[Identification and biological characterization of pathogen causing black spot of Pseudostellaria heterophylla in Fujian province]

In Zherong county, Fujian province, the black spot of Pseudostellaria heterophylla often breaks out in the rainy season from April to June every year. As one of the main leaf diseases of P. heterophylla, black spot seriously affects the yield and quality of the medicinal material. To identify and characterize the pathogens causing black spot, we isolated the pathogens, identified them as a species of Alternaria according to Koch's postulates, and then tested their pathogenicity and biological characteristics. The results showed that the pathogens causing P. heterophylla black spot were A. gaisen, as evidenced by the similar colony morphology, spore characteristics, sporulation phenotype, and the same clade with A. gaisen on the phylogenetic tree(the maximum likelihood support rate of 100% and the Bayesian posterior probability of 1.00) built based on the tandem sequences of ITS, tef1, gapdh, endoPG, Alta1, OPA10-2, and KOG1077. The optimum conditions for mycelial growth of the pathogen were 25 ℃, pH 5-8, and 24 h dark culture. The lethal conditions for mycelia and spores were both treatment at 50 ℃ for 10 min. We reported for the first time the A. gaisen-caused black spot of P. heterophylla. The results could provide a theoretical basis for the diagnosis and control of P. heterophylla leaf spot diseases.

Root-Associated Bacteria Community Characteristics of Antarctic Plants: Deschampsia antarctica and Colobanthus quitensis-a Comparison

Colobanthus quitensis (Kunth) Bartl. and Deschampsia antarctica Desv. are the only Magnoliophyta to naturally colonize the Antarctic region. The reason for their sole presence in Antarctica is still debated as there is no definitive consensus on how only two unrelated flowering plants managed to establish breeding populations in this part of the world. In this study, we have explored and compared the rhizosphere and root-endosphere dwelling microbial community of C. quitensis and D. antarctica specimens sampled in maritime Antarctica from sites displaying contrasting edaphic characteristics. Bacterial phylogenetic diversity (high-throughput 16S rRNA gene fragment targeted sequencing) and microbial metabolic activity (Biolog EcoPlates) with a geochemical soil background were assessed. Gathered data showed that the microbiome of C. quitensis root system was mostly site-dependent, displaying different characteristics in each of the examined locations. This plant tolerated an active bacterial community only in severe conditions (salt stress and nutrient deprivation), while in other more favorable circumstances, it restricted microbial activity, with a possibility of microbivory-based nutrient acquisition. The microbial communities of D. antarctica showed a high degree of similarity between samples within a particular rhizocompartment. The grass' endosphere was significantly enriched in plant beneficial taxa of the family Rhizobiaceae, which displayed obligatory endophyte characteristics, suggesting that at least part of this community is transmitted vertically. Ultimately, the ecological success of C. quitensis and D. antarctica in Antarctica might be largely attributed to their associations and management of root-associated microbiota.

Long-Read Genome Sequence Resource of Ascochyta versabilis Causing Leaf Spot Disease in Pseudostellaria heterophylla

Ascochyta versabilis is the fungal pathogen that causes the severe leaf spot disease of Pseudostellaria heterophylla (Miq.) Pax, a vital Chinese herbal plant. Here, we deployed PacBio single-molecule real-time long-read sequencing technology to generate a near-complete genome assembly for the A. versabilis KC1 strain and obtained a total of 9.80 Gb raw reads. These reads were processed into a 41.05 Mb genome assembly containing 95 contigs with N50 of 1.70 Mb and a maximum length of 3.93 Mb. A total of 10,457 gene models, of which 1,004 encode putatively secreted proteins, were identified in the genome. This high-quality genome assembly and gene annotation resource will facilitate the institution of functional genetic studies aimed at providing a better insight into the infection mechanisms of A. versabilis to support the development of effective control strategies for leaf spot disease of P. heterophylla.

