Functions of the cell wall in the interactions of plant cells: Analysis using carrot cultured cells

Interactions between cells and between tissues are important in the development and morphogenesis of higher plants. Attempts to characterize the role of the cell wall in such interactions have benefited from the use of carrot (Daucus carota L.) cultured cells in vitro as a model system. The development of carrot cells in culture can be divided into three processes: the acquisition of embryogenic competence; the development of the embryo; and the maturation and dormancy of the embryo. Induction of non-embryogenic callus is accompanied by weakened intercellular attachment, decreased levels of endogenous ABA and a decrease in responsiveness to exogenous ABA. Cell wall polysaccharides are known to be involved in various developmental and morphogenetic events. In carrot cultured cells, possible roles in intercellular attachment have been proposed for arabinan and xylose in the neutral sugar regions of pectins, and various extracellular proteins have been shown to be involved in somatic embryogenesis in vitro. Some of these proteins are also present around and/or in zygotic embryos, possibly being involved in the formation and functions of zygotic embryos and seeds. A 57-kDa extracellular soluble glycoprotein that binds to insulin-like peptides and an 18-kDa extracellular insoluble cystatin that inhibits the proteinases of germinating seeds of carrot might be involved in cellular signal transduction and intertissue interaction, respectively, in carrot seeds.