Jasmonic acid does not increase oxidative defense mechanisms or common defense-related enzymes in postharvest sugarbeet roots

Jasmonic acid (JA) treatment significantly reduces rot due to several sugarbeet (Beta vulgaris L.) storage pathogens. However, the mechanisms by which JA protects postharvest sugarbeet roots from disease are unknown. In other plant species and organs, alterations in antioxidant defense mechanisms and elevations in common pathogenesis-related defense enzymes have been implicated in jasmonate-induced disease resistance. To investigate whether these mechanisms are involved in JA-induced disease resistance in stored sugarbeet roots, the activities of several reactive oxygen species (ROS)-scavenging and pathogenesis-related defense enzymes and the total concentration of antioxidant compounds were determined in harvested sugarbeet roots in the 60 d following treatment with JA. ROS-scavenging and pathogenesis-related defense enzymes and the concentration of antioxidant compounds were largely unaffected by JA as JA-treated roots exhibited small declines in superoxide dismutase (SOD) and chitinase activities, and were generally unaltered in ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), β-1,3-glucanase (β-Gluc), polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) activities or antioxidant compounds concentration. The lack of increase in enzyme activities or metabolites related to defense against oxidative stress or pathogens suggests that JA-induced disease resistance in postharvest sugarbeet roots does not arise from a direct increase in any of the ROS-scavenging and defense-related enzymes examined, or the concentration of total antioxidant compounds. However, ROS-scavenging enzymes and pathogenesis-related defense enzymes were affected by storage duration with POD, SOD, β-Gluc, chitinase, and PPO activities elevated and APX and CAT activities reduced in roots stored for 10 d or more. Storage-related changes in activities of ROS-scavenging enzymes and defense-related enzymes provide further evidence that these enzymes are uninvolved in sugarbeet root disease resistance during storage since many of these enzymes increased in activity after prolonged storage when disease resistance generally declines.