Transcript accumulation of polygalacturonase inhibiting protein (PGIP) following pathogen infections in soybean

Proteins inhibiting fungal endo-polygalacturonase (PGIP) are constitutively expressed and localized on cell walls of most plant species. Induction of pgip transcripts following pathogen infection would demonstrate a role of PGIP in active plant defence mechanisms. We investigated pgip expression in hypocotyls of soybean seedlings (cvs 'Sapporo' and 'Kure') infected with the fungal pathogens Diaporthe phaseolorum var. caulivora, Sclerotinia sclerotiorum and an avirulent (race 1) and virulent (race 20) races of Phytophthora sojae. Two pgip transcripts were transiently expressed in all types of interaction with similar timing and maximum accumulation at 16-24 h after infection. However, soybean seedlings of both cvs infected with the virulent race 20 of P. sojae showed higher levels of pgip expression than seedlings infected with the avirulent race 1. A delay of pgip accumulation was observed when plants were inoculated with zoospores in place of mycelium of P. sojae. The presence of endo-polygalacturonase (endo-PG) in infected tissue was monitored both by immunological detection and by determining PG activity. PG was detected only in soybean seedlings infected with S. sclerotiorum and not in those infected with D. phaseolorum var. caulivora or P. sojae. The apparent inability in vivo of the virulent and avirulent races of P. sojae to produce PG suggests that PG-PGIP interaction is not required for the resistance response of soybean against this pathogen.