Coordinated activation of metabolic pathways for antioxidants and defence compounds by jasmonates and their roles in stress tolerance in Arabidopsis

Jasmonic acid (JA) and methyl jasmonate (MeJA), collectively termed jasmonates, are ubiquitous plant signalling compounds. Several types of stress conditions, such as wounding and pathogen infection, cause endogenous JA accumulation and the expression of jasmonate-responsive genes. Although jasmonates are important signalling components for the stress response in plants, the mechanism by which jasmonate signalling contributes to stress tolerance has not been clearly defined. A comprehensive analysis of jasmonate-regulated metabolic pathways in Arabidopsis was performed using cDNA macroarrays containing 13516 expressed sequence tags (ESTs) covering 8384 loci. The results showed that jasmonates activate the coordinated gene expression of factors involved in nine metabolic pathways belonging to two functionally related groups: (i) ascorbate and glutathione metabolic pathways, which are important in defence responses to oxidative stress, and (ii) biosynthesis of indole glucosinolate, which is a defence compound occurring in the Brassicaceae family. We confirmed that JA induces the accumulation of ascorbate, glutathione and cysteine and increases the activity of dehydroascorbate reductase, an enzyme in the ascorbate recycling pathway. These antioxidant metabolic pathways are known to be activated under oxidative stress conditions. Ozone (O3) exposure, a representative oxidative stress, is known to cause activation of antioxidant metabolism. We showed that O3 exposure caused the induction of several genes involved in antioxidant metabolism in the wild type. However, in jasmonate-deficient Arabidopsis 12-oxophytodienoate reductase 3 (opr3) mutants, the induction of antioxidant genes was abolished. Compared with the wild type, opr3 mutants were more sensitive to O3 exposure. These results suggest that the coordinated activation of the metabolic pathways mediated by jasmonates provides resistance to environmental stresses.     

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.     

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.     

Constitutive activation of the jasmonate signaling pathway enhances the production of secondary metabolites in tomato

The phytohormone jasmonic acid (JA) regulates the synthesis of secondary metabolites in a wide range of plant species. Here, we show that exogenous methyl-JA (MeJA) elicits massive accumulation of caffeoylputrescine (CP) in tomato leaves. A mutant (jai1) that is defective in jasmonate perception failed to accumulate CP in flowers and MeJA-treated leaves. Conversely, a transgenic tomato line (called 35S::PS) that exhibits constitutive JA signaling accumulated high levels of leaf CP in the absence of jasmonate treatment. RNA blot analysis showed that genes encoding enzymes in the phenylpropanoid and polyamine pathways for CP biosynthesis are upregulated in MeJA-treated wild-type plants and in untreated 35S::PS plants. These results indicate that CP accumulation in tomato is tightly controlled by the jasmonate signaling pathway, and provide proof-of-concept that the production of some plant secondary metabolites can be enhanced by transgenic manipulation of endogenous JA levels.     

Salicylate-mediated suppression of jasmonate-responsive gene expression in Arabidopsis is targeted downstream of the jasmonate biosynthesis pathway

Jasmonates (JAs) and salicylic acid (SA) are plant hormones that play pivotal roles in the regulation of induced defenses against microbial pathogens and insect herbivores. Their signaling pathways cross-communicate providing the plant with a regulatory potential to finely tune its defense response to the attacker(s) encountered. In Arabidopsis thaliana, SA strongly antagonizes the jasmonic acid (JA) signaling pathway, resulting in the downregulation of a large set of JA-responsive genes, including the marker genes PDF1.2 and VSP2. Induction of JA-responsive marker gene expression by different JA derivatives was equally sensitive to SA-mediated suppression. Activation of genes encoding key enzymes in the JA biosynthesis pathway, such as LOX2, AOS, AOC2, and OPR3 was also repressed by SA, suggesting that the JA biosynthesis pathway may be a target for SA-mediated antagonism. To test this, we made use of the mutant aos/dde2, which is completely blocked in its ability to produce JAs because of a mutation in the ALLENE OXIDE SYNTHASE gene. Mutant aos/dde2 plants did not express the JA-responsive marker genes PDF1.2 or VSP2 in response to infection with the necrotrophic fungus Alternaria brassicicola or the herbivorous insect Pieris rapae. Bypassing JA biosynthesis by exogenous application of methyl jasmonate (MeJA) rescued this JA-responsive phenotype in aos/dde2. Application of SA suppressed MeJA-induced PDF1.2 expression to the same level in the aos/dde2 mutant as in wild-type Col-0 plants, indicating that SA-mediated suppression of JA-responsive gene expression is targeted at a position downstream of the JA biosynthesis pathway.     

