FUNGAL PATHOGENS CAN ELUDE THE INHIBITION BY HOST PLANT POLYGALACTURONASE INHIBITING PROTEINS PGIPs BY DIFFERENT MECHANISMS
To infect and colonize successfully host plants, fungal pathogens have evolved specific strategies. Many fungi overcome the defensive barrier represented by the cell wall by producing large amount of cell wall degrading enzymes (CWDE) during infection. Among CWDE, pectinases, and especially endo-polygalacturonases (endo- PG), play an important role since they depolymerise the pectic component of the cell wall and middle lamella. Polygalacturonase-inhibiting proteins (PGIPs) are plant defence molecules able to block the polygalacturonase activity of fungal pathogens. Therefore, PGIP could be used both in traditional and innovative breeding programmes to defeat those pathogens that express endo-PG activity to assist with plant infection. However, fungi have evolved strategies to overcome PGIP inhibition. For example, Sclerotinia sclerotiorum in the early stage of soybean infection produces considerable amount of a basic PG encoded by the Sspgb gene. By quantitative RT-PCR it was shown that, initially, the transcript of this pg gene largely overcomes that of the soybean pgip gene. Therefore, at this stage, the PGIP level would be not enough to significantly counteract fungal PG activity.