Leafy gall formation by Rhodococcus fascians

Genetic analysis of acyl-CoA carboxylases involved in lipid accumulation in Rhodococcus jostii RHA1

In actinomycetes, the acyl-CoA carboxylases, including the so-called acetyl-CoA carboxylases (ACCs), are biotin-dependent enzymes that exhibit broad substrate specificity and diverse domain and subunit arrangements. Bioinformatic analyses of the Rhodococcus jostii RHA1 genome found that this microorganism contains a vast arrange of putative acyl-CoA carboxylases domains and subunits. From the thirteen putative carboxyltransferase domains, only the carboxyltransferase subunit RO01202 and the carboxyltransferase domain present in the multidomain protein RO04222 are highly similar to well-known essential ACC subunits from other actinobacteria. Mutant strains in each of these genes showed that none of these enzymes is essential for R. jostii growth in rich or in minimal media with high nitrogen concentration, presumably because of their partial overlapping activities. A mutant strain in the ro04222 gene showed a decrease in triacylglycerol and mycolic acids accumulation in rich and minimal medium, highlighting the relevance of this multidomain ACC in the biosynthesis of these lipids. On the other hand, RO01202, a carboxyltransferase domain of a putative ACC complex, whose biotin carboxylase and biotin carboxyl carrier protein domain were not yet identified, was found to be essential for R. jostii growth only in minimal medium with low nitrogen concentration. The results of this study have identified a new component of the TAG-accumulating machinery in the oleaginous R. jostii RHA1. While non-essential for growth and TAG biosynthesis in RHA1, the activity of RO04222 significantly contributes to lipogenesis during single-cell oil production. Furthermore, this study highlights the high functional diversity of ACCs in actinobacteria, particularly regarding their essentiality under different environmental conditions. KEY POINTS: • R. jostii possess a remarkable heterogeneity in their acyl-carboxylase complexes. • RO04222 is a multidomain acetyl-CoA carboxylase involved in lipid accumulation. • RO01202 is an essential carboxyltransferase only at low nitrogen conditions.

Development of Efficient Genome-Reduction Tool Based on Cre/loxP System in Rhodococcus erythropolis

Rhodococcus has been extensively studied for its excellent ability to degrade artificial chemicals and its capability to synthesize biosurfactants and antibiotics. In recent years, studies have attempted to use Rhodococcus as a gene expression host. Various genetic tools, such as plasmid vectors, transposon mutagenesis, and gene disruption methods have been developed for use in Rhodococcus; however, no effective method has been reported for performing large-size genome reduction. Therefore, the present study developed an effective plasmid-curing method using the levansucrase-encoding sacB gene and a simple two-step genome-reduction method using a modified Cre/loxP system. For the results, R. erythropolis JCM 2895 was used as the model; a mutant strain that cured all four plasmids and deleted seven chromosomal regions was successfully obtained in this study. The total DNA deletion size was >600 kb, which corresponds mostly to 10% of the genome size. Using this method, a genome-structure-stabilized and unfavorable gene/function-lacking host strain can be created in Rhodococcus. This genetic tool will help develop and improve Rhodococcus strains for various industrial and environmental applications.

The C-terminal stretch of glycine-rich proline-rich protein (SbGPRP1) from Sorghum bicolor serves as an antimicrobial peptide by targeting the bacterial outer membrane protein

The C-terminal stretch in SbGPRP1 (Sorghum glycine-rich proline-rich protein) acts as an antimicrobial peptide in the host innate defense mechanism. Cationic antimicrobial proteins or peptides can either bind to the bacterial membrane or target a specific protein on the bacterial membrane thus leading to membrane perturbation. The 197 amino acid polypeptide of SbGPRP1 showed disordered structure at the N-terminal end and ordered conformation at the C-terminal end. In the present study, the expression of N-SbGPRP1, C-SbGPRP1, and ∆SbGPRP1 followed by antimicrobial assays showed potential antimicrobial property of the C-terminal peptide against gram-positive bacteria Bacillus subtilis and phytopathogen Rhodococcus fascians. The SbGPRP1 protein loses its antimicrobial property when the 23 amino acid sequence (GHGGHGVFGGGYGHGGYGHGYGG) from position 136 to 158 is deleted from the protein. Thus, it can be concluded that the 23 amino acid sequence is vital for the said antimicrobial property. NPN assay, SEM analysis, and electrolyte leakage assays showed potent antimicrobial activity for C-SbGPRP1. Overexpression of the C-SbGPRP1 mutant protein in tobacco followed by infection with Rhodococcus fascians inhibited bacterial growth as shown by SEM analysis. To determine if C-SbGPRP1 might target any protein on the bacterial membrane we isolated the bacterial membrane protein from both Bacillus subtilis and Rhodococcus fascians. Bacterial membrane protein that interacted with the column-bound C-SbGPRP1 was eluted and subjected to LC-MS/MS. LC-MS/MS data analysis showed peptide hit with membrane protein YszA from Bacillus subtilis and a membrane protein from Rhodococcus fascians. Isolated bacterial membrane protein from Bacillus subtilis or Rhodococcus fascians was able to reduce the antimicrobial activity of C-SbGPRP1. Furthermore, BiFC experiments showed interactions between C-SbGPRP1 and YszA protein from Bacillus subtilis leading to the conclusion that bacterial membrane protein was targeted in such membrane perturbation leading to antimicrobial activity.

Structure-activity correlations for peptaibols obtained from clade Longibrachiatum of Trichoderma: A combined experimental and computational approach

Integrated disease management and plant protection have been discussed with much fervor in the past decade due to the rising environmental concerns of using industrially produced pesticides. Members of the genus Trichoderma are a subject of considerable research today due to their several properties as biocontrol agents. In our study, the peptaibol production of Trichoderma longibrachiatum SZMC 1775, T. longibrachiatum f. bissettii SZMC 12546, T. reesei SZMC 22616, T. reesei SZMC 22614, T. saturnisporum SZMC 22606 and T. effusum SZMC 22611 were investigated to elucidate structure-activity relationships (SARs) between the properties of peptaibols and their 3D structures. The effects of peptaibol mixtures obtained from every Trichoderma strain were examined against nine commonly known bacteria. The lowest minimum inhibitory concentrations (MIC, mg ml^-1) were exerted by T. longibrachiatum f. bissettii SZMC 12546 against Gram-positive bacteria, which was also able to inhibit the plant pathogenic Gram-negative Rhizobium radiobacter. Accelerated molecular dynamics (aMD) simulations were performed in aqueous solvent to explore the folding dynamics of 12 selected peptaibol sequences. The most characteristic difference between the peptaibols from group A and B relies in the 'Gly-Leu-Aib-Pro' and 'Gly-Aib-Aib-Pro' motifs ('Aib' stands for α-aminoisobutyric acid), which imparted a significant effect on the folding dynamics in water and might be correlated with their expressed bioactivity. In our aMD simulation experiments, Group A peptaibols showed more restricted folding dynamics with well-folded helical conformations as the most stable representative structures. This structural stability and dynamics may contribute to their bioactivity against the selected bacterial species.

