In the past decades the critical importance of Jasmonic acid (JA) for defense responses in a number of plant species was frequently reported. However in monocots it is poorly documented. In this study to investigate the roles of JA to regulate defense response against insects and pathogens, a maize mutant opr7opr8 was generated in which the JA biosynthesis responsible enzymes—oxo-phytodienoic acid reductases (ZmOPR7& ZmOPR8) were mutated by Mutator transposon. opr7opr8 showed significantly reduced JA level in all the organs tested and displayed highly susceptible to Spodoptera exigua, indicating that JA signal in maize is required for defense immunity against insect attack. Reverse Northern Blot (Macroarrays) Analysis found that a bunch of JA-dependent genes such as LOX2, LOX3, LOX8, AOS, AOC, JAZ7, JAZ8, HPL, PR5, MPI, WRKY46, PAL2 and PAL3 are substantially down regulated in the opr7opr8 versus wild type in response to wounding, verifying that JA signaling network regulates defense response against insect herbivores in maize. For pathogen attack opr7opr8 mutant exhibited a high mortality rate (~90%) when grown in non-sterile soil or 100% mortality when in the field and the death was found due to root rots caused by Pythium spp., indicating that endogenous JA level is crucial factor for maize plant to struggle with soil-borne pathogens. Gene expression profiling demonstrated that JA signal regulates hundreds of signaling-/defense-related genes, which severely down-regulated in opr7opr8 comparing with wild type. In conclusion JA is an indispensable signal in maize for immunity against insects and soil-borne pathogens.