Regulation of jasmonic acid biosynthesis by silicon application during physical injury to Oryza sativa L.

We investigated the effects of silicon (Si) application on rice plants (Oryza sativa L.) and its responses in the regulation of jasmonic acid (JA) during wounding stress. Endogenous JA was significantly higher in wounded rice plants than in non-wounded. In contrast, Si treatment significantly reduced JA synthesis as compared to non-Si applications under wounding stress. mRNA expression of O. sativa genes showed down-regulation of lipoxygenase, allene oxide synthase 1, allene oxide synthase 2, 12-oxophytodienoate reductase 3, and allene oxide cyclase upon Si application and wounding stress as compared to non-Si-treated wounded rice plants. The physical injury-induced-oxidative stress was modulated by Si treatments, which resulted in higher catalase, peroxidase, and polyphenol oxidase activities as compared with non-Si-treated plants under wounding stress. The higher Si accumulation in rice plants also reduced the level of lipid peroxidation, which helped the rice plants to protect it from wounding stress. In conclusion, Si accumulation in rice plants mitigated the adverse effects of wounding through regulation of antioxidants and JA.     

Nitric oxide negatively modulates wound signaling in tomato plants

Synthesis of proteinase inhibitor I protein in response to wounding in leaves of excised tomato (Lycopersicon esculentum) plants was inhibited by NO donors sodium nitroprusside and S-nitroso-N-acetyl-penicillamine. The inhibition was reversed by supplying the plants with the NO scavenger 2-(4-carboxiphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. NO also blocked the hydrogen peroxide (H(2)O(2)) production and proteinase inhibitor synthesis that was induced by systemin, oligouronides, and jasmonic acid (JA). However, H(2)O(2) generated by glucose oxidase and glucose was not blocked by NO, nor was H(2)O(2)-induced proteinase inhibitor synthesis. Although the expression of proteinase inhibitor genes in response to JA was inhibited by NO, the expression of wound signaling-associated genes was not. The inhibition of wound-inducible H(2)O(2) generation and proteinase inhibitor gene expression by NO was not due to an increase in salicylic acid, which is known to inhibit the octadecanoid pathway. Instead, NO appears to be interacting directly with the signaling pathway downstream from JA synthesis, upstream of H(2)O(2) synthesis. The results suggest that NO may have a role in down-regulating the expression of wound-inducible defense genes during pathogenesis.     

Role of Tomato Lipoxygenase D in Wound-Induced Jasmonate Biosynthesis and Plant Immunity to Insect Herbivores

In response to insect attack and mechanical wounding, plants activate the expression of genes involved in various defense-related processes. A fascinating feature of these inducible defenses is their occurrence both locally at the wounding site and systemically in undamaged leaves throughout the plant. Wound-inducible proteinase inhibitors (PIs) in tomato (Solanum lycopersicum) provide an attractive model to understand the signal transduction events leading from localized injury to the systemic expression of defense-related genes. Among the identified intercellular molecules in regulating systemic wound response of tomato are the peptide signal systemin and the oxylipin signal jasmonic acid (JA). The systemin/JA signaling pathway provides a unique opportunity to investigate, in a single experimental system, the mechanism by which peptide and oxylipin signals interact to coordinate plant systemic immunity. Here we describe the characterization of the tomato suppressor of prosystemin-mediated responses8 (spr8) mutant, which was isolated as a suppressor of (pro)systemin-mediated signaling. spr8 plants exhibit a series of JA-dependent immune deficiencies, including the inability to express wound-responsive genes, abnormal development of glandular trichomes, and severely compromised resistance to cotton bollworm (Helicoverpa armigera) and Botrytis cinerea. Map-based cloning studies demonstrate that the spr8 mutant phenotype results from a point mutation in the catalytic domain of TomLoxD, a chloroplast-localized lipoxygenase involved in JA biosynthesis. We present evidence that overexpression of TomLoxD leads to elevated wound-induced JA biosynthesis, increased expression of wound-responsive genes and, therefore, enhanced resistance to insect herbivory attack and necrotrophic pathogen infection. These results indicate that TomLoxD is involved in wound-induced JA biosynthesis and highlight the application potential of this gene for crop protection against insects and pathogens.     

Wound-induced RNaseLE expression is jasmonate and systemin independent and occurs only locally in tomato (Lycopersicon esculentum cv. Lukullus)

Tomato RNaseLE is induced by phosphate deficiency and wounding and may play a role in macromolecular recycling as well as wound healing. Here, we analyzed the role of jasmonate and systemin in the wound-induced RNaseLE activation. The rapid expression of RNaseLE upon wounding of leaves leading to maximal RNase activity within 10 h, appeared only locally. Jasmonic acid (JA) or its molecular mimic ethyl indanoyl isoleucine conjugate did not induce RNaseLE expression. Correspondingly, RNaseLE was expressed upon wounding of 35S::allene oxide cyclase antisense plants known to be JA deficient. RNaseLE was not expressed upon systemin treatment, but was locally expressed in the spr1 mutant which is affected in systemin perception. In tomato plants carrying a PromLE::uidA construct, GUS activity could be detected upon wounding, but not following treatment with JA or systemin. The data indicate a locally acting wound-inducible systemin- and JA-independent signaling pathway for RNaseLE expression.     

