Influence of environmental pH modulation on efficiency of apoplastic PR proteins during Fusarium culmorum - wheat seedling interaction

Variation of apoplastic pH by Fusarium culmorum and its influence on the production, activity and isoenzymes patterns of the pathogenesis-related (PR) proteins beta 1,3-glucanase, chitinase and peroxidase enzymes were detected in apoplastic fluids (AFs) from infected wheat seedlings. The time course in the 24-48 h interval post infection was characterized by an increase in activity and isoenzymatic differential induction of the selected PR proteins and by a concomitant rise of apoplastic pH. Chitinase attained maximum activity at pH 8.0 in the case of inoculated seedlings. Optimal beta 1,3-glucanase activity in the pH range 6.0-8.0, was observed at pH 7.0. Peroxidase was strongly affected by pH, with enzyme activity having a maximum rate at pH 6.0 and thereafter rapidly declining at higher pH. Maximum peroxidase activity paralleled the appearance of the complete isoenzymatic pattern. In order to investigate the biological role of PR proteins in AFs, the in vitro antifungal activity was evaluated. In the interval 0-6 h, pH of macroconidia suspensions rose up to 7.2. AFs revealed inhibitory activity against germinating macroconidia of F. culmorum by decreasing the germination efficiency of macroconidia apical compartments, while this effect was compensated by an increased germination capacity of middle compartments. Present results suggest that during infection of wheat seedlings by F. culmorum the pH modulation favours host colonization by enhancing the activity of pectin lyase, and simultaneously inhibits the capacity of the host to oppose the pathogen by interfering with peroxidases which represent an important component of the defence arsenal.