Accumulation of salicylic acid,jasmonic acid and phytoalexins in rice, <Emphasis Type="Italic">Oryza sativa</Emphasis>, infested by the white-backed planthopper, <Emphasis Type="Italic">Sogatella furcifera</Emphasis> (Hemiptera: Delphacidae)

In order to clarify the mechanism of induced resistance to blast disease in rice, Oryza sativa, that had been previously infested by the white-backed planthopper, Sogatella furcifera Horváth, we first investigated the accumulation of salicylic acid (SA) and jasmonic acid (JA) in rice plants infested by the planthopper. The results confirmed that infestation of S. furcifera strongly stimulates the production of SA and JA in rice. These results indicate that both salicylate- and jasmonate-mediated pathways (SA and JA pathways), which are involved in the general defense system in plants, were activated in rice infested by S. furcifera. Further results confirmed that S. furcifera infestation induces accumulation of a major rice diterpenoid phytoalexin, momilactone A, and a flavonoid phytoalexin, sakuranetin, which are well known as antimicrobial chemicals, particularly in blast disease caused by the blast fungus, Magnaporthe oryzae B. Couch. All these results strongly suggest the following hypothetical mechanism of induced-resistance to M. oryzae in rice infested by S. furcifera. First, S. furcifera releases some elicitor-active compounds, which might be produced in the salivary glands, into the rice plant during feeding. Next, the defense signal systems, SA- and JA-mediated pathways, are activated by the elicitor. Finally, phytoalexins are induced in rice as antimicrobial compounds mainly through activation of the JA-mediated pathway.     

Ozone sensitivity in hybrid poplar correlates with insensitivity to both salicylic acid and jasmonic acid. The role of programmed cell death in lesion formation

Our earlier studies demonstrated that the ozone-sensitive hybrid poplar clone NE-388 displays an attenuated level of ozone-, wound-, and phytopathogen-induced defense gene expression. To determine if this reduced gene activation involves signal transduction pathways dependent on salicylic acid (SA) and/or jasmonic acid (JA), we compared the responses of NE-388 and an ozone-tolerant clone, NE-245, to these signal molecules. JA levels increased in both clones in response to ozone, but only minimal increases in SA levels were measured for either clone. Treatment with SA and methyl jasmonate induced defense gene expression only in NE-245, indicating that NE-388 is insensitive to these signal molecules. DNA fragmentation, an indicator of programmed cell death (PCD), was detected in NE-245 treated with either ozone or an avirulent phytopathogen, but was not detected in NE-388. We conclude that these clones undergo two distinct mechanisms of ozone-induced lesion formation. In NE-388, lesions appear to be due to toxic cell death resulting from a limited ability to perceive and subsequently activate SA- and/or JA-mediated antioxidant defense responses. In NE-245, SA-dependent PCD precedes lesion formation via a process related to the PCD pathway activated by phytopathogenic bacteria. These results support the hypothesis that ozone triggers a hypersensitive response.     

Jasmonic acid and salicylic acid activate a common defense system in rice

Jasmonic acid (JA) and salicylic acid (SA) play important roles in plant defense systems. JA and SA signaling pathways interact antagonistically in dicotyledonous plants, but, the status of crosstalk between JA and SA signaling is unknown in monocots. Our rice microarray analysis showed that more than half of the genes upregulated by the SA analog BTH are also upregulated by JA, suggesting that a major portion of the SA-upregulated genes are regulated by JA-dependent signaling in rice. A common defense system that is activated by both JA and SA is thus proposed which plays an important role in pathogen defense responses in rice.     

