Conditioning as a factor in parasitoid host plant preference

The relative effects of larval and adult conditioning on Diglyphus isaea host plant preference and subsequent parasitism were investigated. Parasitoid larvae were reared on Chromatomyia syngenesiae on lettuce, chrysanthemum or in isolation, and female parasitoids were exposed to hosts on either lettuce or chrysanthemum. Hosts on lettuce and chrysanthemum were then presented to assess parasitoid preference. Larval and adult D. isaea conditioning only slightly affected the females selection of host plant type on which the host larvae were located. The major effect of conditioning was a reduction in the number of eggs laid after larvae and adults were conditioned on chrysanthemum and particularly when both stages were conditioned on this host plant.     

Rearing history affects behaviour and performance of two virulent Nasonovia ribisnigri populations on two lettuce cultivars

Many aphid species have become virulent to host‐plant resistance, which limits the sustainability of insect resistance breeding. However, when this adaptation to resistant plants is associated with fitness costs for the aphids, virulence can be lost in the absence of resistant plants. For two populations of the lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae), we evaluated whether virulence to Nr‐gene‐based resistance was lost on a susceptible lettuce, Lactuca sativa L. (Asteraceae), and assessed possible costs of virulence. The feeding behaviour and performance of these aphids, reared and tested on susceptible and resistant lettuce, were investigated. The rearing plant affected feeding behaviour and performance of the aphids. Temporary reduction and long‐term loss of virulence were found. The total duration of phloem intake was shorter after being reared on susceptible lettuce and tested on resistant lettuce. In addition, one population had a lower survival on resistant lettuce after being reared on susceptible lettuce. There were also indications of fitness costs of the virulence in both populations.     

The survival on chrysanthemum roots of epiphytic mycelium of Mycosphaerella ligulicola

Mycosphaerella ligulicola has been shown to survive as epiphytic mycelium on the root surface of chrysanthemum cuttings: such survival could continue throughout the life of the glasshouse crop. Symptomless surface colonization of roots of cuttings could be induced in non-sterile soil from an inoculum of (a) mycelium and sclerotia or (b) conidia (Ascochyta state); the colonization could spread upwards over the root surface. After 12 weeks survival as an epiphyte on chrysanthemum roots the fungus was still pathogenic to unrooted cuttings. Although the root surfaces of twelve other plants could be colonized by M. ligulicola the fungus survived on these roots for not more than 8 weeks.     

Selection of an Isolate of the Insect Pathogenic Fungus Metarhizium anisopliae Virulent to the Lettuce Root Aphid, Pemphigus bursarius

The virulence of hyphomycete entomopathogenic fungi was measured in laboratory bioassays against the lettuce root aphid, Pemphigus bursarius, a serious pest of field lettuce grown in the UK. Of 25 isolates of fungi examined, only one isolate, Metarhizium anisopliae 391.93, killed lettuce root aphids consistently. This fungus was isolated originally from the closely related saltmarsh aphid, P. trehernei. The median lethal concentration of conidia at 10 days post6- 1 inoculation estimated from five independent bioassays was 2.45 × 10⁶ conidia ml^-1. The fungus had no significant effect on the mean number of offspring/aphid produced, but it sporulated 6 profusely on host cadavers, producing approximately 4 × 10⁶ conidia/cadaver 14 days after treatment, and diseased aphids died attached to plant roots. It thus has the potential to spread through densely packed colonies of P. bursarius feeding on the roots of susceptible or partially resistant plants.     

Survey for psychrotrophic bacterial pathogens in minimally processed lettuce

A total of 120 minimally processed, cut and packaged lettuce samples were purchased from retail supermarkets or provided by a salad production facility over an 8-month period. The samples were tested for total aerobic plate counts and for the presence of potentially pathogenic species belonging to the genera of Listeria, Aeromonas and Yersinia. The aerobic plate counts ranged from 103 to 109 colony forming units (cfu) g-1. Most samples (76%) contained between 105 and 107 cfu g-1 total aerobic bacteria. Listeria monocytogenes was isolated from three samples, Aeromonas hydrophila or Aeromonas caviae from 66 samples, and Yersinia enterocolitica from 71 samples. The pathogenic potential of Y. enterocolitica isolates was determined by screening for an array of biochemical, serological and genetic traits (heat-stable enterotoxin gene, the attachment and invasion gene locus, the invasin gene locus and the virulence plasmid). The Y. enterocolitica isolates lacked many of the phenotypic and genetic markers associated with virulence in primary pathogenic strains. As the roles of the reputed virulence factors of Aeromonas spp. in human infection are uncertain, the pathogenic potential of the Aeromonas isolates in lettuce remains unclear.     

