Cell wall microstructure and plant cell separation

When plant products are used by humans - whether for food or fibre - the capacity of cells to remain attached is an important attribute. Eating an apple in which cells separate will seem mealy and dry. Eating an apple in which cells stay attached will lead to cell bursting upon impact and a juicy sensation. All plant cells have a cell wall and this imposes distinctive mechanisms upon development. Plant cells are attached together by means of their cell walls and at cell division daughter cells are attached together by means of a new cell wall and generally remain attached being neighbours for life. In certain cases plant cells can separate either to release leaves or fruit or to change the mechanical properties of a plant tissue or organ. Little is known of the mechanisms whereby plant cells are attached together or of the mechanisms whereby attachments are lost in a controlled way. Using monoclonal antibodies we have identified two cell wall pectic polysaccharides that are specific to two distinct cell separation processes. The first of these processes is the formation of intercellular space in pith tissues and the second is the detachment of single cells from an organ - such as the sloughing off of root cap cells. We will study the nature of links between plant cells and isolate cell separation-specific pectins to determine their structure and properties. Understanding the mechanisms whereby plant cells can modify their attachments to each other will be essential for the development of effective use of plant materials in terms of the texture of food products and mechanical properties of plant products such as fibres. In addition it will be important knowledge for plant biology in general.

The molecular nature of middle lamellae (or intercellular matrices) that are the junctions between all cells in developing and mature plant organs is very poorly understood. A key facet of these zones of plant cell walls is that in certain circumstances they are specifically dismantled to allow various modes of cell separation. Pectic polymers of the homogalacturonan (HG) class are presumed to be involved and some evidence supports this. Some observations also support the involvement of pectic polymers rhamnogalacturonan-I and rhamnogalacturonan-II. We have recently identified two monoclonal antibodies that bind to pectic epitopes that are specific to two of these modes of cell separation. Antibody LM7 binds to a partially methylesterified epitope of HG that occurs specifically at the corners (junction between adhered and non-adhered cell faces) of developing and mature intercellular spaces in all parenchyma systems. Antibody LM8 binds to a xylogalacturonan (XG) that is specific to cell walls of all cells that are in process of detaching from organs, such as root cap cells, in all angiosperm systems so far examined. We will use these antibodies to dissect cell separation processes in these two systems. We will identify linkages holding these specific pectins at their specific locations in cell walls. Using immunoaffinity procedures we will isolate these specific pectins (LM7/HG and LM8/XG) for characterisation of their structure and in vitro properties in pectate gels and synthetic cell wall composites. This information will greatly extend our understanding of the cell wall microstructure and specifically as it relates to cell attachment and its loss. In addition to its importance for plant biology in general this will be important underpinning biology for food and fibre technology.