10.4 Breakdown of polysaccharide: pectins
Pectins consist of chains of β1→4 linked galacturonic acids, in which about 20 to 60% of the carboxyl groups are esterified with methanol. They occur primarily in the middle lamella between plant cells. As this represents only a small proportion of the plant wall they are correspondingly of little importance as a component of the bulk of plant litter. However, extensive breakdown of the middle lamella of living plants is brought about by necrotrophic parasites. Pectinases, therefore, are of great importance during fungal invasion of plant tissue (Byrde, 1982).
Polygalacturonases and pectin lyases attack the true pectins, while arabanases and galactanases degrade the neutral sugar polymers associated with them. These activities have drastic effects on the structural integrity of the tissues which may extend to death of the cell due to osmotic stresses imposed by damage to the wall. It seems likely that the products of pectinase activity will be absorbed as nutrient by the fungus, but these enzymes are better considered to be part of the machinery by which plant defences are breached than as being concerned primarily with nutrient supply.
Updated December 17, 2016