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.

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.

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.

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.

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.

Comparative sensitivity to gamma radiation at the organismal, cell and DNA level in young plants of Norway spruce, Scots pine and Arabidopsis thaliana

Persistent DNA damage in gamma-exposed Norway spruce, Scots pine and Arabidopsis thaliana, but persistent adverse effects at the organismal and cellular level in the conifers only. Gamma radiation emitted from natural and anthropogenic sources may have strong negative impact on plants, especially at high dose rates. Although previous studies implied different sensitivity among species, information from comparative studies under standardized conditions is scarce. In this study, sensitivity to gamma radiation was compared in young seedlings of the conifers Scots pine and Norway spruce and the herbaceous Arabidopsis thaliana by exposure to ⁶⁰Co gamma dose rates of 1-540 mGy h^-1 for 144 h, as well as 360 h for A. thaliana. Consistent with slightly less prominent shoot apical meristem, in the conifers growth was significantly inhibited with increasing dose rate ≥ 40 mGy h^-1. Post-irradiation, the conifers showed dose-rate-dependent inhibition of needle and root development consistent with increasingly disorganized apical meristems with increasing dose rate, visible damage and mortality after exposure to ≥ 40 mGy h^-1. Regardless of gamma duration, A. thaliana showed no visible or histological damage or mortality, only delayed lateral root development after ≥ 100 mGy h^-1 and slightly, but transiently delayed post-irradiation reproductive development after ≥ 400 mGy h^-1. In all species dose-rate-dependent DNA damage occurred following ≥ 1-10 mGy h^-1 and was still at a similar level at day 44 post-irradiation. In conclusion, the persistent DNA damage (possible genomic instability) following gamma exposure in all species may suggest that DNA repair is not necessarily mobilized more extensively in A. thaliana than in Norway spruce and Scots pine, and the far higher sensitivity at the organismal and cellular level in the conifers indicates lower tolerance to DNA damage than in A. thaliana.

The epigenetic memory of temperature during embryogenesis modifies the expression of bud burst-related genes in Norway spruce epitypes

Epigenetic memory affects the timing of bud burst phenology and the expression of bud burst-related genes in genetically identical Norway spruce epitypes in a manner usually associated with ecotypes. In Norway spruce, a temperature-dependent epigenetic memory established during embryogenesis affects the timing of bud burst and bud set in a reproducible and predictable manner. We hypothesize that the clinal variation in these phenological traits, which is associated with adaptation to growth under frost-free conditions, has an epigenetic component. In Norway spruce, dehydrins (DHNs) have been associated with extreme frost tolerance. DHN transcript levels decrease gradually prior to flushing, a time when trees are highly sensitive to frost. Furthermore, EARLY BUD BREAK 1 genes (EBB1) and the FT-TFL1-LIKE 2-gene (PaFTL2) were previously suggested to be implied in control of bud phenology. Here we report an analysis of transcript levels of 12 DHNs, 3 EBB1 genes and FTL2 in epitypes of the same genotype generated at different epitype-inducing temperatures, before and during spring bud burst. Earlier flushing of epitypes originating from embryos developed at 18 °C as compared to 28 °C, was associated with differential expression of these genes between epitypes and between buds and last year's needles. The majority of these genes showed significantly different expressions between epitypes in at least one time point. The general trend in DHN expression pattern in buds showed the expected reduction in transcript levels when approaching flushing, whereas, surprisingly, transcript levels peaked later in needles, mainly at the moment of bud burst. Collectively, our results demonstrate that the epigenetic memory of temperature during embryogenesis affects bud burst phenology and expression of the bud burst-related DHN, EBB1 and FTL2 genes in genetically identical Norway spruce epitypes.

Transcriptional profiling of epigenetic regulators in somatic embryos during temperature induced formation of an epigenetic memory in Norway spruce

