Role of methyl jasmonate in the expression of mycorrhizal induced resistance against Fusarium oxysporum in tomato plants

Arbuscular mycorrhiza (AM) colonization led to a decrease in the severity of fusarium wilt disease caused by Fusarium oxysporum f. sp. lycopersici in tomato plants. The involvement of two plant defense hormones, namely methyl jasmonate (MeJA) and salicylic acid (SA), in the expression of mycorrhiza induced resistance (MIR) against this vascular pathogen was studied in the AM colonized and non-colonized (controls) plants. Activity of lipoxygenase (LOX), which plays a role in jasmonic acid (JA) biosynthesis, as well as levels of methyl jasmonate (MeJA) increased in AM colonized plants as compared to controls, but did not show any further changes in response to F. oxysporum inoculation. On the other hand, activity of phenylalanine ammonia lyase (PAL), which is an enzyme from salicylic acid (SA) biosynthetic pathway, as well as SA levels, increased in both controls and AM colonized plants in response to application of F. oxysporum spores. Hence the JA and not the SA signalling pathway appeared to play a role in the expression of MIR against this vascular pathogen. The resistance observed in AM colonized plants was completely compromised when plants were treated with the JA biosynthesis inhibitor salicylhydroxamic acid (SHAM). This confirmed that the AM-induced increase in JA levels was involved in the expression of resistance toward F. oxysporum. The SA response gene pathogenesis-related 1 (PR1) showed an increased expression in response to F. oxysporum infection in SHAM treated AM colonized plants as compared to plants that were not treated with this JA inhibitor. This suggested the possibility that JA inhibited SA responses, at least in the roots. AM colonization therefore appeared to prime plants for improved tolerance against the vascular pathogen F. oxysporum, which was mediated through the JA signalling pathway.     

Transgenic expression of a sorghum gene (SbLRR2) encoding a simple extracellular leucine-rich protein enhances resistance against necrotrophic pathogens in Arabidopsis

Plant leucine-rich repeat (LRR) domain-containing proteins are known to play important roles in signaling transduction and defense responses. In sorghum, SbLRR2 is pathogen-inducible gene encoding a simple extracellular LRR protein. Here, we demonstrated an earlier and stronger expression of SbLRR2 in a sorghum resistant genotype in comparison to a susceptible genotype following inoculation with the anthracnose pathogen (Colletotrichum sublineolum). In addition, SbLRR2 expression was found to be induced strongly by methyl-jasmonate treatment. Functional analysis was performed in SbLRR2 over-expression (OE) Arabidopsis plants, which showed enhanced resistance against the necrotrophic pathogens Botrytis cinerea and Alternaria brassicicola. In addition, the OE lines were found to have elevated expression of several jasmonate acid (JA)-associated genes and higher endogenous JA contents. Hence, the SbLRR2-mediated defense responses in transgenic Arabidopsis are likely to be dependent on JA-signaling through increased JA production. On the other hand, the OE lines remained susceptible to Pseudomonas syringae pv. tomato like the wild type plants. Consistently, there was no up-regulation of salicylic acid (SA) defense marker gene expression or SA levels in the OE lines. Our results suggested that SbLRR2 is potentially useful for enhancing resistance against necrotrophic pathogens in transgenic dicot crops.     

Systemic resistance to bacterial leaf speck pathogen in Arabidopsis thaliana induced by the culture filtrate of a plant growth-promoting fungus (PGPF) Phoma sp. GS8-1

The culture filtrate (CF) from the plant growth-promoting fungus Phoma sp. GS8-1 was found to induce systemic resistance in Arabidopsis thaliana against the bacterial leaf speck pathogen Pseudomonas syringae pv. tomato DC3000 (Pst), and the underlying mechanism was studied. Roots of A. thaliana were treated with CF from GS8-1, and plants expressed a clear resistance to subsequent Pst infection; disease severity was reduced, and proliferation of pathogen was suppressed. Various mutants of A. thaliana were used to test whether the CF induced resistance through one of the known signaling pathways: salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). The CF was fully protective against Pst in Arabidopsis mutants jar1 and ein2 similar to wild-type plants. However, its efficacy was reduced in plants containing transgene NahG. Examination of systemic gene expression revealed that CF modulates the expression of SA-inducible PR-1, PR-2 and PR-5 genes, the JA/ET-inducible ChitB gene, and the ET-inducible Hel gene. Moreover, the expression of these genes was further enhanced upon subsequent stimulation after attack by Pst. Our data suggest that in addition to a partial requirement for SA, the signals JA and ET may also play a role in defense signaling in Arabidopsis.     

