Jasmonic acid(JA) and related metabolites play a key role in plant defense and growth. JA carboxyl methyltransferase(JMT) may be involved in plant defense and development by methylating JA to methyl jasmonate(Me JA) and thus influencing the concentrations of JA and related metabolites. However, no JMT gene has been well characterized in monocotyledon defense and development at the molecular level. After we cloned a rice JMT gene,Os JMT1, whose encoding protein was localized in the cytosol, we found that the recombinant Os JMT1 protein catalyzed JA to Me JA. Os JMT1 is up-regulated in response to infestation with the brown planthopper(BPH; Nilaparvata lugens). Plants in which Os JMT1 had been overexpressed(oeJMT plants) showed reduced height and yield. These oe-JMT plants also exhibited increased Me JA levels but reduced levels of herbivore-induced JA and jasmonoyl-isoleucine(JAIle). The oe-JMT plants were more attractive to BPH female adults but showed increased resistance to BPH nymphs,probably owing to the different responses of BPH female adults and nymphs to the changes in levels of H_2O_2 and Me JA in oe-JMT plants. These results indicate that Os JMT1,by altering levels of JA and related metabolites, plays a role in regulating plant development and herbivore-induced defense responses in rice. JA carboxyl methyltransferase (JMT) may be involved in plant defense and development by methylating JA to methyl jasmonate (Me JA) and thus influencing the concentrations of JA and related metabolites. However, no JMT gene has been well characterized in monocotyledon defense and development at the molecular level. After we cloned a rice JMT gene, Os JMT1, whose encoding protein was localized in the cytosol, we found that the recombinant Os JMT1 protein catalyzed JA to Me JA. Os JMT1 is up-regulated in response to infestation with the brown planthopper (BPH; Nilaparvata lugens). Plants in which Os JMT1 had been overexpressed (oeJMT plants) showed reduced height and yield. These oe-JMT plants Also showed increased levels of JA levels but reduced levels of herbivore-induced JA and jasmonoyl-isoleucine (JAIle). The oe-JMT plants were more attractive to BPH female adults but showed increased resistance to BPH nymph s, probably due to the different responses of BPH female adults and nymphs to the changes in levels of H 2 O 2 and Me JA in oe-JMT plants. These results indicate that Os JMT1, by altering levels of JA and related metabolites, plays a role in regulating plant development and herbivore-induced defense responses in rice.