Responses to Ecopollutants and Pathogenization Risks of Saprotrophic Rhodococcus Species

Under conditions of increasing environmental pollution, true saprophytes are capable of changing their survival strategies and demonstrating certain pathogenicity factors. Actinobacteria of the genus Rhodococcus, typical soil and aquatic biotope inhabitants, are characterized by high ecological plasticity and a wide range of oxidized organic substrates, including hydrocarbons and their derivatives. Their cell adaptations, such as the ability of adhering and colonizing surfaces, a complex life cycle, formation of resting cells and capsule-like structures, diauxotrophy, and a rigid cell wall, developed against the negative effects of anthropogenic pollutants are discussed and the risks of possible pathogenization of free-living saprotrophic Rhodococcus species are proposed. Due to universal adaptation features, Rhodococcus species are among the candidates, if further anthropogenic pressure increases, to move into the group of potentially pathogenic organisms with "unprofessional" parasitism, and to join an expanding list of infectious agents as facultative or occasional parasites.

Detection of Rhodococcus fascians, the Causative Agent of Lily Fasciation in South Korea

Rhodococcus fascians is an important pathogen that infects various herbaceous perennials and reduces their economic value. In this study, we examined R. fascians isolates carrying a virulence gene from symptomatic lily plants grown in South Korea. Phylogenetic analysis using the nucleotide sequences of 16S rRNA, vicA, and fasD led to the classification of the isolates into four different strains of R. fascians. Inoculation of Nicotiana benthamiana with these isolates slowed root growth and resulted in symptoms of leafy gall. These findings elucidate the diversification of domestic pathogenic R. fascians and may lead to an accurate causal diagnosis to help reduce economic losses in the bulb market.

Norwegian Soils and Waters Contain Mesophilic, Plastic-Degrading Bacteria

Plastic pollution has become one of the most critical environmental issues, as rapidly increasing production, compounded by persistence of plastic wastes in the environment, are outpacing efforts to keep ecosystems plastic-free. A switch to plastics more amenable to microbial attack is one of several possible responses. Against this background, the current study describes the isolation, enumeration and polyphasic characterization of plastic-degrading bacteria present in Norwegian terrestrial and aquatic habits. It shows that these bacteria are present in relatively high numbers, and that plastic-degrading capabilities are found in several taxa, most especially Streptomyces. Some isolates wereable to degrade several plastics. Notably, a Rhodococcus sp. and a Streptomyces sp. degraded, respectively, four and six of the eight plastics investigated and a number of other polymers relevant for plastic blends. The paper also has a methodological aspect, presenting various approaches for assaying plastic-degrading properties and a PCR/sequencing-based approach for the identification of potential polyethylene terephthalate-degrading genes. A candidate gene was detected in several Streptomyces isolates. The study shows that Norwegian environments are a rich source of bacteria with the ability to degrade bioplastics possibly representing a natural remediation capacity, as well as a potential source of useful enzymes.

A sustained CYCLINB1;1 and STM expression in the neoplastic tissues induced by Rhodococcus fascians on Arabidopsis underlies the persistence of the leafy gall structure

is a gram-positive phytopathogen that infects a wide range of plant species. The actinomycete induces the formation of neoplastic growths, termed leafy galls, that consist of a gall body covered by small shoots of which the outgrowth is arrested due to an extreme form of apical dominance. In our previous work, we demonstrated that in the developing gall, auxin drives the transdifferentiation of parenchyma cells into vascular elements. In this work, with the use of transgenic Arabidopsis thaliana plants carrying molecular reporters for cell division (pCYCB1;1:GUS) and meristematic activity (pSTM:GUS), we analyzed the fate of cells within the leafy gall. Our results indicate that the size of the gall body is determined by ongoing mitotic cell divisions as illustrated by strong CYCB1;1 expression combined with the de novo formation of new meristematic areas triggered by STM expression. The shoot meristems that develop in the peripheral parts of the gall are originating from high ectopic STM expression. Altogether the presented data provide further insight into the cellular events that accompany the development of leafy galls in response to R. fascians infection.

Comparative Genomics of the Rhodococcus Genus Shows Wide Distribution of Biodegradation Traits

The genus Rhodococcus exhibits great potential for bioremediation applications due to its huge metabolic diversity, including biotransformation of aromatic and aliphatic compounds. Comparative genomic studies of this genus are limited to a small number of genomes, while the high number of sequenced strains to date could provide more information about the Rhodococcus diversity. Phylogenomic analysis of 327 Rhodococcus genomes and clustering of intergenomic distances identified 42 phylogenomic groups and 83 species-level clusters. Rarefaction models show that these numbers are likely to increase as new Rhodococcus strains are sequenced. The Rhodococcus genus possesses a small “hard” core genome consisting of 381 orthologous groups (OGs), while a “soft” core genome of 1253 OGs is reached with 99.16% of the genomes. Models of sequentially randomly added genomes show that a small number of genomes are enough to explain most of the shared diversity of the Rhodococcus strains, while the “open” pangenome and strain-specific genome evidence that the diversity of the genus will increase, as new genomes still add more OGs to the whole genomic set. Most rhodococci possess genes involved in the degradation of aliphatic and aromatic compounds, while short-chain alkane degradation is restricted to a certain number of groups, among which a specific particulate methane monooxygenase (pMMO) is only found in Rhodococcus sp. WAY2. The analysis of Rieske 2Fe-2S dioxygenases among rhodococci genomes revealed that most of these enzymes remain uncharacterized.

Plants make galls to accommodate foreigners: some are friends, most are foes

At the colonization site of a foreign entity, plant cells alter their trajectory of growth and development. The resulting structure - a plant gall - accommodates various needs of the foreigner, which are phylogenetically diverse: viruses, bacteria, protozoa, oomycetes, true fungi, parasitic plants, and many types of animals, including rotifers, nematodes, insects, and mites. The plant species that make galls also are diverse. We assume gall production costs the plant. All is well if the foreigner provides a gift that makes up for the cost. Nitrogen-fixing nodule-inducing bacteria provide nutritional services. Gall wasps pollinate fig trees. Unfortunately for plants, most galls are made for foes, some of which are deeply studied pathogens and pests: Agrobacterium tumefaciens, Rhodococcus fascians, Xanthomonas citri, Pseudomonas savastanoi, Pantoea agglomerans, 'Candidatus' phytoplasma, rust fungi, Ustilago smuts, root knot and cyst nematodes, and gall midges. Galls are an understudied phenomenon in plant developmental biology. We propose gall inception for discovering unifying features of the galls that plants make for friends and foes, talk about molecules that plants and gall-inducers use to get what they want from each other, raise the question of whether plants colonized by arbuscular mycorrhizal fungi respond in a gall-like manner, and present a research agenda.

