Epigenetic stress memory in gymnosperms

Gymnosperms are long-lived, cone-bearing seed plants that include some of the most ancient extant plant species. These relict land plants have evolved to survive in habitats marked by chronic or episodic stress. Their ability to thrive in these environments is partly due to their phenotypic flexibility, and epigenetic regulation likely plays a crucial part in this plasticity. We review the current knowledge on abiotic and biotic stress memory in gymnosperms and the possible epigenetic mechanisms underlying long-term phenotypic adaptations. We also discuss recent technological improvements and new experimental possibilities that likely will advance our understanding of epigenetic regulation in these ancient and hard-to-study plants.

Effects of combined drought and pathogen stress on growth, resistance and gene expression in young Norway spruce trees

Drought-induced mortality is a major direct effect of climate change on tree health, but drought can also affect trees indirectly by altering their susceptibility to pathogens. Here, we report how a combination of mild or severe drought and pathogen infection affected the growth, pathogen resistance and gene expression in potted 5-year-old Norway spruce trees [Picea abies (L.) Karst.]. After 5 weeks of drought, trees were inoculated with the fungal pathogen Endoconidiophora polonica. Combined drought-pathogen stress over the next 8 weeks led to significant reductions in the growth of drought-treated trees relative to well-watered trees and more so in trees subjected to severe drought. Belowground, growth of the smallest fine roots was most affected. Aboveground, shoot diameter change was most sensitive to the combined stress, followed by shoot length growth and twig biomass. Both drought-related and some resistance-related genes were upregulated in bark samples collected after 5 weeks of drought (but before pathogen infection), and gene expression levels scaled with the intensity of drought stress. Trees subjected to severe drought were much more susceptible to pathogen infection than well-watered trees or trees subjected to mild drought. Overall, our results show that mild drought stress may increase the tree resistance to pathogen infection by upregulating resistance-related genes. Severe drought stress, on the other hand, decreased tree resistance. Because drought episodes are expected to become more frequent with climate change, combined effects of drought and pathogen stress should be studied in more detail to understand how these stressors interactively influence tree susceptibility to pests and pathogens.

Warmer temperature during asexual reproduction induce methylome, transcriptomic, and lasting phenotypic changes in Fragaria vesca ecotypes

Plants must adapt with increasing speed to global warming to maintain their fitness. One rapid adaptation mechanism is epigenetic memory, which may provide organisms sufficient time to adapt to climate change. We studied how the perennial Fragaria vesca adapted to warmer temperatures (28°C vs. 18°C) over three asexual generations. Differences in flowering time, stolon number, and petiole length were induced by warmer temperature in one or more ecotypes after three asexual generations and persisted in a common garden environment. Induced methylome changes differed between the four ecotypes from Norway, Iceland, Italy, and Spain, but shared methylome responses were also identified. Most differentially methylated regions (DMRs) occurred in the CHG context, and most CHG and CHH DMRs were hypermethylated at the warmer temperature. In eight CHG DMR peaks, a highly similar methylation pattern could be observed between ecotypes. On average, 13% of the differentially methylated genes between ecotypes also showed a temperature-induced change in gene expression. We observed ecotype-specific methylation and expression patterns for genes related to gibberellin metabolism, flowering time, and epigenetic mechanisms. Furthermore, we observed a negative correlation with gene expression when repetitive elements were found near (±2 kb) or inside genes. In conclusion, lasting phenotypic changes indicative of an epigenetic memory were induced by warmer temperature and were accompanied by changes in DNA methylation patterns. Both shared methylation patterns and transcriptome differences between F. vesca accessions were observed, indicating that DNA methylation may be involved in both general and ecotype-specific phenotypic variation.

Epitype-inducing temperatures drive DNA methylation changes during somatic embryogenesis in the long-lived gymnosperm Norway spruce

An epigenetic memory of the temperature sum experienced during embryogenesis is part of the climatic adaptation strategy of the long-lived gymnosperm Norway spruce. This memory has a lasting effect on the timing of bud phenology and frost tolerance in the resulting epitype trees. The epigenetic memory is well characterized phenotypically and at the transcriptome level, but to what extent DNA methylation changes are involved have not previously been determined. To address this, we analyzed somatic epitype embryos of Norway spruce clones produced at contrasting epitype-inducing conditions (18 and 28°C). We screened for differential DNA methylation in 2744 genes related mainly to the epigenetic machinery, circadian clock, and phenology. Of these genes, 68% displayed differential DNA methylation patterns between contrasting epitype embryos in at least one methylation context (CpG, CHG, CHH). Several genes related to the epigenetic machinery (e.g., DNA methyltransferases, ARGONAUTE) and the control of bud phenology (FTL genes) were differentially methylated. This indicates that the epitype-inducing temperature conditions induce an epigenetic memory involving specific DNA methylation changes in Norway spruce.

Major transcriptomic differences are induced by warmer temperature conditions experienced during asexual and sexual reproduction in Fragaria vesca ecotypes

A major challenge for plants in a rapidly changing climate is to adapt to rising temperatures. Some plants adapt to temperature conditions by generating an epigenetic memory that can be transmitted both meiotically and mitotically. Such epigenetic memories may increase phenotypic variation to global warming and provide time for adaptation to occur through classical genetic selection. The goal of this study was to understand how warmer temperature conditions experienced during sexual and asexual reproduction affect the transcriptomes of different strawberry (Fragaria vesca) ecotypes. We let four European F. vesca ecotypes reproduce at two contrasting temperatures (18 and 28°C), either asexually through stolon formation for several generations, or sexually by seeds (achenes). We then analyzed the transcriptome of unfolding leaves, with emphasis on differential expression of genes belonging to the epigenetic machinery. For asexually reproduced plants we found a general transcriptomic response to temperature conditions but for sexually reproduced plants we found less significant responses. We predicted several splicing isoforms for important genes (e.g. a SOC1, LHY, and SVP homolog), and found significantly more differentially presented splicing event variants following asexual vs. sexual reproduction. This difference could be due to the stochastic character of recombination during meiosis or to differential creation or erasure of epigenetic marks during embryogenesis and seed development. Strikingly, very few differentially expressed genes were shared between ecotypes, perhaps because ecotypes differ greatly both genetically and epigenetically. Genes related to the epigenetic machinery were predominantly upregulated at 28°C during asexual reproduction but downregulated after sexual reproduction, indicating that temperature-induced change affects the epigenetic machinery differently during the two types of reproduction.

Methyl jasmonate, salicylic acid, and oxalic acid affects growth, inducible defenses, and pine weevil resistance in Norway spruce

The large pine weevil (Hylobius abietis) is a major regeneration pest in commercial forestry. Pesticide application has historically been the preferred control method, but pesticides are now being phased out in several countries for environmental reasons. There is, thus, a need for alternative plant protection strategies. We applied methyl jasmonate (MeJA), salicylic acid (SA) or oxalic acid (OxA) on the stem of 2-year-old Norway spruce (Picea abies) plants to determine effects on inducible defenses and plant growth. Anatomical examination of stem cross-sections 9 weeks after application of 100 mM MeJA revealed massive formation of traumatic resin ducts and greatly reduced sapwood growth. Application of high concentrations of SA or OxA (500 and 200 mM, respectively) induced much weaker physiological responses than 100 mM MeJA. All three treatments reduced plant height growth significantly, but the reduction was larger for MeJA (~55%) than for SA and OxA (34-35%). Lower MeJA concentrations (5-50 mM) induced comparable traumatic resin duct formation as the high MeJA concentration but caused moderate (and non-significant) reductions in plant growth. Two-year-old spruce plants treated with 100 mM MeJA showed reduced mortality after exposure to pine weevils in the field, and this enhanced resistance-effect was statistically significant for three years after treatment.

