Novel rice MAP kinases OsMSRMK3 and OsWJUMK1 involved in encountering diverse environmental stresses and developmental regulation

We report isolation of two novel rice (Oryza sativa L.) mitogen-activated protein kinases (MAPKs), OsMSRMK3 (multiple stress responsive) and OsWJUMK1 (wound- and JA-uninducible) that most likely exist as single copy genes in its genome. OsMSRMK3 and OsWJUMK1 encode 369 and 569 amino acid polypeptides having the MAPK family signature and phosphorylation activation motifs TEY and TDY, respectively. Steady state mRNA analyses of these MAPKs with constitutive expression in leaves of two-week-old seedlings revealed that OsMSRMK3 was up-regulated upon wounding (by cut), jasmonic acid (JA), salicylic acid (SA), ethylene, abscisic acid, hydrogen peroxide (H(2)O(2)), protein phosphatase inhibitors, chitosan, high salt/sugar, and heavy metals, whereas OsWJUMK1 not induced by either wounding, JA or SA, showed up-regulation only by H(2)O(2), heavy metals, and cold stress (12 degrees C). Moreover, these MAPKs were developmentally regulated. These results strongly suggest a role for OsMSRMK3 and OsWJUMK1 in both stress-signalling pathways and development in rice.     

Rice OsSIPK and its orthologs: A “central master switch” for stress responses

Mitogen-activated protein kinase (MAPK) plays a central role in controlling a vast array of plant biochemical and physiological processes. It is regulated by a characteristic phosphorelay system in which a series of three kinases phosphorylate and activate each other. Over the past years, several plants MAPKs have been identified and characterized. Of these, rice OsSIPK (Salicylic acid (SA)-Induced Protein Kinase) and its orthologs in other plants are of particular interest. A large body of evidence demonstrates the involvement of SIPKs in fine-tuned regulation of the plant responses to ozone, wounding, SA, and jasmonic acid (JA). Interestingly, their function appears to be conserved across reference plants, such as rice, tobacco, and Arabidopsis. In this minireview, we discuss the recent progress on rice OsSIPK and its orthologs as a “central master switch” for mediating plant responses against ozone, wounding, and JA as examples.     

Molecular cloning and mRNA expression analysis of a novel rice (Oryzasativa L.) MAPK kinase kinase,OsEDR1, an ortholog of Arabidopsis AtEDR1, reveal its role in defense/stress signalling pathways and development

Mitogen-activated protein kinase (MAPK) cascade(s) is important for plant defense/stress responses. Though MAPKs have been identified and characterized in rice (Oryza sativa L.), a monocot cereal crop research model, the first upstream component of the kinase cascade, namely MAPK kinase kinase (MAPKKK) has not yet been identified. Here we report the cloning of a novel rice gene encoding a MAPKKK, OsEDR1, designated based on its homology with the Arabidopsis MAPKKK, AtEDR1. OsEDR1, a single copy gene in the genome of rice, encodes a predicted protein with molecular mass of 113046.13 and a pI of 9.03. Using our established two-week-old rice seedling in vitro model system, we show that OsEDR1 has a constitutive expression in seedling leaves and is further up-regulated within 15 min upon wounding by cut, treatment with the global signals jasmonic acid (JA), salicylic acid (SA), ethylene (ethephon, ET), abscisic acid, and hydrogen peroxide. In addition, protein phosphatase inhibitors, fungal elicitor chitosan, drought, high salt and sugar, and heavy metals also dramatically induce its expression. Moreover, OsEDR1 expression was altered by co-application of JA, SA, and ET, and required de novo synthesized protein factor(s) in its transient regulation. Furthermore, using an in vivo system we also show that OsEDR1 responds to changes in temperature and environmental pollutants-ozone and sulfur dioxide. Finally, OsEDR1 expression varied significantly in vegetative and reproductive tissues. These results suggest a role for OsEDR1 in defense/stress signalling pathways and development.     

Characterization of a novel rice gene OsATX and modulation of its expression by components of the stress signalling pathways

In our search to identify gene(s) involved in the rice self-defense responses, we cloned a novel rice ( Oryza sativa L. cv. Nipponbare) gene, OsATX , a single copy gene, from the JA treated rice seedling leaves cDNA library. This gene encodes a 69 amino acid polypeptide with a predicted molecular mass of 7649.7 and a pI of 5.6. OsATX was responsive to cutting (wounding by cutting the excised leaf), over its weak constitutive expression in the healthy leaves. The critical signalling molecules, jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), and hydrogen peroxide, together with protein phosphatase inhibitors, effectively up-regulated the OsATX expression with time, over the excised leaf cut control, whereas ethylene had no affect. Furthermore, copper, a heavy metal, also up-regulated OsATX expression. Moreover, induced expression of OsATX mRNA was influenced by light signal(s), and showed a requirement for de novo synthesized protein factors. Additionally, co-application of either JA or ABA with SA drastically suppressed the induced OsATX mRNA level. Finally, the blast pathogen, Magnaporthe grisea , triggered OsATX mRNA accumulation. These results strongly suggest a function/role(s) for OsATX in defense/stress responses in rice.     

