillar arrangement: experiments with nitella.

In cylindrical cells growing throughout their length, over-all transverse reinforcement of the wall by microfibrils is believed to be required for cell elongation. The multinet theory states that in such cells microfibrils are deposited at the inner surface of the wall with transverse orientation and are then passively reoriented toward the longitudinal direction by the predominant longitudinal strain (surface expension). In the present study young Nitella cells were physically forced to grow in highly abnormal patterns: in length only, in girth only, or with localized suppression of growth. Subsequent gradients of microfibrillar arrangement within the wall cross-section were measured with polarized light and interference microscopes. The novel wall structures produced were in all cases explainable by passive reorientation, i.e. by the multinet theory. The study also showed that orientation of synthesis remains insensitive to several of the physical manipulations that strongly influence the passive behavior of wall microfibrils. Only the localized complete suppression of surface growth led to the deposition of nontransverse cellulose. These results suggest that the presence of strain is needed for continued oriented synthesis, but that the directional aspect of strain is not an "instructional" agent continuously guiding the orientation of synthesis, once this orientation has been established.

Journal Article↗
PubMed1974

Growth rate and turgor pressure: auxin effect studies with an automated apparatus for single coleoptiles.

Because turgor pressure is regarded as the driving force for cell extension, any general theory of plant growth requires quantitative information on the relationship between steady irreversible growth rate and turgor pressure. To investigate contrasting views of this relation an automated apparatus was constructed which perfused both the outer and inner epidermis of a single coleoptile while its growth rate was continuously recorded. Turgor was altered abruptly by perfusing with solutions of varying tonicity. With specially grown rye coleoptiles the half-time of the osmo-elastic response was reduced to 2 minutes or less. After decay of this response, however, rate continued to change (so as to partially compensate the effects of the turgor shift in question) for 30 to 60 minutes. Only then could a steady rate be taken. A characterization of steady rate versus turgor covering five turgor values for a single coleoptile thus required many hours. The conclusions are as follows. (a) The change in steady rate, per unit change in turgor, was much greater +IAA than -IAA. (b) Both auxin and turgor act to reset an apparent stabilizing system whose presence is shown in the partial compensation of the initial response to turgor shifts. The above "extensibility" changes are operational only. They need not reflect changes in the immediate physical extensibility of the wall; they could reflect changes in a process acting on the wall. (c) The growth rate versus turgor relation shows some hysteresis.

Journal Article↗
PubMed1971

Metabolic and physical control of cell elongation rate: in vivo studies in nitella.

Several levels of control of elongation rate are revealed through the detailed study of responses of the Nitella internode to abrupt shifts in turgor. The immediate response, which apparently reflects the physical state of the cell, is approximately described by the equation r = (P - Y)m where r is rate, P is pressure, Y is the wall's yielding threshold, and m is related to the wall's apparent fluidity (reciprocal viscosity). Because P and Y are in the range 5 to 6 atmospheres, and (P - Y) is roughly 0.2 atmosphere, elongation rate is initially extremely sensitive to changes in P. A small step-down in turgor (0.7 atmosphere) stops growth, and a similar rise greatly accelerates it. These initial responses are, however, soon (15 minutes) compensated by changes in Y. An apparent metabolism-dependent reaction (azide-sensitive) lowers Y; strain hardening (azide-insensitive) raises it. These two opposing processes, acting on Y, serve as a governor on (P - Y), tending to maintain it at a given value despite changes in P. This ability to compensate is itself a function of turgor. Turgor step-downs are less and less well compensated, leading to lower rate, as turgor falls from 5 atmospheres to about 2 atmospheres where growth appears not to resume. This is the lowest attainable yield value, Y(1). The turgor dependency of compensation reflects a turgor requirement of the Y-lowering ("wall-softening") process. Thus the relation between steady state, r(s), and turgor is an indirect one, derived from time-dependent alterations of the cell wall. This relationship superficially resembles the instantaneously valid one in that, roughly, r(s) = (P - Y(1))m(s). Y(1) and m(s), however, have much lower values than Y and m. The duality of the elongation rate versus turgor relation and the prominent role of Y in regulating rate are the major features of growth control in Nitella.

Journal Article↗