水稻中水杨酸和茉莉酸特异诱导蛋白激酶基因OsSJMK1的分离和鉴定

裂原激活蛋白激酶（MAPK）级联系统负责把接受自胞外或胞内的信号进一步传递和放大而最终作用于特异的转录因子,从而启动或调控基因的表达。MAPK信号级联系统在细胞分裂、分化、生物胁迫和非生物胁迫等多种信号传递途径中起着十分重要的作用。该文以一个茉莉酸（JA）诱导的表达序列为基础,在水稻中分离到了一个裂原激活蛋白激酶基因OsSJMK1的全长cDNA。序列比较分析表明该基因编码498个氨基酸,蛋白质等电点为8．43,包括完整的MAPK家族的蛋白激酶结构域。OsSJMK1与所有物种的MAPK一样包括蛋白激酶的全部11个次级结构域,在Ⅶ和Ⅷ次级结构域间一个双磷酸化位点;该位点苏氨酸（T）和酪氨酸（Y）残基之间为天冬氨酸（D）,而不是其他MAPK中常见的谷氨酸（E）、脯氨酸（P）或甘氨酸（G）。除了典型的MAPK激酶功能域外,在羧基端还有一段长约150个氨基酸残基的可能参与蛋白互作的结构域。以上这些结构特征表明OsSJMK1属于植物中第v类MAPK家族成员。蛋白激酶结构域序列比较表明OsSJMK1与报道的稻瘟病菌和机械伤害诱导的BWMK1的序列相似性高达81％,而且基因内含子和外显子的组成也非常相似,属于同一亚类,但在蛋白质序列的C端差异却很大。与BWMK1不同,OsSJMK1的表达不受伤害诱导,而受稻瘟病轻微诱导,但在JA和SA（水杨酸）处理早期表达量却迅速升高。在JA处理后1h,OsSJMK1转录水平升高到最大,而12h后回落到处理前的本底水平;在SA处理后30min转录水平就开始上升,2h达到最高值,而随后开始下降到处理前的本底水平。SA类似物BTH也能诱导OsSJMK1的表达。其他一些激素处理（如ABA）和非生物胁迫（如干旱、盐胁迫）都不对基因的表达产生任何影响,而且在植物大部分组织中的表达量都非常低。这些结果说明OsSJMK1可能特异性的参与JA和SA介导的防卫反应。     

Disease resistance against Magnaporthe grisea is enhanced in transgenic rice with suppression of omega-3 fatty acid desaturases

Linolenic acid (18:3) is the most abundant fatty acid in plant membrane lipids and is a source for various oxidized metabolites, called oxylipins. 18:3 and oxylipins play important roles in the induction of defense responses to pathogen infection and wound stress in Arabidopsis. However, in rice, endogenous roles for 18:3 and oxylipins in disease resistance have not been confirmed. We generated 18:3-deficient transgenic rice plants (F78Ri) with co-suppression of two omega-3 fatty acid desaturases, OsFAD7 and OsFAD8. that synthesize 18:3. The F78Ri plants showed enhanced resistance to the phytopathogenic fungus Magnaporthe grisea. A typical 18:3-derived oxylipin, jasmonic acid (JA), acts as a signaling molecule in defense responses to fungal infection in Arabidopsis. However, in F78Ri plants, the expression of JA-responsive pathogenesis-related genes, PBZ1 and PR1b, was induced after inoculation with M. grisea, although the JA-mediated wound response was suppressed. Furthermore, the application of JA methyl ester had no significant effect on the enhanced resistance in F78Ri plants. Taken together, our results indicate that, although suppression of fatty acid desaturases involves the concerted action of varied oxylipins via diverse metabolic pathways, 18:3 or 18:3-derived oxylipins, except for JA, may contribute to signaling on defense responses of rice to M. grisea infection.     

Interacting signal pathways control defense gene expression in Arabidopsis in response to cell wall-degrading enzymes from Erwinia carotovora

We have characterized the role of salicylic acid (SA)-independent defense signaling in Arabidopsis thaliana in response to the plant pathogen Erwinia carotovora subsp. carotovora. Use of pathway-specific target genes as well as signal mutants allowed us to elucidate the role and interactions of ethylene, jasmonic acid (JA), and SA signal pathways in this response. Gene expression studies suggest a central role for both ethylene and JA pathways in the regulation of defense gene expression triggered by the pathogen or by plant cell wall-degrading enzymes (CF) secreted by the pathogen. Our results suggest that ethylene and JA act in concert in this regulation. In addition, CF triggers another, strictly JA-mediated response inhibited by ethylene and SA. SA does not appear to have a major role in activating defense gene expression in response to CF. However, SA may have a dual role in controlling CF-induced gene expression, by enhancing the expression of genes synergistically induced by ethylene and JA and repressing genes induced by JA alone.     