Methylome, transcriptome, and phenotype changes induced by temperature conditions experienced during sexual reproduction in Fragaria vesca

Temperature conditions experienced during embryogenesis and seed development may induce epigenetic changes that increase phenotypic variation in plants. Here we investigate if embryogenesis and seed development at two different temperatures (28 vs. 18°C) result in lasting phenotypic effects and DNA methylation changes in woodland strawberry (Fragaria vesca). Using five European ecotypes from Spain (ES12), Iceland (ICE2), Italy (IT4), and Norway (NOR2 and NOR29), we found statistically significant differences between plants from seeds produced at 18 or 28°C in three of four phenotypic features investigated under common garden conditions. This indicates the establishment of a temperature-induced epigenetic memory-like response during embryogenesis and seed development. The memory effect was significant in two ecotypes: in NOR2 flowering time, number of growth points and petiole length were affected, and in ES12 number of growth points was affected. This indicates that genetic differences between ecotypes in their epigenetic machinery, or other allelic differences, impact this type of plasticity. We observed statistically significant differences between ecotypes in DNA methylation marks in repetitive elements, pseudogenes, and genic elements. Leaf transcriptomes were also affected by embryonic temperature in an ecotype-specific manner. Although we observed significant and lasting phenotypic change in at least some ecotypes, there was considerable variation in DNA methylation between individual plants within each temperature treatment. This within-treatment variability in DNA methylation marks in F. vesca progeny may partly be a result of allelic redistribution from recombination during meiosis and subsequent epigenetic reprogramming during embryogenesis.

Long-lasting memory of jasmonic acid-dependent immunity requires DNA demethylation and ARGONAUTE1

Stress can have long-lasting impacts on plants. Here we report the long-term effects of the stress hormone jasmonic acid (JA) on the defence phenotype, transcriptome and DNA methylome of Arabidopsis. Three weeks after transient JA signalling, 5-week-old plants retained induced resistance (IR) against herbivory but showed increased susceptibility to pathogens. Transcriptome analysis revealed long-term priming and/or upregulation of JA-dependent defence genes but repression of ethylene- and salicylic acid-dependent genes. Long-term JA-IR was associated with shifts in glucosinolate composition and required MYC2/3/4 transcription factors, RNA-directed DNA methylation, the DNA demethylase ROS1 and the small RNA (sRNA)-binding protein AGO1. Although methylome analysis did not reveal consistent changes in DNA methylation near MYC2/3/4-controlled genes, JA-treated plants were specifically enriched with hypomethylated ATREP2 transposable elements (TEs). Epigenomic characterization of mutants and transgenic lines revealed that ATREP2 TEs are regulated by RdDM and ROS1 and produce 21 nt sRNAs that bind to nuclear AGO1. Since ATREP2 TEs are enriched with sequences from IR-related defence genes, our results suggest that AGO1-associated sRNAs from hypomethylated ATREP2 TEs trans-regulate long-lasting memory of JA-dependent immunity.

Transcriptional profiling of defense responses to Botrytis cinerea infection in leaves of Fragaria vesca plants soil-drenched with β-aminobutyric acid

Grey mold caused by the necrotrophic fungal pathogen Botrytis cinerea can affect leaves, flowers, and berries of strawberry, causing severe pre- and postharvest damage. The defense elicitor β-aminobutyric acid (BABA) is reported to induce resistance against B. cinerea and many other pathogens in several crop plants. Surprisingly, BABA soil drench of woodland strawberry (Fragaria vesca) plants two days before B. cinerea inoculation caused increased infection in leaf tissues, suggesting that BABA induce systemic susceptibility in F. vesca. To understand the molecular mechanisms involved in B. cinerea susceptibility in leaves of F. vesca plants soil drenched with BABA, we used RNA sequencing to characterize the transcriptional reprogramming 24 h post-inoculation. The number of differentially expressed genes (DEGs) in infected vs. uninfected leaf tissue in BABA-treated plants was 5205 (2237 upregulated and 2968 downregulated). Upregulated genes were involved in pathogen recognition, defense response signaling, and biosynthesis of secondary metabolites (terpenoid and phenylpropanoid pathways), while downregulated genes were involved in photosynthesis and response to auxin. In control plants not treated with BABA, we found a total of 5300 DEGs (2461 upregulated and 2839 downregulated) after infection. Most of these corresponded to those in infected leaves of BABA-treated plants but a small subset of DEGs, including genes involved in 'response to biologic stimulus', 'photosynthesis' and 'chlorophyll biosynthesis and metabolism', differed significantly between treatments and could play a role in the induced susceptibility of BABA-treated plants.

Transcriptomic changes during the establishment of long-term methyl jasmonate-induced resistance in Norway spruce

Norway spruce (Picea abies) is an economically and ecologically important tree species that grows across northern and central Europe. Treating Norway spruce with jasmonate has long-lasting beneficial effects on tree resistance to damaging pests, such as the European spruce bark beetle Ips typographus and its fungal associates. The (epi)genetic mechanisms involved in such long-lasting jasmonate induced resistance (IR) have gained much recent interest but remain largely unknown. In this study, we treated 2-year-old spruce seedlings with methyl jasmonate (MeJA) and challenged them with the I. typographus vectored necrotrophic fungus Grosmannia penicillata. MeJA treatment reduced the extent of necrotic lesions in the bark 8 weeks after infection and thus elicited long-term IR against the fungus. The transcriptional response of spruce bark to MeJA treatment was analysed over a 4-week time course using mRNA-seq. This analysis provided evidence that MeJA treatment induced a transient upregulation of jasmonic acid, salicylic acid and ethylene biosynthesis genes and downstream signalling genes. Our data also suggests that defence-related genes are induced while genes related to growth are repressed by methyl jasmonate treatment. These results provide new clues about the potential underpinning mechanisms and costs associated with long-term MeJA-IR in Norway spruce.

A highly-contiguous genome assembly of the Eurasian spruce bark beetle, Ips typographus, provides insight into a major forest pest

Conifer-feeding bark beetles are important herbivores and decomposers in forest ecosystems. These species complete their life cycle in nutritionally poor substrates and some can kill enormous numbers of trees during population outbreaks. The Eurasian spruce bark beetle (Ips typographus) can destroy >100 million m3 of spruce in a single year. We report a 236.8 Mb I. typographus genome assembly using PacBio long-read sequencing. The final phased assembly has a contig N50 of 6.65 Mb in 272 contigs and is predicted to contain 23,923 protein-coding genes. We reveal expanded gene families associated with plant cell wall degradation, including pectinases, aspartyl proteases, and glycosyl hydrolases. This genome sequence from the genus Ips provides timely resources to address questions about the evolutionary biology of the true weevils (Curculionidae), one of the most species-rich animal families. In forests of today, increasingly stressed by global warming, this draft genome may assist in developing pest control strategies to mitigate outbreaks.

