Turnip mosaic virus induces expression of the LRR II subfamily genes and regulates the salicylic acid signaling pathway in non-heading Chinese cabbage

In order to study the defense response to turnip mosaic virus (TuMV) infection in non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino), we cloned the LRR II subfamily genes which comprises six members. They were high homologous to the function-known LRR II genes of Arabidopsis. We investigated their expression through quantitative real-time PCR analysis. TuMV infection induced the expression of these genes locally and systematically, and regulated the endogenous accumulation of salicylic acid (SA). Exogenous SA spraying was able to induce resistance to the susceptibility of the TuMV-infected plants, which might function via inhibiting the viral duplication. Though TuMV-induced SA accumulation was not the determinant in regulating gene expression, it mediated the reaction oxygen species (ROS) burst as a channel of defense.     

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.     

Colonization of <Emphasis Type="Italic">Arabidopsis</Emphasis> roots by <Emphasis Type="Italic">Trichoderma atroviride</Emphasis> promotes growth and enhances systemic disease resistance through jasmonic acid/ethylene and salicylic acid pathways

Trichoderma spp. are common soil fungi used as biocontrol agents due to their capacity to produce antibiotics, induce systemic resistance in plants and parasitize phytopathogenic fungi of major agricultural importance. The present study investigated whether colonization of Arabidopsis thaliana seedlings by Trichoderma atroviride affected plant growth and development. Here it is shown that T. atroviride promotes growth in Arabidopsis. Moreover, T. atroviride produced indole compounds in liquid cultures. These results suggest that indoleacetic acid-related indoles (IAA-related indoles) produced by T. atroviride may have a stimulatory effect on plant growth. In addition, whether colonization of Arabidopsis roots by T. atroviride can induce systemic protection against foliar pathogens was tested. Arabidopsis roots inoculation with T. atroviride provided systemic protection to the leaves inoculated with bacterial and fungal pathogens. To investigate the possible pathway involved in the systemic resistance induced by T. atroviride, the expression profile of salicylic acid, jasmonic acid/ethylene, oxidative burst and camalexin related genes was assessed in Arabidopsis. T. atroviride induced an overlapped expression of defence-related genes of SA and JA/ET pathways, and of the gene involved in the synthesis of the antimicrobial phytoalexin, camalexin, both locally and systemically. This is the first report where colonization of Arabidopsis roots by T. atroviride induces the expression of SA and JA/ET pathways simultaneously to confer resistance against hemibiotrophic and necrotrophic phytopathogens. The beneficial effects induced by the inoculation of Arabidopsis roots with T. atroviride and the induction of the plant defence system suggest a molecular dialogue between these organisms.     

Rice <Emphasis Type="Italic">OsPBL1</Emphasis> (ORYZA SATIVA ARABIDOPSIS PBS1-LIKE 1) enhanced defense of <Emphasis Type="Italic">Arabidopsis</Emphasis> against <Emphasis Type="Italic">Pseudomonas syringae</Emphasis> DC3000

The receptor-like cytoplasmic kinases (RLCK family VII) are required for plant defense against various pathogens. Previously, OsPBL1 (ORYZA SATIVA ARABIDOPSIS PBS1-LIKE 1) was isolated from rice as a potential RSV (rice stripe virus) resistant factor, but its physiological roles in plant defense are yet to be investigated. In this study, we demonstrated that OsPBL1increased defense against P. syringae in transgenic Arabidopsis. To ascertain the role of OsPBL1 gene in plant defense, OsPBL1 tagged with HA (i.e. Hemagglutinin) was overexpressed in Arabidopsis and examined for the resistance against Pseudomonas syringae pv. tomato DC3000 (i.e. Pst DC3000). At 3 dpi of Pst DC3000, transgenic Arabidopsis lines exhibited the reduced chlorotic lesion and propagation of P. syringae, compared to wild type. Elevated pathogen resistance of transgenic lines was correlated with increased H₂O₂ accumulation and callose deposition on the infected leaves. It was also revealed that expression levels of salicylic acid dependent genes such as PR1, PR2, and PR5, were induced higher in transgenic lines than wild type. Taken together, our data suggested that OsPBL1 exerted the role in defense against pathogen attacks in plant via mainly facilitating salicylic acid dependent pathway.     

