Jasmonoyl-l-isoleucine is required for the production of a flavonoid phytoalexin but not diterpenoid phytoalexins in ultraviolet-irradiated rice leaves

Rice produces low-molecular-weight antimicrobial compounds known as phytoalexins, in response to not only pathogen attack but also abiotic stresses including ultraviolet (UV) irradiation. Rice phytoalexins are composed of diterpenoids and a flavonoid. Recent studies have indicated that endogenous jasmonyl-l-isoleucine (JA-Ile) is not necessarily required for the production of diterpenoid phytoalexins in blast-infected or CuCl₂-treated rice leaves. However, JA-Ile is required for the accumulation of the flavonoid phytoalexin, sakuranetin. Here, we investigated the roles of JA-Ile in UV-induced phytoalexin production. We showed that UV-irradiation induces the biosynthesis of JA-Ile and its precursor jasmonic acid. We also showed that rice jasmonate biosynthesis mutants produced diterpenoid phytoalexins but not sakuranetin in response to UV, indicating that JA-Ile is required for the production of sakuranetin but not diterpenoid phytoalexins in UV-irradiated rice leaves.     

Methionine-induced phytoalexin production in rice leaves

The application of methionine on wounded rice leaves induced the production of rice phytoalexins, sakuranetin and momilactone A. This induction resulted from stimulation of phenylalanine ammonia-lyase and naringenin 7-O-methyltransferase activity. Jasmonic acid, ethylene, and active oxygen species are important as signal transducers in disease resistance mechanisms. However, although the endogenous level of jasmonic acid rapidly increased in reaction to wound, methionine treatment could not induced endogenous JA production. Ethylene induced the production of the flavonoid phytoalexin, sakuranetin, but did not induce the production of a terpenoid phytoalexin, momilactone A. On the other hand, a free radical scavenger, Tiron, counteracted the induction of both sakuranetin and momilactone A production in methionine-treated leaves. Active oxygen species may be important in methionine-induced production of phytoalexins.     

Coronatine elicits phytoalexin production in rice leaves (Oryza sativa L.) in the same manner as jasmonic acid

The phytotoxin coronatine induced the accumulation of the flavonoid phytoalexins sakuranetin and momilactone A in rice leaves. Coronatine-inducible sakuranetin production was under the control of kinetin and ascorbic acid (AsA), as observed with jasmonic acid (JA). The effects of kinetin and AsA on the activity of coronatine indicated that coronatine might elicit sakuranetin production in a manner similar to JA. The similarity of both their structures and the manner of elicitation of coronatine and JA suggest that they might interact at the same active site(s) to lead to phytoalexin production.     

OsJAR1 Contributes Mainly to Biosynthesis of the Stress-Induced Jasmonoyl-Isoleucine Involved in Defense Responses in Rice

Jasmonate plays key roles in plant growth and stress responses, as in defense against pathogen attack. Jasmonoyl-isoleucine (JA-Ile), a major active form of jasmonates, is thought to play a pivotal role in plant defense responses, but the involvement of JA-Ile in rice defense responses, including phytoalexin production, remains largely unknown. Here we found that OsJAR1 contributes mainly to stress-induced JA-Ile production by the use of an osjar1 Tos17 mutant. The osjar1 mutant was impaired in JA-induced expression of JA-responsive genes and phytoalexin production, and these defects were restored genetically. Endogenous JA-Ile was indispensable to the production of a flavonoid phytoalexin, sakuranetin, but not to that of diterpenoid phytoalexins in response to heavy metal stress and the rice blast fungus. The osjar1 mutant was also found to be more susceptible to the blast fungus than the parental wild type. These results suggest that JA-Ile production makes a contribution to rice defense responses with a great impact on stress-induced sakuranetin production.     

Purification and identification of naringenin 7-O-methyltransferase, a key enzyme in biosynthesis of flavonoid phytoalexin sakuranetin in rice

Sakuranetin, the major flavonoid phytoalexin in rice, is induced by ultraviolet (UV) irradiation, CuCl(2) treatment, jasmonic acid treatment, and infection by phytopathogens. It was recently demonstrated that sakuranetin has anti-inflammatory activity, anti-mutagenic activity, anti-pathogenic activities against Helicobacter pylori, Leishmania, and Trypanosoma and contributes to the maintenance of glucose homeostasis in animals. Thus, sakuranetin is a useful compound as a plant antibiotic and a potential pharmaceutical agent. Sakuranetin is biosynthesized from naringenin by naringenin 7-O-methyltransferase (NOMT). In previous research, rice NOMT (OsNOMT) was purified to apparent homogeneity from UV-treated wild-type rice leaves, but the purified protein, named OsCOMT1, exhibited caffeic acid O-methyltransferase (COMT) activity and not NOMT activity. In this study, we found that OsCOMT1 does not contribute to sakuranetin production in rice in vivo, and we purified OsNOMT using the oscomt1 mutant. A crude protein preparation from UV-treated oscomt1 leaves was subjected to three sequential purification steps, resulting in a 400-fold purification from the crude enzyme preparation. Using SDS-PAGE, the purest enzyme preparation showed a minor band at an apparent molecular mass of 40 kDa. Two O-methyltransferase-like proteins, encoded by Os04g0175900 and Os12g0240900, were identified from the 40-kDa band by MALDI-TOF/TOF analysis. Recombinant Os12g0240900 protein showed NOMT activity, but the recombinant Os04g0175900 protein did not. Os12g0240900 expression was induced by jasmonic acid treatment in rice leaves prior to sakuranetin accumulation, and the Os12g0240900 protein showed reasonable kinetic properties to OsNOMT. On the basis of these results, we conclude that Os12g0240900 encodes an OsNOMT.     