Activation of the Jasmonic Acid Pathway by Depletion of the Hydroperoxide Lyase OsHPL3 Reveals Crosstalk between the HPL and AOS Branches of the Oxylipin Pathway in Rice

The allene oxide synthase (AOS) and hydroperoxide lyase (HPL) branches of the oxylipin pathway, which underlie the production of jasmonates and aldehydes, respectively, function in plant responses to a range of stresses. Regulatory crosstalk has been proposed to exist between these two signaling branches; however, there is no direct evidence of this. Here, we identified and characterized a jasmonic acid (JA) overproduction mutant, cea62, by screening a rice T-DNA insertion mutant library for lineages that constitutively express the AOS gene. Map-based cloning was used to identify the underlying gene as hydroperoxide lyase OsHPL3. HPL3 expression and the enzyme activity of its product, (E)-2-hexenal, were depleted in the cea62 mutant, which resulted in the dramatic overproduction of JA, the activation of JA signaling, and the emergence of the lesion mimic phenotype. A time-course analysis of lesion formation and of the induction of defense responsive genes in the cea62 mutant revealed that the activation of JA biosynthesis and signaling in cea62 was regulated in a developmental manner, as was OsHPL3 activity in the wild-type plant. Microarray analysis showed that the JA-governed defense response was greatly activated in cea62 and this plant exhibited enhanced resistance to the T1 strain of the bacterial blight pathogen Xanthomonasoryzaepvoryzae (Xoo). The wounding response was attenuated in cea62 plants during the early stages of development, but partially recovered when JA levels were elevated during the later stages. In contrast, the wounding response was not altered during the different developmental stages of wild-type plants. These findings suggest that these two branches of the oxylipin pathway exhibit crosstalk with regards to biosynthesis and signaling and cooperate with each other to function in diverse stress responses.     

GTR1 is a jasmonic acid and jasmonoyl-l-isoleucine transporter in Arabidopsis thaliana

Jasmonates are major plant hormones involved in wounding responses. Systemic wounding responses are induced by an electrical signal derived from damaged leaves. After the signaling, jasmonic acid (JA) and jasmonoyl-l-isoleucine (JA-Ile) are translocated from wounded to undamaged leaves, but the molecular mechanism of the transport remains unclear. Here, we found that a JA-Ile transporter, GTR1, contributed to these translocations in Arabidopsis thaliana. GTR1 was expressed in and surrounding the leaf veins both of wounded and undamaged leaves. Less accumulations and translocation of JA and JA-Ile were observed in undamaged leaves of gtr1 at 30 min after wounding. Expressions of some genes related to wound responses were induced systemically in undamaged leaves of gtr1. These results suggested that GTR1 would be involved in the translocation of JA and JA-Ile in plant and may be contributed to correct positioning of JA and JA-Ile to attenuate an excessive wound response in undamaged leaves.     

OsMPK3 positively regulates the JA signaling pathway and plant resistance to a chewing herbivore in rice

Key message

Silencing OsMPK3 decreased elicited JA levels, which subsequently reduced levels of herbivore-induced trypsin protease inhibitors (TrypPIs) and improved the performance of SSB larvae, but did not influence BPH.

Abstract

Mitogen-activated protein kinases (MPKs) are known to play an important role in plant defense by transferring biotic and abiotic signals into programmed cellular responses. However, their functions in the herbivore-induced defense response in rice remain largely unknown. Here, we identified a MPK3 gene from rice, OsMPK3, and found that its expression levels were up-regulated in response to infestation by the larvae of the striped stem borer (SSB) (Chilo suppressalis), to mechanical wounding and to treatment with jasmonic acid (JA), but not to infestation by the brown planthopper (BPH) Nilaparvata lugens or to treatment with salicylic acid. Moreover, mechanical wounding and SSB infestation induced the expression of OsMPK3 strongly and quickly, whereas JA treatment induced the gene more weakly and slowly. Silencing OsMPK3 (ir-mpk3) reduced the expression of the gene by 50–70 %, decreased elicited levels of JA and diminished the expression of a lipoxygenase gene OsHI-LOX and an allene oxide synthase gene OsAOS1. The reduced JA signaling in ir-mpk3 plants decreased the levels of herbivore-induced trypsin protease inhibitors (TrypPIs) and improved the performance of SSB larvae, but did not influence BPH. Our findings suggest that the gene OsMPK3 responds early in herbivore-induced defense and can be regulated by rice plants to activate a specific and appropriate defense response to different herbivores.     