Comparative Genomic Analysis of Rhodococcus equi: An Insight into Genomic Diversity and Genome Evolution

Rhodococcus equi, a member of the Rhodococcus genus, is a gram-positive pathogenic bacterium. Rhodococcus possesses an open pan-genome that constitutes the basis of its high genomic diversity and allows for adaptation to specific niche conditions and the changing host environments. Our analysis further showed that the core genome of R. equi contributes to the pathogenicity and niche adaptation of R. equi. Comparative genomic analysis revealed that the genomes of R. equi shared identical collinearity relationship, and heterogeneity was mainly acquired by means of genomic islands and prophages. Moreover, genomic islands in R. equi were always involved in virulence, resistance, or niche adaptation and possibly working with prophages to cause the majority of genome expansion. These findings provide an insight into the genomic diversity, evolution, and structural variation of R. equi and a valuable resource for functional genomic studies.

Evaluation and entomopathogenicity of gut bacteria associated with dauer juveniles of Oscheius chongmingensis (Nematoda: Rhabditidae)

The nematodes of genus Oscheius are insect parasites with a potential role as biological control agents. The composition of gut microbiota and its potential assistant role in the complex pathogenic mechanism of nematodes have been poorly illustrated. In this study, the intestinal bacteria associated with dauer juveniles of the nematode Oscheius chongmingensis Tumian were classified by 16S rDNA high-throughput sequencing. The raw reads were assigned to 845 operational taxonomic units (OTUs) after quality filtering. The results showed that the genus Ochrobactrum, with a proportion of 59.82%, was the most abundant genus, followed by 7.13% Bacillus, 4.7% Albidiferax, 4.26% Acinetobacter, and 3.09% Rhodococcus. The two dominant bacteria, Ochrobactrum and Bacillus, were further isolated by culturing on NBTA and LB medium respectively, and then identified as Ochrobactrum tritici and Bacillus cereus by morphological and 16S rDNA sequence analysis. Furthermore, the entomopathogenicity of these two bacterial species was studied. The results showed that O. tritici caused 93.33% mortality within 144 hr in the 4th -instar larvae of Galleria mellonella treated with 2 × 109  CFU/ml, whereas B. cereus showed 100% mortality at a concentration of 3.3 × 107  CFU/ml within 48 hr. These findings, especially the presence of O. tritici, which had not been found in other nematode species in the genus Oscheius, indicate that the associated nematode O. chongmingensis may have particular utility as a biocontrol agent.

Virulence mechanisms of plant-pathogenic Streptomyces species: an updated review

Gram-positive Actinobacteria from the genus Streptomyces are best known for their morphological complexity and for their ability to produce numerous bioactive specialized metabolites with useful applications in human and veterinary medicine and in agriculture. In contrast, the ability to infect living plant tissues and to cause diseases of root and tuber crops such as potato common scab (CS) is a rare attribute among members of this genus. Research on the virulence mechanisms of plant-pathogenic Streptomyces spp. has revealed the importance of the thaxtomin phytotoxins as key pathogenicity determinants produced by several species. In addition, other phytotoxic specialized metabolites may contribute to the development or severity of disease caused by Streptomyces spp., along with the production of phytohormones and secreted proteins. A thorough understanding of the molecular mechanisms of plant pathogenicity will enable the development of better management procedures for controlling CS and other plant diseases caused by the Streptomyces.

The Cytokinin Complex Associated With Rhodococcus fascians: Which Compounds Are Critical for Virulence?

Virulent strains of Rhodococcus fascians cause a range of disease symptoms, many of which can be mimicked by application of cytokinin. Both virulent and avirulent strains produce a complex of cytokinins, most of which can be derived from tRNA degradation. To test the three current hypotheses regarding the involvement of cytokinins as virulence determinants, we used PCR to detect specific genes, previously associated with a linear virulence plasmid, including two methyl transferase genes (mt1 and mt2) and fas4 (dimethyl transferase), of multiple strains of R. fascians. We inoculated Pisum sativum (pea) seeds with virulent and avirulent strains of R. fascians, monitored the plants over time and compared these to mock-inoculated controls. We used RT-qPCR to monitor the expression of mt1, mt2, and fas4 in inoculated tissues and LC-MS/MS to obtain a comprehensive picture of the cytokinin complement of inoculated cotyledons, roots and shoots over time. The presence and expression of mt1 and mt2 was associated with those strains of R. fascians classed as virulent, and not those classed as avirulent. Expression of mt1, mt2, and fas4 peaked at 9 days post-inoculation (dpi) in cotyledons and at 15 dpi in shoots and roots developed from seeds inoculated with virulent strain 602. Pea plants inoculated with virulent and avirulent strains of R. fascians both contained cytokinins likely to have been derived from tRNA turnover including the 2-methylthio cytokinins and cis-zeatin-derivatives. Along with the isopentenyladenine-type cytokinins, the levels of these compounds did not correlate with virulence. Only the novel 1- and 2-methylated isopentenyladenine cytokinins were uniquely associated with infection by the virulent strains and are, therefore, the likely causative factors of the disease symptoms.

Comment on "Evolutionary transitions between beneficial and phytopathogenic Rhodococcus challenge disease management"

I would like to report significant issues of concern regarding this paper (Savory et al., 2017).

Neovascularization during leafy gall formation on Arabidopsis thaliana upon Rhodococcus fascians infection

Extensive de novo vascularization of leafy galls emerging upon Rhodococcus fascians infection is achieved by fascicular/interfascicular cambium activity and transdifferentiation of parenchyma cells correlated with increased auxin signaling. A leafy gall consisting of fully developed yet growth-inhibited shoots, induced by the actinomycete Rhodococcus fascians, differs in structure compared to the callus-like galls induced by other bacteria. To get insight into the vascular development accompanying the emergence of the leafy gall, the anatomy of infected axillary regions of the inflorescence stem of wild-type Arabidopsis thaliana accession Col-0 plants and the auxin response in pDR5:GUS-tagged plants were followed in time. Based on our observations, three phases can be discerned during vascularization of the symptomatic tissue. First, existing fascicular cambium becomes activated and interfascicular cambium is formed giving rise to secondary vascular elements in a basipetal direction below the infection site in the main stem and in an acropetal direction in the entire side branch. Then, parenchyma cells in the region between both stems transdifferentiate acropetally towards the surface of the developing symptomatic tissue leading to the formation of xylem and vascularize the hyperplasia as they expand. Finally, parenchyma cells in the developing gall also transdifferentiate to vascular elements without any specific direction resulting in excessive vasculature disorderly distributed in the leafy gall. Prior to any apparent anatomical changes, a strong auxin response is mounted, implying that auxin is the signal that controls the vascular differentiation induced by the infection. To conclude, we propose the "sidetracking gall hypothesis" as we discuss the mechanisms driving the formation of superfluous vasculature of the emerging leafy gall.

