Polyphenoloxidase is induced by methyljasmonate and Meloidogyne javanica in soybean roots but is not involved in resistance

Abstract

Abstract The role of the enzyme polyphenoloxidase (PPO) in the response of two soybean varieties, Hartwig (resistant) and Cristalina (susceptible), to Meloidogyne javanica was studied in plants where root systems were exposed to the known PPO inducer, methyljasmonate. Chlorogenic acid was the best substrate for root PPO. Treatment of both varieties with 100 and 400 μM methyljasmonate solutions induced a similar increase in enzyme activity 72 h after treatment. Inoculation of roots with second-stage juveniles (J2) induced PPO increase in cv. Cristalina but not in cv. Hartwig. Moreover, combined treatment of methyljasmonate and J2 inoculation enhanced PPO activity in both varieties. Two PPO cDNAs were isolated from the roots of the resistant variety 48 h after J2 inoculation, and Southern blot experiments in both varieties, using GmPPOJH1 and GmPPOJH2 cDNAs as probes, indicated that PPOs are represented by a multigene family in soybean. RT-PCR assays showed more GmPPOJH1 transcripts in plants treated with methyljasmonate and infected with nematodes, where high PPO activity was also observed. Plants treated with methyljasmonate and infected with J2 showed a marked decrease of nematode population 35 days after inoculation. These findings suggest that methyljasmonate triggers a resistant response in soybean roots to M. javanica but PPO is not involved in the resistance process.