Cell expansion in the epidermis: microtubules, cellulose orientation and wall loosening enzymes

Two models of isolated epidermis were used to demonstrate that the net orie ntation of cellulose microfibrils in the cell wall is related to mechanical properties of the tissue, and can be used as an indicator for wall anisotr opy. In the developing plant epidermis, cells expand in one or two directio ns in the plane of the plant surface. In epidermis cells actively expanding in one direction (elongation), the orientation of cortical microtubules cl osely matches the net cellulose orientation. In epidermis cells expanding i n two directions, the orientation of the parallel microtubules does not coi ncide with the net cellulose orientation in the adjacent cell wall. The ori entation of cortical microtubules is thus not always a reliable indicator o f wall characteristics. In both types of epidermis, a high rate of expansio n correlates with a high activity of xyloglucan endotransglycosylase (XET), as determined in situ. This high activity alone cannot explain unidirectio nal wall expansion.