Cell adhesion, cell separation and plant morphogenesis

A simplified and useful view of plant cell development is that it involves the spatial and temporal organization of cell division, cell expansion and cell differentiationall taking place within the constraints of a lack of cell migration.A facet of such development that has often been neglected, is understanding how the cells of a developing structure hold fast or adhere.Subsequently in development the adhered cells of apices or embryos give rise to more loosely attached cells in mature structures where the degree of cell separation and intercellular space formation is developmentally regulated.Recent cell biological studies, most significantly in the derivation of monoclonal antibody probes forthe plant cell surface (Knox, 1992;Roberts, 1990), have increased our awareness of the local modifications of cell walls and rekindled interest in aspects of plant cell wall adhesion and intercellular space formation.It seems timely to examine our current understanding of the mechanisms of cell adhesion and separation and their importance for plant morphogenesis.Molecular probes for cell surface components offer immense potential, not only for the specific study of the local wall modifications such as those resulting in cell separation but also as probes of molecular markers of cell identity relating to position and context.This essay attempts to draw some of the threads together regarding both the nature and importance of cell adhesion and cell separation in plant tissues. Plant and animal cell adhesionFor animal cells, cell adhesion is an extremely important and dynamic process, involving cell recognition which is mediated by a large number of distinct adhesive systems.Diverse molecular mechanisms are utilized to aid the sorting and aggregation of appropriate cells during the development of tissues and structures.Recognition occurs between the plasma membranes of adjacent cells, mediated by cell surface glycoproteins such as cadherins, and also between cells and the extracellular matrix.Examples of the latter class of interaction include the adhesive glycoproteins of the extracellular matrix such as fibronectin and vitronectin that are recognized by plasma membrane