Genes and plant cell walls: a difficult relationship

Chemical information? carried by genes, is one of several types of informat ion important for the functioning of cells and organisms. While genes gover n the two-dimensional flow of information, the cell walls are at the basis of a structural, three-dimensional framework of plant form and growth. Rece nt data show the walls to be a cellular 'organelle' undergoing dynamic chan ges in response to a plethora of stimuli. In this review, an integrated app roach, rooted in the organismal perspective, is taken to consider the role of cell walls in the biology of plants. First, the complexity of molecular and biochemical events leading to the biosynthesis of wall components is de scribed within the framework of its spatial cellular organisation, and the major regulatory check-points are characterised. Second, cell walls form a structural and functional continuum within the whole plant and thus could b e defined in relation to the protoplasts that produce them and in relation to the plant itself. Model systems of suspension-cultured cells are used to reveal the existence of a bidirectional exchange of information between th e protoplast and its walls. The 'plasticity' of plant cell reactions, seen in defence responses or in changes in wall composition, to e.g. stress, pla nt growth regulators or chemical agents as well as the role of cell walls a nd/or wall components in somatic embryogenesis are also discussed. Third. b eing a continuum within the plant body, the walls fulfil vital functions in plant growth and development, The examples characterised include the deter mination of cellular polarity and the plane of cell division, cytokinesis, and the role of plasmodesmata in cell-to-cell communication and the formati on of functional symplastic domains. Fourth, the exocellular control of mor phogenetic processes is described and the potential of cell walls as determ inants or reservoirs of positional information is indicated. Particular emp hasis is put on the (bio)chemical signals coming through or derived from ce ll walls as well as the mechanical properties of the walls. Based on those data, the 'plant body' concept is formulated. The plant is thus treated as a unit filled with intertwining networks: (1) symplastic, (2) the endomembr ane system and (3) cytoskeletal, with cell walls providing an architectural scaffolding and communication ports formed within (4) the cytoskelelon-pla sma membrane-cell wall continuum.