Jasmonic acid and abscisic acid play important roles in host–pathogen interaction between Fusarium graminearum and wheat during the early stages of fusarium head blight

The effect of phytohormones on the defense response of wheat against Fusarium graminearum infection was investigated. Infection of heads with F. graminearum induced accumulation of salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and indole acetic acid (IAA). Exogenous phytohormone treatments showed crosstalk between them and a complex effect on expression of the genes ATB2, ExpB6, LEA Td16, PR1, Pdf1.2, PR4. JA treatment reduced F. graminearum growth and fusarium head blight (FHB) symptoms while an increase in FHB was observed with ABA. Transient down-regulation of allene oxide synthase (AOS) supports a complex role for JA in wheat head.     

Investigating interactions of salicylic acid and jasmonic acid signaling pathways in monocots wheat

Upon pathogen or insect attack, plants respond with production of a specific blend of the alarm signals salicylic acid (SA) and jasmonic acid (JA), which are recognized as key players in the regulation of the signaling pathways involved. SA and JA responsive genes and SA/JA cross talk were well characterized in dicotyledonous species, but little is known in monocotyledonous plants. Using qRT-PCR, the expression profiles of SA and JA responsive genes were investigated after SA and JA treatments in monocots wheat. The results showed that Glu2 and PR-2 responded almost exclusively to SA, PR-3 and LOX2 responded positively to methyljasmonate (MeJA) treatment, while Lipase and PR-1.1 were induced in response to treatment with SA or MeJA. Furthermore, either by pathogen infection or exogenous application of hormones can activate the antagonistic effect between SA and JA in wheat, which has been well elucidated in dicotyledonous species. The outcomes of SA-JA interactions could be affected by the relative concentration of each hormone. This study shed light on marker genes that can represent SA and JA pathways in wheat and provided some clues for better understanding their interactions in monocot.     

TaULP5 contributes to the compatible interaction of adult plant resistance wheat seedlings-stripe rust pathogen

Adult plant resistance indicates that plant is susceptible to pathogen at seedling stage, but resistant at adult stage. Understanding the mechanism of the interactions between APR wheat plants and Puccinia striiformis f. sp. tritici (Pst) is important for the creation of strategies to improve cultivar disease resistance. In this study, a full-length cDNA was isolated from APR wheat cultivar Xingzi 9104 (XZ), and was designated as ubiquitin-like protein 5 (TaULP5). TaULP5 was likely to be located in the cytoplasm, with a percentage of 75.9% Arabidopsis protoplasts number. The expression of TaULP5 was largely induced in the compatible interaction of wheat seedlings to Pst, while no obvious change was found in the incompatible interaction of wheat adult plants to Pst. Moreover, when TaULP5 was knocked down, the wheat resistance at seedling stage to Pst was improved. In addition, knockdown of TaULP5 increased the expression levels of some biotic stress-related genes, such as PR1 and PR2. It is the first time to confirm that ubiquitin-like protein could contribute to the compatible interaction of XZ to Pst, and the results will lay a foundation for understanding the mechanisms of different interactions between APR wheat plants and Pst at post-translational level.     