Nocardia zhihengii sp. nov., an actinobacterium isolated from rhizosphere soil of Psammosilene tunicoides

A Nocardia-like actinobacterial strain, designated YIM TG2190^T, was isolated from rhizosphere soil of Psammosilene tunicoides collected from Gejiu, Yunnan province, China. The cells of strain YIM TG2190^T were observed to be Gram-stain positive and non-motile. The strain forms extensively branched substrate mycelia that fragments into rod-shaped elements. The 16S rRNA gene sequence analysis showed that strain YIM TG2190^T is closely related to Nocardia nova (97.5%), Nocardia jiangxiensis (97.1%) and Nocardia miyunensis (96.8%). Growth occurs at 4-30 °C (optimum 28 °C), pH 6.0-8.0 (optimum pH 7.0) and the strain can tolerate NaCl (w/v) up to 3% (optimum 0-1%). The cell walls were found to contain meso-diaminopimelic acid. The whole-cell sugars were identified as glucose, mannose, ribose, galactose, arabinose and fucose. The polar lipids were identified as diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol mannosides, phosphatidylglycerol and an unidentified phospholipid. The menaquinones detected were MK-9 (H₂) and MK-8 (H₄). The major fatty acids (> 5%) were found to be C_16:0 (33.9%), summed feature 3 (21.7%), C_18:0 10-methyl TBSA (13.7%) and C_18:1ω9c (7.0%). The DNA G+C content was determined to be 61.1 mol%. DNA-DNA relatedness between the strain YIM TG2190^T and N. nova CGMCC 4.1705^T, N. jiangxiensis CGMCC 4.1905^T and N. miyunensis CGMCC 4.1904^T were 46.9 ± 2.6, 36.8 ± 1.3, and 35.7 ± 2.6%, respectively, values which are less than the threshold value (70%) for the delineation of prokaryotic genomic species. The phenotypic, chemotaxonomic and phylogenetic data indicates that strain YIM TG2190^T represents a novel species of the genus Nocardia, for which the name Nocardia zhihengii sp. nov. is proposed. The type strain is YIM TG2190^T (=KCTC 39596^T = DSM 100515^T).

Transcriptome analysis of Pseudostellaria heterophylla in response to the infection of pathogenic Fusarium oxysporum

BACKGROUND

Pseudostellaria heterophylla (P. heterophylla), a herbaceous perennial, belongs to Caryophyllaceae family and is one of the Chinese herbal medicine with high pharmacodynamic value. It can be used to treat the spleen deficiency, anorexia, weakness after illness and spontaneous perspiration symptoms. Our previous study found that consecutive monoculture of Pseudostellaria heterophylla could lead to the deterioration of the rhizosphere microenvironment. The specialized forms of pathogenic fungus Fusarium oxysporum f.Sp. heterophylla (F. oxysporum) in rhizosphere soils of P. heterophylla plays an important role in the consecutive monoculture of P. heterophylla.

RESULTS

In this study, F. oxysporum was used to infect the tissue culture plantlets of P. heterophylla to study the responding process at three different infection stages by using RNA-sequencing. We obtained 127,725 transcripts and 47,655 distinct unigenes by de novo assembly and obtained annotated information in details for 25,882 unigenes. The Kyoto Encyclopedia of Genes and Genomes pathway analysis and the real-time quantitative PCR results suggest that the calcium signal system and WRKY transcription factor in the plant-pathogen interaction pathway may play an important role in the response process, and all of the WRKY transcription factor genes were divided into three different types. Moreover, we also found that the stimulation of F. oxysporum may result in the accumulation of some phenolics in the plantlets and the programmed cell death of the plantlets.

CONCLUSIONS

This study has partly revealed the possible molecular mechanism of the population explosion of F. oxysporum in rhizosphere soils and signal response process, which can be helpful in unraveling the role of F. oxysporum in consecutive monoculture problems of P. heterophylla.

Biological Interactions and Simulated Climate Change Modulates the Ecophysiological Performance of Colobanthus quitensis in the Antarctic Ecosystem

Most climate and environmental change models predict significant increases in temperature and precipitation by the end of the 21st Century, for which the current functional output of certain symbioses may also be altered. In this context we address the following questions: 1) How the expected changes in abiotic factors (temperature, and water) differentially affect the ecophysiological performance of the plant Colobanthus quitensis? and 2) Will this environmental change indirectly affect C. quitensis photochemical performance and biomass accumulation by modifying its association with fungal endophytes? Plants of C. quitensis from King George Island in the South Shetland archipelago (62°09' S), and Lagotellerie Island in the Antarctic Peninsula (65°53' S) were put under simulated abiotic conditions in growth chambers following predictive models of global climate change (GCC). The indirect effect of GCC on the interaction between C. quitensis and fungal endophytes was assessed in a field experiment carried out in the Antarctica, in which we eliminated endophytes under contemporary conditions and applied experimental watering to simulate increased precipitation input. We measured four proxies of plant performance. First, we found that warming (+W) significantly increased plant performance, however its effect tended to be less than watering (+W) and combined warming and watering (+T°+W). Second, the presence of fungal endophytes improved plant performance, and its effect was significantly decreased under experimental watering. Our results indicate that both biotic and abiotic factors affect ecophysiological performance, and the directions of these influences will change with climate change. Our findings provide valuable information that will help to predict future population spread and evolution through using ecological niche models under different climatic scenarios.