Wilt and vascular root rot (<i>Pythium tracheiphilum</i>) of lettuce in Bahía Blanca,Argentina

A new disease of lettuce has been observed in the surroundings of Bahía Blanca, Argentina. The symptoms include dwarfing, general chlorosis, wilting, root rot, and leaf blight, sometimes followed by plant death. Pythium tracheiphilum Matta was found to be consistently associated with those symptoms. The morphological and morphometric characteristics of this oomycete are described. Isolates obtained from field-infected lettuce plants were inoculated to lettuce and caused similar symptoms as those found in natural infections. Seedling emergence was also severely affected following experimental inoculation. Koch’s Postulates were fulfilled by recovering the fungus from inoculated plants. The new disease shows a high destructive potential but currently has a limited prevalence and incidence.     

Melanin in <Emphasis Type="Italic">Fonsecaea pedrosoi</Emphasis>: a trap for oxidative radicals

Background  

The pathogenic fungus Fonsecaea pedrosoi constitutively produces the pigment melanin, an important virulence factor in fungi. Melanin is incorporated in the cell wall structure and provides chemical and physical protection for the fungus.     

The STRIPAK complex orchestrates cell wall integrity signalling to govern the fungal development and virulence of Fusarium graminearum

Striatin-interacting phosphatases and kinases (STRIPAKs) are evolutionarily conserved supramolecular complexes that control various important cellular processes such as signal transduction and development. However, the role of the STRIPAK complex in pathogenic fungi remains elusive. In this study, the components and function of the STRIPAK complex were investigated in Fusarium graminearum, an important plant-pathogenic fungus. The results obtained from bioinformatic analyses and the protein–protein interactome suggested that the fungal STRIPAK complex consisted of six proteins: Ham2, Ham3, Ham4, PP2Aa, Ppg1, and Mob3. Deletion mutations of individual components of the STRIPAK complex were created, and observed to cause a significant reduction in fungal vegetative growth and sexual development, and dramatically attenuae virulence, excluding the essential gene PP2Aa. Further results revealed that the STRIPAK complex interacted with the mitogen-activated protein kinase Mgv1, a key component in the cell wall integrity pathway, subsequently regulating the phosphorylation level and nuclear accumulation of Mgv1 to control the fungal stress response and virulence. Our results also suggested that the STRIPAK complex was interconnected with the target of rapamycin pathway through Tap42-PP2A cascade. Taken together, our findings revealed that the STRIPAK complex orchestrates cell wall integrity signalling to govern the fungal development and virulence of F. graminearum and highlighted the importance of the STRIPAK complex in fungal virulence.     

Detection of double stranded RNA in phytopathogenic <Emphasis Type="Italic">Macrophomina phaseolina</Emphasis> causing charcoal rot in <Emphasis Type="Italic">Cyamopsis tetragonoloba</Emphasis>

One hundred one isolates of Macrophomina phaseolina from various hosts and eco-geographical locations were employed for elucidating relationships among genetic diversity and virulence. Highly pathogenic, moderately pathogenic, and hypovirulent cluster bean specific isolates were identified. In order to correlate respective phenotypes of plant pathogenic fungus multiple and complex patterns of dsRNA elements were analyzed. Double-stranded ribonucleic acids (dsRNA) are ubiquitous in all major groups and most of them have vast potential as biological control agents for fungi. Rate of virulence and its further association could ascertain by host plant and their fungal genotypes. Variability of the fungal genotypes decides the link between the complexity of dsRNA with different variants and the change in virulence pattern. Double-stranded RNA was identified in approximately 21.7% of M. phaseolina isolates from charcoal rot infected cluster bean varieties. After recurrent laboratory transfer on culture media, the preponderance of the isolates harboring dsRNAs developed degenerate culture phenotypes and showed reduced virulence (hypovirulence) to cluster bean. Macrophomina has successfully showed diversified and reproducible banding profile in dsRNA containing/free isolates. This is the first report of hypovirulence and detection of dsRNA in Macrophomina phaseolina isolates of cluster bean origin.     