Jasmonic and salicylic acid-induced resistance in sorghum against the stem borer Chilo partellus

Induced resistance was studied in three sorghum genotypes (IS2205, ICSV1 and ICSV700) against Chilo partellus (Swinhoe) (Lepidoptera; Pyralidae) infestation and jasmonic acid (JA) and salicylic acid (SA) application. The activity of plant defensive enzymes [peroxidase (POD), polyphenol oxidase (PPO), superoxide dismutase (SOD), and catalase (CAT)], and the amounts of total phenols, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and proteins were recorded at 6 days after infestation. The induction of enzyme activities and the amounts of secondary metabolites varied among the genotypes and treatments. The genotype IS2205 showed a stronger effect than that of ICSV1 or ICSV 700. Treatment with JA followed by insect infestation induced greater levels of enzymes and secondary metabolites. The results suggest that JA induces greater levels of resistance components in sorghum plants against insect pests. Thus, pretreatment of plants with elicitors including JA and SA could provide a greater opportunity for plant defense against herbivores.     

Mapping of QTL lengthening the latent period of Puccinia striiformis in winter wheat at the tillering growth stage

The winter wheat lines Luke and AQ24788-83 are respectively susceptible and slow-rusting at tillering stage to yellow (stripe) rust, caused by Puccinia striiformis f. sp. tritici (Pst). A mapping population consisting of 206 recombinant inbred lines was developed from the cross Luke?×?AQ24788-83. These lines were evaluated at the tillering stage in the field trials for infection type (IT) and disease incidence (DI) and in greenhouse trials for IT and latent period (LP). A significant negative correlation was found between LP and DI. A genetic map with 473 marker loci was constructed and used for identifying QTL associated with LP and IT. Two QTL, QYr.cau-1BS and QYr.cau-5AS, were mapped on 1BS and 5AS respectively, explaining collectively up to 46.4 % of LP phenotypic variance. QYr.cau-5AS was clearly distinct, in terms of mapping position, from all six yellow rust resistance genes/QTL previously reported on 5A. QYr.cau-1BS could not be spatially differentiated from three (i.e. YrAlp, Yr15, and YrH52) of the six genes/QTL known on 1BS and centromere-vicinity regions, but was determined to be different from these three genes based on phenotype. The two QTL identified here, therefore, are likely to be novel to the currently known Pst resistance genes/QTL. A minor QTL on 3AL was detected to be associated with both IT and LP. Expression of quantitative resistance at early wheat growth stages and usefulness of the QTL are discussed for the wheat-Pst system.     

The role of jasmonic acid signalling in wheat (Triticum aestivum L.) powdery mildew resistance reaction

Jasmonic acid (JA) signalling plays an important role in plant resistance to pathogens. Previously, JA has been found to play a role in induced disease resistance to necrotrophic pathogens in various plant species, but current researches showed that JA also enhanced resistance to biotrophic pathogens. However, its role in wheat (Triticum aestivum L.) powdery mildew (Blumeria graminis f. sp. tritici, Bgt) resistance reaction is largely unknown. To settle this issue, several typical powdery mildew resistant and susceptible wheat varieties were employed. The sensitivity to exogenous methyl jasmonate (MeJA) to wheat powdery mildew resistance, the concentration fluctuation of endogenous JAs after Bgt inoculation, and the expression profiles of nine pathogenesis-related protein genes (PR genes) after MeJA and Bgt treatments were studied systematically. Exogenous MeJA significantly enhanced the powdery mildew resistance of the susceptible varieties. After inoculation with Bgt, endogenous JAs accumulated rapidly, reached the maxima at 2 to 5 h post-inoculation (hpi), then decreased rapidly, and the concentration was almost the same as that of un-inoculated control at 96 hpi. The expression levels of the nine PRs were measured by real time quantitative RT-PCR (qRT-PCR) at different time points after MeJA application or Bgt inoculation respectively. The MeJA and Bgt strongly activated PR1, PR2, PR3, PR4, PR5, PR9, PR10 and Ta-JA2, but almost didn’t affect Ta-GLP2a. The induced powdery mildew resistance was positively correlated with the activated PR genes. JA plays a positive role in defence against Bgt. JA is a signalling molecule in wheat powdery mildew resistance and future manipulation of this pathway may improve powdery mildew resistance in wheat breeding.     