Global regulatory networks control the hrp regulon of the gall-forming bacterium Pantoea agglomerans pv. gypsophilae

Gall formation by Pantoea agglomerans pv. gypsophilae is dependent on the hypersensitive response and pathogenicity (hrp) system. Previous studies demonstrated that PagR and PagI, regulators of the quorum-sensing system, induce expression of the hrp regulatory cascade (i.e., hrpXY, hrpS, and hrpL) that activates the HrpL regulon. Here, we isolated the genes of the Gac/Rsm global regulatory pathway (i.e., gacS, gacA, rsmB, and csrD) and of the post-transcriptional regulator rsmA. Our results demonstrate that PagR and PagI also upregulate expression of the Gac/Rsm pathway. PagR acts as a transcriptional activator of each of the hrp regulatory genes and gacA in a N-butanoyl-L-homoserine lactone-dependent manner as shown by gel shift experiments. Mutants of the Gac/Rsm genes or overexpression of rsmA significantly reduced Pantoea agglomerans virulence and colonization of gypsophila. Overexpression of rsmB sRNA abolished gall formation, colonization, and hypersensitive reaction on nonhost plants and prevented transcription of the hrp regulatory cascade, indicating a lack of functional type III secretion system. Expression of rsmB sRNA in the background of the csrD null mutant suggests that CsrD may act as a safeguard for preventing excessive production of rsmB sRNA. Results presented indicate that the hrp regulatory cascade is controlled directly by PagR and indirectly by RsmA, whereas deficiency in RsmA activity is epistatic to PagR induction.

Polar auxin transport is essential for gall formation by Pantoea agglomerans on Gypsophila

The virulence of the bacterium Pantoea agglomerans pv. gypsophilae (Pag) on Gypsophila paniculata depends on a type III secretion system (T3SS) and its effectors. The hypothesis that plant-derived indole-3-acetic acid (IAA) plays a major role in gall formation was examined by disrupting basipetal polar auxin transport with the specific inhibitors 2,3,5-triiodobenzoic acid (TIBA) and N-1-naphthylphthalamic acid (NPA). On inoculation with Pag, galls developed in gypsophila stems above but not below lanolin rings containing TIBA or NPA, whereas, in controls, galls developed above and below the rings. In contrast, TIBA and NPA could not inhibit tumour formation in tomato caused by Agrobacterium tumefaciens. The colonization of gypsophila stems by Pag was reduced below, but not above, the lanolin-TIBA ring. Following Pag inoculation and TIBA treatment, the expression of hrpL (a T3SS regulator) and pagR (a quorum-sensing transcriptional regulator) decreased four-fold and that of pthG (a T3SS effector) two-fold after 24 h. Expression of PIN2 (a putative auxin efflux carrier) increased 35-fold, 24 h after Pag inoculation. However, inoculation with a mutant in the T3SS effector pthG reduced the expression of PIN2 by two-fold compared with wild-type infection. The results suggest that pthG might govern the elevation of PIN2 expression during infection, and that polar auxin transport-derived IAA is essential for gall initiation.

The type III effector HsvG of the gall-forming Pantoea agglomerans mediates expression of the host gene HSVGT

The type III effector HsvG of the gall-forming Pantoea agglomerans pv. gypsophilae is a DNA-binding protein that is imported to the host nucleus and involved in host specificity. The DNA-binding region of HsvG was delineated to 266 amino acids located within a secondary structure region near the N-terminus of the protein but did not display any homology to canonical DNA-binding motifs. A binding site selection procedure was used to isolate a target gene of HsvG, named HSVGT, in Gypsophila paniculata. HSVGT is a predicted acidic protein of the DnaJ family with 244 amino acids. It harbors characteristic conserved motifs of a eukaryotic transcription factor, including a bipartite nuclear localization signal, zinc finger, and leucine zipper DNA-binding motifs. Quantitative real-time polymerase chain reaction analysis demonstrated that HSVGT transcription is specifically induced in planta within 2 h after inoculation with the wild-type P. agglomerans pv. gypsophilae compared with the hsvG mutant. Induction of HSVGT reached a peak of sixfold at 4 h after inoculation and progressively declined thereafter. Gel-shift assay demonstrated that HsvG binds to the HSVGT promoter, indicating that HSVGT is a direct target of HsvG. Our results support the hypothesis that HsvG functions as a transcription factor in gypsophila.