Herbivore-induced rice resistance against rice blast mediated by salicylic acid

In agro-ecosystems,plants are important mediators of interactions between their associated herbivorous insects and microbes,and any change in plants induced by one species may lead to cascading effects on interactions with other species.Often,such effects are regulated by phytohormones such as jasmonic acid(JA)and salicylic acid(SA).Here,we investigated the tripartite interactions among rice plants,three insect herbivores(Chilo suppressalis,Cnaphalocrocis medinalis or Nilapai-vata lugens),and the causal agent of rice blast disease,the fungus Magnaporthe oryzae.We found that pre-infestation of rice by C.suppressalis or N.lugens but not by C.medinalis conferred resistance to M.oryzae.For C.suppressalis and N.lugens,insect infestation without fungal inoculation induced the accumulation of both JA and SA in rice leaves.In contrast,infestation by C.medinalis increased JA levels but reduced SA levels.The exogenous application of SA but not of JA conferred resistance against M.oryzae.These results suggest that preinfestation by C suppressalis or N.lugens conferred resistance against M.oryzae by increasing SA accumulation.These findings enhance our understanding of the interactions among rice plant,insects and pathogens,and provide valuable information for developing an ecologically sound strategy for controlling rice blast.     

Cross talk between signaling pathways in pathogen defense

Plant defense in response to microbial attack is regulated through a complex network of signaling pathways that involve three signaling molecules: salicylic acid (SA), jasmonic acid (JA) and ethylene. The SA and JA signaling pathways are mutually antagonistic. This regulatory cross talk may have evolved to allow plants to fine-tune the induction of their defenses in response to different plant pathogens.     

Probenazole induces systemic acquired resistance in Arabidopsis with a novel type of action

Probenazole (PBZ; 3-allyloxy-1,2-benzisothiazole-1,1-dioxide), which is the active ingredient in Oryzemate, has been used widely in Asia to protect rice plants against the rice blast fungus Magnaporthe grisea. To study PBZ's mode of action, we analyzed its ability, as well as that of its active metabolite 1, 2-benzisothiazol-3 (2H)-one 1,1-dioxide (BIT) to induce defense gene expression and resistance in Arabidopsis mutants that are defective in various defense signaling pathways. Wild-type Arabidopsis treated with PBZ or BIT exhibited increased expression of several pathogenesis-related genes, increased levels of total salicylic acid (SA), and enhanced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC 3000 and the oomycete pathogen Peronospora parasitica Emco5. The role of several defense signaling hormones, such as SA, ethylene and jasmonic acid (JA), in activating resistance following PBZ or BIT treatment was analyzed using NahG transgenic plants and etr1-1 and coi1-1 mutant plants, respectively. In addition, the involvement of NPR1, a key component in the SA signaling pathway leading to defense responses, was assessed. PBZ or BIT treatment did not induce disease resistance or PR-1 expression in NahG transgenic or npr1 mutant plants, but it did activate these phenomena in etr1-1 and coi 1-1 mutant plants. Thus SA and NPR1 appear to be required for PBZ- and BIT-mediated activation of defense responses, while ethylene and JA are not. Furthermore, our data suggest that PBZ and BIT comprise a novel class of defense activators that stimulate the SA/NPR1-mediated defense signaling pathway upstream of SA.     