AlgU contributes to the virulence of <Emphasis Type="Italic">Pseudomonas syringae</Emphasis> pv. <Emphasis Type="Italic">tomato</Emphasis> DC3000 by regulating production of the phytotoxin coronatine

Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), which causes bacterial speck disease of tomato, has been used as a model pathogen to investigate the molecular basis of plant–pathogen interactions. The function of many potential virulence factors encoded in the Pst DC3000 genome and their modes of action are not fully understood. P. syringae is known to produce the exopolysaccharide alginate. Although AlgU, a sigma factor, is known to regulate the expression of genes such as algD related to alginate biosynthesis, the molecular mechanisms of AlgU in the virulence of Pst DC3000 is still unclear. To investigate the function of AlgU and alginate in plant–bacterial pathogen interactions, we generated ΔalgU and ΔalgD mutants. After inoculation with ΔalgU but not ΔalgD, host plants of Pst DC3000 including tomato and Arabidopsis had milder disease symptoms and reduced bacterial populations. Expression profiles of Pst DC3000 genes revealed that AlgU can regulate not only the expression of genes encoding alginate biosynthesis, but also the expression of genes related to type III effectors and the phytotoxin coronatine (COR). We also demonstrated that the ΔalgU mutant showed full virulence in the Arabidopsis fls2 efr1 double mutant, which is compromised in the recognition of PAMPs. Further, the application of COR was able to restore the phenotype of the ΔalgU mutant in the stomatal response. These results suggest that AlgU has an important role in the virulence of Pst DC3000 by regulating COR production.     

Systemic resistance induced in Arabidopsis thaliana by Trichoderma asperellum SKT-1, a microbial pesticide of seedborne diseases of rice

BACKGROUND: Trichoderma asperellum SKT-1 is a microbial pesticide of seedborne diseases of rice. To investigate the mechanisms of disease suppression in SKT-1, the ability to induce systemic resistance by SKT-1, or its cell-free culture filtrate (CF), was tested using Arabidopsis thaliana Col-0 plants. RESULTS: Both SKT-1 and its CF elicit an induced systemic resistance against the bacterial leaf speck pathogen Pseudomonas syringae pv. tomato DC3000 in Col-0 plants. Involvement of plant hormones in the induced resistance by SKT-1 and CF was assessed using Arabidopsis genotypes such as the jasmonic acid (JA)-resistant mutant jar1, the ethylene (ET)-resistant mutant etr1, the plant impaired in salicylic acid (SA) signalling transgenic NahG and the mutant npr1 impaired in NPR1 activity. In soil experiments using SKT-1, no significant disease suppression effect was observed in NahG transgenic plants or npr1 mutant plants. Expression levels of SA-inducible genes such as PR-1, PR-2 and PR-5 increased substantially in the leaves of Col-0 plants. Expression levels of JA/ET-induced genes such as PDF1.2a, PR-3, PR-4 and AtVsp1 were also induced, but the levels were not as high as for SA-inducible genes. In a hydroponic experiment using CF from SKT-1, all Arabidopsis genotypes showed an induced systemic resistance by CF and increased expression levels of JA/ET- and SA-inducible genes in leaves of CF-treated plants. CONCLUSION: The SA signalling pathway is important in inducing systemic resistance to colonisation by SKT-1, and both SA and JA/ET signalling pathways combine in the signalling of induced resistance by CF. These results indicate that the response of A. thaliana is different from that found in root treatments with barley grain inoculum and CF from SKT-1. Copyright © 2011 Society of Chemical Industry     

Signals induced by exogenous nitric oxide and their role in controlling brown rot disease caused by <Emphasis Type="Italic">Monilinia fructicola</Emphasis> in postharvest peach fruit

