Production of pectin-degrading enzymes by ericoid mycorrhizal fungi

Production of enzymes which degrade plant cell wall macromolecules has been studied in relatively few ericoid fungal isolates, although these polymers are a major component of the organic litter and an important source of nutrients for these fungi. Our aims were to investigate whether the ability to degrade the wall pectic component, only reported for one isolate, is a general feature of ericoid fungi. Of about 35 isolates from different geographic regions, all were capable of growing on pectin as the sole carbon source. Polygalacturonase (PG) activity was detected to a different degree in the culture filtrates and independently of the fungal growth rate. Solid and liquid isoelectric focusing allowed separation and identification of several polygalacturonase isoforms. Among the fungal isolates investigated, those from the northern hemisphere produced mostly acidic isoforms, whereas isolates from South Africa secreted more abundantly basic isoforms. However, purification and biochemical characterization of several PG isoforms from the different isolates revealed an optimal activity in the acidic pH range for all the PG enzymes tested. Polygalacturonase enzymes seem to be an important component of the enzymatic arsenal secreted by ericoid fungi during their saprotrophic life. In addition, they could also play a role during root colonization, since penetration across the plant cell wall is a prerequisite for the establishment of endomycorrhizal symbiosis.