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.     

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.     

Bioluminescence reporter assay system to monitor Arabidopsis MPK3 gene expression in response to infection by Botrytis cinerea

The Arabidopsis MPK3 gene product participates in disease resistance mediated by the MAP kinase cascade. The expression of the MPK3 gene is induced by pathogen inoculation and treatment with chemicals such as salicylic acid (SA) and methyl jasmonate (JA), but the detailed expression pattern of the MPK3 gene has been largely unknown. To investigate MPK3 gene expression in response to disease stress, we fused the MPK3 promoter to the firefly luciferase gene to create a real-time monitoring system for regulated gene expression in planta. The results of an in vivo reporter assay using transgenic Arabidopsis plants harboring MPK3::Fluc showed that the MPK3 promoter activity was induced by treatment with chemicals such as SA and benzo(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), that induce defense gene expression. Inoculation with the fungal pathogen Botrytis cinerea resulted in systemic induction of MPK3::Fluc.     

Induced resistance in tomato plants to the toxin-dependent necrotrophic pathogen Alternaria alternata

A necrotrophic pathogen, the tomato pathotype of Alternaria alternata (Aa) causes Alternaria stem canker on tomato. Its pathogenicity depends on the production of host-specific AAL-toxin. Pre-inoculation with nonpathogenic Aa or pretreatment an elicitor prepared from Aa reduced disease symptoms by the pathogen. Salicylic acid (SA)- and jasmonic acid (JA)-dependent defense responses in tomato are not involved in the resistance to the pathogen induced by nonpathogenic Aa. The results suggest that an alternative and unknown signaling pathway independent of SA- and JA-signaling might modulate the induced resistance by activating the expression of the multiple defense genes.     

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.     

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.     

Victoria Blight,defense turned upside down

Genes that confer disease resistance to biotrophic pathogens typically encode nucleotide-binding, leucine-rich-repeat proteins (NB-LRRs). These proteins confer resistance by detecting the presence of virulence effectors secreted by biotrophic pathogens. Recognition triggers NB-LRR activation and subsequently, the defense response which often includes localized host cell death. The fungus, Cochliobolus victoriae, is a necrotrophic pathogen that causes a disease called Victoria Blight. Virulence of this fungus is dependent on its production of a peptide called “victorin” that has been traditionally described as a toxin. Only plants that respond to victorin are susceptible to Cochliobolus victoriae whereas those that do not are resistant to the fungus. Genetic and molecular analyses have revealed that victorin functions like a biotrophic effector recognized by a NB-LRR resistance protein in Arabidopsis. Further, numerous plant species express victorin sensitivity suggesting there are numerous NB-LRRs that recognize victorin. Thus, through expression of victorin, C. victoriae is able to exploit plant defense to cause disease and is capable of evoking this response in an array of different plants.     

Host‐induced gene silencing in the necrotrophic fungal pathogen Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic fungus that causes a devastating disease called white mould, infecting more than 450 plant species worldwide. Control of this disease with fungicides is limited, so host plant resistance is the preferred alternative for disease management. However, due to the nature of the disease, breeding programmes have had limited success. A potential alternative to developing necrotrophic fungal resistance is the use of host‐induced gene silencing (HIGS) methods, which involves host expression of dsRNA‐generating constructs directed against genes in the pathogen. In this study, the target gene chosen was chitin synthase (chs), which commands the synthesis of chitin, the polysaccharide that is a crucial structural component of the cell walls of many fungi. Tobacco plants were transformed with an interfering intron‐containing hairpin RNA construct for silencing the fungal chs gene. Seventy‐two hours after inoculation, five transgenic lines showed a reduction in disease severity ranging from 55·5 to 86·7% compared with the non‐transgenic lines. The lesion area did not show extensive progress over this time (up to 120 h). Disease resistance and silencing of the fungal chs gene was positively correlated with the presence of detectable siRNA in the transgenic lines. It was demonstrated that expression of endogenous genes from the very aggressive necrotrophic fungus S. sclerotiorum could be prevented by host induced silencing. HIGS of the fungal chitin synthase gene can generate white mould‐tolerant plants. From a biotechnological perspective, these results open new prospects for the development of transgenic plants resistant to necrotrophic fungal pathogens.     

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.