Recent evidence suggests that nitric oxide (NO) signaling plays an important role in plant–pathogen interactions and that aconitase is a major target of NO. In the present study on the signaling role of NO in the elicitation of defense responses in peach fruit against Monilinia fructicola and subsequent effect on brown rot disease, 15 μM NO solution induced disease resistance in harvested peaches. As a potentiated elicitor, NO induced high levels of endogenous NO and superoxide (O₂ ^?), hydrogen peroxide (H₂O₂), and NADPH oxidase and Ca²⁺-ATPase activity in the fruit. Aconitase activity in peach fruit was inhibited by NO. Activity of partially purified aconitase was inhibited in vitro by sodium nitroprusside (SNP) and H₂O₂; however, the inhibition could be relieved by carboxy-2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (cPTIO) or catalase (CAT), indicating that the defense response and signals induced by NO transduction depend on aconitase and conditions leading to elevated levels of NO; otherwise, H₂O₂ would inactivate aconitase directly in fruit. Treatment with NO resulted in salicylic acid (SA) accumulating during storage. Higher levels of jasmonic acid (JA) were detected in NO-treated fruit 48 h after the treatment. But after NO was removed, the level of SA and JA were lower than in the control. The results suggest that exogenous NO enhances resistance of harvested peach fruit against the fungus by inducing signals such as endogenous NO, reactive oxygen species (ROS), SA and JA and by inhibiting aconitase activity.     

Suppressive effect of abscisic acid on systemic acquired resistance in tobacco plants

Recent studies have indicated that the phytohormone abscisic acid (ABA), induced in response to a variety of environmental stresses, plays an important role in modulating diverse plant–pathogen interactions. In Arabidopsis thaliana, we previously clarified that ABA suppressed the induction of systemic acquired resistance (SAR), a plant defense system induced by pathogen infection through salicylic acid (SA) accumulation. We investigated the generality of this suppressive effect by ABA on SAR using tobacco plants. For SAR induction, we used 1,2-benzisothiazole-3(2H)-one 1,1-dioxide (BIT) and benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) that activate upstream and downstream of SA in the SAR signaling pathway, respectively. Wild-type tobacco plants treated with BIT or BTH exhibited enhanced disease resistance against Tobacco mosaic virus (TMV) and tobacco wildfire bacterium, Pseudomonas syringae pv. tabaci (Pst), however, which was suppressed by pretreatment of plants with ABA. Pretreatment with ABA also suppressed the expression of SAR-marker genes by BIT and BTH, indicating that ABA suppressed the induction of SAR. ABA suppressed BTH-induced disease resistance and pathogenesis-related (PR) gene expression in NahG-transgenic plants that are unable to accumulate SA. The accumulation of SA in wild-type plants after BIT treatment was also suppressed by pretreatment with ABA. These data suggest that ABA suppresses both upstream and downstream of SA in the SAR signaling pathway in tobacco.     

Host status of barley to Puccinia coronata from couch grass and P. striiformis from wheat and brome

The pathogenicity and identity was studied of a field sample (PcE) of crown rust fungus Puccinia coronata collected in Hungary on wild couch grass (Elymus repens) and of a field sample (Psb) of stripe rust (P. striiformis) collected in the Netherlands on California brome (Bromus carinatus). We focused on the analysis of the host susceptibility of cultivated barley (Hordeum vulgare) to both pathogen samples, in direct comparison with an isolate of P. striiformis f.sp. tritici (Pst) and P. striiformis f.sp. hordei (Psh). At the seedling stage 83 % of the barley accessions were susceptible to PcE. At the adult plant stage, cultivated barley was predominantly resistant to this crown rust sample. Morphology, pathogenicity on Hordeum and Bromus and ITS DNA sequence analysis determined that PcE represents a European specimen of P. coronata f.sp. hordei. Sample Psb was pathogenic on 47 % of the cultivated barley at the seedling stage. This is intermediate between the isolates Psh (90 %) and Pst (10 %). Psb and Psh were pathogenic to grasses representing several genera. The host range of the Pst isolate was more narrow. We conclude that Psb is unlikely to represent a genotype of Psh that happened to have colonized B. carinatus, and is likely a distinct form of P. striiformis.     

