Jasmonic Acid-mediated Defense Suppresses Brassinosteroid-mediated Susceptibility To Rice Black Streaked Dwarf Virus Infection In Rice

Plant hormones play vital roles in plant immune responses.However,little information is known about hormone-mediated defense systems against Rice black-streaked dwarf virus(RBSDV)infection.In this study,we used a high-throughput sequencing approach to compare the global gene expression of RBSDV-infected rice plants with that of healthy plants.Ultra high performance liquid chromatography-triple quadrupole mass spectrometry(UPLC-MS/MS)was used to determine endogenous hormone content in rice plants.Exogenous hormone applications and transgenic rice were used to examine RBSDV infectivity and pathogenicity.The main results are summarized as followed:1.According to our transcriptome data,many defense-or stress-related genes were up-regulated after RBSDV infection.10 PR-related genes and 11 stress-related transcription factors were selected to verify the accuracy of the transcriptome data by Real time quantitative PCR(RT-qPCR).The expression levels of the selected genes in RBSDV-infected rice were increased at both 30 and 60 day post infection(dpi)compared with that in healthy plants.2.Expression of jasmonic acid(JA)synthesis-related genes was significantly increased after RBSDV infection both in the transcriptome data and when tested by RT-qPCR.The JA-responsive PR gene JiOsPR10 was significantly up-regulated at both 30 and 60 dpi.The JA level in leaves of RBSDV-infected plants was higher than in healthy plants,with an increase of almost 4.1-fold.The disease incidence of RBSDV in plants treated with methyl jasmonate(MeJA)was significantly decreased when compared to that of control plants treated with 0.1%Triton X-100.The infection rate and the transcript level of the RBSDV coat protein(CP)were both higher in virus-inoculated OsCOI1-RNAi(coil-13)mutant rice plants compared to those of wild-type Nipponbare plants.These results imply that JA-mediated defense systems are involved in the suppression of RBSDV infection.3.Our transcriptome data showed that expression of brassinosteroid(BR)synthesis-related genes was significantly decreased after RBSDV infection.RT-qPCR analysis showed that both the BR biosynthetic genes and BR signaling genes were down-regulated after virus infection at both 30 and 60 dpi.The castasterone(CS)level in leaves of RBSDV-infected plants was lower than in those of healthy plants,with a decrease of almost 0.45-fold.The disease incidence of RBSDV in plants treated with epibrassinolide(BL)was significantly increased when compared to 0.1%Triton X-100 treated control plants,while brassinazole(BRZ,a triazole that specifically inhibits BR biosynthesis)treatment resulted in a significant reduction in the proportion of RBSDV-infected plants compared with control plants.The transcript level of the RBSDV CP and the disease incidence were both decreased in RBSDV-infected OsGSK2 overexpression(Go)mutant rice plants compared to those of wild-type Zhonghua 11(Zh11)plants.These results imply that the BR pathway is associated with susceptibility of rice to RBSDV infection.4.A combination of MeJA and BL treatment resulted in a significant reduction in RBSDV infection compared with control treatment.The transcript levels of JA pathway-related genes were significantly increased while the BR pathway-related genes were significantly decreased at 6 and 12 h after foliar MeJA application compared with the non-treated plants.In wild-type Nipponbare plants,the expression of BR biosynthesis and signaling genes was significantly decreased in the treated plants compared to the controls at 12 h after foliar MeJA application.However,in coi1-13 plants,the transcript levels of BR pathway genes were little affected after MeJA-treatment,similar to the result seen in untreated plants.These results indicate that JA-mediated plant defense can suppress BR-mediated susceptibility to RBSDV infection,and that this suppression is regulated by the JA co-receptor OsCOI1.In summary,our results demonstrate that many defense-or stress-related genes are up-regulated,and that the transcription of JA pathway and BR pathway genes are inversely regulated in response to RBSDV infection.Studies with mutants indicated that JA-mediated plant defense can suppress BR-mediated susceptibility to RBSDV infection,and this inhibition is regulated by co-receptor OsCOI1.