The Significance of Mycoparasitism by Streptomyces sp. MBCN152-1 for Its Biocontrol Activity against Alternaria brassicicola

Streptomyces sp. strain MBCN152-1, isolated from cabbage, has potential as a biocontrol agent for Alternaria brassicicola on cabbage seedlings. The present study examined its mode of action. Light microscopy showed that appressorium formation by A. brassicicola was significantly suppressed on cabbage seedlings bacterized with MBCN152-1. Furthermore, scanning electron microscopy revealed that the mycelia of MBCN152-1, which were epiphytically growing on the cotyledon leaves of cabbage seedlings, intensively coiled around the germinating conidia of A. brassicicola. In vitro co-culture experiments demonstrated that MBCN152-1 is an aggressive mycoparasite of A. brassicicola, but not of A. brassicae or Colletotrichum higginsianum. Biocontrol experiments indicated that MBCN152-1 did not control diseases caused by A. brassicae or C. higginsianum. These results suggest that mycoparasitism is the primary mode of action for MBCN152-1. This is the first study to clearly demonstrate the significance of mycoparasitism in the biocontrol efficacy of endophytic Streptomyces.

Transcriptome Analysis and Identification of a Transcriptional Regulatory Network in the Response to H2O2

Hydrogen peroxide (H2O2) is a common signal molecule initiating transcriptional responses to all the known biotic and abiotic stresses of land plants. However, the degree of involvement of H2O2 in these stress responses has not yet been well studied. Here we identify time-dependent transcriptome profiles stimulated by H2O2 application in Arabidopsis (Arabidopsis thaliana) seedlings. Promoter prediction based on transcriptome data suggests strong crosstalk among high light, heat, and wounding stress responses in terms of environmental stresses and between the abscisic acid (ABA) and salicylic acid (SA) responses in terms of phytohormone signaling. Quantitative analysis revealed that ABA accumulation is induced by H2O2 but SA is not, suggesting that the implied crosstalk with ABA is achieved through ABA accumulation while the crosstalk with SA is different. We identified potential direct regulatory pairs between regulator transcription factor (TF) proteins and their regulated TF genes based on the time-course transcriptome analysis for the H2O2 response, in vivo regulation of the regulated TF by the regulator TF identified by expression analysis of mutants and overexpressors, and in vitro binding of the regulator TF protein to the target TF promoter. These analyses enabled the establishment of part of the transcriptional regulatory network for the H2O2 response composed of 15 regulatory pairs of TFs, including five pairs previously reported. This regulatory network is suggested to be involved in a wide range of biotic and abiotic stress responses in Arabidopsis.

Identification of a freshness marker metabolite in stored soybean sprouts by comprehensive mass-spectrometric analysis of carbonyl compounds

The objective of this study was to identify metabolites that quantitatively indicate degrees of freshness of soybean sprouts. Self-cultivated soybean sprouts were stored at 5 °C, 10 °C or 20 °C, and respiratory CO2 production rates were monitored using gas chromatography during storage. Carbonyl compounds (CCs) were analyzed comprehensively using mass-spectroscopic metabolomics analyses. CCs were derivatized using dansyl hydrazine (DH) and were then analyzed using high performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-MS/MS) with multiplexed multiple reaction monitoring (MRM). In the MRM chromatogram, 171 to 358 peaks were observed from stored soybean sprouts. Principle component analysis and discriminant analysis (PCA-DA) selected the CC-DH derivative ion with a m/z 512 at a retention time of 9.34 min as the most significant metabolite. Searching online metabolomics databases and matching fragment patterns of product ion mass spectra of an authentic standard revealed abscisic acid is a freshness marker of soybean sprouts.

Biocontrol Potential of an Endophytic Streptomyces sp. Strain MBCN152-1 against Alternaria brassicicola on Cabbage Plug Seedlings

In the present study, 77 strains of endophytic actinomycetes isolated from cabbage were screened in order to assess their biocontrol potential against Alternaria brassicicola on cabbage seedlings. In the first and second screening trials, cabbage seedlings pretreated with mycelial suspensions of each isolate were spray-inoculated with A. brassicicola. Strain MBCN152-1, which exhibited the best protection in screening trials and had no adverse effects on seedling growth, was selected for the greenhouse trial. In the greenhouse trial, cabbage seedlings, which had been grown in plug trays filled with soil mix containing spores of MBCN152-1 (1×108 spores g-1 of soil mix), were spray-inoculated with A. brassicicola and grown in greenhouse conditions. MBCN152-1 reduced disease incidence and significantly increased the number of viable seedlings. The efficacy of MBCN152-1 against damping-off caused by seed-borne A. brassicicola was then evaluated. Cabbage seeds, artificially infested with A. brassicicola, were sown in soil mix containing MBCN152-1 spores. The disease was completely suppressed when infested seeds were sown in a soil mix blended with MBCN152-1 at 1.5×107 spores g-1 of soil mix. These results strongly suggest that MBCN152-1 has the potential to control A. brassicicola on cabbage plug seedlings. MBCN152-1 was identified as a Streptomyces humidus-related species based on 16S rDNA sequencing. Scanning electron microscopy showed that the hyphae of MBCN152-1 multiplied on the surface of the seedlings and penetrated their epidermal cells. In conclusion, strain MBCN152-1 is a promising biocontrol agent against A. brassicicola on cabbage plug seedlings.

