Plant age affects elicitation of polyphenol oxidase activity by neem extract in Solanum lycopersicum against Pseudomonas syringae pv. tomato

Abstract

Polyphenol oxidase (PPO) is a key defense enzyme widely distributed in the plant kingdom. The enzyme has been reported to be inducible by biotic inducers. The present study instigated the impact of plant age on induction of activity of PPO and pattern of isoenzymes by aqueous fruit extracts of Azadirachta indica (neem). The study involved the host–pathogen system of tomato (F1 tomato, Roopsi variety, Century seeds) and Pseudomonas syringae pv. tomato. Plants of 6, 8, 10 and 12 weeks of age were used for the study. Samples were collected 0, 24, 48, 72, 96 hours and 2 weeks post treatment. The results demonstrated that plant age impacts the expression and inducibility of PPO in tomato. Neem extract does not significantly inhibit the growth of Pseudomonas syringae pv. tomato. Therefore, the reduction in disease severity in neem-treated plants is related to changes in the activity of PPO in the host plant. The 10-week-old plants had significantly highest PPO activity as compared to 12-week-old plants, which had the lowest enzyme activity. Consequently, the 10-week-old plants were more resistant to the pathogen among all the ages studied. The distal untreated leaves or the leaves emerging after neem treatment were also significantly resistant to the pathogen. Treatment with neem prior to pathogen inoculation was more effective in priming the plants. The neem extract induced the defense response in tomato primarily through upregulation of the PPO gene.