Green leaf volatiles and jasmonic acid enhance susceptibility to anthracnose diseases caused by Colletotrichum graminicola in maize

Colletotrichum graminicola is a hemibiotrophic fungus that causes anthracnose leaf blight (ALB) and anthracnose stalk rot (ASR) in maize. Despite substantial economic losses caused by these diseases, the defence mechanisms against this pathogen remain poorly understood. Several hormones are suggested to aid in defence against C. graminicola, such as jasmonic acid (JA) and salicylic acid (SA), but supporting genetic evidence was not reported. Green leaf volatiles (GLVs) are a group of well-characterized volatiles that induce JA biosynthesis in maize and are known to function in defence against necrotrophic pathogens. Information regarding the role of GLVs and JA in interactions with (hemi)biotrophic pathogens remains limited. To functionally elucidate GLVs and JA in defence against a hemibiotrophic pathogen, we tested GLV- and JA-deficient mutants, lox10 and opr7 opr8, respectively, for resistance to ASR and ALB and profiled jasmonates and SA in their stalks and leaves throughout infection. Both mutants were resistant and generally displayed elevated levels of SA and low amounts of jasmonates, especially at early stages of infection. Pretreatment with GLVs restored susceptibility of lox10 mutants, but not opr7 opr8 mutants, which coincided with complete rescue of JA levels. Exogenous methyl jasmonate restored susceptibility in both mutants when applied before inoculation, whereas methyl salicylate did not induce further resistance in either of the mutants, but did induce mutant-like resistance in the wild type. Collectively, this study reveals that GLVs and JA contribute to maize susceptibility to C. graminicola due to suppression of SA-related defences.     

Jasmonoyl isoleucine accumulation is needed for abscisic acid build‐up in roots of Arabidopsis under water stress conditions

Phytohormones are central players in sensing and signalling numerous environmental conditions like drought. In this work, hormone profiling together with gene expression of key enzymes involved in abscisic acid (ABA) and jasmonate biosynthesis were studied in desiccating Arabidopsis roots. Jasmonic acid (JA) content transiently increased after stress imposition whereas progressive and concomitant ABA and Jasmonoyl Isoleucine (JA‐Ile) accumulations were detected. Molecular data suggest that, at least, part of the hormonal regulation takes place at the biosynthetic level. These observations also point to a possible involvement of jasmonates on ABA biosynthesis under stress. To test this hypothesis, mutants impaired in jasmonate biosynthesis (opr3, lox6 and jar1‐1) and in JA‐dependent signalling (coi1) were employed. Results showed that the early JA accumulation leading to JA‐Ile build up was necessary for an ABA increase in roots under two different water stress conditions. Signal transduction between water stress‐induced JA‐Ile accumulation and COI1 is necessary for a full induction of the ABA biosynthesis pathway and subsequent hormone accumulation in roots of Arabidopsis plants. The present work adds a level of interaction between jasmonates and ABA at the biosynthetic level.     

Monitoring of methyl jasmonate-responsive genes in Arabidopsis by cDNA macroarray: self-activation of jasmonic acid biosynthesis and crosstalk with other phytohormone signaling pathways.

Jasmonates mediate various physiological events in plant cells such as defense responses, flowering, and senescence through intracellular and intercellular signaling pathways, and the expression of a large number of genes appears to be regulated by jasmonates. In order to obtain information on the regulatory network of jasmonate-responsive genes (JRGs) in Arabidopsis thaliana (Arabidopsis), we screened 2880 cDNA clones for jasmonate responsiveness by a cDNA macroarray procedure. Since many of the JRGs reported so far have been identified in leaf tissues, the cDNA clones used were chosen from a non-redundant EST library that was prepared from above-ground organs. Hybridization to the filters was achieved using alpha-33P-labeled single-strand DNAs synthesized from mRNAs obtained from methyl jasmonate (MeJA)-treated and untreated Arabidopsis seedlings. Data analysis identified 41 JRGs whose mRNA levels were changed by more than three fold in response to MeJA. This was confirmed by Northern blot analysis by using eight representatives. Among the 41 JRGs identified, 5 genes were JA biosynthesis genes and 3 genes were involved in other signaling pathways (ethylene, auxin, and salicylic acid). These results suggest the existence of a positive feedback regulatory system for JA biosynthesis and the possibility of crosstalk between JA signaling and other signaling pathways.     