Ligionella pneumophila: Avirulent to virulent conversion through passage in cultured human embryonic lung fibroblasts

Legionella pneumophila isolated in guinea pigs from human lung tissues was highly virulent as determined by its infectivity and lethality in guinea pigs. Repeated passages of the bacteria on agar media resulted in the loss of virulence in guinea pigs. Virulence, however, was restored by cultivating the avirulent bacteria in cell cultures of human embryonic lung fibroblasts. Death of the host animals was the result of infection; no lethal toxin was detected in the cultural filtrate. These findings indicate that the virulent form ofL. pneumophilia is capable of surviving inside the host cells either through its endogenous resistance to environmental factors within the host cells or by host cell selection. Intracellular multiplication of the virulent bacteria followed by destruction of host cells appears to be an important pathogenic mechanism of Legionnaires' disease.     

Studies on the influence of leaf washings on infection by Mycosphaerella ligulicola

Infection of chrysanthemum leaf discs by spores of Mycosphaerella ligulicola was increased in the presence of concentrated leaf washings obtained from five plant species tested. On separation of leaf washings by dialysis the residue stimulated infection and caused aggregation of hyphae on leaf and cellophane surfaces. In the presence of the diffusible fraction an extensive branching network of hyphae developed but there was no increase in infection except in the presence of the chrysanthemum diffusate. Infection of leaf discs of certain plants normally resistant to M. ligulicola occurred in the presence of concentrated leaf washings. Leaf washings from a resistant variety of chrysanthemum had the same effects as those from a susceptible variety. An extract of chrysanthemum leaves was less effective in stimulating infection than was a leaf washing. Defined nutrient media and potato extract stimulated infection but caused the development of different growth habits on leaf surfaces. The active components of the dialysis residue of chrysanthemum leaf washings were heat-stable but apparently volatile. Eluates from strips of certain regions of paper chromatographs of leaf washings and potato extract caused increased infection together with development of characteristic growth habits of the fungus on the leaf.     

A Nitrogen Response Pathway Regulates Virulence Functions in Fusarium oxysporum via the Protein Kinase TOR and the bZIP Protein MeaB

During infection, fungal pathogens activate virulence mechanisms, such as host adhesion, penetration and invasive growth. In the vascular wilt fungus Fusarium oxysporum, the mitogen-activated protein kinase Fmk1 is required for plant infection and controls processes such as cellophane penetration, vegetative hyphal fusion, or root adhesion. Here, we show that these virulence-related functions are repressed by the preferred nitrogen source ammonium and restored by treatment with l-methionine sulfoximine or rapamycin, two specific inhibitors of Gln synthetase and the protein kinase TOR, respectively. Deletion of the bZIP protein MeaB also resulted in nitrogen source–independent activation of virulence mechanisms. Activation of these functions did not require the global nitrogen regulator AreA, suggesting that MeaB-mediated repression of virulence functions does not act through inhibition of AreA. Tomato plants (Solanum lycopersicum) supplied with ammonium rather than nitrate showed a significant reduction in vascular wilt symptoms when infected with the wild type but not with the ΔmeaB strain. Nitrogen source also affected invasive growth in the rice blast fungus Magnaporthe oryzae and the wheat head blight pathogen Fusarium graminearum. We propose that a conserved nitrogen-responsive pathway might operate via TOR and MeaB to control virulence in plant pathogenic fungi.     

The endosomal sorting complex required for transport machinery influences haem uptake and capsule elaboration in Cryptococcus neoformans

Iron availability is a key determinant of virulence in the pathogenic fungus Cryptococcus neoformans. Previous work revealed that the ESCRT (endosomal sorting complex required for transport) protein Vps23 functions in iron acquisition, capsule formation and virulence. Here, we further characterized the ESCRT machinery to demonstrate that defects in the ESCRT‐II and III complexes caused reduced capsule attachment, impaired growth on haem and resistance to non‐iron metalloprotoporphyrins. The ESCRT mutants shared several phenotypes with a mutant lacking the pH‐response regulator Rim101, and in other fungi, the ESCRT machinery is known to activate Rim101 via proteolytic cleavage. We therefore expressed a truncated and activated version of Rim101 in the ESCRT mutants and found that this allele restored capsule formation but not growth on haem, thus suggesting a Rim101‐independent contribution to haem uptake. We also demonstrated that the ESCRT machinery acts downstream of the cAMP/protein kinase A pathway to influence capsule elaboration. Defects in the ESCRT components also attenuated virulence in macrophage survival assays and a mouse model of cryptococcosis to a greater extent than reported for loss of Rim101. Overall, these results indicate that the ESCRT complexes function in capsule elaboration, haem uptake and virulence via Rim101‐dependent and independent mechanisms.     

