Endoproteolytic activities in pea roots inoculated with the arbuscular mycorrhizal fungus Glomus mosseae and/or Aphanomyces euteiches in relation to bioprotection

Arbuscular mycorrhizal (AM) symbioses are known to play a role in increased resistance of plants against soilborne pathogens. Mechanisms involved in t his phenomenon are not yet well understood. This work investigates possible roles of endoproteolytic activities in bioprotection of Pisum sativum root s by Glomus mosseae against Aphanomyces euteiches. First, it is demonstrate d that bioprotection occurs only in pre-mycorrhizal plants. Second, endopro teolytic activities were analysed qualitatively and quantitatively during A M symbiosis, in plants infected with either zoospores or mycelium of A. eut eiches, and in mycorrhizal plants infected with the pathogen. In mycorrhiza l symbiosis a progressive increase in endoproteolytic activities was observ ed following root colonization by G. mosseae. By contrast, in roots inocula ted with A. euteiches, a drastic increase in endoproteolytic activities was observed which was correlated with the amount of pathogen occurring in roo ts. Qualitative differences were seen among the endoproteolytic activities detected in roots inoculated with zoospores or mycelium. The constitutive a s well as mycorrhizal and pathogen-induced activities were further characte rized as 'trypsin-like' serine endoproteases. Interestingly, in a situation of bioprotection, only low levels of the activities normally associated wi th the infection by A. euteiches were detected, suggesting that the synthes is of these proteins is directly linked to the growth or virulence of the p athogen.