Coordinate expression of AOS genes and JA accumulation: JA is not required for initiation of closing layer in wound healing tubers

Wounding induces a series of coordinated physiological responses essential for protection and healing of the damaged tissue. Wound-induced formation of jasmonic acid (JA) is important in defense responses in leaves, but comparatively little is known about the induction of JA biosynthesis and its role(s) in tuber wound-healing. In this study, the effects of wounding on JA content, expression of JA biosynthetic genes, and the involvement of JA in the initiation of closing layer formation in potato tubers were determined. In addition, the role of abscisic acid (ABA) in wound-induced JA accumulation was examined. The basal JA content in non-wounded tuber tissues was low (<3 ng g⁻¹ FW). Two hours after wounding, the JA content increased by >5-fold, reached a maximum between 4 and 6 h after wounding, and declined to near-basal levels thereafter. Tuber age (storage duration) had little effect on the pattern of JA accumulation. The expressions of the JA biosynthetic genes (StAOS2, StAOC, and StOPR3) were greatly increased by wounding reaching a maximum 2-4 h after wounding and declining thereafter. A 1-h aqueous wash of tuber discs immediately after wounding resulted in a 94% inhibition of wound-induced JA accumulation. Neither JA treatment nor inhibition of JA accumulation affected suberin polyphenolic accumulation during closing layer development indicating that JA was not essential for the initiation of primary suberization. ABA treatment did not restore JA accumulation in washed tuber tissues suggesting that leaching of endogenous ABA was either not involved or not solely involved in this loss of JA accumulation by washing. Collectively, these results indicate that JA is not required for the induction of processes essential to the initiation of suberization during closing layer development, but do not exclude the possibility that JA may be involved in other wound related responses.     

The tomato mutant spr1 is defective in systemin perception and the production of a systemic wound signal for defense gene expression

Wound-induced systemic expression of defensive proteinase inhibitor (PI) genes in tomato plants requires the action of systemin and its precursor protein prosystemin. Although it is well established that systemin induces PI expression through the octadecanoid pathway for jasmonic acid (JA) biosynthesis, relatively little is known about how systemin and JA interact to promote long-distance signaling between damaged and undamaged leaves. Here, this question was addressed by characterizing a systemin-insensitive mutant (spr1) that was previously identified as a suppressor of prosystemin-mediated responses. In contrast to JA biosynthetic or JA signaling mutants that lack both local and systemic PI expression in response to wounding, spr1 plants were deficient mainly in the systemic response. Consistent with this phenotype, spr1 plants exhibited normal PI induction in response to oligosaccharide signals that are thought to play a role in the local wound response. Moreover, spr1 abolished JA accumulation in response to exogenous systemin, and reduced JA accumulation in wounded leaves to approximately 57% of wild-type (WT) levels. Analysis of reciprocal grafts between spr1 and WT plants showed that spr1 impedes systemic PI expression by blocking the production of the long-distance wound signal in damaged leaves, rather than inhibiting the recognition of that signal in systemic undamaged leaves. These experiments suggest that Spr1 is involved in a signaling step that couples systemin perception to activation of the octadecanoid pathway, and that systemin acts at or near the site of wounding (i.e. in rootstock tissues) to increase JA synthesis to a level that is required for the systemic response. It was also demonstrated that spr1 plants are not affected in the local or systemic expression of a subset of rapidly induced wound-response genes, indicating the existence of a systemin-independent pathway for wound signaling.     

OsNPR1 negatively regulates herbivore‐induced JA and ethylene signaling and plant resistance to a chewing herbivore in rice

NPR1 (a non‐expressor of pathogenesis‐related genes1) has been reported to play an important role in plant defense by regulating signaling pathways. However, little to nothing is known about its function in herbivore‐induced defense in monocot plants. Here, using suppressive substrate hybridization, we identified a NPR1 gene from rice, OsNPR1, and found that its expression levels were upregulated in response to infestation by the rice striped stem borer (SSB) Chilo suppressalis and rice leaf folder (LF) Cnaphalocrocis medinalis, and to mechanical wounding and treatment with jasmonic acid (JA) and salicylic acid (SA). Moreover, mechanical wounding induced the expression of OsNPR1 quickly, whereas herbivore infestation induced the gene more slowly. The antisense expression of OsNPR1 (as‐npr1), which reduced the expression of the gene by 50%, increased elicited levels of JA and ethylene (ET) as well as of expression of a lipoxygenase gene OsHI‐LOX and an ACC synthase gene OsACS2. The enhanced JA and ET signaling in as‐npr1 plants increased the levels of herbivore‐induced trypsin proteinase inhibitors (TrypPIs) and volatiles, and reduced the performance of SSB. Our results suggest that OsNPR1 is an early responding gene in herbivore‐induced defense and that plants can use it to activate a specific and appropriate defense response against invaders by modulating signaling pathways.