Multiple phytohormones and phytoalexins are involved in disease resistance to Magnaporthe oryzae invaded from roots in rice

Blast, caused by the fungus Magnaporthe oryzae, is one of the most devastating diseases of rice worldwide. Phenylalanine ammonia lyase (PAL) is a key enzyme in the phenylpropanoid pathway, which leads to the biosynthesis of defense‐related phytohormone salicylic acid (SA) and flavonoid‐type phytoalexins sakuranetin and naringenin. However, the roles and biochemical features of individual rice PALs in defense responses to pathogens remain unclear. Here, we report that rice OsPAL06, which can catalyze the formation of trans‐cinnamate using l ‐phenylalanine, is involved in rice root–M. oryzae interaction. OsPAL06‐knockout mutant showed increased susceptibility to M. oryzae invaded from roots and developed typical leaf blast symptoms, accompanied by nearly complete disappearance of sakuranetin and naringenin and a two‐third reduction of the SA level in roots. This mutant also showed compensatively induced expression of chalcone synthase, which is involved in flavonoid biosynthesis, isochorismate synthase 1, which is putatively involved in SA synthesis via another pathway, reduced jasmonate content and increased ethylene content. These results suggest that OsPAL06 is a positive regulator in preventing M. oryzae infection from roots. It may regulate defense by promoting both phytoalexin accumulation and SA signaling that synergistically and antagonistically interacts with jasmonate‐ and ethylene‐dependent signaling, respectively.     

Uncovering the complex metabolic network underlying diterpenoid phytoalexin biosynthesis in rice and other cereal crop plants

Rice (Oryza sativa) is a staple food crop and serves as a model cereal crop plant for scientific study. Phytochemical investigations of the agronomically devastating rice blast disease have identified a number of rice phytoalexins exhibiting significant direct anti-fungal activity against the causative agent, Magneporthe grisea. Current evidence strongly indicates that these phytoalexins, largely a family of labdane-related diterpenoids, are important as general antibiotics, and that similar phytoalexins are produced more broadly throughout the cereal crop family. From the extensive sequence information available for rice it has been possible to functionally identify the genes for the enzymes catalyzing the two consecutive cyclization reactions that initiate biosynthesis of these labdane-related diterpenoid phytoalexins. This has led to several insights into the underlying evolution of diterpene biosynthesis throughout the cereal crop family. The hydrocarbon olefins resulting from cyclization must be further elaborated to form bioactive natural products and, because not much is currently known, necessarily speculative biosynthetic pathways for these processes are presented. Given the significant antibiotic activity of the labdane-related diterpenoid phytoalexins from rice, and the presence of similar secondary metabolism throughout the cereal crop plant family, study of this type of biosynthesis will continue to be an area of active investigation.     

A rice fungal MAMP‐responsive MAPK cascade regulates metabolic flow to antimicrobial metabolite synthesis

Plants recognize potential microbial pathogens through microbial‐associated molecular patterns (MAMPs) and activate a series of defense responses, including cell death and the production of reactive oxygen species (ROS) and diverse anti‐microbial secondary metabolites. Mitogen‐activated protein kinase (MAPK) cascades are known to play a pivotal role in mediating MAMP signals; however, the signaling pathway from a MAPK cascade to the activation of defense responses is poorly understood. Here, we found in rice that the chitin elicitor, a fungal MAMP, activates two rice MAPKs (OsMPK3 and OsMPK6) and one MAPK kinase (OsMKK4). OsMPK6 was essential for the chitin elicitor‐induced biosynthesis of diterpenoid phytoalexins. Conditional expression of the active form of OsMKK4 (OsMKK4^DD) induced extensive alterations in gene expression, which implied dynamic changes of metabolic flow from glycolysis to secondary metabolite biosynthesis while suppressing basic cellular activities such as translation and cell division. OsMKK4^DD also induced various defense responses, such as cell death, biosynthesis of diterpenoid phytoalexins and lignin but not generation of extracellular ROS. OsMKK4^DD‐induced cell death and expression of diterpenoid phytoalexin pathway genes, but not that of phenylpropanoid pathway genes, were dependent on OsMPK6. Collectively, the OsMKK4–OsMPK6 cascade plays a crucial role in reprogramming plant metabolism during MAMP‐triggered defense responses.     