Fungal Interactions and Host Tree Preferences in the Spruce Bark Beetle Ips typographus

The spruce bark beetle Ips typographus is the most damaging pest in European spruce forests and has caused great ecological and economic disturbances in recent years. Although native to Eurasia, I. typographus has been intercepted more than 200 times in North America and could establish there as an exotic pest if it can find suitable host trees. Using in vitro bioassays, we compared the preference of I. typographus for its coevolved historical host Norway spruce (Picea abies) and two non-coevolved (naïve) North American hosts: black spruce (Picea mariana) and white spruce (Picea glauca). Additionally, we tested how I. typographus responded to its own fungal associates (conspecific fungi) and to fungi vectored by the North American spruce beetle Dendroctonus rufipennis (allospecific fungi). All tested fungi were grown on both historical and naïve host bark media. In a four-choice Petri dish bioassay, I. typographus readily tunneled into bark medium from each of the three spruce species and showed no preference for the historical host over the naïve hosts. Additionally, the beetles showed a clear preference for bark media colonized by fungi and made longer tunnels in fungus-colonized media compared to fungus-free media. The preference for fungus-colonized media did not depend on whether the medium was colonized by conspecific or allospecific fungi. Furthermore, olfactometer bioassays demonstrated that beetles were strongly attracted toward volatiles emitted by both con- and allospecific fungi. Collectively, these results suggest that I. typographus could thrive in evolutionary naïve spruce hosts if it becomes established in North America. Also, I. typographus could probably form and maintain new associations with local allospecific fungi that might increase beetle fitness in naïve host trees.

A highly contiguous genome assembly of a major forest pest, the Eurasian spruce bark beetleIps typographus.. [PMID: 0 DOI: 10.1101/2020.11.28.401976]

The Eurasian spruce bark beetle (Ips typographus[L.]), is a major killer of spruce forests across the Palearctic. During epidemics, it can destroy over 100 million cubic meters of spruce trees in a single year. Here we report a 236 Mb, highly contiguousI. typographusgenome assembly using PacBio long-read sequencing. The final phased assembly had a contig N50of 6.65 Mb in 272 contigs and was predicted to contain 23,923 protein-coding genes. Comparative genomic analysis revealed expanded gene families associated with plant cell wall degradation, including pectinases, aspartyl proteases, and glycosyl hydrolases. In today’s forests, increasingly stressed by global warming, this resource can assist in mitigating bark beetle outbreaks by developing novel pest control strategies. Further, this first whole-genome sequence from the genusIpsprovides timely resources to address important questions about the evolutionary biology and ecology of Curculionidae, the true weevils, one of the largest animal families.

Molecular underpinnings of methyl jasmonate-induced resistance in Norway spruce

In response to various stimuli, plants acquire resistance against pests and/or pathogens. Such acquired or induced resistance allows plants to rapidly adapt to their environment. Spraying the bark of mature Norway spruce (Picea abies) trees with the phytohormone methyl jasmonate (MeJA) enhances resistance to tree-killing bark beetles and their associated phytopathogenic fungi. Analysis of spruce chemical defenses and beetle colonization success suggests that MeJA treatment both directly induces immune responses and primes inducible defenses for a faster and stronger response to subsequent beetle attack. We used metabolite and transcriptome profiling to explore the mechanisms underlying MeJA-induced resistance in Norway spruce. We demonstrated that MeJA treatment caused substantial changes in the bark transcriptional response to a triggering stress (mechanical wounding). Profiling of mRNA expression showed a suite of spruce inducible defenses are primed following MeJA treatment. Although monoterpenes and diterpene resin acids increased more rapidly after wounding in MeJA-treated than control bark, expression of their biosynthesis genes did not. We suggest that priming of inducible defenses is part of a complex mixture of defense responses that underpins the increased resistance against bark beetle colonization observed in Norway spruce. This study provides the most detailed insights yet into the mechanisms underlying induced resistance in a long-lived gymnosperm.

Priming of inducible defenses protects Norway spruce against tree-killing bark beetles

Plants can form an immunological memory known as defense priming, whereby exposure to a priming stimulus enables quicker or stronger response to subsequent attack by pests and pathogens. Such priming of inducible defenses provides increased protection and reduces allocation costs of defense. Defense priming has been widely studied for short-lived model plants such as Arabidopsis, but little is known about this phenomenon in long-lived plants like spruce. We compared the effects of pretreatment with sublethal fungal inoculations or application of the phytohormone methyl jasmonate (MeJA) on the resistance of 48-year-old Norway spruce (Picea abies) trees to mass attack by a tree-killing bark beetle beginning 35 days later. Bark beetles heavily infested and killed untreated trees but largely avoided fungus-inoculated trees and MeJA-treated trees. Quantification of defensive terpenes at the time of bark beetle attack showed fungal inoculation induced 91-fold higher terpene concentrations compared with untreated trees, whereas application of MeJA did not significantly increase terpenes. These results indicate that resistance in fungus-inoculated trees is a result of direct induction of defenses, whereas resistance in MeJA-treated trees is due to defense priming. This work extends our knowledge of defense priming from model plants to an ecologically important tree species.

Surviving in a Hostile World: Plant Strategies to Resist Pests and Diseases

As primary producers, plants are under constant pressure to defend themselves against potentially deadly pathogens and herbivores. In this review, we describe short- and long-term strategies that enable plants to cope with these stresses. Apart from internal immunological strategies that involve physiological and (epi)genetic modifications at the cellular level, plants also employ external strategies that rely on recruitment of beneficial organisms. We discuss these strategies along a gradient of increasing timescales, ranging from rapid immune responses that are initiated within seconds to (epi)genetic adaptations that occur over multiple plant generations. We cover the latest insights into the mechanistic and evolutionary underpinnings of these strategies and present explanatory models. Finally, we discuss how knowledge from short-lived model species can be translated to economically and ecologically important perennials to exploit adaptive plant strategies and mitigate future impacts of pests and diseases in an increasingly interconnected and changing world.

Convergent evolution of semiochemicals across Kingdoms: bark beetles and their fungal symbionts

Convergent evolution of semiochemical use in organisms from different Kingdoms is a rarely described phenomenon. Tree-killing bark beetles vector numerous symbiotic blue-stain fungi that help the beetles colonize healthy trees. Here we show for the first time that some of these fungi are able to biosynthesize bicyclic ketals that are pheromones and other semiochemicals of bark beetles. Volatile emissions of five common bark beetle symbionts were investigated by gas chromatography-mass spectrometry. When grown on fresh Norway spruce bark the fungi emitted three well-known bark beetle aggregation pheromones and semiochemicals (exo-brevicomin, endo-brevicomin and trans-conophthorin) and two structurally related semiochemical candidates (exo-1,3-dimethyl-2,9-dioxabicyclo[3.3.1]nonane and endo-1,3-dimethyl-2,9-dioxabicyclo[3.3.1]nonane) that elicited electroantennogram responses in the spruce bark beetle Ips typographus. When grown on malt agar with ¹³C d-Glucose, the fungus Grosmannia europhioides incorporated ¹³C into exo-brevicomin and trans-conophthorin. The enantiomeric compositions of the fungus-produced ketals closely matched those previously reported from bark beetles. The production of structurally complex bark beetle pheromones by symbiotic fungi indicates cross-kingdom convergent evolution of signal use in this system. This signaling is susceptible to disruption, providing potential new targets for pest control in conifer forests and plantations.