[Establishment of induction and culture system for hairy roots of Psammosilene tunicoides]

OBJECTIVE

To establish a culture system for Psammosilene tunicoides hairy roots, and provide technological aid for the large-scale production of P. tunicoides material.

METHOD

The young leaves and stem segments of sterile plantlets were infected with ACCC10060 strain, and subsequently a culture system suitable for hairy roots growth was further established.

RESULT

When explants were co-cultured with ACCC10060 (A600 0.8) on B5 media containing 20 mg x L(-1) Acetosyringo (AS) for 48 h, the hairy roots could be successfully induced, and it could achieve a higher induction rate using young leaves as explants than that of stem segments. The transfected hairy roots possessed the ability of kanamycin resistance and growth on hormone-free media, and synthesis of opines. All above results demonstrated that the present hairy roots originated in the infection of P. tunicoides tissues by ACCC10060 strains. After 35 d culture in liquid hormone-free MS (1/2 strength), the biomass of hairy roots increased 14.11 times (fresh weight) and 8. 39 times (dry weight), respectively, and the content of total saponins in hairy roots reached to 0.857% (DW), by contrast, it's only 0.388% and 0.217% in callus and seedlings respectively.

CONCLUSION

Establishment of hairy roots culture of P. tunicoides provided a foundation for industrial production of active components from P. tunicoides culture.

Distribution of the anther-smut pathogen Microbotryum on species of the Caryophyllaceae

*Understanding disease distributions is of fundamental and applied importance, yet few studies benefit from integrating broad sampling with ecological and phylogenetic data. Here, anther-smut disease, caused by the fungus Microbotryum, was assessed using herbarium specimens of Silene and allied genera of the Caryophyllaceae. *A total of 42,000 herbarium specimens were examined, and plant geographical distributions and morphological and life history characteristics were tested as correlates of disease occurrence. Phylogenetic comparative methods were used to determine the association between disease and plant life-span. *Disease was found on 391 herbarium specimens from 114 species and all continents with native Silene. Anther smut occurred exclusively on perennial plants, consistent with the pathogen requiring living hosts to overwinter. The disease was estimated to occur in 80% of perennial species of Silene and allied genera. The correlation between plant life-span and disease was highly significant while controlling for the plant phylogeny, but the disease was not correlated with differences in floral morphology. *Using resources available in natural history collections, this study illustrates how disease distribution can be determined, not by restriction to a clade of susceptible hosts or to a limited geographical region, but by association with host life-span, a trait that has undergone frequent evolutionary transitions.

Induced production of antifungal naphthoquinones in the pitchers of the carnivorous plant Nepenthes khasiana

Nepenthes spp. are carnivorous plants that have developed insect capturing traps, evolved by specific modification of the leaf tips, and are able to utilize insect degradation products as nutritional precursors. A chitin-induced antifungal ability, based on the production and secretion to the trap liquid of droserone and 5-O-methyldroserone, is described here. Such specific secretion uniquely occurred when chitin injection was used as the eliciting agent and probably reflects a certain kind of defence mechanism that has been evolved for protecting the carnivory-based provision of nutritional precursors. The pitcher liquid containing droserone and 5-O-methyldroserone at 3:1 or 4:1 molar ratio, as well as the purified naphthoquinones, exerted an antifungal effect on a wide range of plant and human fungal pathogens. When tested against Candida and Aspergillus spp., the concentrations required for achieving inhibitory and fungicidal effects were significantly lower than those causing cytotoxicity in cells of the human embryonic kidney cell line, 293T. These naturally secreted 1,4-naphthoquinone derivatives, that are assumed to act via semiquinone enhancement of free radical production, may offer a new lead to develop alternative antifungal drugs with reduced selectable pressure for potentially evolved resistance.

