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

Abstract 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 H 2 O 2 -scavenging enzymes, guaiacol peroxidase (GPX), ascorbate peroxidase (APX) and catalase (CAT). The mechanisms responsible for MeJA-induced H 2 O 2 accumulation was investigated further by studying both the production and scavenging of H 2 O 2 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 H 2 O 2 detected. The results indicated that H 2 O 2 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 H 2 O 2 generation and degradation. It is concluded that exogenous MeJA may be involved in the oxidative stress processes by regulating antioxidant enzyme activities.