Bark Beetle-Associated Blue-Stain Fungi Increase Antioxidant Enzyme Activities and Monoterpene Concentrations in Pinus yunnanensis

Yunnan pine is the most important tree species in SW China in both economical and ecological terms, but it is often killed by pine shoot beetles (Tomicus spp.). Tomicus beetles are secondary pests in temperate regions and the aggressiveness of the beetles in SW China is considered to be due to the warm subtropical climates as well as the beetles' virulent fungal associates. Here, we assessed the virulence of three blue-stain fungi (Leptographium wushanense, L. sinense and Ophiostoma canum) associated with pine shoot beetles to Yunnan pine (Pinus yunnanensis) in SW China. Following fungal inoculation, we measured necrotic lesion lengths, antioxidant enzyme activities and monoterpene concentrations in the stem phloem of Yunnan pine. Leptographium wushanense induced twice as long lesions as L. sinense and O. canum, and all three fungi induced significantly longer lesions than sterile agar control inoculations. The activity of three tested antioxidant enzymes (peroxidase, polyphenol oxidase, and superoxide dismutase) increased after both fungal inoculation and control inoculation. However, L. wushanense and L. sinense generally caused a greater increase in enzyme activities than O. canum and the control treatment. Fungal inoculation induced stronger increases in six major monoterpenes than the control treatment, but the difference was significant only for some fungus-monoterpene combinations. Overall, our results show that L. wushanense and L. sinense elicit stronger defense responses and thus are more virulent to Yunnan pine than O. canum. The two Leptographium species may thus contribute to the aggressiveness of their beetle vectors and could damage Yunnan pine across SW China if they spread from the restricted geographical area they have been found in so far.

Effects of prolonged drought on the anatomy of sun and shade needles in young Norway spruce trees

Predicted increases in the frequency and duration of drought are expected to negatively affect tree vitality, but we know little about how water shortage will influence needle anatomy and thereby the trees' photosynthetic and hydraulic capacity. In this study, we evaluated anatomical changes in sun and shade needles of 20-year-old Norway spruce trees exposed to artificial drought stress. Canopy position was found to be important for needle structure, as sun needles had significantly higher values than shade needles for all anatomical traits (i.e., cross-sectional needle area, number of tracheids in needle, needle hydraulic conductivity, and tracheid lumen area), except proportion of xylem area per cross-sectional needle area. In sun needles, drought reduced all trait values by 10-40%, whereas in shade needles, only tracheid maximum diameter was reduced by drought. Due to the relatively weaker response of shade needles than sun needles in drought-stressed trees, the difference between the two needle types was reduced by 25% in the drought-stressed trees compared to the control trees. The observed changes in needle anatomy provide new understanding of how Norway spruce adapts to drought stress and may improve predictions of how forests will respond to global climate change.

Using laser micro-dissection and qRT-PCR to analyze cell type-specific gene expression in Norway spruce phloem

The tangentially oriented polyphenolic parenchyma (PP) and radially organized ray parenchyma in the phloem are central in the defense of conifer stems against insects and pathogens. Laser micro-dissection enables examination of cell-specific defense responses. To examine induced defense responses in Norway spruce stems inoculated with the necrotrophic blue-stain fungus Ceratocystis polonica, RNA extracted from laser micro-dissected phloem parenchyma and vascular cambium was analyzed using real-time RT-PCR (qRT-PCR) to profile transcript levels of selected resistance marker genes. The monitored transcripts included three pathogenesis-related proteins (class IV chitinase (CHI4), defensin (SPI1), peroxidase (PX3), two terpene synthesis related proteins (DXPS and LAS), one ethylene biosynthesis related protein (ACS), and a phenylalanine ammonia-lyase (PAL). Three days following inoculation, four genes (CHI4, PAL, PX3, SPI1) were differentially induced in individual cell and tissue types, both close to the inoculation site (5 mm above) and, to a lesser degree, further away (10 mm above). These resistance marker genes were all highly induced in ray parenchyma, supporting the important role of the rays in spruce defense propagation. CHI4 and PAL were also induced in PP cells and in conducting secondary phloem tissues. Our data suggests that different cell types in the secondary phloem of Norway spruce have overlapping but not fully redundant roles in active host defense. Furthermore, the study demonstrates the usefulness of laser micro-dissection coupled with qRT-PCR to characterize gene expression in different cell types of conifer bark.

Conifer stored resources and resistance to a fungus associated with the spruce bark beetle Ips typographus

Bark beetles and associated fungi are among the greatest natural threats to conifers worldwide. Conifers have potent defenses, but resistance to beetles and fungal pathogens may be reduced if tree stored resources are consumed by fungi rather than used for tree defense. Here, we assessed the relationship between tree stored resources and resistance to Ceratocystis polonica, a phytopathogenic fungus vectored by the spruce bark beetle Ips typographus. We measured phloem and sapwood nitrogen, non-structural carbohydrates (NSC), and lipids before and after trees were attacked by I. typographus (vectoring C. polonica) or artificially inoculated with C. polonica alone. Tree resistance was assessed by measuring phloem lesions and the proportion of necrotic phloem around the tree's circumference following attack or inoculation. While initial resource concentrations were unrelated to tree resistance to C. polonica, over time, phloem NSC and sapwood lipids declined in the trees inoculated with C. polonica. Greater resource declines correlated with less resistant trees (trees with larger lesions or more necrotic phloem), suggesting that resource depletion may be caused by fungal consumption rather than tree resistance. Ips typographus may then benefit indirectly from reduced tree defenses caused by fungal resource uptake. Our research on tree stored resources represents a novel way of understanding bark beetle-fungal-conifer interactions.

Localization of phenolics in phloem parenchyma cells of Norway spruce (Picea abies)

Norway spruce (Picea abies) bark contains specialized phloem parenchyma cells that swell and change their contents upon attack by the bark beetle Ips typographus and its microbial associate, the blue stain fungus Ceratocystis polonica. These cells exhibit bright autofluorescence after treatment with standard aldehyde fixatives, and so have been postulated to contain phenolic compounds. Laser microdissection of spruce bark sections combined with cryogenic NMR spectroscopy demonstrated significantly higher concentrations of the stilbene glucoside astringin in phloem parenchyma cells than in adjacent sieve cells. After infection by C. polonica, the flavonoid (+)-catechin also appeared in phloem parenchyma cells and there was a decrease in astringin content compared to cells from uninfected trees. Analysis of whole-bark extracts confirmed the results obtained from the cell extracts and revealed a significant increase in dimeric stilbene glucosides, both astringin and isorhapontin derivatives (piceasides A to H), in fungus-infected versus uninfected bark that might explain the reduction in stilbene monomers. Phloem parenchyma cells thus appear to be a principal site of phenolic accumulation in spruce bark.