Regulatory interactions between quorum-sensing, auxin, cytokinin, and the Hrp regulon in relation to gall formation and epiphytic fitness of Pantoea agglomerans pv. gypsophilae

Gall formation by Pantoea agglomerans pv. gypsophilae is controlled by hrp/hrc genes, phytohormones, and the quorum-sensing (QS) regulatory system. The interactions between these three components were investigated. Disruption of the QS genes pagI and pagR and deletion of both substantially reduced the transcription levels of the hrp regulatory genes hrpXY, hrpS, and hrpL, as determined by quantitative reverse-transcriptase polymerase chain reaction. Expression of hrpL in planta was inhibited by addition of 20 microM or higher concentrations of the QS signal C(4)-HSL. The pagR and hrpL mutants caused an equivalent reduction of 1.3 orders in bacterial multiplication on bean leaves, suggesting possible mediation of the QS effect on epiphytic fitness of P. agglomerans pv. gypsophilae by the hrp regulatory system. indole-3-acetic acid (IAA) and cytokinin significantly affected the expression of the QS and hrp regulatory genes. Transcription of pagI, pagR, hrpL, and hrpS in planta was substantially reduced in iaaH mutant (disrupted in IAA biosynthesis via the indole-3-acetamide pathway) and etz mutant (disrupted in cytokinin biosynthesis). In contrast, the ipdC mutant (disrupted in IAA biosynthesis via the indole-3-pyruvate pathway) substantially increased expression of pagI, pagR, hrpL, and hrpS. Results presented suggest the involvement of IAA and cytokinins in regulation of the QS system and hrp regulatory genes.

Complexity of semiarid gypsophilous shrub communities mediates the AMF biodiversity at the plant species level

The community composition of arbuscular mycorrhizal fungi (AMF) was analyzed in roots of Gypsophila struthium growing in gypsum soils under semiarid conditions. In order to investigate the effect of plant community degradation on the AMF biodiversity at the single species level, on the basis of the plant community complexity level, we selected four areas affected by degradation and shrub species spatial heterogeneity. The AM fungal community colonizing G. struthium was investigated from the morphological and molecular points of view. All plants were well colonized and showed a high level of infective AM propagules. Roots were analyzed by polymerase chain reaction, restriction fragment length polymorphism screening, and sequence analyses of the ribosomal DNA small subunit region. Four AM fungal types were identified and clustered into the AM fungal family: Glomeraceae, Glomus being the only taxon present. One fungal type was present in all the selected areas. Two fungal types are distinct from any previously published sequences and could be specific to gypsum soils. The chemical-physical properties of the soil were not correlated with the AMF diversity in roots. Our data show vegetation cover complexity-dependent differences in the AM fungal community composition.

Quorum-sensing system affects gall development incited by Pantoea agglomerans pv. gypsophilae

The quorum-sensing (QS) regulatory system of the gall-forming Pantoea agglomerans pv. gypsophilae was identified. Mass spectral analysis, together with signal-specific biosensors, demonstrated that P. agglomerans pv. gypsophilae produced N-butanoyl-l-homoserine lactone (C4-HSL) as a major and N-hexanoyl-l-homoserine lactone (C6-HSL) as a minor QS signal. Homologs of luxI and luxR regulatory genes, pagI and pagR, were characterized in strain P. agglomerans pv. gypsophilae Pag824-1 and shown to be convergently transcribed and separated by 14 bp. The deduced PagI (23.8 kDa) and PagR (26.9 kDa) show high similarity with SmaI (41% identity) and SmaR (43% identity), respectively, of Serratia sp. American Type Culture Collection 39006. PagR possesses characteristic autoinducer binding and a helix-turn-helix DNA-binding domain. Gall formation by P. agglomerans pv. gypsophilae depends on a plasmid-borne hrp/hrc gene cluster, type III effectors, and phytohormones. Disruption of pagI, pagR, or both genes simultaneously in Pag824-1 reduced gall size in gypsophila cuttings by 50 to 55% when plants were inoculated with 10(6) CFU/ml. Higher reductions in gall size (70 to 90%) were achieved by overexpression of pagI or addition of exogenous C4-HSL. Expression of the hrp/hrc regulatory gene hrpL and the type III effector pthG in the pagI mutant, as measured with quantitative reverse-transcriptase polymerase chain reaction, was reduced by 5.8 and 6.6, respectively, compared with the wild type, suggesting an effect of the QS system on the Hrp regulon.

