Methyl jasmonate-induced antioxidant defence in root apoplast from sunflower seedlings

We have investigated the physiological functions of the rapid generation of reactive oxygen species (ROS) and the implication of the antioxidant enzymes in the apoplast and symplast of roots of sunflower (Helianthus annuus L.) seedlings exposed to methyl jasmonate (MeJA, 50 μM). MeJA-elicited roots showed a fast increase in ROS content, followed by a marked increase in the activity of H2O2-scavenging enzymes, guaiacol peroxidase (GPX), ascorbate peroxidase (APX) and catalase (CAT). The mechanisms responsible for MeJA-induced H2O2 accumulation was investigated further by studying both the production and scavenging of H2O2 in the extracellular matrix. Peroxidases active against (2,2′-azino-bis-[3-ethylbenzthiazoline-6-sulfonic acid], ABTS) and guaiacol were found in the apoplastic fluid, and proved to be ionically and covalently associated with sunflower cell walls, although only the peroxidase activities of the soluble apoplastic fractions and those ionically linked to the cell wall were correlated with the accumulation of the H2O2 detected. The results indicated that H2O2 accumulation is a complex and highly regulated event requiring the time-dependent stimulation and down-regulation of differently located enzymes, some of which are involved in H2O2 generation and degradation. It is concluded that exogenous MeJA may be involved in the oxidative stress processes by regulating antioxidant enzyme activities.