Relationships among endo-polygalacturonase, oxalate, pH, and plant polygalacturonase-inhibiting protein (PGIP) in the interaction between Sclerotinia sclerotiorum and soybean

The necrotrophic fungal pathogen Sclerotinia sclerotiorum secretes oxalic acid and endo-polygalacturonase (endo-PG) in host plants. Oxalic acid acidifies the plant tissue to values more suitable to endo-PG activity. However, we observed that the soybean infected seedlings possessed a pH of 3.8 which is below that optimal for endo-PG activity (4.5-5.0). We investigated, therefore, the effects of pH (from 5.0 to 3.6) and oxalate (5-20 mM) on the activity of the major basic endo-PG (PGb) and towards an acidic endo-PG (PGa), secreted by S. sclerotiorum during soybean infection. We verified that only PGb activity is stimulated by oxalate while, at the lowest pHs, PGa escapes the inhibition of a soybean polygalacturonase-inhibiting protein (PGIP). These results, performed on polygalacturonic acid, were apparently consistent with data obtained from soybean hypocotyl segments, where PGb activity was increased by oxalate and PGa maintained its activity also at pH 3.6, possibly because at this pH the PGIP contained in the plant tissue is inactive. RT-PCR analysis showed that, during soybean infection, the expression of the putative pga gene is delayed in comparison to the basic one. The different temporal expression of the two endo-PGs and their different response to pH, oxalate and PGIP seem to be consistent with a possible maximisation of the fungal PG activity in the host tissue.