Cophylogeny of the anther smut fungi and their caryophyllaceous hosts: prevalence of host shifts and importance of delimiting parasite species for inferring cospeciation

BACKGROUND

Using phylogenetic approaches, the expectation that parallel cladogenesis should occur between parasites and hosts has been validated in some studies, but most others provided evidence for frequent host shifts. Here we examine the evolutionary history of the association between Microbotryum fungi that cause anther smut disease and their Caryophyllaceous hosts. We investigated the congruence between host and parasite phylogenies, inferred cospeciation events and host shifts, and assessed whether geography or plant ecology could have facilitated the putative host shifts identified. For cophylogeny analyses on microorganisms, parasite strains isolated from different host species are generally considered to represent independent evolutionary lineages, often without checking whether some strains actually belong to the same generalist species. Such an approach may mistake intraspecific nodes for speciation events and thus bias the results of cophylogeny analyses if generalist species are found on closely related hosts. A second aim of this study was therefore to evaluate the impact of species delimitation on the inferences of cospeciation.

RESULTS

We inferred a multiple gene phylogeny of anther smut strains from 21 host plants from several geographic origins, complementing a previous study on the delimitation of fungal species and their host specificities. We also inferred a multi-gene phylogeny of their host plants, and the two phylogenies were compared. A significant level of cospeciation was found when each host species was considered to harbour a specific parasite strain, i.e. when generalist parasite species were not recognized as such. This approach overestimated the frequency of cocladogenesis because individual parasite species capable of infecting multiple host species (i.e. generalists) were found on closely related hosts. When generalist parasite species were appropriately delimited and only a single representative of each species was retained, cospeciation events were not more frequent than expected under a random distribution, and many host shifts were inferred.Current geographic distributions of host species seemed to be of little relevance for understanding the putative historical host shifts, because most fungal species had overlapping geographic ranges. We did detect some ecological similarities, including shared pollinators and habitat types, between host species that were diseased by closely related anther smut species. Overall, genetic similarity underlying the host-parasite interactions appeared to have the most important influence on specialization and host-shifts: generalist multi-host parasite species were found on closely related plant species, and related species in the Microbotryum phylogeny were associated with members of the same host clade.

CONCLUSION

We showed here that Microbotryum species have evolved through frequent host shifts to moderately distant hosts, and we show further that accurate delimitation of parasite species is essential for interpreting cophylogeny studies.

Phylogenetic evidence of host-specific cryptic species in the anther smut fungus

Cryptic structure of species complexes confounds an accurate accounting of biological diversity in natural systems. Also, cryptic sibling species often become specialized to different ecological conditions, for instance, with host specialization by cryptic parasite species. The fungus Microbotryum violaceum causes anther smut disease in plants of Caryophyllaceae, and the degree of specialization and gene flow between strains on different hosts have been controversial in the literature. We conducted molecular phylogenetic analyses on M. violaceum from 23 host species and different geographic origins using three single-copy nuclear genes (beta-tub, gamma-tub, and Ef1alpha). Congruence between the phylogenies identified several lineages that evolved independently for a long time. The lineages had overlapping geographic ranges but were highly specialized on different hosts. These results thus suggest that M. violaceum is a complex of highly specialized sibling species. Two incongruencies between the individual gene phylogenies and one intragene recombination event were detected at basal nodes, suggesting ancient introgression events or speciation events via hybridizations. However, incongruencies and recombination were not detected among terminal branches, indicating that the potentials for cross-infection and experimental hybridization are often not sufficient to suggest that introgressions would likely persist in nature.

Domain switching and host recognition

The collective function of secreted pathogen effector molecules is to enhance the virulence and avirulence activity of the pathogen during the infection of its host. While the activity of a majority of pathogen effectors is unknown, several classes of effector molecules have been well characterized. Among these include proteins which function to modulate host defences either through proteolysis, post-translational modifications, or by directly manipulating the host transcriptional machinery that regulates the induction of defence responses. In recent years, several key advances have been made in the characterization of the latter class of effector molecules. Among these include research characterizing the processes associated with host nuclear import and the targeting of host transcriptional defences. While current research is beginning to reveal the biochemical and genetic mechanisms controlling the induction of host resistance, the signalling events that control host specificity remain largely unknown. In this issue of Molecular Microbiology, work by Nissan et al. sheds light onto the molecular-genetic patterns involved in determining host specificity and pathogen virulence in the Pantoea-gypsophila interaction.