Inducibility of chemical defenses in Norway spruce bark is correlated with unsuccessful mass attacks by the spruce bark beetle

Secondary attraction to aggregation pheromones plays a central role in the host colonization behavior of the European spruce bark beetle Ips typographus. However, it is largely unknown how the beetles pioneering an attack locate suitable host trees, and eventually accept or reject them. To find possible biomarkers for host choice by I. typographus, we analyzed the chemistry of 58 Norway spruce (Picea abies) trees that were subsequently either (1) successfully attacked and killed, (2) unsuccessfully attacked, or (3) left unattacked. The trees were sampled before the main beetle flight in a natural Norway spruce-dominated forest. No pheromones were used to attract beetles to the experimental trees. To test the trees' defense potential, each tree was treated in a local area with the defense hormone methyl jasmonate (MeJ), and treated and untreated bark were analyzed for 66 different compounds, including terpenes, phenolics and alkaloids. The chemistry of MeJ-treated bark correlated strongly with the success of I. typographus attack, revealing major chemical differences between killed trees and unsuccessfully attacked trees. Surviving trees produced significantly higher amounts of most of the 39 analyzed mono-, sesqui-, and diterpenes and of 4 of 20 phenolics. Alkaloids showed no clear pattern. Differences in untreated bark were less pronounced, where only 1,8-cineole and (-)-limonene were significantly higher in unsuccessfully attacked trees. Our results show that the potential of individual P. abies trees for inducing defense compounds upon I. typographus attack may partly determine tree resistance to this bark beetle by inhibiting its mass attack.

Local and systemic changes in expression of resistance genes, NB-LRR genes and their putative microRNAs in Norway spruce after wounding and inoculation with the pathogen Ceratocystis polonica

BACKGROUND

NB-LRR resistance proteins are involved in recognizing pathogens and other exogenous stressors in plants. Resistance proteins are the first step in induced defence responses and a better understanding of their regulation is important to understand the mechanisms of plant defence. Much of the post-transcriptional regulation in plants is controlled by microRNAs (miRNA). We examined the expression of five Norway spruce miRNA that may regulate NB-LRR related transcripts in secondary phloem (bark) of resistant Norway spruce after wounding and inoculation with the necrotrophic blue stain fungus Ceratocystis polonica.

RESULTS

The plants of this clone recovered from both the pathogen inoculations and wounding alone. We found local and systemic induction of the resistance marker genes PaChi4, PaPAL and PaPX3 indicative of an effective induced host defence response. There were minor local and systemic changes in the expression of five miRNAs and 21 NB-LRRs between healthy and treated plants. Only five putative NB-LRRs (PaLRR1, PaLRR3, PaLRR14, PaLRR15 and PaLRR16) showed significant increases greater than two-fold as a local response to C. polonica. Of all NB-LRRs only PaLRR3, the most highly differentially regulated NB-LRR, showed a significant increase also due to wounding. The five miRNAs showed indications of an initial local and systemic down-regulation at day 1, followed by a later increase up to and beyond the constitutive levels at day 6. However, the initial down-regulation was significant only for miR3693 and miR3705.

CONCLUSIONS

Overall, local and systemic expression changes were evident only for the established resistance marker genes and PaLRR3. The minor expression changes observed both for the followed miRNAs and their predicted NB-LRR targets suggest that the expression of most NB-LRR genes are maintained close to their constitutive levels in stressed and healthy Norway spruce plants.

Induction of isoprenyl diphosphate synthases, plant hormones and defense signalling genes correlates with traumatic resin duct formation in Norway spruce (Picea abies)

Norway spruce (Picea abies) defends itself against herbivores and pathogens by formation of traumatic resin ducts filled with terpenoid-based oleoresin. An important group of enzymes in terpenoid biosynthesis are the short-chain isoprenyl diphosphate synthases which produce geranyl diphosphate (C(10)), farnesyl diphosphate (C(15)), and geranylgeranyl diphosphate (C(20)) as precursors of monoterpenes, sesquiterpenes, and diterpene resin acids, respectively. After treatment with methyl jasmonate (MJ) we investigated the expression of all isoprenyl diphosphate synthase genes characterized to date from Norway spruce and correlated this with formation of traumatic resin ducts and terpene accumulation. Formation of traumatic resin ducts correlated with higher amounts of monoterpenes, sesquiterpenes and diterpene resin acids and an upregulation of isoprenyl diphosphate synthase genes producing geranyl diphosphate or geranylgeranyl diphosphate. Among defense hormones, jasmonate and jasmonate-isoleucine conjugate accumulated to higher levels in trees with extensive traumatic resin duct formation, whereas salicylate did not. Jasmonate and ethylene are likely to both be involved in formation of traumatic resin ducts based on elevated transcripts of genes encoding lipoxygenase and 1-aminocyclopropane-1-carboxylic acid oxidase associated with resin duct formation. Other genes involved in defense signalling in other systems, mitogen-activated protein kinase3 and nonexpressor of pathogenesis-related gene1, were also associated with traumatic resin duct formation. These responses were detected not only at the site of MJ treatment, but also systemically up to 60 cm above the site of treatment on the trunk.

Induced terpene accumulation in Norway spruce inhibits bark beetle colonization in a dose-dependent manner

Background

Tree-killing bark beetles (Coleoptera, Scolytinae) are among the most economically and ecologically important forest pests in the northern hemisphere. Induction of terpenoid-based oleoresin has long been considered important in conifer defense against bark beetles, but it has been difficult to demonstrate a direct correlation between terpene levels and resistance to bark beetle colonization.

Methods

To test for inhibitory effects of induced terpenes on colonization by the spruce bark beetle Ips typographus (L.) we inoculated 20 mature Norway spruce Picea abies (L.) Karsten trees with a virulent fungus associated with the beetle, Ceratocystis polonica (Siem.) C. Moreau, and investigated induced terpene levels and beetle colonization in the bark.

Results

Fungal inoculation induced very strong and highly variable terpene accumulation 35 days after inoculation. Trees with high induced terpene levels (n = 7) had only 4.9% as many beetle attacks (5.1 vs. 103.5 attacks m⁻²) and 2.6% as much gallery length (0.029 m m⁻² vs. 1.11 m m⁻²) as trees with low terpene levels (n = 6). There was a highly significant rank correlation between terpene levels at day 35 and beetle colonization in individual trees. The relationship between induced terpene levels and beetle colonization was not linear but thresholded: above a low threshold concentration of ∼100 mg terpene g⁻¹ dry phloem trees suffered only moderate beetle colonization, and above a high threshold of ∼200 mg terpene g⁻¹ dry phloem trees were virtually unattacked.

Conclusion/Significance

This is the first study demonstrating a dose-dependent relationship between induced terpenes and tree resistance to bark beetle colonization under field conditions, indicating that terpene induction may be instrumental in tree resistance. This knowledge could be useful for developing management strategies that decrease the impact of tree-killing bark beetles.

