Jasmonic acid (JA)-mediating MYB transcription factor1, JMTF1, coordinates the balance between JA and auxin signalling in the rice defence response.

The plant hormone jasmonic acid (JA) is a signalling compound involved in the regulation of cellular defence and development in plants. In this study, we investigated the roles of a JA-responsive MYB transcription factor, JMTF1, in the JA-regulated defence response against rice bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo). JMTF1 did not interact with any JASMONATE ZIM-domain (JAZ) proteins. Transgenic rice plants overexpressing JMTF1 showed a JA-hypersensitive phenotype and enhanced resistance against Xoo. JMTF1 upregulated the expression of a peroxidase, OsPrx26, and monoterpene synthase, OsTPS24, which are involved in the biosynthesis of lignin and antibacterial monoterpene, γ-terpinene, respectively. OsPrx26 was mainly expressed in the vascular bundle. Transgenic rice plants overexpressing OsPrx26 showed enhanced resistance against Xoo. In addition to the JA-hypersensitive phenotype, the JMTF1-overexpressing rice plants showed a typical auxin-related phenotype. The leaf divergence and shoot gravitropic responses were defective, and the number of lateral roots decreased significantly in the JMTF1-overexpressing rice plants. JMTF1 downregulated the expression of auxin-responsive genes but upregulated the expression of OsIAA13, a suppressor of auxin signalling. The rice gain-of-function mutant Osiaa13 showed high resistance against Xoo. Transgenic rice plants overexpressing OsEXPA4, a JMTF1-downregulated auxin-responsive gene, showed increased susceptibility to Xoo. JMTF1 is selectively bound to the promoter of OsPrx26 in vivo. These results suggest that JMTF1 positively regulates disease resistance against Xoo by coordinating crosstalk between JA- and auxin-signalling in rice.

Rice MEDIATOR25, OsMED25, is an essential subunit for jasmonate-mediated root development and OsMYC2-mediated leaf senescence.

The Mediator multiprotein complex acts as a universal adaptor between transcription factors (TFs) and RNA polymerase II. MEDIATOR25 (MED25) has an important role in jasmonic acid (JA) signaling in Arabidopsis. However, no research has been conducted on the role of MED25 in JA signaling in rice, which is one of the most important food crops globally and is a model plant for molecular studies in other monocotyledonous species. In the present study, we isolated the loss-of function mutant of MED25, osmed25, through the map-based cloning and phenotypic complementation analysis by the introduction of OsMED25 and investigated the role of OsMED25 in JA signaling in rice. The osmed25 mutants had longer primary (seminal) roots than those of the wild-type (WT) and exhibited JA-insensitive phenotypes. S-type lateral root densities in osmed25 mutants were lower than those in the WT, whereas L-type lateral root densities in osmed25 mutants were higher than those in the WT. Furthermore, the osmed25 mutants retarded JA-regulated leaf senescence under dark-induced senescence. Mutated osmed25 protein could not interact with OsMYC2, which is a positive TF in JA signaling in rice. The expression of JA-responsive senescence-associated genes was not upregulated in response to JA in the osmed25 mutants. The results suggest that OsMED25 participates in JA-mediated root development and OsMYC2-mediated leaf senescence in rice.

The overexpression of OsSRO1a, which encodes an OsNINJA1- and OsMYC2-interacting protein, negatively affects OsMYC2-mediated jasmonate signaling in rice.

KEY MESSAGE: OsNINJA1-interacting protein, OsSRO1a, acts as a mediator that suppresses OsMYC2 activity in response to JA. Jasmonic acid (JA) is an important plant hormone for the stable growth and development of higher plants. The rice gene NOVEL INTERACTOR OF JAZ1 (OsNINJA1) interacts with Jasmonate ZIM-domain (JAZ) proteins and is a repressor of JA signaling. In this study, we identified several OsNINJA1-interacting proteins in rice from a yeast two-hybrid screen. Among the newly identified genes, we focused on SIMILAR TO RCD ONE1a (OsSRO1a) and investigated its role in JA signaling. Full-length OsSRO1a interacted with OsNINJA1 in plant cells but not in yeast cells. OsSRO1a also interacted with OsMYC2, a positive transcription factor in JA signaling, in both plant and yeast cells. The expression of OsSRO1a was upregulated at a late phase after JA treatment. Transgenic rice plants overexpressing OsSRO1a exhibited JA-insensitive phenotypes. In wild-type plants, JA induces resistance against rice bacterial blight, but this phenotype was suppressed in the OsSRO1a-overexpressing plants. Furthermore, the degradation of chlorophyll under dark-induced senescence conditions and the JA-induced upregulation of OsMYC2-responsive genes were suppressed in the OsSRO1a-overexpressing plants. These results suggest that OsSRO1a is a negative regulator of the OsMYC2-mediated JA signaling pathway in rice.

Jasmonic Acid-Induced VQ-Motif-Containing Protein OsVQ13 Influences the OsWRKY45 Signaling Pathway and Grain Size by Associating with OsMPK6 in Rice.

Jasmonic acid (JA) is a plant hormone that plays an important role in the defense response and stable growth of rice. In this study, we investigated the role of the JA-responsive valine-glutamine (VQ)-motif-containing protein OsVQ13 in JA signaling in rice. OsVQ13 was primarily located in the nucleus and cytoplasm. The transgenic rice plants overexpressing OsVQ13 exhibited a JA-hypersensitive phenotype and increased JA-induced resistance to Xanthomonas oryzae pv. oryzae (Xoo), which is the bacteria that causes rice bacterial blight, one of the most serious diseases in rice. Furthermore, we identified a mitogen-activated protein kinase, OsMPK6, as an OsVQ13-associating protein. The expression of genes regulated by OsWRKY45, an important WRKY-type transcription factor for Xoo resistance that is known to be regulated by OsMPK6, was upregulated in OsVQ13-overexpressing rice plants. The grain size of OsVQ13-overexpressing rice plants was also larger than that of the wild type. These results indicated that OsVQ13 positively regulated JA signaling by activating the OsMPK6-OsWRKY45 signaling pathway in rice.

Overexpression of OsNINJA1 negatively affects a part of OsMYC2-mediated abiotic and biotic responses in rice.

The plant hormone jasmonic acid (JA) plays an important role in defense response and plant development. Jasmonate ZIM-domain (JAZ) proteins act as transcriptional repressors of plant responses to JA. In this study, we found that OsNINJA1, which is a JAZ-interacting adaptor protein, plays an important role in JA signaling that is positively regulated by the transcription factor OsMYC2 in rice. The expression of OsNINJA1 was upregulated at an early phase after JA treatment, and OsNINJA1 interacted with several OsJAZ proteins in a C domain-dependent manner. Transgenic rice plants overexpressing OsNINJA1 exhibited a JA-insensitive phenotype and were more susceptible to rice bacterial blight caused by Xanthomonas oryzae pv. oryzae, which is one of the most serious diseases affecting rice. Furthermore, OsNINJA1 negatively affected JA-regulated leaf senescence under dark-induced senescence conditions. Finally, the expression of OsMYC2-responsive pathogenesis-related (PR) genes and senescence-associated genes (SAGs) tended to be downregulated in the OsNINJA1-overexpressing rice plants. These results indicate that OsNINJA1 acts as a negative regulator of OsMYC2-mediated JA signaling in rice.