DIfferential induction of peroxidase and polyphenol oxidase enzyme activities in downy mildew resistant and susceptible genotypes of maize during interaction with Peronosclerospora sorghi

The changes in peroxidase (PO) and polyphenol oxidase (PPO) enzyme activities in downy mildew resistant (MAI 756) and susceptible (CM 500) genotypes of maize (Zea mays L.) during interaction with Peronosclerospora sorghi, the maize downy mildew pathogen were studied. Maize seedlings at two-leaf stage were sprayed with conidial suspension of P. sorghi (1×10⁵ conidia ml^-1) and leaf samples were collected at different time intervals after inoculation and analyzed for PO and PPO enzyme activities by gel electrophoresis. PO isozyme analysis revealed that control leaves of CM 500 expressed two isozymes (PO-1 and PO-2) constitutively. No alterations in the PO enzyme activities were observed up to 72 h after inoculation with P. sorghi. However, at 96 h after inoculation the activity of PO-1 decreased. In MAI 756, the activity of PO-2 decreased 48 h after inoculation and at 72 h after inoculation both PO1 and PO2 decreased considerably. PPO isozyme analysis revealed constitutive expression of PPO-3 in control leaves of CM 500. The activity of PPO-3 increased 24 h after inoculation with P. sorghi and elevated level of PPO-3 enzyme activity was detected throughout the experimental period of 96 h. In MAI 756, constitutive expression of four PPO isozymes (PPO-1, PPO-2, PPO-3 and PPO-4) was detected. The activity of PPO-3 increased 24 h after inoculation and the activity of another PPO enzyme (PPO-4) increased 72 h after inoculation with P. sorghi.