Insight into plant defense strategies by studying PGIPs, polygalacturonases and oligogalacturonides

Polygalacturonase-inhibiting proteins (PGIPs) counteract fungal polygalacturonases (PGs) and reduce fungal disease symptoms by hampering the invasion process and the release of nutrients necessary for fungal growth. PGIPs are encoded by gene families the members of which are not merely functionally redundant. Polymorphism of their sequences is associated to different regulation and recognition specificities likely to be instrumental against different phytopathogenic fungi. The recognition ability of PGIPs resides in their leucine-rich repeat (LRR) protein structure. We determined the crystal structure of both PG and PGIP. The interesting features of the PGIP structure may help to elucidate its interactive properties and its mechanism of recognition. The interaction between fungal PGs and plant PGIPs is thought to favour the accumulation of oligogalacturonides (OGs). The ability of OGs of up-regulating a wide number of defense genes has been analysed by full genome mRNA expression profiling. Many of the OG-regulated genes are also induced during fungal infection, including the expression of PGIPs, suggesting that OGs mediate part of the expression changes occurring in response to pathogen attack.