Endomembrane system cell wall

The major polymers that make up the wall are polysaccharides and lignin. These occur together with more minor but very important constituents such as protein and lipid. Water constitutes a major and very important material of young, primary walls (2). The lignin is transported in the form of its building units (these may be present as glucosides) and is polymerized within the wall. Those polysaccharides which make up the matrix of the wall (hemicelluloses and pectin material) are polymerized in the endomembrane system and are secreted in a preformed condition to the outside of the cell. Further modifications of the polysaccharides (such as acetylation) may occur within the wall after deposition. Cellulose is polymerized at the cell surface by a complex enzyme system transported to the plasma membrane (3). 

The formation of pectin and its assembly for final export across the plasmamembrane will be considered here as a production line to indicate the various limiting steps which are controlled to monitor its production. The various channels for the movement and synthesis will be described separately so that the control points can be assessed. The channels which operate for production and movement of the polymers within the cytoplasm and endomembrane system of the cells are - Channel 1. Production and movement of nucleotide sugar donors. Channel 2. Synthesis and compartmentalization of the pectin polymers within the endomembrane system. Channel 3. Movement of vesicles and fusion with the plasmamembrane for assembly and deposition within the wall. 

Fig. 1. Proposed process by which an infection thread passes through a cell wall between two plant cells, in this case between the root hair cell and an adjacent cortical cell. (1) The membrane at the tip of the infection thread has fused with the root hair cell plasma membrane to form a pore. Tonoplast membrane, TM infection thread membrane ITM rhizobia, R infection thread wall, ITW root hair cell cytoplasm, RHCC root hair cell plasma membrane, RHCP plant cell wall, PCW cortex cell plasma membrane, CCP cortex cell cytoplasm, CCC. The arrows and vesicles represent intense activity of the endomembrane system involved in the synthesis of membranes and wall material. (2) The rhizobia have forced their way, by dividing, into the region between the two cells although they are still enclosed by wall material. (3) An intercellular infection thread is formed by degradation and resyntheses of cell wall material in the vicinity of the dividing rhizobia. (4) The infection thread has entered the cortical cell by promoting invagination and growth of the plasma membrane and wall of that cell, in front of the dividing bacteria.

---