How phenology influences physiology in deciduous forest spring ephemerals

The protein phosphatase inhibitor cantharidin activates defense responses in rice leaves when applied exogenously at concentrations ranging from 100 to 500 μ M . Responses include the accumulation of the major rice phenolic phytoalexin sakuranetin and the lactone phytoalexin momilactone A. Accumulation of sakuranetin was preceded by an induction of phenylalanine ammonia lyase (PAL) activity and an increase in the activity of naringenin 7- O -methyltransferase (NOMT), the key enzyme in sakuranetin biosynthesis. Cantharidin also strongly induced accumulation of the probenazole (PBZ)-inducible protein (PBZ1) and two novel, related proteins named PBZ2 and PBZ3. Endothall, a herbicide and potent protein phosphatase inhibitor, but not its inactive analog (1,4-dimethylendothall) also induced sakuranetin accumulation, increased activity of NOMT and accumulation of the 3 PBZ proteins. In contrast, two other protein phosphatase inhibitors, calyculin A and microcystin LR, did not activate these defense responses. Induction of NOMT and PAL activity, and sakuranetin accumulation, was completely blocked by cycloheximide. Leaf segments treated with cantharidin and endothall showed brownish and orange colored lesions, respectively, similar to the lesion mimic mutants of rice. These results indicate a direct role for protein phosphorylation/dephosphorylation events in the activation of defense responses in rice, in particular on the accumulation of antifungal phytoalexins and the PBZ proteins.     

Chitosan activates defense/stress response(s) in the leaves of Oryza sativa seedlings

In this study, we examined the response(s) of rice (Oryza sativa L. japonica-type cv. Nipponbare) seedling leaves treated with a fungal elicitor chitosan (CT). Small brownish necrotic spots (streaks) appeared in the interveinal regions on the leaf surface after treatment by 0.1% CT, over the cut control. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis further revealed strong induction of ascorbate peroxidase, and changes in “phytocystatins” (cysteine proteinase inhibitors). Using two-dimensional polyacrylamide gel electrophoresis, evidence is provided for the accumulation of two major classes of pathogenesis-related (PR) proteins, namely OsPR5 and OsPR10 in the leaves. In parallel, northern analyses revealed potent accumulation of the OsPR5 and OsPR10 mRNAs; a time- and dose-dependent expression, and a requirement for de novo protein synthesis was observed. Furthermore, CT-elicited changes were also accompanied by production of anti-fungal phytoalexins, the flavonoid sakuranetin and the diterpenoid lactone momilactone A, as determined by liquid chromatography-mass spectrometry/mass spectrometry analysis. Present results reveal for the first time the potency of CT in initiating multiple events linked with defense/stress response(s) in the leaves of whole rice plants.     

Identification of a biosynthetic gene cluster in rice for momilactones

Rice diterpenoid phytoalexins such as momilactones and phytocassanes are produced in suspension-cultured rice cells treated with a chitin oligosaccharide elicitor and in rice leaves irradiated with UV light. The common substrate geranylgeranyl diphosphate is converted into diterpene hydrocarbon precursors via a two-step sequential cyclization and then into the bioactive phytoalexins via several oxidation steps. It has been suggested that microsomal cytochrome P-450 monooxygenases (P-450s) are involved in the downstream oxidation of the diterpene hydrocarbons leading to the phytoalexins and that a dehydrogenase is involved in momilactone biosynthesis. However, none of the enzymes involved in the downstream oxidation of the diterpene hydrocarbons have been identified. In this study, we found that a putative dehydrogenase gene (AK103462) and two functionally unknown P-450 genes (CYP99A2 and CYP99A3) form a chitin oligosaccharide elicitor- and UV-inducible gene cluster, together with OsKS4 and OsCyc1, the diterpene cyclase genes involved in momilactone biosynthesis. Functional analysis by heterologous expression in Escherichia coli followed by enzyme assays demonstrated that the AK103462 protein catalyzes the conversion of 3beta-hydroxy-9betaH-pimara-7,15-dien-19,6beta-olide into momilactone A. The double knockdown of CYP99A2 and CYP99A3 specifically suppressed the elicitor-inducible production of momilactones, strongly suggesting that CYP99A2, CYP99A3, or both are involved in momilactone biosynthesis. These results provide strong evidence for the presence on chromosome 4 of a gene cluster involved in momilactone biosynthesis.     

Comprehensive profiling and natural variation of flavonoids in rice

Flavonoids constitute a major group of plant phenolic compounds. While extensively studied in Arabidopsis, profiling and natural y occurring variation of these compounds in rice (Oryza sativa), the mon...