The type III effectors HsvG and HsvB of gall-forming Pantoea agglomerans determine host specificity and function as transcriptional activators

Pantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and a hypersensitive response on beet, whereas P. agglomerans pv. betae (Pab) induces galls on both beet and gypsophila. The pathogenicity of both pathovars is dependent on the presence of a plasmid harbouring type III secretion system (TTSS) components and effectors. The HsvG TTSS effectors of Pag (HsvG-Pag) and Pab (HsvG-Pab) determine the host specificity of both pathovars on gypsophila. Here we describe a novel HsvG homologue, HsvB, which determines the host specificity of Pag and Pab on beet. HsvG requires two direct amino acid repeats for pathogenicity on gypsophila, whereas one repeat in HsvB is sufficient for pathogenicity on beet. Exchanging repeats between HsvG-Pag and HsvB-Pab resulted in a switch of host specificities. Transient expression of GFP-HsvG or GFP-HsvB fusions in gypsophila, beet or melon leaves showed that HsvG and HsvB were localized to the nuclei of host and non-host plants. A yeast one-hybrid assay revealed that a single repeat of HsvG or HsvB was sufficient to activate transcription. By employing random binding-site selection and gel-shift assay HsvG was demonstrated to be a double-stranded DNA-binding protein with an ACACC/aAA consensus binding site. These results suggest that HsvG and HsvB are host-specificity determinants and bear the potential to affect the host transcriptional machinery.

Comparative anatomy of gall development on Gypsophila paniculata induced by bacteria with different mechanisms of pathogenicity

Galls induced on Gypsophila paniculata by Pantoea agglomerans pv. gypsophilae (Pag) and Agrobacterium tumefaciens (At), bacteria with different mechanisms of pathogenicity, were compared morphologically and anatomically. The pathogenicity of Pag is dependent on the presence of an indigenous plasmid that harbors hrp gene cluster, genes encoding Hop virulence proteins and biosynthetic genes for auxin (IAA) and cytokinins (CKs), whereas that of At involves host transformation. The Pag-induced gall was rough, brittle and exhibited limited growth, in contrast to the smooth, firm appearance and continuous growth of the At-induced gall. Anatomical analysis revealed the presence of cells with enlarged nuclei and multiple nucleoli, giant cells and suberin deposition in Pag that were absent from At-induced galls. Although circular vessels were observed in both gall types, they were more numerous and the vascular system was more organized in At. An aerenchymal tissue was observed in the upper part of the galls. Ethylene emission from Pag galls, recorded 6 days after inoculation, was eight times as great as that from non-infected controls. In contrast, a significant decrease in ethylene production was observed in Gypsophila cuttings infected with Pag mutants deficient in IAA and CK production. The results presented are best accounted for by the two pathogens having distinct pathogenicity mechanisms that lead to their differential recognition by the host as non-self (Pag) and self (At).

Symbiotic bacteria as a determinant of plant community structure and plant productivity in dune grassland

Symbiotic interactions are thought to play a key role in ecosystems. Empirical evidence for the impact of symbiotic bacteria on plant communities is, however, extremely scarce because of experimental constraints. Here, in three complementary experiments, we show that nitrogen-fixing rhizobia bacteria act as a determinant of plant community structure and diversity. Grassland microcosms inoculated with a mixture of rhizobia had a higher above-ground plant productivity (+35%), contained more nitrogen (+85%) and had significant higher community evenness (+34%) than control microcosms without rhizobia. Moreover, three of the four studied legume species required rhizobia to successfully coexist with other plant species. In contrast, the growth and survival of three grass and five forb species were not affected by the presence or absence of rhizobia. Finally, our results also showed that the legume species largely relied on symbiotically fixed nitrogen, both in the field and in the microcosms. This indicates that results in the microcosms are indicative for processes occurring in the field. It is concluded that symbiotic interactions between plants and prokaryotes can contribute to plant productivity, plant community structure and acquisition of limiting resources in legume-rich grassland communities.

