Michael T. McManus

Flexibility and commitment in plant cells during development

Publisher Summary Developmental biologists speculate on the similarities and differences among the mechanisms regulating differentiation in plants and animals. Because cells in tissues from differentiated parts of plants can be induced to undergo cell division and then express a complete spectrum of morphogenetic characters (and hence gene products) not expressed by the original parent tissue, plants have been thought to be more plastic in their development than animals. Because tissue cultures derived from leaves or root cells can, for example, be induced to produce whole new individuals, plant cells have been considered to be totipotent. New individuals cannot be regenerated in this way in animals. It is, therefore, frequently assumed that there is some basic underlying difference between these two life forms. The molecular mechanisms underlying differentiation in plants and animals are intrinsically similar, but in plant tissues, the forward progression of different cells along their differentiation pathways is not closely synchronized in time. As a result, it is possible to reconcile the considerable heterogeneity of cell commitment that persists in mature plant tissues with some cells remaining essentially uncommitted, while others are arrested at stages permitting a fewer variety of options (flexibility) for further change in cell determination. Unlike most animal cells, few plant cells, if any, appear to reach a phase of commitment in which, in culture, they will faithfully reproduce daughter cells of only their own committed phenotype.

Abscission and the Recognition of Zone Specific Target Cells. The Role of Ethylene

Abscission in plants is a process of cell separation that occurs at precisely defined positions and leads to the shedding of organs distal to the site of separation [1,2]. In dicotyledons, we know that abscission is preceded by the enlargement of cells at the separation zone and by the localized synthesis of novel proteins, some of which are secreted through the plasmamembrane to the cell wall. Certain of these proteins are presumed to play an essential role in the enzymatic loosening of the polysaccharide and protein matrix of the wall (thereby decreasing the yield stress component in wall extensibility) and in the dissolution of the middle lamellae (thereby reducing the cohesive force that holds the cells in these regions together). One of the earliest developmental effects of ethylene to be recorded noted that abscission at these precise positions could be induced at will by exposing plants to low concentrations of the gas for several hours or more and all the present evidence points to ethylene as the natural hormonal inducer of the abscission process [3]. Normally, abscission occurs when tissues distal to abscission zones increase their ethylene production during ripening or senescence or when plants are subjected to environmental conditions that are sufficiently stressful to cause their basal levels of ethylene production to rise above a critical level.

Hormones, Signals and Target Cells in Plant Development: Terminally Committed Cell Types and the Target Status

The term target cell, first conceived in animal biology, is generally taken to mean a cell that has a pre-determined competence to respond in a defined way to a specific hormone signal. In this volume, the authors present the theory that every plant cell is a target cell for one or more of the plant hormones or other regulatory signals. The different types of cells seen in a mature plant arise as a result of selective responses of meristematic cells to chemical inputs. In this context, the authors provide an overview of plant signals as well as evidence for both shortand long-distance cell-to-cell communication. An interpretation of the target cell concept at the biochemical and molecular levels is then presented using a wide range of examples. This volume will serve as a valuable reference for those working in the field of plant developmental biology.

Hormones, Signals and Target Cells in Plant Development: Cell-to-Cell Signalling: Short and Long Distance

Preface 1. Introduction 2. Hormones and signals 3. Cell-to-cell signalling - long distance and short distance 4. Population diversity of cell types and target identification in higher plants 5. Flexibility of cell types and the target cell status 6. Terminally committed cell types and the target status 7. The mechanisms of target cell perception and response to specific signals 8. Hormone action and the relief of repression 9. The phenomenon of hormonal crosstalk References.