A significant number of epigenetic regulators were differentially expressed during embryogenesis at different epitype-inducing conditions. Our results support that methylation of DNA and histones, as well as sRNAs, are pivotal for the establishment of the epigenetic memory. As a forest tree species with long generation times, Norway spruce is remarkably well adapted to local environmental conditions despite having recently, from an evolutionary perspective, recolonized large areas following the last glaciation. In this species, there is an enigmatic epigenetic memory of the temperature conditions during embryogenesis that allows rapid adaptation to changing environment. We used a transcriptomic approach to investigate the molecular mechanisms underlying the formation of the epigenetic memory during somatic embryogenesis in Norway spruce. Nine mRNA libraries were prepared from three epitypes of the same genotype resulting from exposure to epitype-inducing temperatures of 18, 23 and 28 °C. RNA-Seq analysis revealed more than 10,000 differentially expressed genes (DEGs). The epitype-inducing conditions during SE were accompanied by marked transcriptomic changes for multiple gene models related to the epigenetic machinery. Out of 735 putative orthologs of epigenetic regulators, 329 were affected by the epitype-inducing temperatures and differentially expressed. The majority of DEGs among the epigenetic regulators was related to DNA and histone methylation, along with sRNA pathways and a range of putative thermosensing and signaling genes. These genes could be the main epigenetic regulators involved in formation of the epigenetic memory. We suggest considerable expansion of gene families of epigenetic regulators in Norway spruce compared to orthologous gene families in Populus and Arabidopsis. Obtained results provide a solid basis for further genome annotation and studies focusing on the importance of these candidate genes for the epigenetic memory formation.

Transcriptional responses of Norway spruce (Picea abies) inner sapwood against Heterobasidion parviporum

The white-rot fungus Heterobasidion parviporum Niemelä & Korhonen establishes a necrotrophic interaction with Norway spruce (Picea abies (L.) H.Karst.) causing root and butt rot and growth losses in living trees. The interaction occurs first with the bark and the outer sapwood, as the pathogen enters the tree via wounds or root-to-root contacts. Later, when the fungus reaches the heartwood, it spreads therein creating a decay column, and the interaction mainly occurs in the inner sapwood where the tree creates a reaction zone. While bark and outer sapwood interactions are well studied, little is known about the nature of the transcriptional responses leading to the creation of a reaction zone. In this study, we sampled bark and sapwood both proximal and distal to the reaction zone in artificially inoculated and naturally infected trees. We quantified gene expression levels of candidate genes in secondary metabolite, hormone biosynthesis and signalling pathways using quantitative polymerase chain reaction. An up-regulation of mainly the phenylpropanoid pathway and jasmonic acid biosynthesis was found at the inoculation site, when inoculations were compared with wounding. We found that transcriptional responses in inner sapwood were similar to those reported upon infection through the bark. Our data suggest that the defence mechanism is induced due to direct fungal contact irrespective of the tissue type. Understanding the nature of these interactions is important when considering tree breeding-based resistance strategies to reduce the spread of the pathogen between and within trees.

DNA quantification of basidiomycetous fungi during storage of logging residues

The demand for bioenergy caused an increased use of logging residues, branches and treetops that were previously left on the ground after harvesting. Residues are stored outdoors in piles and it is unclear to what extent fungi transform this material. Our objective was to quantify the amount of wood degrading fungi during storage using quantitative real-time PCR (qPCR) to detect basidiomycetous DNA in logging residues, a novel approach in this field. We found that the qPCR method was accurate in quantifying the fungal DNA during storage. As the moisture content of the piled logging residues decreased during the storage period, the fungal DNA content also decreased. Scots pine residues contained more fungal DNA than residues from Norway spruce. Loose piles had generally more fungal DNA than bundled ones.

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.

Dehydrin accumulation and extreme low-temperature tolerance in Siberian spruce (Picea obovata)

To investigate the role of dehydrins (DHNs) in extreme low-temperature (LT) tolerance, we sampled needle tissue of Siberian spruce (Picea obovata Ledeb.) from trees growing in an arboretum in Trondheim, Norway from August 2006 to April 2007 and tracked changes in LT tolerance via relative electrolyte leakage. We used western blotting to estimate relative amounts of proteins binding a DHN K-segment antibody, measured relative amounts of nine transcripts for small (<25 kDa) DHNs by quantitative reverse transcription-polymerase chain reaction (PCR) using primers developed for DHN transcripts in a closely related species, Picea abies (L.) Karsten, and isolated and sequenced PCR products for five P. obovata DHNs. Three protein bands of 53, 35 and 33 kDa were detected on western blots of SDS-PAGE-separated protein extracts. The 53-kDa DHN was already present late in the growing season, but accumulated during acclimation, and levels decreased rapidly during deacclimation. The 33- and 35-kDa proteins, identified as Picg5 class DHNs by mass spectrometry, first appeared in detectable amounts late in the acclimation process and remained at detectable levels throughout the period of maximum LT tolerance. Levels of the 53-kDa DHN correlated with two LT tolerance parameters, while results for the 33- and 35-kDa proteins were equivocal due to limited sample size and variation in LT tolerance during the mid-winter period. Three additional bands of 30, 28 and 26 kDa were detected in extracts from needles collected in November 2010 using an immunity-purified antibody. Immunoblotting of two-dimensional gel electrophoresis gels loaded with proteins extracted from October and November samples corroborated the results obtained by SDS-PAGE western blots. One large spot in the 53 kDa range and two trains of spots in the same size range as the 33 and 35 kDa DHNs were detected using the K-segment antibody. Eight of the nine DHN transcripts closely tracked LT tolerance parameters, whereas the ninth DHN transcripts followed a reverse pattern, decreasing during winter and increasing again during deacclimation. Multiple regression models using principal components of the transcripts to predict two different LT tolerance parameters suggest separate but overlapping functions for different DHNs in establishing and maintaining extreme LT tolerance.