Host and non-host pathogens elicit different jasmonate/ethylene responses in Arabidopsis

Arabidopsis does not support the growth and asexual reproduction of the barley pathogen, Blumeria graminis f. sp. hordei Bgh). A majority of germlings fail to penetrate the epidermal cell wall and papillae. To gain additional insight into this interaction, we determined whether the salicylic acid (SA) or jasmonate (JA)/ethylene (ET) defence pathways played a role in blocking barley powdery mildew infections. Only the eds1 mutant and NahG transgenics supported a modest increase in penetration success by the barley powdery mildew. We also compared the global gene expression patterns of Arabidopsis inoculated with the non-host barley powdery mildew to those inoculated with a virulent, host powdery mildew, Erysiphe cichoracearum. Genes repressed by inoculations with non-host and host powdery mildews relative to non-inoculated control plants accounted for two-thirds of the differentially expressed genes. A majority of these genes encoded components of photosynthesis and general metabolism. Consistent with this observation, Arabidopsis growth was inhibited following inoculation with Bgh, suggesting a shift in resource allocation from growth to defence. A number of defence-associated genes were induced during both interactions. These genes likely are components of basal defence responses, which do not effectively block host powdery mildew infections. In addition, genes encoding defensins, anti-microbial peptides whose expression is under the control of the JA/ET signalling pathway, were induced exclusively by non-host pathogens. Ectopic activation of JA/ET signalling protected Arabidopsis against two biotrophic host pathogens. Taken together, these data suggest that biotrophic host pathogens must either suppress or fail to elicit the JA/ET signal transduction pathway.     

Ectopic expression of AtJMT in Nicotiana attenuata: creating a metabolic sink has tissue-specific consequences for the jasmonate metabolic network and silences downstream gene expression

To create a metabolic sink in the jasmonic acid (JA) pathway, we generated transgenic Nicotiana attenuata lines ectopically expressing Arabidopsis (Arabidopsis thaliana) jasmonic acid O-methyltransferase (35S-jmt) and additionally silenced in other lines the N. attenuata methyl jasmonate esterase (35S-jmt/ir-mje) to reduce the deesterification of methyl jasmonate (MeJA). Basal jasmonate levels did not differ between transgenic and wild-type plants; however, after wounding and elicitation with Manduca sexta oral secretions, the bursts of JA, jasmonoyl-isoleucine (JA-Ile), and their metabolites that are normally observed in the lamina, midvein, and petiole of elicited wild-type leaves were largely absent in both transformants but replaced by a burst of endogenous MeJA that accounted for almost half of the total elicited jasmonate pools. In these plants, MeJA became a metabolic sink that affected the jasmonate metabolic network and its spread to systemic leaves, with major effects on 12-oxo-phytodieonic acid, JA, and hydroxy-JA in petioles and on JA-Ile in laminas. Alterations in the size of jasmonate pools were most obvious in systemic tissues, especially petioles. Expression of threonine deaminase and trypsin proteinase inhibitor, two JA-inducible defense genes, was strongly decreased in both transgenic lines without influencing the expression of JA biosynthesis genes that were uncoupled from the wounding and elicitation with M. sexta oral secretions-elicited JA-Ile gradient in elicited leaves. Taken together, this study provides support for a central role of the vasculature in the propagation of jasmonates and new insights into the versatile spatiotemporal characteristics of the jasmonate metabolic network.     

Myzus persicae (green peach aphid) salivary components induce defence responses in Arabidopsis thaliana

Myzus persicae (green peach aphid) feeding on Arabidopsis thaliana induces a defence response, quantified as reduced aphid progeny production, in infested leaves but not in other parts of the plant. Similarly, infiltration of aphid saliva into Arabidopsis leaves causes only a local increase in aphid resistance. Further characterization of the defence-eliciting salivary components indicates that Arabidopsis recognizes a proteinaceous elicitor with a size between 3 and 10 kD. Genetic analysis using well-characterized Arabidopsis mutants shows that saliva-induced resistance against M. persicae is independent of the known defence signalling pathways involving salicylic acid, jasmonate and ethylene. Among 78 Arabidopsis genes that were induced by aphid saliva infiltration, 52 had been identified previously as aphid-induced, but few are responsive to the well-known plant defence signalling molecules salicylic acid and jasmonate. Quantitative PCR analyses confirm expression of saliva-induced genes. In particular, expression of a set of O -methyltransferases, which may be involved in the synthesis of aphid-repellent glucosinolates, was significantly up-regulated by both M. persicae feeding and treatment with aphid saliva. However, this did not correlate with increased production of 4-methoxyindol-3-ylmethylglucosinolate, suggesting that aphid salivary components trigger an Arabidopsis defence response that is independent of this aphid-deterrent glucosinolate.     