Age-related resistance to Pseudomonas syringae pv. tomato is associated with the transition to flowering in Arabidopsis and is effective against Peronospora parasitica

As plants mature it has been observed that some become more resistant to normally virulent pathogens. The ability to manifest the Age-Related Resistance (ARR) response in Arabidopsis to Pseudomonas syringae pathovars tomato (Pst) coincided with the transition to flowering in plants both delayed and accelerated in the transition to flowering. ARR was also associated with a change in PR-1 gene expression, such that young plants expressed PR-1 abundantly at 3 days post inoculation (dpi) while mature plants expressed much less. The Arabidopsis ARR response requires SA accumulation via isochorismate synthase (ICS1) [24]. ICS1 was expressed one dpi with virulent and avirulent Pst in both young and mature plants. The ARR response was also effective versus avirulent Pst providing an additional 4-fold limitation in bacterial growth. Arabidopsis ARR was found to be ineffective against two necrotrophs, Erwinia carotovora subspecies carotovora (bacterium) and Botrytis cinerea (fungus) and one obligate biotroph, Erysiphe cichoracearum (fungus). However, mature wild type, SA-deficient sid2 and NahG plants supported little growth of the obligate biotrophic oomycete, Peronospora parasitica. Therefore ARR to P. parasitica appears to be SA-independent, however the level of ARR resistance was somewhat reduced in these mutants in some experiments. Thus, there may be numerous defence pathways that contribute to adult plant resistance in Arabidopsis.     

Differential response of phytohormone signalling network determines nonhost resistance in rice during wheat stem rust (Puccinia graminis f. sp. tritici) colonization

Stem rust caused by Puccinia graminis f. sp. tritici is one of the most devastating diseases of wheat. Breakdown of host resistance under field conditions triggered by the evolution of new pathogenic races and pathotypes is a perennial threat for wheat cultivation. Rice, often grown in a rice–wheat cropping system, is immune to rust infection. Our microscopic studies revealed that P. graminis f. sp. tritici, although displaying nearly identical uredospore germination, stomatal entry, and epi- and endophytic mycelial growth in rice and wheat, failed to sporulate to cause rust disease in rice. We identified 18 key defence signalling genes in rice and unravelled their elicitation dynamics in time-course studies during infection. ICS1, NPR1-3, PRs, EDS1, PAD4, FMO1 (salicylic acid [SA] signalling), and ethylene-related genes (ACO4 and ACS6) were strongly elicited in rice. However, genes from the jasmonic acid (JA) signalling pathway (LOX2, AOS2, MYC2, PDF2.2, JAZ8, JAZ10) showed a delayed response during colonization in rice compared to an early or no induction in wheat. However, the JA/ethylene marker gene PDF2.2 was strongly induced in wheat as early as 12 hr postinoculation. Furthermore, rice and wheat displayed specific profiles of accumulation of various phenolic acids during P. graminis f. sp. tritici 40A infection. We propose a model where a differential modulation of the SA/JA-dependent defence network may modulate nonhost resistance. A deeper understanding of the molecular mechanism governing differential elicitation of defence signalling may provide a novel resistance mechanism for the sustainable management of rust diseases.     

Putative effector genes detected in Fusarium oxysporum from natural ecosystems of Australia

The Fusarium oxysporum species complex (FOSC) causes disease in plants and animals, but is also widely dispersed in natural ecosystems without evidence of disease. The present study screened a population representing natural ecosystems across the Australian continent for the putative effector genes pisatin demethylase 1 (PDA1), pectate lyase (pelD), secreted gene expression (SGE1) and secreted in xylem (SIX). The genes pelD and SGE1 were prevalent in the natural isolates, PDA1 was present at an intermediate level, whereas SIX genes were detected at low levels. Phylogenies of these putative effector genes were compared to the EF‐1α species phylogeny to determine the likely modes of gene transmission: vertical gene transfer (VGT) and horizontal gene transfer (HGT). There was evidence of both modes of gene transmission within the F. oxysporum isolates. PDA1, pelD and SGE1 were likely to be only vertically inherited, whereas the SIX genes had evidence for both VGT and HGT. The phylogenetic relationships of SIX genes in isolates from natural ecosystems and formae speciales from agro‐ecosystems were also established. These findings have important implications for the evolution of effectors in the FOSC.