Genes associated with lignin degradation in the polyphagous white-rot pathogen Heterobasidion irregulare show substrate-specific regulation

The pathogenic white-rot basidiomycete Heterobasidion irregulare is able to remove lignin and hemicellulose prior to cellulose during the colonization of root and stem xylem of conifer and broadleaf trees. We identified and followed the regulation of expression of genes belonging to families encoding ligninolytic enzymes. In comparison with typical white-rot fungi, the H. irregulare genome has exclusively the short-manganese peroxidase type encoding genes (6 short-MnPs) and thereby a slight contraction in the pool of class II heme-containing peroxidases, but an expansion of the MCO laccases with 17 gene models. Furthermore, the genome shows a versatile set of other oxidoreductase genes putatively involved in lignin oxidation and conversion, including 5 glyoxal oxidases, 19 quinone-oxidoreductases and 12 aryl-alcohol oxidases. Their genetic multiplicity and gene-specific regulation patterns on cultures based on defined lignin, cellulose or Norway spruce lignocellulose substrates suggest divergent specificities and physiological roles for these enzymes. While the short-MnP encoding genes showed similar transcript levels upon fungal growth on heartwood and reaction zone (RZ), a xylem defense tissue rich in phenolic compounds unique to trees, a subset of laccases showed higher gene expression in the RZ cultures. In contrast, other oxidoreductases depending on initial MnP activity showed generally lower transcript levels on RZ than on heartwood. These data suggest that the rate of fungal oxidative conversion of xylem lignin differs between spruce RZ and heartwood. It is conceivable that in RZ part of the oxidoreductase activities of laccases are related to the detoxification of phenolic compounds involved in host-defense. Expression of the several short-MnP enzymes indicated an important role for these enzymes in effective delignification of wood by H. irregulare.

Host responses in Norway spruce roots induced to the pathogen Ceratocystis polonica are evaded or suppressed by the ectomycorrhizal fungus Laccaria bicolor

The outcome of a compatible mycorrhizal interaction is different from that in a compatible plant-pathogen interaction; however, it is not clear what mechanisms are used to evade or suppress the host defence. The aim of this work is to reveal differences between the interaction of Norway spruce roots to the pathogen Ceratocystis polonica and the ectomycorrhizal Laccaria bicolor, examine if L. bicolor is able to evade inducing host defence responses typically induced by pathogens, and test if prior inoculation with the ectomycorrhizal fungus affects the outcome of a later challenge with the pathogen. The pathogen was able to invade the roots and caused extensive necrosis, leading to seedling death, with or without prior inoculation with L. bicolor. The ectomycorrhizal L. bicolor colonised primary roots of the Norway spruce seedlings by partly covering, displacing and convoluting the cells of the outer root cortex, leaving the seedlings healthy. We detected increased total peroxidase activity, and staining indicating increased lignification in roots as a response to C. polonica. In L. bicolor inoculated roots there was no increase in total peroxidase activity, but an additional highly acidic peroxidase isoform appeared that was not present in healthy roots, or in roots invaded by the pathogen. Increased protease activity was detected in roots colonised by C. polonica, but little protease activity was detected in L. bicolor inoculated roots. These results suggest that the pathogen efficiently invades the roots despite the induced host defence responses, while L. bicolor suppresses or evades inducing such host responses in this experimental system.