Possible Involvement of MYB44-Mediated Stomatal Regulation in Systemic Resistance Induced by Penicillium simplicissimum GP17-2 in Arabidopsis

The plant growth-promoting fungus (PGPF), Penicillium simplicissimum GP17-2 (GP17-2), induces systemic resistance against Pseudomonas syringae pv. tomato DC3000 (Pst) in Arabidopsis thaliana. The molecular mechanisms underlying induced systemic resistance (ISR) by GP17-2 were investigated in the present study. Microscopic observations revealed that stomatal reopening by Pst was restricted by elicitation with the culture filtrate (CF) from GP17-2. A gene expression analysis of MYB44, which enhances abscisic acid signaling and consequently closes stomata, revealed that the gene was activated by CF. CF-elicited myb44 mutant plants failed to restrict stomatal reopening and showed lower resistance to Pst than wild-type plants. These results indicate that stomatal resistance by GP17-2 is mediated by the gene activation of MYB44. We herein revealed that the MYB44-mediated prevention of penetration through the stomata is one of the components responsible for GP17-2-elicited ISR.

Systemic resistance induced by volatile organic compounds emitted by plant growth-promoting fungi in Arabidopsis thaliana

Volatile organic compounds (VOC) were extracted and identified from plant growth-promoting fungi (PGPF), Phoma sp., Cladosporium sp. and Ampelomyces sp., using gas chromatography-mass spectrometry (GC-MS). Among the three VOC extracted, two VOC blends (emitted from Ampelomyces sp. and Cladosporium sp.) significantly reduced disease severity in Arabidopsis plants against Pseudomonas syringae pv. tomato DC3000 (Pst). Subsequently, m-cresol and methyl benzoate (MeBA) were identified as major active volatile compounds from Ampelomyces sp. and Cladosporium sp., respectively, and found to elicit induced systemic resistance (ISR) against the pathogen. Molecular signaling for disease suppression by the VOC were investigated by treating different mutants and transgenic Arabidopsis plants impaired in salicylic acid (SA) or Jasmonic acid (JA)/ethylene (ET) signaling pathways with m-cresol and MeBA followed by challenge inoculation with Pst. Results show that the level of protection was significantly lower when JA/ET-impaired mutants were treated with MeBA, and in SA-, and JA/ET-disrupted mutants after m-cresol treatment, indicating the involvement of these signal transduction pathways in the ISR primed by the volatiles. Analysis of defense-related genes by real-time qRT-PCR showed that both the SA-and JA-signaling pathways combine in the m-cresol signaling of ISR, whereas MeBA is mainly involved in the JA-signaling pathway with partial recruitment of SA-signals. The ET-signaling pathway was not employed in ISR by the volatiles. Therefore, this study identified two novel volatile components capable of eliciting ISR that may be promising candidates in biological control strategy to protect plants from diseases.

ppdb: plant promoter database version 3.0

ppdb (http://ppdb.agr.gifu-u.ac.jp) is a plant promoter database that provides information on transcription start sites (TSSs), core promoter structure (TATA boxes, Initiators, Y Patches, GA and CA elements) and regulatory element groups (REGs) as putative and comprehensive transcriptional regulatory elements. Since the last report in this journal, the database has been updated in three areas to version 3.0. First, new genomes have been included in the database, and now ppdb provides information on Arabidopsis thaliana, rice, Physcomitrella patens and poplar. Second, new TSS tag data (34 million) from A. thaliana, determined by a high throughput sequencer, has been added to give a ∼200-fold increase in TSS data compared with version 1.0. This results in a much higher coverage of ∼27 000 A. thaliana genes and finer positioning of promoters even for genes with low expression levels. Third, microarray data-based predictions have been appended as REG annotations which inform their putative physiological roles.

Analysis of environmental stress in plants with the aid of marker genes for H2O2 responses

Hydrogen peroxide acts as a signaling molecule mediating the acquisition of tolerance to both biotic and abiotic stresses. Identification of marker genes for H2O2 response could help to intercept the signaling network of stress response of plants. Here, we describe application of marker genes for H2O2 responses to monitoring several abiotic stress responses. Arabidopsis plants were treated with UV-B, high light, and cold stresses, where involvement of H2O2-mediated signaling is known or suggested. Monitoring of these stress responses with molecular markers using quantitative real-time RT-PCR can detect landmark events in the sequential stress responses. These methods can be used for analysis of mutants and transgenic plants to examine natural H2O2 responses that are involved in environmental adaptation.

Analysis of volatile organic compounds emitted by plant growth-promoting fungus Phoma sp. GS8-3 for growth promotion effects on tobacco

We extracted volatile organic compounds (VOCs) emitted by a plant growth-promoting fungus (PGPF) Phoma sp. GS8-3 by gas chromatography and identified them by mass spectrometry. All of the identified compounds belonged to C4-C8 hydrocarbons. Volatiles varied in number and quantity by the culture period of the fungus (in days). 2-Methyl-propanol and 3-methyl-butanol formed the main components of the volatile blends for all the culture periods of fungus. Growth-promoting effects of the identified synthetic compounds were analyzed individually and in blends using tobacco plants. We found that the mixture of volatiles extracted from 3-day-old culture showed significant growth promotion in tobacco in vitro. The volatile blend showed better growth promotion at lower than higher concentrations. Our results confirm the potential role of volatile organic compounds in the mechanism of growth enhancement by GS8-3.

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.