Siderophore accumulation and phytopathogenicity in Microbotryum violaceum

We investigated the role of the siderophore rhodotorulic acid (RA) accumulation in the host-pathogen interaction of Microbotryum violaceum and Silene latifolia. While the wild-type M. violaceum accumulated RA in the culture medium in response to iron stress, the monogenic mutant 45 of M. violaceum did not accumulate detectable amounts of RA under limiting iron supply. Genetic analysis showed the mutant 45 locus to be 3.8 cM from the centromere, endorsing the pericentric gene clustering of M. violaceum. Crosses of mutant and wild-type strains demonstrated that the lack of siderophore accumulation in mutant 45 did not preclude pathogenesis, hence fungal reproduction.

Analysis of promoters recognized by HrpL, an alternative sigma-factor protein from Pantoea agglomerans pv. gypsophilae

HrpL, an alternative sigma factor, activates the transcription of the Hrp regulon by its binding to a common "hrp box" promoter. Based on computational techniques, the hrp box previously was defined as a consensus bipartite cis element, 5'-GGAACC-N(15-16)-CCACNNA-3'. The present report combines a quantitative in vivo assay for measuring Hrp promoter activity with site-specific mutagenesis to analyze the effect of consensus and nonconsensus nucleotides on promoter activity. The analysis was carried out with Hop effectors of the tumorigenic bacterium Pantoea agglomerans pv. gypsophilae, in which HrpL is indispensable for gall formation. Mutational analysis indicates that the hrp box consensus can be divided into crucial and noncrucial nucleotides. The first 5 nucleotides (nt) of the--35 consensus motif (GGAAC) and the 3 nt of the--10 motif (ACNNA) are crucial, whereas other consensus and adjacent nonconsensus nucleotides exert a significant effect on the promoter's strength. With spacing of 13 or 17 nt between the two motifs, significant activity was still retained. Gel shift assays indicated that deletion of GG from the--35 consensus motif eliminated HrpL binding, whereas mutations in the--10 consensus motif or modification of the spacing, which eliminates promoter activity, did not elicit any effect. The degeneracy in Hrp promoters of four hrp and type III effector genes of P agglomerans pv. gypsophilae indicated significant differences in promoter activity, whereas increasing the promoter strength of the Hop effector, HsvG, resulted in overexpression of gall formation.

Phylogenetic relationships of Peronospora and related genera based on nuclear ribosomal ITS sequences

In order to investigate phylogenetic relationships of selected members of Peronosporaceae to the genera Pythium, Halophkytophthora, Phytophthora, and Peronophythora, Bayesian analysis of partial sequences of the ITS1-5.8S-ITS2 region was performed. In addition, sequences of the complete ITS1-5.8S-ITS2 region were analysed for 101 collections belonging to the genera Peronospora, Hyaloperonospora, Perofascia, Pseudoperonospora, Phytophthora and Peronophythora, using Bayesian inference of phylogeny and maximum parsimony. The results confirm a close relationship of these genera. The strongly supported Peronosporaceae clade is located within the paraphyletic genus Phytophthora (including Peronophythora). Monophyly of the genera Pseudoperonospora and Hyaloperonospora are strongly supported, but monophyly of Peronospora s. str. can neither be confirmed nor rejected. Within Peronospora s. str., basic relationships often remain unclear; however, some groups form highly supported monophyletic clades. Peronospora species parasitising the same host families or orders are only partly resolved as monophyletic, indicating frequent host-jumping also between distantly related host families. All species inhabiting flowers of different host families form a strongly supported monophyletic group.

Molecular diagnostic procedures for production of pathogen-free propagation material

Production of disease-free propagation material is a major means of controlling most bacterial diseases of plants, particularly when neither resistant clones nor effective chemical treatments are available. For this purpose sensitive, specific and rapid detection methods are required. The advent of molecular biology and, in particular, the polymerase chain reaction (PCR) has opened new ways for the characterization and identification of plant pathogens and the development of disease-management strategies. PCR-based detection methods rely on the development of primers for the specific detection of the pathogen. The use of pathogenicity genes as targets for primer design is the preferred procedure for obtaining specific primers but other procedures may also be useful for this purpose. In the present review we describe four examples of procedures for detecting four important bacterial pathogens in Israel: Erwinia herbicola pv gypsophilae in gypsophila, Xanthomonas campestris pv pelargonii in geranium, Agrobacterium tumefaciens in asters and roses, and Xanthomonas campestris pv campestris in crucifers. Procedures for constructing specific PCR primers for each bacterium are illustrated and discussed as well as the combination of PCR with other methods.