Jasmonate-induced gene expression of barley (Hordeum vulgare) leaves -- the link between jasmonate and abscisic acid

In barley leaves a group of genes is expressed in response to treatment with jasmonates and abscisic acid (ABA) [21]. One of these genes coding for a jasmonate-induced protein of 23 kDa (JIP-23) was analyzed to find out the link between ABA and jasmonates by recording its expression upon modulating independently, the endogenous level of both of them. By use of inhibitors of JA synthesis and ABA degradation, and the ABA-deficient mutant Az34, as well as of cultivar-specific differences, it was shown that endogenous jasmonate increases are necessary and sufficient for expression of this gene. The endogenous rise of ABA did not induce synthesis of JIP-23, whereas exogenous ABA did not act via jasmonates. Different signalling pathways are suggested and discussed.     

Analysis of oxidative signalling induced by ozone in Arabidopsis thaliana

We are using acute ozone as an elicitor of endogenous reactive oxygen species (ROS) to understand oxidative signalling in Arabidopsis. Temporal patterns of ROS following a 6 h exposure to 300 nL L(-1) of ozone in ozone-sensitive Wassilewskija (Ws-0) ecotype showed a biphasic ROS burst with a smaller peak at 4 h and a larger peak at 16 h. This was accompanied by a nitric oxide (NO) burst that peaked at 9 h. An analysis of antioxidant levels showed that both ascorbate (AsA) and glutathione (GSH) were at their lowest levels, when ROS levels were high in ozone-stressed plants. Whole genome expression profiling analysis at 1, 4, 8, 12 and 24 h after initiation of ozone treatment identified 371 differentially expressed genes. Early induction of proteolysis and hormone-responsive genes indicated that an oxidative cell death pathway was triggered rapidly. Down-regulation of genes involved in carbon utilization, energy pathways and signalling suggested an inefficient defense response. Comparisons with other large-scale expression profiling studies indicated some overlap between genes induced by ethylene and ozone, and a significant overlap between genes repressed by ozone and methyl jasmonate treatment. Further, analysis of cis elements in the promoters of ozone-responsive genes also supports the view that phytohormones play a significant role in ozone-induced cell death.     

Genome-wide expression-monitoring of jasmonate-responsive genes of Arabidopsis using cDNA arrays

Jasmonates are generally considered to mediate signalling, such as defence responses, flowering and senescence. However, factors involved in the jasmonate signal-transduction pathway remain unclear. To clarify the functions and signalling mechanisms of jasmonates on a genome-wide level, we adopted a cDNA macroarray technique. We prepared nylon filters of a cDNA macroarray on which 2880 independent expressed sequence tag clones of Arabidopsis were blotted, and hybridized (33)P-labelled single-strand DNAs synthesized from mRNAs of methyl jasmonate (MeJA)-treated and untreated Arabidopsis plants to the nylon filters. By analysing the data from the cDNA macroarray, we identified many function-known and unknown genes as MeJA-responsive genes, and confirmed that the profiles of the expression showed good agreement with Northern-blot analysis. These results demonstrate the efficiency of the cDNA macroarray for systematically analysing jasmonate-responsive genes on a genome-wide scale.     

Jasmonates induce Nod factor production by Bradyrhizobium japonicum

Jasmonates are signaling molecules involved in induced systemic resistance, wounding and stress responses of plants. We have previously demonstrated that jasmonates can induce nod genes of Bradyrhizobium japonicum when measured by beta-galactosidase activity. In order to test whether jasmonates can effectively induce the production and secretion of Nod factors (lipo-chitooligosaccharides, LCOs) from B. japonicum, we induced two B. japonicum strains, 532C and USDA3, with jasmonic acid (JA), methyl jasmonate (MeJA) and genistein (Ge). As genistein is well characterized as an inducer of nod genes it was used a positive control. The high-performance liquid chromatography (HPLC) profile of LCOs isolated following treatment with jasmonates or genistein showed that both JA and MeJA effectively induced nod genes and caused production of LCOs from bacterial cultures. JA and MeJA are more efficacious inducers of LCO production than genistein. Genistein plus JA or MeJA resulted in greater LCO production than either alone. A soybean root hair deformation assay showed that jasmonate induced LCOs were as effective as those induced by genistein. This is the first report that jasmonates induce Nod factor production by B. japonicum. This report establishes the role of jasmonates as a new class of signaling molecules in the Bradyrhizobium-soybean symbiosis.     