The pathogenic white-rot fungus Heterobasidion parviporum responds to spruce xylem defense by enhanced production of oxalic acid

Pathogen challenge of tree sapwood induces the formation of reaction zones with antimicrobial properties such as elevated pH and cation content. Many fungi lower substrate pH by secreting oxalic acid, its conjugate base oxalate being a reductant as well as a chelating agent for cations. To examine the role of oxalic acid in pathogenicity of white-rot fungi, we conducted spatial quantification of oxalate, transcript levels of related fungal genes, and element concentrations in heartwood of Norway spruce challenged naturally by Heterobasidion parviporum. In the pathogen-compromised reaction zone, upregulation of an oxaloacetase gene generating oxalic acid coincided with oxalate and cation accumulation and presence of calcium oxalate crystals. The colonized inner heartwood showed trace amounts of oxalate. Moreover, fungal exposure to the reaction zone under laboratory conditions induced oxaloacetase and oxalate accumulation, whereas heartwood induced a decarboxylase gene involved in degradation of oxalate. The excess level of cations in defense xylem inactivates pathogen-secreted oxalate through precipitation and, presumably, only after cation neutralization can oxalic acid participate in lignocellulose degradation. This necessitates enhanced production of oxalic acid by H. parviporum. This study is the first to determine the true influence of white-rot fungi on oxalate crystal formation in tree xylem.

Xylem defense wood of Norway spruce compromised by the pathogenic white-rot fungus Heterobasidion parviporum shows a prolonged period of selective decay

Heterobasidion parviporum, a common pathogenic white-rot fungus in managed Norway spruce forests in northern and central Europe, causes extensive decay columns within stem heartwood of the host tree. Infected trees combat the lateral spread of decay by bordering the heartwood with a fungistatic reaction zone characterized by elevated pH and phenol content. To examine the mode of fungal feeding in the reaction zone of mature Norway spruce trees naturally infected by H. parviporum, we conducted spatial profiling of pectin and hemicellulose composition, and established transcript levels of candidate fungal genes encoding enzymes involved in degradation of the different cell wall components of wood. Colonized inner heartwood showed pectin and hemicellulose concentrations similar to those of healthy heartwood, whereas the carbohydrate profiles of compromised reaction zone, irrespective of the age of fungal activity in the tissue, indicated selective fungal utilization of galacturonic acid, arabinose, xylose and mannose. These data show that the rate of wood decay in the reaction zone is slow. While the up-regulation of genes encoding pectinases and hemicellulases preceded that of the endoglucanase gene during an early phase of fungal interaction with xylem defense, the manganese peroxidase gene showed similar transcript levels during different phases of wood colonization. It seems plausible that the reaction zone components of Norway spruce interfere with both lignin degradation and the associated co-hydrolysis of hemicelluloses and pectin, resulting in a prolonged phase of selective decay.

Indications of heightened constitutive or primed host response affecting the lignin pathway transcripts and phenolics in mature Norway spruce clones

Two mature clones of Norway spruce (Picea abies (L.) Karst.) that have previously been shown to have differential degrees of resistance towards the necrotrophic pathogen Heterobasidion parviporum (Niemelä & Korhonen) were compared with respect to the primed defense expression of transcripts related to biosynthesis of lignin, stilbenes and other phenolic compounds from one year to the next. The host's response to physical wounding and pathogen inoculation was examined in the initial year, whereas indications of heightened basal defense level or primed response, and responses to re-wounding, were examined the following year. The responses of the two clones to wounding and pathogen inoculation, examined in the initial year, differed; the increases in lignin and phenolics were more distinct in response to the pathogen than to wounding alone. The more resistant clone 589 had higher initial lignin concentrations in the cell walls when compared with clone 409, and these remained higher in clone 589 over both years and increased after the treatments. Both clones responded at the transcriptional and chemical levels to wounding; changes were evident both in the initial wounds and when re-wounded the following year. There were distinct differences in the basal transcript levels of the lignin pathway-related genes, phenolics and total lignin levels in healthy tissue from the initial year to the following year indicative of a primed host response or at least altered constitutive level of defense expression.

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.

Insight into trade-off between wood decay and parasitism from the genome of a fungal forest pathogen

Parasitism and saprotrophic wood decay are two fungal strategies fundamental for succession and nutrient cycling in forest ecosystems. An opportunity to assess the trade-off between these strategies is provided by the forest pathogen and wood decayer Heterobasidion annosum sensu lato. We report the annotated genome sequence and transcript profiling, as well as the quantitative trait loci mapping, of one member of the species complex: H. irregulare. Quantitative trait loci critical for pathogenicity, and rich in transposable elements, orphan and secreted genes, were identified. A wide range of cellulose-degrading enzymes are expressed during wood decay. By contrast, pathogenic interaction between H. irregulare and pine engages fewer carbohydrate-active enzymes, but involves an increase in pectinolytic enzymes, transcription modules for oxidative stress and secondary metabolite production. Our results show a trade-off in terms of constrained carbohydrate decomposition and membrane transport capacity during interaction with living hosts. Our findings establish that saprotrophic wood decay and necrotrophic parasitism involve two distinct, yet overlapping, processes.