Lantibiotics produced by Actinobacteria and their potential applications (a review)

The phylum Actinobacteria, which comprises a great variety of Gram-positive bacteria with a high G+C content in their genomes, is known for its large production of bioactive compounds, including those with antimicrobial activity. Among the antimicrobials, bacteriocins, ribosomally synthesized peptides, represent an important arsenal of potential new drugs to face the increasing prevalence of resistance to antibiotics among microbial pathogens. The actinobacterial bacteriocins form a heterogeneous group of substances that is difficult to adapt to most proposed classification schemes. However, recent updates have accommodated efficiently the diversity of bacteriocins produced by this phylum. Among the bacteriocins, the lantibiotics represent a source of new antimicrobials to control infections caused mainly by Gram-positive bacteria and with a low propensity for resistance development. Moreover, some of these compounds have additional biological properties, exhibiting activity against viruses and tumour cells and having also potential to be used in blood pressure or inflammation control and in pain relief. Thus, lantibiotics already described in Actinobacteria exhibit potential practical applications in medical settings, food industry and agriculture, with examples at different stages of pre-clinical and clinical trials.

Knots Untie: Molecular Determinants Involved in Knot Formation Induced by Pseudomonas savastanoi in Woody Hosts

The study of the molecular basis of tree diseases is lately receiving a renewed attention, especially with the emerging perception that pathogens require specific pathogenicity and virulence factors to successfully colonize woody hosts. Pathosystems involving woody plants are notoriously difficult to study, although the use of model bacterial strains together with genetically homogeneous micropropagated plant material is providing a significant impetus to our understanding of the molecular determinants leading to disease. The gammaproteobacterium Pseudomonas savastanoi belongs to the intensively studied Pseudomonas syringae complex, and includes three pathogenic lineages causing tumorous overgrowths (knots) in diverse economically relevant trees and shrubs. As it occurs with many other bacteria, pathogenicity of P. savastanoi is dependent on a type III secretion system, which is accompanied by a core set of at least 20 effector genes shared among strains isolated from olive, oleander, and ash. The induction of knots of wild-type size requires that the pathogen maintains adequate levels of diverse metabolites, including the phytohormones indole-3-acetic acid and cytokinins, as well as cyclic-di-GMP, some of which can also regulate the expression of other pathogenicity and virulence genes and participate in bacterial competitiveness. In a remarkable example of social networking, quorum sensing molecules allow for the communication among P. savastanoi and other members of the knot microbiome, while at the same time are essential for tumor formation. Additionally, a distinguishing feature of bacteria from the P. syringae complex isolated from woody organs is the possession of a 15 kb genomic island (WHOP) carrying four operons and three other genes involved in degradation of phenolic compounds. Two of these operons mediate the catabolism of anthranilate and catechol and, together with another operon, are required for the induction of full-size tumors in woody hosts, but not in non-woody micropropagated plants. The use of transposon mutagenesis also uncovered a treasure trove of additional P. savastanoi genes affecting virulence and participating in diverse bacterial processes. Although there is still much to be learned on what makes a bacterium a successful pathogen of trees, we are already untying the knots.

Next-generation systematics: An innovative approach to resolve the structure of complex prokaryotic taxa

Prokaryotic systematics provides the fundamental framework for microbiological research but remains a discipline that relies on a labour- and time-intensive polyphasic taxonomic approach, including DNA-DNA hybridization, variation in 16S rRNA gene sequence and phenotypic characteristics. These techniques suffer from poor resolution in distinguishing between closely related species and often result in misclassification and misidentification of strains. Moreover, guidelines are unclear for the delineation of bacterial genera. Here, we have applied an innovative phylogenetic and taxogenomic approach to a heterogeneous actinobacterial taxon, Rhodococcus, to identify boundaries for intrageneric and supraspecific classification. Seven species-groups were identified within the genus Rhodococcus that are as distantly related to one another as they are to representatives of other mycolic acid containing actinobacteria and can thus be equated with the rank of genus. It was also evident that strains assigned to rhodococcal species-groups are underspeciated with many misclassified using conventional taxonomic criteria. The phylogenetic and taxogenomic methods used in this study provide data of theoretical value for the circumscription of generic and species boundaries and are also of practical significance as they provide a robust basis for the classification and identification of rhodococci of agricultural, industrial and medical/veterinary significance.

Differential proteomics reveals novel insights into Nosema-honey bee interactions

Host manipulation is a common strategy by parasites to reduce host defense responses, enhance development, host exploitation, reproduction and, ultimately, transmission success. As these parasitic modifications can reduce host fitness, increased selection pressure may result in reciprocal adaptations of the host. Whereas the majority of studies on host manipulation have explored resistance against parasites (i.e. ability to prevent or limit an infection), data describing tolerance mechanisms (i.e. ability to limit harm of an infection) are scarce. By comparing differential protein abundance, we provide evidence of host-parasite interactions in the midgut proteomes of N. ceranae-infected and uninfected honey bees from both Nosema-tolerant and Nosema-sensitive lineages. We identified 16 proteins out of 661 protein spots that were differentially abundant between experimental groups. In general, infections of Nosema resulted in an up-regulation of the bee's energy metabolism. Additionally, we identified 8 proteins that were differentially abundant between tolerant and sensitive honey bees regardless of the Nosema infection. Those proteins were linked to metabolism, response to oxidative stress and apoptosis. In addition to bee proteins, we also identified 3 Nosema ceranae proteins. Interestingly, abundance of two of these Nosema proteins were significantly higher in infected Nosema-sensitive honeybees relative to the infected Nosema-tolerant lineage. This may provide a novel candidate for studying the molecular interplay between N. ceranae and its honey bee host in more detail.