植物的环境信号分子茉莉酸及其生物学功能

茉莉酸信号分子参与植物生长发育众多生理过程的调控,尤其是作为环境信号分子能有效地介导植物对生物及非生物胁迫的防御反应。迄今已知具有信号分子生理功能的至少包括茉莉酸(jasmonic acid,JA)以及茉莉酸甲酯(methyl jasmonate,Me JA)和茉莉酸-异亮氨酸复合物(jasmonoyl-isoleucine,JA-Ile)等茉莉酸衍生物,统称为茉莉酸类化合物(jasmonates,JAs)。从环境信号分子角度介绍了茉莉酸信号的启动(环境信号感知与转导、茉莉酸类化合物合成)、传递(局部传递、维管束传输、空气传播)和生物学功能(茉莉酸信号受体、调控的转录因子、参与的生物学过程)。     

Jasmonate biosynthesis and the allene oxide cyclase family of Arabidopsis thaliana

In biosynthesis of octadecanoids and jasmonate (JA), the naturally occurring enantiomer is established in a step catalysed by the gene cloned recently from tomato as a single-copy gene (Ziegler et al., 2000). Based on sequence homology, four full-length cDNAs were isolated from Arabidopsis thaliana ecotype Columbia coding for proteins with AOC activity. The expression of AOCgenes was transiently and differentially up-regulated upon wounding both locally and systemically and was induced by JA treatment. In contrast, AOC protein appeared at constitutively high basal levels and was slightly increased by the treatments. Immunohistochemical analyses revealed abundant occurrence of AOC protein as well as of the preceding enzymes in octadecanoid biosynthesis, lipoxygenase (LOX) and allene oxide synthase (AOS), in fully developed tissues, but much less so in 7-day old leaf tissues. Metabolic profiling data of free and esterified polyunsaturated fatty acids and lipid peroxidation products including JA and octadecanoids in wild-type leaves and the jasmonate-deficient mutant OPDA reductase 3 (opr3) revealed preferential activity of the AOS branch within the LOX pathway. 13-LOX products occurred predominantly as esterified derivatives, and all 13-hydroperoxy derivatives were below the detection limits. There was a constitutive high level of free 12-oxo-phytodienoic acid (OPDA) in untreated wild-type and opr3 leaves, but an undetectable expression of AOC. Upon wounding opr3 leaves exhibited only low expression of AOC, wounded wild-type leaves, however, accumulated JA and AOC mRNA. These and further data suggest regulation of JA biosynthesis by OPDA compartmentalization and a positive feedback by JA during leaf development.     

Fatty acid-derived signals in plants

Plants synthesize many fatty acid derivatives, several of which play important regulatory roles. Jasmonates are the best characterized examples. Jasmonate-insensitive mutants and mutants with a constitutive jasmonate response have given us new insights into jasmonate signalling. The jasmonate biosynthesis mutant opr3 allowed the dissection of cyclopentanone and cyclopentenone signalling, thus defining specific roles for these molecules. Jasmonate signalling is a complex network of individual signals and recent findings on specific activities of methyl jasmonate and (Z)-jasmone add to this picture. In addition, there are keto, hydroxy and hydroperoxy fatty acids that might be involved in cell death and the expression of stress-related genes. Finally, there are bruchins and volicitin, signal molecules from insects that are perceived by plants in the picomole to femtomole range. They highlight the importance of fatty acid-derived molecules in interspecies communication and in plant defence.     

Nuclear genes that promote splicing of group I introns in the chloroplast 23S rRNA and psbA genes in Chlamydomonas reinhardtii

Defence against pathogens in Arabidopsis is orchestrated by at least three signalling molecules: salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). The hrl1 (hypersensitive response-like lesions 1) mutant of Arabidopsis is characterized by spontaneous necrotic lesions, accumulation of reactive oxygen species, constitutive expression of SA- and ET/JA-responsive defence genes, and enhanced resistance to virulent bacterial and oomycete pathogens. Epistasis analyses of hrl1 with npr1, etr1, coi1 and SA-depleted nahG plants revealed novel interactions between SA and ET/JA signalling pathways in regulating defence gene expression and cell death. RNA gel-blot analysis of RNA isolated separately from the lesion+ and the lesion- leaves of double mutants of hrl1 revealed different signalling requirements for the expression of defence genes in these tissues. Expression of the ET/JA-responsive PDF1.2 gene was markedly reduced in hrl1 npr1 and in SA-depleted hrl1 nahG plants. In hrl1 nahG plants, expression of PDF1.2 was regulated by benzathiadiazole in a concentration-dependent manner: induced at low concentration and suppressed at high concentration. The hrl1 etr1 plants lacked systemic PR-1 expression, and exhibited compromised resistance to virulent Pseudomonas syringae and Peronospora parasitica. Inhibiting JA responses in hrl1 coi1 plants lead to exaggerated cell death and severe stunting of plants. Finally, the hrl1 mutation lead to elevated expression of AtrbohD, which encodes a major subunit of the NADPH oxidase complex. Our results indicate that defence gene expression and resistance against pathogens in hrl1 is regulated synergistically by SA and ET/JA defence pathways.