Abstract: 【Objective】The purpose of the study was to reveal the effects of exogenous jasmonic acid(JA) on defense response of lipoxygenase gene LOX3 knockout rice plants, and to provide basic data for the use of exogenous JA and other exciters to improve the reduced defense response of rice caused by the loss of resistance related genes in crops.【Method】The susceptible japonica rice Lijiang Xintuan Heigu(LTH) and its LOX3 knockout rice lines (Δlox3 #1, Δlox3#10 and Δlox3#20) were used as materials. Phenotypic analysis including plant height, tiller number and 100-grain weight, detection of the antimicrobial compound activity(content) of peroxidase(POD), reactive oxygen species(ROS) and lignin, as well as analysis of the expression of defense-related genes were carried out. Further, the tested rice was inoculated with Magnaporthe oryzae, the rice blast symptoms of the tested rice were investigated, the activity(content) of antimicrobial compounds and expression of defense-related genes in M. oryzae inoculated rice plants were examined, and the inoculation process of M. oryzae in rice was observed. On this basis, exogenous JA was applied to M. oryzae inoculated Δlox3 knockout rice lines and LTH at 36 and 48 hpi. The effects of JA on rice blast symptom, activity(content) of antimicrobial compounds and expression of defense-related genes in M. oryzae inoculated rice were analyzed, and the effects of JA on the inoculation process of M. oryzae were also observed. 【Result】The seeds of Δlox3 knockout rice plants exhibited faster germination speed than that of the wild type, but the plant height, tiller number and 100-grain weight were similar to those of the wild type. The more serious blast symptom appeared on M. oryzae inoculated Δlox3 knockout rice plants than that on M. oryzae inoculated wild type rice. M. oryzae inoculated Δlox3 knockout rice plants exhibited lower number of callose, POD activity and lignin content, and higher ROS content and number of dead cells, and the magnitude of the down-regulation of OsWRKY45, OsPR1a, and OsPOX1 genes and the amount of M. oryzae hyphae colonization were greatly higher than that of M. oryzae inoculated wild type rice. Exogenous JA effectively alleviated rice blast symptom in Δlox3 knockout rice plants inoculated with M. oryzae for 36 and 48 hpi. Moreover, JA enhanced the number of callose, POD activity, lignin content and expression levels of defense-related genes OsWRKY45, OsPR1a and OsPOX1 in Δlox3 knockout rice plants inoculated with M. oryzae for 36 and 48 hpi, decreased expression of OsRbohB, ROS content and number of dead cells, and inhibited expansion of M. oryzae hyphae. The effects of JA on the alleviation of rice blast symptoms in Δlox3 knockout rice plants inoculated with M. oryzae for 48 hpi were better than those of JA on Δlox3 knockout rice plants inoculated with M. oryzae for 36 hpi. It was mainly reflected in the fact that JA induced a better increase in antimicrobial compounds, expression levels of defense-related genes, number of cell deaths, and inhibition of M. oryzae hyphae expansion in the Δlox3 knockout rice plants inoculated with M. oryzae for 48 hpi than in the Δlox3knockout rice plants inoculated with M. oryzae for 36 hpi under JA treatment. 【Conclusion】LOX3 gene knockout enhances rice seed germination speed and decreases rice blast resistance, while exogenous JA alleviates the decrease of rice defense response caused by LOX3 knockout and enhances rice blast resistance. JA has a potential application prospect in mitigating reduction or loss of crop resistance resulted from short durability of resistance genes.