Diversity of retrievable heterotrophic bacteria in Kongsfjorden, an Arctic fjord

The diversity and abundance of retrievable pelagic heterotrophic bacteria in Kongsfjorden, an Arctic fjord, was studied during the summer of 2011 (June, August, and September). Retrievable bacterial load ranged from 10³ to 10⁷ CFU L⁻¹ in June, while it was 10⁴–10⁶ CFU L⁻¹ in August and September. Based on 16S rRNA gene sequence similarities, a higher number of phylotypes was observed during August (22 phylotypes) compared to that during June (6 phylotypes) and September (12 phylotypes). The groups were classified into four phyla: Firmicutes, Actinobacteria, Proteobacteria, and Bacteroidetes. Bacteroidetes was represented only by a single member Leewenhoekiella aequorea during the three months and was dominant (40%) in June. However, this dominance changed in August to a well-known phytopathogenic species Rhodococcus fascians (32%), which could be a result of decrease in the phytoplankton biomass following the secondary bloom. It is the first report of Halomonas titanicae isolation from the Arctic waters. It showed an increase in its abundance with the intrusion of Atlantic water into Kongsfjorden. Increased abundance of Psychrobacter species in the late summer months coincided with the presence of cooler waters. Thus, the composition and function of heterotrophic bacterial community was fundamentally different in different months. This could be linked to the changes in the water masses and/or phytoplankton bloom dynamics occurring in Arctic summer.

Influence of Plasmid Type on the Replication of Rhodococcus equi in Host Macrophages

The soil-dwelling, saprophytic actinomycete Rhodococcus equi is a multihost, facultative intracellular pathogen of macrophages. When inhaled by susceptible foals, it causes severe bronchopneumonia. It is also a pathogen of pigs, which may develop submaxillary lymphadenitis upon exposure. R. equi isolates obtained from foals and pigs possess conjugative plasmids housing a pathogenicity island (PAI) containing a novel family of genes of unknown function called the virulence-associated protein or vap family. The PAI regions of the equine and swine plasmids differ in vap gene composition, with equine isolates possessing six vap genes, including the major virulence determinant vapA, while the PAIs of swine isolates house vapB and five other unique vap genes. Possession of the pVAPA-type virulence plasmid by equine isolates bestows the capacity for intramacrophage replication essential for disease development in vivo. Swine isolates of R. equi are largely unstudied. Here, we show that R. equi isolates from pigs, carrying pVAPB-type plasmids, are able to replicate in a plasmid-dependent manner in macrophages obtained from a variety of species (murine, swine, and equine) and anatomical locations. Similarly, equine isolates carrying pVAPA-type plasmids are capable of replication in swine macrophages. Plasmid swapping between equine and swine strains through conjugation did not alter the intracellular replication capacity of the parental strain, indicating that coevolution of the plasmid and chromosome is not crucial for this attribute. These results demonstrate that while distinct plasmid types exist among R. equi isolates obtained from equine and swine sources, this tropism is not determined by host species-specific intramacrophage replication capabilities. IMPORTANCE This work greatly advances our understanding of the opportunistic pathogen Rhodococcus equi, a disease agent of animals and immunocompromised people. Clinical isolates from diseased foals carry a conjugative virulence plasmid, pVAPA1037, that expresses Vap proteins, including VapA, essential for intramacrophage replication and virulence in vivo. The understudied R. equi isolates from pigs carry a related but different plasmid, pVAPB, expressing distinct Vap proteins, including VapB. In this work, we document for the first time that R. equi isolates carrying pVAPB-type plasmids are capable of intramacrophage replication. Moreover, we show that R. equi isolates carrying either plasmid type can replicate in both equine and swine macrophages, indicating that host species tropism is not due to species-specific intramacrophage replication capabilities defined by plasmid type. Furthermore, plasmid swapping between equine and swine strains did not alter intracellular replication capacity, indicating that coevolution of the plasmid and chromosome is not essential for intracellular growth.

Rhodococcus fascians infection after haematopoietic cell transplantation: not just a plant pathogen?

Introduction:

Rhodococcus spp. have been implicated in a variety of infections in immunocompromised and immunocompetent hosts. Rhodococcus equi is responsible for the majority of reported cases, but Rhodococcus erythropolis, Rhodococcusgordoniae and Rhodococcusruber infections have been described. There are no prior reports of human infection with Rhodococcus fascians.

Case presentation:

We describe the unexpected finding of R. fascians in liver lesions incidentally noted at autopsy in an immunosuppressed patient status after bone-marrow transplant for acute lymphoblastic leukaemia who died of unrelated causes (septic shock due to Clostridium difficile colitis). At autopsy, an otherwise unremarkable liver contained several dozen well-demarcated sclerotic-appearing lesions measuring 0.1–0.3 cm in size. The absence of other bacterial or fungal DNA in the setting of histologically visible organisms argues against its presence as a contaminant and raises the consideration that R. fascians represents a human pathogen for the immunocompromised.

Conclusion:

Whether it represents the sole infectious agent responsible for the miliary lesions or a partially treated co-infection is impossible to determine, but our finding continues to reinforce the importance of molecular techniques in associating organisms with sites of infection and optimizing treatment of infectious diseases.

Taxonomy, Physiology, and Natural Products of Actinobacteria

Actinobacteria are Gram-positive bacteria with high G+C DNA content that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems. Many Actinobacteria have a mycelial lifestyle and undergo complex morphological differentiation. They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds. Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture. Actinobacteria play diverse roles in their associations with various higher organisms, since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species of Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, and Tropheryma), soil inhabitants (e.g., Micromonospora and Streptomyces species), plant commensals (e.g., Frankia spp.), and gastrointestinal commensals (Bifidobacterium spp.). Actinobacteria also play an important role as symbionts and as pathogens in plant-associated microbial communities. This review presents an update on the biology of this important bacterial phylum.

Rice-gall midge interactions: Battle for survival

Gall midges are insects specialized in maneuvering plant growth, metabolic and defense pathways for their benefit. The Asian rice gall midge and rice share such an intimate relationship that there is a constant battle for survival by either partner. Diverse responses by the rice host against the midge include necrotic hypersensitive resistance reaction, non-hypersensitive resistance reaction and gall-forming compatible interaction. Genetic studies have revealed that major R (resistance) genes confer resistance to gall midge in rice. Eleven gall midge R genes have been characterized so far in different rice varieties in India. In addition, no single R gene confers resistance against all the seven biotypes of the Asian rice gall midge, and none of the biotypes is virulent against all the resistance genes. Further, the interaction of the plant resistance gene with the insect avirulence gene is on a gene-for-gene basis. Our recent investigations involving suppressive subtraction hybridization cDNA libraries, microarray analyses, gene expression assays and metabolic profiling have revealed several molecular mechanisms, metabolite markers and pathways that are induced, down-regulated or altered in the rice host during incompatible or compatible interactions with the pest. This is also true for some of the pathways studied in the gall midge. Next generation sequencing technology, gene expression studies and conventional screening of gall midge cDNA libraries highlighted molecular approaches adopted by the insect to feed, survive and reproduce. This constant struggle by the midge to overcome the host defenses and the host to resist the pest has provided us with an opportunity to observe this battle for survival at the molecular level.