Effect of thinning on anatomical adaptations of Norway spruce needles

Conifers and other trees are constantly adapting to changes in light conditions, water/nutrient supply and temperatures by physiological and morphological modifications of their foliage. However, the relationship between physiological processes and anatomical characteristics of foliage has been little explored in trees. In this study we evaluated needle structure and function in Norway spruce families exposed to different light conditions and transpiration regimes. We compared needle characteristics of sun-exposed and shaded current-year needles in a control plot and a thinned plot with 50% reduction in stand density. Whole-tree transpiration rates remained similar across plots, but increased transpiration of lower branches after thinning implies that sun-exposed needles in the thinned plot were subjected to higher water stress than sun-exposed needles in the control plot. In general, morphological and anatomical needle parameters increased with increasing tree height and light intensity. Needle width, needle cross-section area, needle stele area and needle flatness (the ratio of needle thickness to needle width) differed most between the upper and lower canopy. The parameters that were most sensitive to the altered needle water status of the upper canopy after thinning were needle thickness, needle flatness and percentage of stele area in needle area. These results show that studies comparing needle structure or function between tree species should consider not only tree height and light gradients, but also needle water status. Unaccounted for differences in needle water status may have contributed to the variable relationship between needle structure and irradiance that has been observed among conifers.

Host resistance elicited by methyl jasmonate reduces emission of aggregation pheromones by the spruce bark beetle, Ips typographus

We treated Norway spruce (Picea abies) stems with methyl jasmonate (MeJA) to determine possible quantitative and qualitative effects of induced tree defenses on pheromone emission by the spruce bark beetle Ips typographus. We measured the amounts of 2-methyl-3-buten-2-ol and (S)-cis-verbenol, the two main components of the beetle's aggregation pheromone, released from beetle entrance holes, along with phloem terpene content and beetle performance in MeJA-treated and untreated Norway spruce logs. As expected, phloem terpene levels were higher and beetle tunnel length was shorter (an indication of poor performance) in MeJA-treated logs relative to untreated logs. Parallel to the higher phloem terpene content and poorer beetle performance, beetles in MeJA-treated logs released significantly less 2-methyl-3-buten-2-ol and (S)-cis-verbenol, and the ratio between the two pheromone components was significantly altered. These results suggest that host resistance elicited by MeJA application reduces pheromone emission by I. typographus and alters the critical ratio between the two main pheromone components needed to elicit aggregation. The results also provide a mechanistic explanation for the reduced performance and attractivity observed in earlier studies when bark beetles colonize trees with elicited host defenses, and extend our understanding of the ecological functions of conifer resistance against bark beetles.

The influence of Ceratocystis polonica inoculation and methyl jasmonate application on terpene chemistry of Norway spruce, Picea abies

Constitutive and inducible terpene production is involved in conifer resistance against bark beetles and their associated fungi. In this study 72 Norway spruce (Picea abies) were randomly assigned to methyl jasmonate (MJ) application, inoculation with the bluestain fungus Ceratocystis polonica, or no-treatment control. We investigated terpene levels in the stem bark of the trees before treatment, 30 days and one year after treatment using GC-MS and two-dimensional GC (2D-GC) with a chiral column, and monitored landing and attack rates of the spruce bark beetle, Ips typographus, on the trees by sticky traps and visual inspection. Thirty days after fungal inoculation the absolute amount and relative proportion of (+)-3-carene, sabinene, and terpinolene increased and (+)-alpha-pinene decreased. Spraying the stems with MJ tended to generally increase the concentration of most major terpenes with minor alteration to their relative proportions, but significant increases were only observed for (-)-beta-pinene and (-)-limonene. Fungal inoculation significantly increased the enantiomeric ratio of (-)-alpha-pinene and (-)-limonene 1 month after treatment, whereas MJ only increased that of (-)-limonene. One year after treatment, both MJ and fungal inoculation increased the concentration of most terpenes relative to undisturbed control trees, with significant changes in (-)-beta-pinene, (-)-beta-phellandrene and some other compounds. Terpene levels did not change in untreated stem sections after treatment, and chemical induction by MJ and C. polonica thus seemed to be restricted to the treated stem section. The enantiomeric ratio of (-)-alpha-pinene was significantly higher and the relative proportions of (-)-limonene were significantly lower in trees that were attractive to bark beetles compared to unattractive trees. One month after fungal inoculation, the total amount of diterpenes was significantly higher in putative resistant trees with shorter lesion lengths than in putative susceptible trees with longer lesions. Thus, terpene composition in the stem bark may be related to resistance of Norway spruce against I. typographus and C. polonica.

Methyl jasmonate and oxalic acid treatment of Norway spruce: anatomically based defense responses and increased resistance against fungal infection

To study the effect of chemical pretreatment on conifer resistance, 13-year-old Norway spruce (Picea abies (L.) Karst.) trees were treated with methyl jasmonate (MJ) or oxalic acid (OxA) on the outer bark and inoculated with the pathogenic blue-stain fungus Ceratocystis polonica (Siem.) C. Moreau 4 weeks later. Both chemicals significantly reduced symptoms of fungal infection, but MJ was more effective than OxA (51 versus 18% reduction in length of necrotic lesions in the phloem relative to untreated control trees). Anatomical examination of treated stem tissues showed that MJ induced extensive formation of traumatic resin ducts in the xylem and extra polyphenolic parenchyma (PP) cells in the secondary phloem between the cambium and the regular annual PP cell layer. No traumatic resin ducts were formed after treatment with OxA, and the coverage of extra PP cells in OxA-treated tissues was not significantly higher than in the controls. The anatomically based defense reactions induced by MJ were similar to the reactions observed after pathogen infection, mechanical wounding and bark beetle attack. Neither MJ nor OxA had apparent phytotoxic effects on Norway spruce at the concentrations used, with needle and stem tissues of all trees appearing normal without visible symptoms of toxicity. However, trees treated with MJ had 30% less radial sapwood growth than control trees. In conclusion, MJ treatment of Norway spruce appears to have practical potential as a tool for increasing plant resistance to fungal infection, but with a modest reduction in sapwood growth.

Methyl jasmonate treatment of mature Norway spruce (Picea abies) trees increases the accumulation of terpenoid resin components and protects against infection by Ceratocystis polonica, a bark beetle-associated fungus

When conifers such as Picea abies Karst. (Norway spruce) are attacked by insects or pathogens, they often respond by producing increased quantities of terpenoid oleoresin. This response can be mimicked in young P. abies seedlings by treatment with methyl jasmonate (MJ). In this study, we determined the effects of MJ on terpenoids and other chemical defenses of mature P. abies, and investigated whether this treatment protected trees against attack by the blue-stain fungus Ceratocystis polonica (Siem.) C. Moreau, the most important fungal associate of the spruce bark beetle Ips typographus L. Methyl jasmonate treatment induced the formation of traumatic resin ducts in the developing xylem, enhanced resin flow and stimulated increased accumulation of monoterpenes, sesquiterpenes and diterpene resin acids. However, only minor changes were detected in terpene composition in response to MJ treatment and no changes in soluble phenolic concentration were measured. There was much variability in the timing and degree of response to MJ among clones. The observed chemical and anatomical changes in response to MJ treatment were correlated with increased resistance to C. polonica, suggesting that terpenoid oleoresin may function in defense against this pathogen.