Phytotoxicity analysis of extracts from compost and their ability to inhibit soil-borne pathogenic fungi and reduce root-knot nematodes

Compost extracts are novel organic amendments, typically applied to suppress soil-borne diseases. This research evaluated the phytotoxicity of compost extracts and analyzed their ability to inhibit pathogenic fungal growth and reduce root-knot nematodes. The physical, chemical and biological characteristics of extracts from a pig manure and straw compost were analyzed. Three types of extracts were tested: direct extracts of compost (DEC), aerated fermentation extracts of compost (AFEC) and non-aerated fermentation extracts of compost (NAFEC). All compost extracts showed low phytotoxicity against lettuce and cress, but AFEC and NAFEC were more phytotoxic than DEC. All compost extracts significantly inhibited pathogenic fungal growth except for the fungus Rhizoctonia solania AG4. For two seasons, tomato root biomass of three compost extracts was 1.25–5.67 times greater than CK (water control), and AFEC and NAFEC showed the best tomato root growth promotion. The reduction ratio of root egg mass and density of soil nematodes were 34.51–87.77% and 30.92–51.37%, when applied with three compost extracts. The microbial population in compost extracts was considered to be the most significant factor of inhibition pathogenic fungal growth. No markedly correlations among bacterial community diversity, the inhibition of pathogenic fungal growth and the reduction of root-knot nematodes were observed. This information adds to the understanding of the growth-promoting and suppression effects of compost extracts and will help to enhance crop production.     

Prp19-associated splicing factor Cwf15 regulates fungal virulence and development in the rice blast fungus

The splicing factor Cwf15 is an essential component of the Prp19-associated component of the spliceosome and regulates intron splicing in several model species, including yeasts and human cells. However, the roles of Cwf15 remain unexplored in plant pathogenic fungi. Here, we report that MoCWF15 in the rice blast fungus Magnaporthe oryzae is non-essential to viability and important to fungal virulence, growth and conidiation. MoCwf15 contains a putative nuclear localization signal (NLS) and is localized into the nucleus. The NLS sequence but not the predicted phosphorylation site or two sumoylation sites was essential for the biological functions of MoCwf15. Importantly, MoCwf15 physically interacted with the Prp19-associated splicing factors MoCwf4, MoSsa1 and MoCyp1, and negatively regulated protein accumulations of MoCyp1 and MoCwf4. Furthermore, with the deletion of MoCWF15, aberrant intron splicing occurred in near 400 genes, 20 of which were important to the fungal development and virulence. Taken together, MoCWF15 regulates fungal growth and infection-related development by modulating the intron splicing efficiency of a subset of genes in the rice blast fungus.     

Long‐distance early endosome motility in Aspergillus fumigatus promotes normal hyphal growth behaviors in controlled microenvironments but is dispensable for virulence

In filamentous fungi, early endosomes are continuously trafficked to, and from, the growing hyphal tip by microtubule‐based motor proteins, serving as platforms for the long‐distance transport of diverse cargos including mRNA, signaling molecules, and other organelles which hitchhike on them. While the cellular machinery for early endosome motility in filamentous fungi is fairly well characterized, the broader physiological significance of this process remains less well understood. We set out to determine the importance of long‐distance early endosome trafficking in Aspergillus fumigatus, an opportunistic human pathogenic fungus that can cause devastating pulmonary infections in immunocompromised individuals. We first characterized normal early endosome motile behavior in A. fumigatus, then generated a mutant in which early endosome motility is severely perturbed through targeted deletion of the gene encoding for FtsA, one of a complex of proteins that links early endosomes to their motor proteins. Using a microfluidics‐based approach we show that contact‐induced hyphal branching behaviors are impaired in ΔftsA mutants, but that FtsA‐mediated early endosome motility is dispensable for virulence in an invertebrate infection model. Overall, our study provides new insight into early endosome motility in an important human pathogenic fungus.     