Quantitative Analysis of Lysobacter Predation

Bacteria of the genus Lysobacter are considered to be facultative predators that use a feeding strategy similar to that of myxobacteria. Experimental data supporting this assumption, however, are scarce. Therefore, the predatory activities of three Lysobacter species were tested in the prey spot plate assay and in the lawn predation assay, which are commonly used to analyze myxobacterial predation. Surprisingly, only one of the tested Lysobacter species showed predatory behavior in the two assays. This result suggested that not all Lysobacter strains are predatory or, alternatively, that the assays were not appropriate for determining the predatory potential of this bacterial group. To differentiate between the two scenarios, predation was tested in a CFU-based bioassay. For this purpose, defined numbers of Lysobacter cells were mixed together with potential prey bacteria featuring phenotypic markers, such as distinctive pigmentation or antibiotic resistance. After 24 h, cocultivated cells were streaked out on agar plates and sizes of bacterial populations were individually determined by counting the respective colonies. Using the CFU-based predation assay, we observed that Lysobacter spp. strongly antagonized other bacteria under nutrient-deficient conditions. Simultaneously, the Lysobacter population was increasing, which together with the killing of the cocultured bacteria indicated predation. Variation of the predator/prey ratio revealed that all three Lysobacter species tested needed to outnumber their prey for efficient predation, suggesting that they exclusively practiced group predation. In summary, the CFU-based predation assay not only enabled the quantification of prey killing and consumption by Lysobacter spp. but also provided insights into their mode of predation.

Prospecting metal-tolerant rhizobia for phytoremediation of mining soils from Morocco using Anthyllis vulneraria L

The aim of this work was using the legume plant Anthyllis vulneraria L. (ecotype metallicolous) as a trap plant, in order to isolate metal-tolerant rhizobial strains from metal-contaminated soils from Morocco, with pollution indexes spanning three orders of magnitude. As bioindicator, soil bacterial density was inversely correlated to the pollution index. Forty-three bulk soil bacteria and sixty two bacteria from nodules were isolated. The resistance of bacteria from nodules to heavy metals was four to ten times higher than that of bulk soil bacteria, reaching high maximum tolerable concentrations for Cd (2 mM), Cu (2 mM), Pb (7 mM), and Zn (3 mM). Besides, some strains show multiple metal-tolerant abilities and great metal biosorption onto the bacterial surface. Amplification and restriction analysis of ribosomal 16S rDNA (ARDRA) and 16S ribosomal DNA (rDNA) sequencing were used to assess biodiversity and phylogenetic position among bacteria present in nodules. Our results suggest that a great diversity of non-rhizobial bacteria (alpha- and gamma-proteobacteria) colonize nodules of Anthyllis plants in contaminated soils. Taking together, our results evidence that, in polluted soils, rhizobia can be displaced by non-rhizobial (and hence, non-fixing) strains from nodules. Thus, the selection of metal-resistant rhizobia is a key step for using A. vulneraria symbioses for in situ phytoremediation.

Why There Are No Essential Genes on Plasmids

Mobile genetic elements such as plasmids are important for the evolution of prokaryotes. It has been suggested that there are differences between functions coded for by mobile genes and those in the "core" genome and that these differences can be seen between plasmids and chromosomes. In particular, it has been suggested that essential genes, such as those involved in the formation of structural proteins or in basic metabolic functions, are rarely located on plasmids. We model competition between genotypically varying bacteria within a single population to investigate whether selection favors a chromosomal location for essential genes. We find that in general, chromosomal locations for essential genes are indeed favored. This is because the inheritance of chromosomes is more stable than that for plasmids. We define the "degradation" rate as the rate at which chance genetic processes, for example, mutation, deletion, or translocation, render essential genes nonfunctioning. The only way in which plasmids can be a location for functioning essential genes is if chromosomal genes degrade faster than plasmid genes. If the two degradation rates are equal, or if plasmid genes degrade faster than chromosomal genes, functioning essential genes will be found only on chromosomes.

Detection and identification of the phytoplasma associated with China ixeris (Ixeridium chinense) fasciation

BACKGROUND

Phytoplasmas are always associated with symptoms in host plants such as stunting of stems, witches'-broom, yellowing of leaves, formation of sterile-deformed flowers, virescence and phyllody. Recently also symptom of fasciation was reported associated with phytoplasma presence. In the present work, China ixeris fasciation was observed associated with phytoplasmas in Guanzhong Area, Shaanxi, China.

RESULTS

Phytoplasma-like bodies were observed under transmission electron microscope in stem tissues of symptomatic samples. The 16S rRNA operon and tuf genes from phytoplasmas were amplified by PCR assays. Phylogenetic trees were calculated respectively based on sequences data of these two genes. The pattern of restriction fragment length polymorphism (RFLP) was generated via digesting the PCR products of 16S rRNA gene with eight restriction enzymes.

CONCLUSION

The presence of phytoplasma in China ixeris exhibiting fasciation symptom was confirmed by the results of TEM observation and PCR testing. Based on sequence data, phylogeny analysis and actual restriction fragment length polymorphism (RFLP) analysis, the associated phytoplasma was classified as related to 16SrI-C subgroup. This was the first record of phytoplasmas in China ixeris.

Threats and opportunities of plant pathogenic bacteria

Plant pathogenic bacteria can have devastating effects on plant productivity and yield. Nevertheless, because these often soil-dwelling bacteria have evolved to interact with eukaryotes, they generally exhibit a strong adaptivity, a versatile metabolism, and ingenious mechanisms tailored to modify the development of their hosts. Consequently, besides being a threat for agricultural practices, phytopathogens may also represent opportunities for plant production or be useful for specific biotechnological applications. Here, we illustrate this idea by reviewing the pathogenic strategies and the (potential) uses of five very different (hemi)biotrophic plant pathogenic bacteria: Agrobacterium tumefaciens, A. rhizogenes, Rhodococcus fascians, scab-inducing Streptomyces spp., and Pseudomonas syringae.

Differential gene expression in gall midge susceptible rice genotypes revealed by suppressive subtraction hybridization (SSH) cDNA libraries and microarray analysis

BACKGROUND

A major pest of rice, the Asian rice gall midge (Orseolia oryzae Wood-Mason), causes significant yield losses in the rice growing regions throughout Asia. Feeding by the larvae induces susceptible plants to produce nutritive tissue to support growth and development. In order to identify molecular signatures during compatible interactions, genome wide transcriptional profiling was performed using SSH library and microarray technology.