Exogenous application of methyl jasmonate elicits defenses in Norway spruce (Picea abies) and reduces host colonization by the bark beetle Ips typographus

The terpenoid and phenolic constituents of conifers have been implicated in protecting trees from infestation by bark beetles and phytopathogenic fungi, but it has been difficult to prove these defensive roles under natural conditions. We used methyl jasmonate, a well-known inducer of plant defense responses, to manipulate the biochemistry and anatomy of mature Picea abies (Norway spruce) trees and to test their resistance to attack by Ips typographus (the spruce bark beetle). Bark sections of P. abies treated with methyl jasmonate had significantly less I. typographus colonization than bark sections in the controls and exhibited shorter parental galleries and fewer eggs had been deposited. The numbers of beetles that emerged and mean dry weight per beetle were also significantly lower in methyl jasmonate-treated bark. In addition, fewer beetles were attracted to conspecifics tunneling in methyl jasmonate-treated bark. Stem sections of P. abies treated with methyl jasmonate had an increased number of traumatic resin ducts and a higher concentration of terpenes than untreated sections, whereas the concentration of soluble phenolics did not differ between treatments. The increased amount of terpenoid resin present in methyl jasmonate-treated bark could be directly responsible for the observed decrease in I. typographus colonization and reproduction.

Anatomical-based defense responses of Scots pine (Pinus sylvestris) stems to two fungal pathogens

We investigated the cellular responses of stem tissues of mature Scots pine (Pinus sylvestris L.) trees to inoculations with two fungal pathogens. The bark beetle vectored fungus, Leptographium wingfieldii Morelet, induced longer lesions in the bark, stronger swelling of polyphenolic parenchyma cells, more polyphenol accumulation and increased ray parenchyma activity compared with the root rot fungus, Heterobasidion annosum (Fr.) Bref., or mechanical wounding. Axial resin ducts in the xylem are a general feature of the preformed defenses of Scots pine, but there was no clear induction of additional traumatic axial resin ducts in response to wounding or fungal infection. The anatomical responses of Scots pine to pathogen infection were localized to the infection site and were attenuated away from bark lesions. The responses observed in Scots pine were compared with published studies on Norway spruce (Picea abies (L.) Karst.) for which anatomically based defense responses have been well characterized.

Are bark beetle outbreaks less synchronous than forest Lepidoptera outbreaks?

Comparisons of intraspecific spatial synchrony across multiple epidemic insect species can be useful for generating hypotheses about major determinants of population patterns at larger scales. The present study compares patterns of spatial synchrony in outbreaks of six epidemic bark beetle species in North America and Europe. Spatial synchrony among populations of the Eurasian spruce bark beetle Ips typographus was significantly higher than for the other bark beetle species. The spatial synchrony observed in epidemic bark beetles was also compared with previously published patterns of synchrony in outbreaks of defoliating forest Lepidoptera, revealing a marked difference between these two major insect groups. The bark beetles exhibited a generally lower degree of spatial synchrony than the Lepidoptera, possibly because bark beetles are synchronized by different weather variables that are acting on a smaller scale than those affecting the Lepidoptera, or because inherent differences in their dynamics leads to more cyclic oscillations and more synchronous spatial dynamics in the Lepidoptera.

Anatomical and chemical defenses of conifer bark against bark beetles and other pests

Conifers are long-lived organisms, and part of their success is due to their potent defense mechanisms. This review focuses on bark defenses, a front line against organisms trying to reach the nutrient-rich phloem. A major breach of the bark can lead to tree death, as evidenced by the millions of trees killed every year by specialized bark-invading insects. Different defense strategies have arisen in conifer lineages, but the general strategy is one of overlapping constitutive mechanical and chemical defenses overlaid with the capacity to up-regulate additional defenses. The defense strategy incorporates a graded response from 'repel', through 'defend' and 'kill', to 'compartmentalize', depending upon the advance of the invading organism. Using a combination of toxic and polymer chemistry, anatomical structures and their placement, and inducible defenses, conifers have evolved bark defense mechanisms that work against a variety of pests. However, these can be overcome by strategies including aggregation pheromones of bark beetles and introduction of virulent phytopathogens. The defense structures and chemicals in conifer bark are reviewed and questions about their coevolution with bark beetles are discussed.

A PCR-RFLP based diagnostic technique to rapidly identify Seiridium species causing cypress canker

Seiridium cardinale, S. cupressi and S. unicorne represent three distinct species of fungi that cause cankers on Cupressus species and the disease collectively known as cypress canker. These fungi cannot be distinguished reliably from each other using morphological characters or ribosomal DNA sequence data. Here we describe a RFLP assay based on digesting β-tubulin amplicons with a single endonuclease, HaeIII, which easily can be used to distinguish among these three species. This RFLP assay provides an inexpensive and simple means of identifying Seiridium species, which include some of the most serious threats to trees in Cupressaceae.

Induced responses to pathogen infection in Norway spruce phloem: changes in polyphenolic parenchyma cells, chalcone synthase transcript levels and peroxidase activity

Polyphenolic parenchyma cells (PP cells) in Norway spruce (Picea abies (L.) Karst.) stem phloem play important roles in constitutive and inducible defenses. To determine whether anatomical and molecular changes in PP cells are correlated with tree resistance, we infected two Norway spruce clones with the pathogenic fungus Ceratocystis polonica (Siem.) C. Moreau. The fungus induced significantly different lesion lengths in the two clones, indicating that one clone was more resistant to the fungus (short lesions) than the other (long lesions). After infection, the cross-sectional area of PP cells and their vacuolar polyphenol bodies increased in the three most recent annual rings of PP cells in both clones. The more resistant clone had larger PP cells with denser polyphenol bodies than the less resistant clone, whereas the less resistant clone accumulated relatively more polyphenols after infection. Compared with the less resistant clone, the more resistant clone contained higher starch concentrations before infection that were reduced more quickly after infection before returning to original values. Low transcript levels of chalcone synthase were detected in uninfected tissues of both clones, but the levels increased dramatically after infection. Transcript levels were higher and peaked 6 days earlier in the more resistant clone than in the less resistant clone. The activity of at least one highly basic peroxidase isoform was greatly enhanced after infection, and this increase occurred earlier in the more resistant clone.

Inducible anatomical defense responses in Norway spruce stems and their possible role in induced resistance

Norway spruce (Picea abies (L.) Karst.) trees were preinoculated with a sublethal dose of the blue-stain fungus Ceratocystis polonica Siem. (C. Moreau) 1 to 52 weeks before they were mass inoculated with the same fungus. Trees pretreated 1 week before mass inoculation had similar, severe symptoms of fungal infection as the control trees. Pretreatment 3, 6 or 9 weeks before mass inoculation resulted in effective protection of the trees, reducing pathogenic symptoms by 63-90% relative to the control trees, whereas pretreatment 52 weeks before mass inoculation gave intermediate protection (44-71% reduction in symptoms). Thus, pretreatment induced resistance to the blue-stain fungus in Norway spruce by a process that requires more than 1 week to become activated and protects trees for at least one year after pretreatment. Pretreatment induced formation of traumatic resin ducts (TDs) in the sapwood and swelling and proliferation of polyphenolic parenchyma cells (PP cells) in the phloem. Trees pretreated 3-9 weeks before mass inoculation had more TDs and showed greater swelling of existing PP cells than control trees or trees pretreated 1 week before mass inoculation. We conclude that induced disease resistance in Norway spruce is probably associated with PP cell activation and TD induction, because resistance was enhanced within the same time frame as the induction of these defense responses.

A model for peak and width of signaling windows: Ips duplicatus and Chilo partellus pheromone component proportions--does response have a wider window than production?