Jasmonate-dependent plant defenses mediate soybean thrips and soybean aphid performance on soybean

Insect herbivores from different feeding guilds induce different signaling pathways in plants. In this study, we examined the effects of salicylic acid (SA)- and jasmonic acid (JA)-mediated defenses on performance of insect herbivores from two different feeding guilds: cell-content feeders, soybean thrips and phloem feeders, soybean aphids. We used a combination of RT-qPCR analysis and elicitor-induced plant resistance to determine induction of SA and JA signaling pathways and the impact on herbivore performance. In the early interaction between the host plant and the two herbivores, SA and JA signaling seems to occur simultaneously. But overall, soybean thrips induced JA-related marker genes, whereas soybean aphids increased SA and ABA-related marker genes over a 24-h period. Populations of both soybean thrips and soybean aphids were reduced (47 and 25 %, respectively) in methyl jasmonate (MeJA)-pretreated soybean plants. SA treatment has no effect on either herbivore performance. A combination pretreatment of SA and MeJA did not impact soybean thrips population but reduced soybean aphid numbers which was comparable with MeJA treatment. Our data suggest that SA–JA antagonism could be responsible for the effect of hormone pretreatment on thrips performance, but not on aphid performance. By linking plant defense gene expression and elicitor-induced resistance, we were able to pinpoint the role for JA signaling pathway in resistance to two herbivores from different feeding guilds.     

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.     

Enhancement of root hydraulic conductivity by methyl jasmonate and the role of calcium and abscisic acid in this process

The role of jasmonic acid in the induction of stomatal closure is well known. However, its role in regulating root hydraulic conductivity (L) has not yet been explored. The objectives of the present research were to evaluate how JA regulates L and how calcium and abscisic acid (ABA) could be involved in such regulation. We found that exogenous methyl jasmonate (MeJA) increased L of Phaseolus vulgaris, Solanum lycopersicum and Arabidopsis thaliana roots. Tomato plants defective in JA biosynthesis had lower values of L than wild‐type plants, and that L was restored by addition of MeJA. The increase of L by MeJA was accompanied by an increase of the phosphorylation state of the aquaporin PIP2. We observed that MeJA addition increased the concentration of cytosolic calcium and that calcium channel blockers inhibited the rise of L caused by MeJA. Treatment with fluoridone, an inhibitor of ABA biosynthesis, partially inhibited the increase of L caused by MeJA, and tomato plants defective in ABA biosynthesis increased their L after application of MeJA. It is concluded that JA enhances L and that this enhancement is linked to calcium and ABA dependent and independent signalling pathways.     

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)。     

Cooperative ethylene and jasmonic acid signaling regulates selenite resistance in Arabidopsis

Selenium (Se) is an essential element for many organisms, but excess Se is toxic. To better understand plant Se toxicity and resistance mechanisms, we compared the physiological and molecular responses of two Arabidopsis (Arabidopsis thaliana) accessions, Columbia (Col)-0 and Wassilewskija (Ws)-2, to selenite treatment. Measurement of root length Se tolerance index demonstrated a clear difference between selenite-resistant Col-0 and selenite-sensitive Ws-2. Macroarray analysis showed more pronounced selenite-induced increases in mRNA levels of ethylene- or jasmonic acid (JA)-biosynthesis and -inducible genes in Col-0 than in Ws-2. Indeed, Col-0 exhibited higher levels of ethylene and JA. The selenite-sensitive phenotype of Ws-2 was attenuated by treatment with ethylene precursor or methyl jasmonate (MeJA). Conversely, the selenite resistance of Col-0 was reduced in mutants impaired in ethylene or JA biosynthesis or signaling. Genes encoding sulfur (S) transporters and S assimilation enzymes were up-regulated by selenite in Col-0 but not Ws-2. Accordingly, Col-0 contained higher levels of total S and Se and of nonprotein thiols than Ws-2. Glutathione redox status was reduced by selenite in Ws-2 but not in Col-0. Furthermore, the generation of reactive oxygen species by selenite was higher in Col-0 than in Ws-2. Together, these results indicate that JA and ethylene play important roles in Se resistance in Arabidopsis. Reactive oxygen species may also have a signaling role, and the resistance mechanism appears to involve enhanced S uptake and reduction.     

Jasmonic acid alleviates cadmium toxicity in Arabidopsis via suppression of cadmium uptake and translocation

Jasmonic acid(JA) is thought to be involved in plant responses to cadmium(Cd) stress, but the underlying molecular mechanisms are poorly understood. Here, we show that Cd treatment rapidly induces the expression of genes promoting endogenous JA synthesis, and subsequently increases the JA concentration in Arabidopsis roots. Furthermore, exogenous methyl jasmonate(MeJA)alleviates Cd-generated chlorosis of new leaves by decreasing the Cd concentration in root cell sap and shoot, and decreasing the expression of the AtIRT1,AtHMA2 and AtHMA4 genes promoting Cd uptake and long-distance translocation, respectively. In contrast,mutation of a key JA synthesis gene, At AOS, greatly enhances the expression of AtIRT1, AtHMA2 and AtHMA4,increases Cd concentration in both roots and shoots, and confers increased sensitivity to Cd. Exogenous Me JA recovers the enhanced Cd-sensitivity of the ataos mutant,but not of atcoi1, a JA receptor mutant. In addition,exogenous Me JA reduces NO levels in Cd-stressed Arabidopsis root tips. Taken together, our results suggest that Cd-induced JA acts via the JA signaling pathway and its effects on NO levels to positively restrict Cd accumulation and alleviates Cd toxicity in Arabidopsis via suppression of the expression of genes promoting Cd uptake and long-distance translocation.