RESULTS

Results revealed up-regulation of genes related to primary metabolism, nutrient relocation, cell organization and DNA synthesis. Concomitantly, defense, secondary metabolism and signaling genes were suppressed. Further, real-time PCR validation of a selected set of 20 genes, in three susceptible rice varieties (TN1, Kavya and Suraksha) during the interaction with the respective virulent gall midge biotypes, also revealed variation in gene expression in Kavya as compared to TN1 and Suraksha.

CONCLUSIONS

These studies showed that virulent insects induced the plants to step up metabolism and transport nutrients to their feeding site and suppressed defense responses. But Kavya rice mounted an elevated defense response during early hours of virulent gall midge infestation, which was over-powered later, resulting in host plant susceptibility.

Three Types of Antibiotics Produced from Rhodococcus erythropolis Strains

A total of 15 Rhodococcus erythropolis strains were characterized as antibiotic producers and classified into three groups according to their antibiotic spectrum and growth compatibility (strains within a group did not inhibit each other's growth). Each of the antibiotic groups exhibited antibiotic activity against a taxonomically different breadth of bacteria: Group I exhibited antibiotic activity against a broad range of Gram-positives; Group II, mainly against the genus Rhodococcus and some other Gram-positives; and Group III, particularly against R. erythropolis. The antibiotic compounds of the strains belonging to Groups I and II were partially purified from liquid culture media. It was found that each group produces distinct antibiotics. In contrast to the diversity of antibiotic activity, the sequence of the 16S rRNA gene in the analyzed 1,440-nt region was found to be identical in all these 15 Rhodococcus strains. In addition to the antibiotic diversity in R. erythropolis strains, we elucidated the diversity in antibiotic-producing species of the genus Rhodococcus. Thus far, only a few antibiotic-producing strains have been reported in Rhodococcus; however, our results demonstrated that the genus comprises diverse antibiotic producers, and is a good source of new antibiotics.

Agrobacterium tumefaciens tumor morphology root plastid localization and preferential usage of hydroxylated prenyl donor is important for efficient gall formation

Upon Agrobacterium tumefaciens infection of a host plant, Tumor morphology root (Tmr) a bacterial adenosine phosphate-isopentenyltransferase (IPT), creates a metabolic bypass in the plastid for direct synthesis of trans-zeatin (tZ) with 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate as the prenyl donor. To understand the biological importance of Tmr function for gall formation, we compared Tmr and Trans-zeatin secretion (Tzs) another agrobacterial IPT that functions within the bacterial cell. Although there is no significant difference in their substrate specificities in vitro, ectopic overexpression of Tzs in Arabidopsis (Arabidopsis thaliana) resulted in the accumulation of comparable amounts of tZ- and N⁶-(Δ²-isopentenyl)adenine (iP)-type cytokinins, whereas overexpression of Tmr resulted exclusively in the accumulation of tZ-type cytokinins. Ectopic expression of Tzs in plant cells yields only small amounts of the polypeptide in plastid-enriched fractions. Obligatory localization of Tzs into Arabidopsis plastid stroma by translational fusions with ferredoxin transit peptide (TP-Tzs) increased the accumulation of both tZ- and iP-type cytokinins. Replacement of tmr on the Ti plasmid with tzs, TP-tzs, or an Arabidopsis plastidic IPT induced the formation of smaller galls than wild-type A. tumefaciens, and they were accompanied by the accumulation of iP-type cytokinins. Tmr is thus specialized for plastid localization and preferential usage of 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate in vivo and is important for efficient gall formation.

Genetic engineering of cytokinin metabolism: prospective way to improve agricultural traits of crop plants

Cytokinins (CKs) are ubiquitous phytohormones that participate in development, morphogenesis and many physiological processes throughout plant kingdom. In higher plants, mutants and transgenic cells and tissues with altered activity of CK metabolic enzymes or perception machinery, have highlighted their crucial involvement in different agriculturally important traits, such as productivity, increased tolerance to various stresses and overall plant morphology. Furthermore, recent precise metabolomic analyses have elucidated the specific occurrence and distinct functions of different CK types in various plant species. Thus, smooth manipulation of active CK levels in a spatial and temporal way could be a very potent tool for plant biotechnology in the future. This review summarises recent advances in cytokinin research ranging from transgenic alteration of CK biosynthetic, degradation and glucosylation activities and CK perception to detailed elucidation of molecular processes, in which CKs work as a trigger in model plants. The first attempts to improve the quality of crop plants, focused on cereals are discussed, together with proposed mechanism of action of the responses involved.

Cytokinins and plant immunity: old foes or new friends?

Cytokinins are plant growth promoting hormones involved in the specification of embryonic cells, maintenance of meristematic cells, shoot formation and development of vasculature. Cytokinins have also emerged as a major factor in plant-microbe interactions during nodule organogenesis and pathogenesis. Microbe-originated cytokinins confer abnormal hypersensitivity of cytokinins to plants, augmenting the sink activity of infected regions. However, recent findings have shed light on a distinct role of cytokinins in plant immune responses. Plant-borne cytokinins systemically induce resistance against pathogen infection. This resistance is orchestrated by endogenous cytokinin and salicylic acid signaling. Here, we discuss how plant- and pathogen-derived cytokinins inversely affect the plant defense response. In addition, we consider the molecular mechanisms underlying plant-derived cytokinin action in plant immunity.

A successful bacterial coup d'état: how Rhodococcus fascians redirects plant development

Rhodococcus fascians is a gram-positive phytopathogen that induces differentiated galls, known as leafy galls, on a wide variety of plants, employing virulence genes located on a linear plasmid. The pathogenic strategy consists of the production of a mixture of six synergistically acting cytokinins that overwhelm the plant's homeostatic mechanisms, ensuring the activation of a signaling cascade that targets the plant cell cycle and directs the newly formed cells to differentiate into shoot meristems. The shoots that are formed upon infection remain immature and never convert to source tissues resulting in the establishment of a nutrient sink that is a niche for the epiphytic and endophytic R. fascians subpopulations. Niche formation is accompanied by modifications of the transcriptome, metabolome, physiology, and morphology of both host and pathogen. Here, we review a decade of research and set the outlines of the molecular basis of the leafy gall syndrome.

Walls around tumours - why plants do not develop cancer

In plants, as in animals, most cells that constitute the organism limit their reproductive potential in order to provide collective support for the immortal germ line. And, as in animals, the mechanisms that restrict the proliferation of somatic cells in plants can fail, leading to tumours. There are intriguing similarities in tumorigenesis between plants and animals, including the involvement of the retinoblastoma pathway as well as overlap with mechanisms that are used for stem cell maintenance. However, plant tumours are less frequent and are not as lethal as those in animals. We argue that fundamental differences between plant and animal development make it much more difficult for individual plant cells to escape communal controls.

