Plants have rigid cell walls that are made up of cell wall polysaccharides that mediate cell-cell adhesion. The primary cell walls of plants consist of two independent and interacting polysaccharide networks: a pectin matrix that embeds the second network comprising cellulose and hemicelluloses.
Pectins are complex heteropolymers mainly composed of negatively-charged α-D-glucopyranosyl uronic acid and some neutral glycosyl residues such as α-L-rhamnopyranose, α-L-arabinofuranose, and β-D-galactopyranose. Pectin networks crosslink one cell’s middle lamella to others, thus acting as the most important tool for cell-cell adhesion.
Adjacent cell walls are connected via membrane-lined channels called plasmodesmata (singular plasmodesma). Each plasmodesma allows the transport of molecules via the connected cytoplasm through contact-dependent signaling, like gap junctions in animals. Unlike gap junctions, plasmodesmata are more flexible because they let molecules pass through the cell wall and membrane. The plasmodesmata also allow direct communication from a single cell to many cells, joining the cells in a symplast network.
Other cellular components such as ferulic acids, xyloglucan-like polysaccharides, and proteins such as specialized wall-associated kinases and extensins also regulate cell adhesion during growth and development, although the exact mechanisms remain unknown.