BWMK1 Responds to Multiple Environmental Stresses and Plant Hormones

Many plant mltogen-actlvated protein klnases （MAPKs） play an important role In regulating responses to both ablotlc and biotic stresses. The first reported rice MAPK gene BWMK1 Is Induced by both rice blast （Magnaporthe grisea） Infection and mechanical wounding. For further analysis of Its response to other environmental cues and plant hormones, such as jasmonlc acid （JA）, salicylic acid （SA）, and benzothladlazole （BTH）, the promoter of BWMKf was fused with the coding region of the β-glucuronldase （GUS） reporter gene. Two promoter-GUS constructs with a 1.0- and 2.5-kb promoter fragment, respectively, were generated and transformed into the Japonica rice cultIvars TP309 and Zhonghua 11. Expression of GUS was Induced in the transgenic lines by cold, drought, dark, and JA. However, light, SA, and BTH treatments suppressed GUS expression. These results demonstrate that BWMK1 Is responsive to multiple ablotlc stresses and plant hormones and may play a role In cross-talk between different signaling pathways.     

Proteomic analysis of differentially expressed proteins induced by rice blast fungus and elicitor in suspension-cultured rice cells

We used two-dimensional electrophoresis (2-DE) and other proteomic approaches to identify proteins expressed in suspension-cultured rice cells in response to the rice blast fungus, Magnaporthe grisea. Proteins were extracted from suspension-cultured cells at 24 and 48 h after rice blast fungus inoculation or treatment with elicitor or other signal molecules such as jasmonic acid (JA), salicylic acid, and H(2)O(2). The proteins were then polyethylene glycol fractionated before separation by 2-DE. Fourteen protein spots were induced or increased by the treatments, which we analyzed by N-terminal or internal amino acid sequencing. Twelve proteins from six different genes were identified. Rice pathogen-related protein class 10 (OsPR-10), isoflavone reductase like protein, beta-glucosidase, and putative receptor-like protein kinase were among those induced by rice blast fungus; these have not previously been reported in suspension-cultured rice cells. Six isoforms of probenazole-inducible protein (PBZ1) and two isoforms of salt-induced protein (SalT) that responded to blast fungus, elicitor, and JA were also resolved on a 2-DE gel and identified by proteome analysis. The expression level of these induced proteins both in suspension-cultured cells and in leaves of whole plants was analyzed by Western blot. PBZ1, OsPR-10, and SalT proteins from incompatible reactions were induced earlier and to a greater extent than those in compatible reactions. Proteome analysis can thus distinguish differences in the timing and amount of protein expression induced by pathogens and other signal molecules in incompatible and compatible interactions.     

A rice isoflavone reductase-like gene, OsIRL, is induced by rice blast fungal elicitor

We have isolated and characterized a rice isoflavone reductase-like gene, OsIRL, whose expression is induced by a fungal elicitor. The OsIRL cDNA contains 1203 bp with an open reading frame of 942 nucleotides encoding 314 amino acids. The deduced amino acid sequence of OsIRL has a putative pyridine nucleotide binding domain and is 68% homologous with the maize isoflavone reductase-like gene. Southern blot analysis revealed that OsIRL belongs to a small multigene family. Expression of OsIRL was induced by treatment with a fungal elicitor and jasmonic acid as well as by inoculation with rice blast fungus. Cycloheximide (1 microM), strongly inhibited the induction of OsIRL by the fungal elicitor, indicating that new protein synthesis is required. The protein kinase inhibitor, staurosporine (1 microM), had little effect, but the phosphatase inhibitor, calyculin A (1 microM), strongly inhibited induction. Treatment with salicylic acid (SA, 5 mM) strongly inhibited expression of OsIRL in response to fungal elicitor and JA, while abscisic acid (ABA, 200 microM) also strongly antagonized OsIRL induction by JA, but had only a weak effect on induction by the fungal elicitor. These results suggest that the expression of OsIRL is positively regulated by phytohormones such as JA, and negatively by phytohormones such as SA, ABA.     

Methyl Salicylate Production and Jasmonate Signaling Are Not Essential for Systemic Acquired Resistance in Arabidopsis

Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.