Unusual conservation among genes encoding small secreted salivary gland proteins from a gall midge

BACKGROUND

In most protein-coding genes, greater sequence variation is observed in noncoding regions (introns and untranslated regions) than in coding regions due to selective constraints. During characterization of genes and transcripts encoding small secreted salivary gland proteins (SSSGPs) from the Hessian fly, we found exactly the opposite pattern of conservation in several families of genes: the non-coding regions were highly conserved, but the coding regions were highly variable.

RESULTS

Seven genes from the SSSGP-1 family are clustered as one inverted and six tandem repeats within a 15 kb region of the genome. Except for SSSGP-1A2, a gene that encodes a protein identical to that encoded by SSSGP-1A1, the other six genes consist of a highly diversified, mature protein-coding region as well as highly conserved regions including the promoter, 5'- and 3'-UTRs, a signal peptide coding region, and an intron. This unusual pattern of highly diversified coding regions coupled with highly conserved regions in the rest of the gene was also observed in several other groups of SSSGP-encoding genes or cDNAs. The unusual conservation pattern was also found in some of the SSSGP cDNAs from the Asian rice gall midge, but not from the orange wheat blossom midge. Strong positive selection was one of the forces driving for diversification whereas concerted homogenization was likely a mechanism for sequence conservation.

CONCLUSION

Rapid diversification in mature SSSGPs suggests that the genes are under selection pressure for functional adaptation. The conservation in the noncoding regions of these genes including introns also suggested potential mechanisms for sequence homogenization that are not yet fully understood. This report should be useful for future studies on genetic mechanisms involved in evolution and functional adaptation of parasite genes.

Agrobacterium-produced and exogenous cytokinin-modulated Agrobacterium-mediated plant transformation

Agrobacterium tumefaciens is a plant pathogenic bacterium that causes neoplastic growths, called 'crown gall', via the transfer and integration of transferred DNA (T-DNA) from the bacterium into the plant genome. We characterized an acetosyringone (AS)-induced tumour-inducing (Ti) plasmid gene, tzs (trans-zeatin synthesizing), that is responsible for the synthesis of the plant hormone cytokinin in nopaline-type A. tumefaciens strains. The loss of Tzs protein expression and trans-zeatin secretions by the tzs frameshift (tzs-fs) mutant is associated with reduced tumorigenesis efficiency on white radish stems and reduced transformation efficiencies on Arabidopsis roots. Complementation of the tzs-fs mutant with a wild-type tzs gene restored wild-type levels of trans-zeatin secretions and transformation efficiencies. Exogenous application of cytokinin during infection increased the transient transformation efficiency of Arabidopsis roots infected by strains lacking Tzs, which suggests that the lower transformation efficiency resulted from the lack of Agrobacterium-produced cytokinin. Interestingly, although the tzs-fs mutant displayed reduced tumorigenesis efficiency on several tested plants, the loss of Tzs enhanced tumorigenesis efficiencies on green pepper and cowpea. These data strongly suggest that Tzs, by synthesizing trans-zeatin at early stage(s) of the infection process, modulates plant transformation efficiency by A. tumefaciens.

Rhodococcus fascians impacts plant development through the dynamic fas-mediated production of a cytokinin mix

The phytopathogenic actinomycete Rhodococcus fascians D188 relies mainly on the linear plasmid-encoded fas operon for its virulence. The bacteria secrete six cytokinin bases that synergistically redirect the developmental program of the plant to stimulate proliferation of young shoot tissue, thus establishing a leafy gall as a niche. A yeast-based cytokinin bioassay combined with cytokinin profiling of bacterial mutants revealed that the fas operon is essential for the enhanced production of isopentenyladenine, trans-zeatin, cis-zeatin, and the 2-methylthio derivatives of the zeatins. Cytokinin metabolite data and the demonstration of the enzymatic activities of FasD (isopentenyltransferase), FasE (cytokinin oxidase/dehydrogenase), and FasF (phosphoribohydrolase) led us to propose a pathway for the production of the cytokinin spectrum. Further evaluation of the pathogenicity of different fas mutants and of fas gene expression and cytokinin signal transduction upon infection implied that the secretion of the cytokinin mix is a highly dynamic process, with the consecutive production of a tom initiation wave followed by a maintenance flow.

H-gene-mediated resistance to Hessian fly exhibits features of penetration resistance to fungi

Features shared by host-specific phytophagous insects and biotrophic plant pathogens include gene-for-gene interactions and the ability to induce susceptibility in plants. The Hessian fly shows both. To protect against Hessian fly, grasses have H genes. Avirulent larvae die on H-gene-containing resistant plants but the cause of death is not known. Imaging techniques were used to examine epidermal cells at larval attack sites, comparing four resistant wheat genotypes (H6, H9, H13, and H26) to a susceptible genotype. Present in both resistant and susceptible plants attacked by larvae were small holes in the tangential cell wall, with the size of the holes (0.1 microm in diameter) matching that of the larval mandible. Absent from attacked resistant plants were signs of induced susceptibility, including nutritive tissue and ruptured cell walls. Present in attacked resistant plants were signs of induced resistance, including cell death and fortification of the cell wall. Both presumably limit larval access to food, because the larva feeds on the leaf surface by sucking up liquids released from ruptured cells. Resistance was associated with several subcellular responses, including elaboration of the endoplasmic reticulum-Golgi complex and associated vesicles. Similar responses are observed in plant resistance to fungi, suggesting that "vesicle-associated penetration resistance" also functions against insects.

Ratios of Cells With and Without Virulence Genes in Rhodococcus fascians Populations Correlate with Degrees of Symptom Development

Rhodococcus fascians, a gram-positive phytopathogenic bacterium, causes fasciation and leafy galls on a wide range of monocotyledonous and dicotyledonous plants for which it requires the plasmid-borne fas operon. Strains isolated from symptomatic plants over a 20-year-period exhibited a high degree of variability when their virulence was assessed on garden pea seedlings. Polymerase chain reaction amplification of the fas-1 and fasR virulence genes from randomly chosen single colonies showed that many strains consisted of two subpopulations, of which one had lost these genes. Inoculation of pea seedlings with mixtures of fas-1-positive and -negative cells that originated from the same strain demonstrated a strong correlation (Pearson's r ≥ 0.9205) between the proportion of cells in the inoculum carrying the fas-1 gene and the severity of disease symptoms. The minimal concentration of fas-1-positive cells required for the development of small lateral shoots on pea seedlings was 2.5 × 10⁴ CFU/ml (P ≤ 0.008), while the overall suppression of main stem growth was observed at 2.5 × 10⁵ CFU/ml (P ≤ 0.019). These observations underline that care should be taken when the virulence of clinical R. fascians strains is evaluated.