The possible involvement of peroxidase in defense of yellow lupine embryo axes against Fusarium oxysporum

Peroxidase activity (EC 1.11.1.7) towards phenolic substrates, i.e. pyrogallol, syringaldazine and guaiacol, and ascorbate peroxidase activity (EC 1.11.1.11) were analyzed in embryo axes of Lupinus luteus L. cv. Polo cultured on Heller medium for 96 h after inoculation with the necrotrophic fungus Fusarium oxysporum f.sp. Schlecht lupini. Four variants were compared: inoculated embryo axes cultured with 60 mM sucrose (+Si) or without it (−Si), and non-inoculated embryo axes cultured with 60 mM sucrose (+Sn) or without it (−Sn). Between 0 and 96 h of culture, peroxidase activity towards the phenolic substrates increased in all variants except −Si, where a decrease was noted in peroxidase activity towards syringaldazine and guaiacol, but not towards pyrogallol. In +Si tissues, a considerable increase in enzyme activity towards these substrates was recorded starting from 72 h of culture. Lignin content of +Si tissues increased already at the first stage of infection, i.e. 24 h after inoculation. Additionally, in +Sn tissues, high ascorbate peroxidase activity was observed during the culture. Its activity increased in +Si tissues, beginning at 72 h after inoculation. However, this was lower than in +Sn tissues. At 72 h after inoculation, a considerably stronger development of the infection was observed in −Si than in +Si tissues during our earlier research [Morkunas, I. et al., 2005. Sucrose-stimulated accumulation of isoflavonoids as a defense response of lupine to Fusarium oxysporum. Plant Physiol Biochem 2005; 43: 363–73]. Both peroxidases assayed towards phenolic substrates and ascorbate peroxidase was less active in −Si tissues than in −Sn tissues. Hydrogen peroxide concentration was much higher in −Si than in +Si tissues. These results indicate that peroxidases may be some of the elements of the defense system that are stimulated by sucrose in yellow lupine embryo axes in response to infection caused by F. oxysporum.