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。     

Expression of a salt-induced protein (SALT) in suspension-cultured cells and leaves of rice following exposure to fungal elicitor and phytohormones

Phytohormones are essential signal compounds in the regulation of stress-related and defense-related genes. However, there is no clear evidence for any effect of these signal molecules and biotic elicitors on the regulation of the SALT gene in suspension-cultured rice cells. We characterized the expression of a SALT gene following treatment with fungal elicitor, phytohormones, cycloheximide, and inhibitors of protein kinase/phosphatases. SALT expression was up-regulated following treatment with a fungal elicitor, jasmonic acid (JA), abscisic acid (ABA), and NaCl. However, salicylic acid (SA) alone or in combination with one of the other elicitors not only strongly inhibited SALT gene expression but also exhibited an antagonistic effect in suspension cells and leaves. Cycloheximide inhibited SALT accumulation in suspension cells and in leaves, but the inhibitors of protein kinase/phosphatase did not. Immunolocalization revealed that SALT protein was present in xylem parenchyma cells of vascular bundles in the major and minor leaf veins.     

The rice MAPKK–MAPK interactome: the biological significance of MAPK components in hormone signal transduction

Mitogen-activated protein kinase (MAPK) signaling cascades are evolutionarily conserved fundamental signal transduction pathways. A MAPK cascade consists of many distinct MAPKKK–MAPKK–MAPK modules linked to various upstream receptors and downstream targets through sequential phosphorylation and activation of the cascade components. These cascades collaborate in transmitting a variety of extracellular signals and in controlling cellular responses and processes such as growth, differentiation, cell death, hormonal signaling, and stress responses. Although MAPK proteins play central roles in signal transduction pathways, our knowledge of MAPK signaling in hormonal responses in rice has been limited to a small subset of specific upstream and downstream interacting targets. However, recent studies revealing direct MAPK and MAPKK interactions have provided the basis for elucidating interaction specificities, functional divergence, and functional modulation during hormonal responses. In this review, we highlight current insights into MAPKK–MAPK interaction patterns in rice, with emphasis on the biological significance of these interacting pairs in SA (salicylic acid), JA (jasmonic acid), ET (ethylene), and ABA (abscisic acid) responses, and discuss the challenges in understanding functional signal transduction networks mediated by these hormones.     

外源水杨酸和茉莉酸对荞麦幼苗耐盐生理特性的效应

以盐敏感荞麦品种‘TQ-0808’幼苗为材料,采用100mmol·L^（-1）NaCl并添加不同浓度水杨酸（sA）和茉莉酸（JA）的处理方法,测定荞麦耐盐生理指标,确定外源SA和JA对荞麦耐盐生理特性的效应。结果表明,外源SA和JA均能明显降低盐胁迫下荞麦叶片质膜透性和丙二醛（MDA）含量,外源SA降低的幅度略高于JA,外源SA最适浓度为0．6mmol·L^（-1）外源SA和JA均能明显增加盐胁迫下荞麦叶片超氧化物歧化酶（SOD）和抗坏血酸过氧化物酶（APX）活性及叶片净光合速率,外源JA增加的幅度明显大于SA,外源JA最适浓度为40gmol·L^（-1）。外源sA和JA明显改善盐胁迫下荞麦的生理特性,对盐胁迫具有较好的缓解作用,说明通过适当浓度的外源SA和JA处理来提高荞麦耐盐性是可行的。     

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.     

Physiological and Molecular Changes of Detached Wheat Leaves in Responding to Various Treatments

Leaf senescence is induced or accelerated when leaves are detached. However, the senescence process and expression pattern of senescence-associated genes (SAGs) when leaves are detached are not clearly understood. To detect senescence-associated physiological changes and SAG expression, wheat (Triticum aestivum L.) leaves were detached and treated with light, darkness, low temperature (4 C), jasmonic acid (JA), abscisic acid (ABA), and salicylic acid (SA). The leaf phenotypes, chlorophyll content, delayed fluorescence (DF), and expression levels of two SAGs, namely, TaSAG3 and TaSAG5, were analyzed. Under these different treatments, the detached leaves turned yellow with different patterns and varying chlorophyll content. DF significantly decreased after the dark, ABA, JA and SA treatments. TaSAG3 and TaSAG5, which are expressed in natural senescent leaves, showed different expression patterns under various treatments. However, both TaSAG3 and TaSAG5 were upregulated after leaf detachment. Our results revealed senescence-associated physiological changes and molecular differences in leaves, which induced leaf senescence during different stress treatments.     

β-Aminobutyric Acid-induced Resistance in Brassica juncea against the Necrotrophic Pathogen Alternaria brassicae

β‐Aminobutyric acid (BABA) pretreatment of Brassica plants protected them against the necrotrophic pathogen Alternaria brassicae. The achieved resistance level was much higher than that seen after salicylic acid (SA) and jasmonic acid (JA) pretreatments. BABA pretreatment to the leaves, 1 day before inoculation, led to an inhibition of the oxidative burst and a decrease in SA levels, but did not influence lipoxygenase activity nor cause callose deposition at the site of inoculation. Expression of two marker genes of the SA and JA pathways, namely PR1 and PDF1.2, was enhanced in response to BABA pretreatment. Our results indicate that BABA‐induced resistance is mediated through an enhanced expression of pathogenesis‐related protein genes, independent of SA and JA accumulation.