Biochemical determinants of plasma membrane to cell wall adhesion in plants

One consequence of adaptation to environmental stress is the formation of a tight adhesion of the plasma membrane to the cell wall. This adhesion is often visualized as concave plasmolysis and has been demonstrated in tobacco suspension culture cells adapted to grow in medium containing 428 mM NaCl, in plant-pathogen interactions (Lee-Stadelmann et al. 1984), and in epidermal cells of several species including onion. The adhesion of the cell wall to the plasma membrane in onion bulb tissue is eliminated by $\mu$M amounts of octyl guanidine but unaffected by mM amounts of EGTA and, thus, Ca$\sp{2+}$ independent. Proteins showing cross-reactivity to antisera against human fibronectin (Fn) and vitronectin (Vn) are enriched in cell walls and membranes of tobacco suspension culture cells adapted to osmotic stress (Zhu et al. 1993) and in onion bulb tissue. In leek and onion bulb tissues, I have also identified a 46 kDa protein (FnBP) that binds human fibronectin a modified Western blot. Although the protein may be a membrane protein that is tightly bound to the wall, the FnBP can be extracted from a purified cell wall fraction using SDS, urea, or CaCl$\sb2$. Using the same modified Western screening technique, clones have been selected from an expression library of leek epidermal tissue. The clones encode novel proteins that retain the Fn-binding activity. Sequence analysis of the gycine-rich proteins (GRPs) indicate that some GRPs may be localized on the cytoplasmic side of the plasma membrane. Two of the Arabidopsis GRP genes (atGRP-3 and atGRP-5) encode proteins with strong hydrophilic regions flanking their putative membrane-spanning domains. Analysis of these hydrophilic domains indicate that the atGRP-3 protein would be translocated at the ER membrane while the protein encoded by atGRP-5 would not, resulting in localization to the cytosolic face of the plasma membrane. These and other structural proteins may form a continuum between the cell wall and the cytoskeleton in higher plants similar to the integrin-extracellular matrix protein complex in animal systems