Variation for Resistance to <i>Alternaria tenuissima</i> and Potential Structural Mechanism among Different Cultivars of <i>Chrysanthemum morifolium</i>

Black spot disease, caused by the necrotrophic fungus Alternaria tenuissima (Fr.) Wiltsh (A. tenuissima), is considered a highly destructive disease of Chrysanthemum (Chrysanthemum morifolium Ramat.). A set of 17 accessions of commercial chrysanthemum cultivars were evaluated for resistance to A. tenuissima by seedling artificial inoculation. It was found that the reaction of the accessions to artificial inoculation ranged from resistant to highly susceptible. Five varieties of chrysanthemum (‘Zhongshan Taogui’, ‘Jinba’, ‘Zhongshan Jinguan’, ‘Jinling Wanhuang’ and ‘Jinling Yangguang’) were resistant; two varieties of chrysanthemum (‘Zhongshan Xinggui’ and ‘Zhongshan Jinkui’) were moderately resistant; and others were susceptible to various degrees, four varieties of chrysanthemum (‘Zhongshan Zihe’, ‘Zhongshan Jiuhong’, ‘Zaoyihong’ and ‘Jinling Jiaohuang’) were highly susceptible, especially. Some leaf morphological features of two resistant and two highly susceptible cultivars were further researched. Trichome density, length, height, gland size and stomata density were found to be associated with plant passive resistance. Resistant varieties that were identified in present study will be promising germplasm for exploitation of breeding programmes aimed at developing A. tenuissima-resistant cultivars and increasing genetic diversity.     

禾谷镰刀菌基因组中含寄主靶向模体分泌蛋白功能的初步分析

基于生物信息学的方法,以SignalP v3.0、TargetP v1.01、Big-PI Predictor和TMHMM v2.0四个分析软件对禾谷镰刀菌(Fusarium graminearum Schw.)全基因组的11 640个蛋白编码基因的N-端氨基酸序列进行信号肽分析,预测出606个潜在的分泌蛋白编码基因.通过对这些潜在的分泌蛋白进行MEME软件分析,发现其中有157个分泌蛋白的剪切点下游120氨基酸残基范围内具有一个保守的RXLX模体,其中有79个分泌蛋白具有可预测的功能性描述,包括FG00023.1具有与草酸盐氧化酶1有关的功能,FG01588.1具有与1,4-α-葡糖苷酶葡聚糖有关的功能,这些基因可作为禾谷镰刀菌致病相关的候选基因.其中FG04097.1编码的具有与丝氨酸型蛋白酶有关的功能在致病疫霉和疟原虫真核寄生物中具有相似保守的寄主靶标模体的效应蛋白中也观察到,但FG09127.1,FG05287.1编码的蛋白尚未被证明参与致病过程的研究报道.深入研究分泌蛋白将有助于明确植物与病原微生物互作的分子机制.利用禾谷镰刀菌基因组学研究成果,结合计算机技术和生物信息学的方法,分析其分泌蛋白组学,将有助于全面掌握其致病因子的结构与功能.对于这些蛋白功能的比较研究,将有利于对禾谷镰刀菌致病性的分子机制的探索,并为设计新的防治措施提供理论依据.     

Phaseollin production in cell suspensions and whole plants of Phaseolus vulgaris

Production of phaseollin was measured in cell suspension cultures and whole plants of Phaseolus vulgaris. In suspension cultures phaseollin appeared when there was no further increase in cell mass. Cells transferred to a medium without auxins yielded three times higher phaseollin concentrations than cells grown in their presence. Addition of autoclaved fungal mycelia or polysaccharides as elicitors resulted in an increased phaseollin concentration in the cell suspension.In whole plants phaseollin could be detected only after the plants were challenged by a fungus which caused lesions (browning) of the upper root neck region, Rhizoctonia solani. Treatment of non-infected plants with autoclaved fungal mycelia or other elicitors did not induce phaseollin production. However, when they were added before or together with the pathogenic fungus, the elicitors further increased phaseollin concentration in the root neck regions of the plants. This indicated that the pathogenic fungus was important for the penetration of the elicitors to inner plant tissues where phaseollin (and probably other phytoalexins) is produced.