Pheromone communication systems have a reliable signal with a restricted window of amounts and ratios released and perceived. We propose a model based on a Gaussian response profile that allows a quantification of the response peak (location of optimum) and a measure of the peak width (response window). Interpreting the Gaussian curve, fitted by nonlinear regression (NLR), as a standard normal distribution, the peak location equals the mean (it) and the window width equals 2 x the standard deviation (2sigma). The NLR procedure can provide an objective measure for both peak location and width for a wide range of data sets. Four empirical data sets as well as 10 literature data sets were analyzed. The double-spined spruce engraver, Ips duplicatus, was field tested in four populations to find the optimum proportion for attraction to the two male aggregation pheromone components, ipsdienol (Id) and (E)-myrcenol(EM), ranging from 0 to 100% of Id. Tests in Norway and the Czech Republic confirmed the preference of western populations for a blend between 50 and 90% Id. A population in Inner Mongolia showed a preference for traps with the 10 and 50% Id baits. The NLR fitted values for response peak and width (mu; 2sigma) were: Norway 0.64, 0.73; Czech Republic 0.53, 0.73; NE China 0.77, 0.29; and Inner Mongolia 0.33, 0.50. The signal produced by Norwegian field-collected males had a narrower window width (2sigma = 0.12). Males of the maize stem borer, Chilo partellus, were tested in a flight tunnel for their response to variation in the two major female sex pheromone gland components, (Z)- l1-hexadecenal and the corresponding alcohol (OH). Variation of the alcohol in seven levels from 2 to 29% OH showed the highest male response for 17% OH. For all behavioral steps, the peak of male response was near mu = 0.14, while the window width fell from 2sigma = 0.5 to 0.2 for eight sequential behavioral steps from take-off to copulation. Female production had a similar peak location (mu = 0.13) but a narrower width, 2sigma = 0.14. Literature data from other moth species showed similar patterns, with a wider male response relative to the female production windows. Literature data on response to enantiomer ratios in a hymenopteran and to pheromone amounts in a dipteran were also described by our model. In a bark beetle population (Ips pini), with two hybridizing enantiomeric strains, the production peaks were narrower (0.1) than the response peaks (0.5). Thus, it in general, seems that in the pheromone systems analyzed, the width of the response window (2sigma = 0.1 to 0.8) is larger than that of the production window (2sigma = 0.03 to 0.14), irrespective of the sex of the sender.

Phloem parenchyma cells are involved in local and distant defense responses to fungal inoculation or bark-beetle attack in Norway spruce (Pinaceae)

The anatomical response of Norway spruce bark polyphenolic parenchyma cells (PP cells) to inoculation with the phytopathogenic fungus Ceratocystis polonica and attack by its bark-beetle vector Ips typographus was examined. Fungal inoculation on the periderm surface had no effect, while inoculation just below the periderm or halfway into the phloem (mid-phloem) generated detectable responses within 3 wk. The responses included increase in PP cell size and in periodic acid-Schiff's staining of PP cell phenolics, wound periderm initiation from PP cells, and cambial zone traumatic resin duct formation. Fungi were not seen in samples 3 wk after subperiderm or mid-phloem inoculation, but were found in some samples 6 and 9 wk after mid-phloem inoculation. In contrast, inoculations into the cambium resulted in partial (3 wk) or complete (6 and 9 wk) fungal colonization and death of tissue in the infected area. This indicates that PP cells have defenses capable of inhibiting fungal growth. Samples taken near bark-beetle galleries had similar anatomical responses as inoculated samples, validating the inoculation approach to studying defense responses in spruce. These results show that PP cells represent not only a constitutive defense system, but are also involved in local and remote inducible defenses against fungal and beetle attack.

Induced resistance to pathogenic fungi in norway spruce

Norway spruce (Picea abies) trees (approximately 16 m high) of a single clone were used to study the effects of fungal infection and wounding on induction of resistance to the bark beetle-associated bluestain fungus Ceratocystis polonica. A dose-response experiment was designed involving three different dosages of fungal (fungus and wound) and sterile agar (wound) pretreatment inoculations (10, 50, or 100 inoculations/m(2) on the stem between 0.8 and 2.0 m high). Three weeks after pretreatment, trees were challenged with a massive C. polonica inoculation (400 inoculations/m(2)). Control trees that received no pretreatment were heavily colonized and killed by the challenge inoculation. The high and medium fungal pretreatments reduced subsequent fungal colonization success by 76% to 97% relative to the control, and fungal pretreatments protected the trees much more efficiently than sterile agar pretreatments. The protection was demonstrated to be local and not systemic in a subsequent experiment, where trees were pretreated with the medium fungal dosage on the lower bole and challenge inoculated further up the stem. Protection was also demonstrated to be pathogen nonspecific, as trees that had been pretreated with a medium dosage of the root rot fungus Heterobasidion annosum showed enhanced resistance to challenge inoculation with C. polonica.

Mechanical injury and fungal infection induce acquired resistance in Norway spruce

Norway spruce trees (Picea abies (L.) Karst.) pretreated by wounding and fungal infection showed highly enhanced resistance to a subsequent challenge inoculation with the pathogenic bluestain fungus Ceratocystis polonica (Siem.) C. Moreau. This is the first time the effectiveness of the constitutive and inducible defenses has been shown to depend on prior wounding and infection in conifers, although such acquired resistance has previously been found in several angiosperms. Trees that were pretreated with a combination of 12 bark wounds (1.6 x 10 cm), four fungal inoculations and four sterile inoculations 1-15 days before mass inoculation with C. polonica at 400 inoculations per square meter over a 0.8 m stem section had significantly shorter necroses in the phloem, less bluestained sapwood, and less dead cambium than untreated control trees. Pretreatment with four fungal or sterile inoculations alone did not lead to enhanced resistance. Pretreatment by bark wounding alone seemed to provide an intermediate degree of resistance compared to bark wounding, fungal inoculations and sterile inoculations combined. All trees had a marked increase in the number of resin ducts in the year of inoculation compared with previous years, suggesting that formation of traumatic resin ducts play an important role in the development and maintainance of enhanced resistance.

Growth and virulence of mountain pine beetle associated blue-stain fungi, Ophiostoma clavigerum and Ophiostoma montium

The mountain pine beetle (Dendroctonus ponderosae) is commonly associated with the blue-stain fungi Ophiostoma clavigerum and Ophiostoma montium. Ophiostoma clavigerum is the primary invader of sapwood after beetle infestation and is thought to be the most virulent of the two fungi. Growth of these fungi was studied under oxygen-deficient conditions on malt agar in test tubes and Petri dishes. In addition, growth was studied in phloem and sapwood of young living shore pines (Pinus contorta var. contorta) and western white pines (Pinus monticola) inoculated with fungus in low densities (eight inoculations per tree). In test tubes with limited oxygen O. clavigerum grew for a longer time than O. montium. Both fungi are fast growing on malt agar (maximum growth 4.4-9.0 mm/day), but O. clavigerum grew better at temperatures below 25°C. The rapid growth and the ability to tolerate low oxygen levels may be important adaptations for O. clavigerum as the primary invader of fresh sapwood. However, although O. clavigerum grew better in the phloem of both tree species, there were no differences between the two fungi in their ability to colonize the sapwood of the inoculated trees.Key words: blue-stain fungi, Dendroctonus ponderosae, growth rate, oxygen deficiency, virulence.