Fig. 1.

Structure of cell wall polysaccharides and their degradation by cell-wall degrading enzymes (CWDEs). Polysaccharides make up most of the cell wall and vary in different species and tissues. Cellulose is the core load-bearing component of cell wall and composed of β-1,4-linked glucan. Pectin provides a matrix that embeds the cellulose-hemicellulose network. The four types of pectin polysaccharides including homogalacturonans, rhamnogalacturonan-I, rhamnogalacturonan-II and xylogalacturonan are abbreviated as HG, RG-I, RG-II and XGA, respectively. Xylan and mixed-linked glucans are the main components of monocots hemicellulose while the xyloglucan is the major hemicellulose in dicots. To overcome the barrier of the plant cell wall, pathogens produce CWDEs to degrade plant cell wall polysaccharides, resulting in the release of cell wall oligosacchrides like cellodextrins and cellobiose from cellulose, oligogalacturonic acids from pectin, the xyloglucan oligosaccharides, 3³-α-L-arabinofuranosyl-xylotetraose (XA3XX) from arabinoinxylan and the D-cellobiosyl-(1,3)-β-D-glucose (MLG43) from mixed-linked glucans