The expression pattern of the Picea glauca Defensin 1 promoter is maintained in Arabidopsis thaliana, indicating the conservation of signalling pathways between angiosperms and gymnosperms

A 1149 bp genomic fragment corresponding to the 5' non-coding region of the PgD1 (Picea glauca Defensin 1) gene was cloned, characterized, and compared with all Arabidopsis thaliana defensin promoters. The cloned fragment was found to contain several motifs specific to defence or hormonal response, including a motif involved in the methyl jasmonate reponse, a fungal elicitor responsive element, and TC-rich repeat cis-acting element involved in defence and stress responsiveness. A functional analysis of the PgD1 promoter was performed using the uidA (GUS) reporter system in stably transformed Arabidopsis and white spruce plants. The PgD1 promoter was responsive to jasmonic acid (JA), to infection by fungus and to wounding. In transgenic spruce embryos, GUS staining was clearly restricted to the shoot apical meristem. In Arabidopsis, faint GUS coloration was observed in leaves and flowers and a strong blue colour was observed in guard cells and trichomes. Transgenic Arabidopsis plants expressing the PgD1::GUS construct were also infiltrated with the hemibiotrophic pathogen Pseudomonas syringae pv. tomato DC3000. It caused a suppression of defensin expression probably resulting from the antagonistic relationship between the pathogen-stimulated salicylic acid pathway and the jasmonic acid pathway. It is therefore concluded that the PgD1 promoter fragment cloned appears to contain most if not all the elements for proper PgD1 expression and that these elements are also recognized in Arabidopsis despite the phylogenetic and evolutionary differences that separates them.

Gene expression changes during short day induced terminal bud formation in Norway spruce

The molecular basis for terminal bud formation in autumn is not well understood in conifers. By combining suppression subtractive hybridization and monitoring of gene expression by qRT-PCR analysis, we aimed to identify genes involved in photoperiodic control of growth cessation and bud set in Norway spruce. Close to 1400 ESTs were generated and their functional distribution differed between short day (SD-12 h photoperiod) and long day (LD-24 h photoperiod) libraries. Many genes with putative roles in protection against stress appeared differentially regulated under SD and LD, and also differed in transcript levels between 6 and 20 SDs. Of these, PaTFL1(TERMINAL FLOWER LIKE 1) showed strongly increased transcript levels at 6 SDs. PaCCCH(CCCH-TYPE ZINC FINGER) and PaCBF2&3(C-REPEAT BINDING FACTOR 2&3) showed a later response at 20 SDs, with increased and decreased transcript levels, respectively. For rhythmically expressed genes such as CBFs, such differences might represent a phase shift in peak expression, but might also suggest a putative role in response to SD. Multivariate analyses revealed strong differences in gene expression between LD, 6 SD and 20 SD. The robustness of the gene expression patterns was verified in 6 families differing in bud-set timing under natural light with gradually decreasing photoperiod.

Differential gene expression related to an epigenetic memory affecting climatic adaptation in Norway spruce

In Norway spruce, the temperature during zygotic embryogenesis appears to adjust an adaptive epigenetic memory in the progeny that may regulate bud phenology and cold acclimation. Conditions colder than normal advance the timing whilst temperatures above normal delay the onset of these processes and altered performance is long lasting in progeny with identical genetic background. As a step toward unraveling the molecular mechanism behind an epigenetic memory, transcriptional analysis was performed on seedlings from seeds of six full-sib families produced at different embryogenesis temperature-cold (CE) vs warm (WE) under long and short day conditions. We prepared two suppressive subtracted cDNA libraries, forward and reverse, representing genes predominantly expressed in plants from seeds obtained after CE and WE embryogenesis following short day treatment (inducing bud set). Sequencing and annotation revealed considerable differences in the transcriptome of WE versus CE originated plants. By using qRT-PCR we studied the expression patterns of 32 selected candidate genes chosen from subtractive cDNA libraries analysis and nine siRNA pathways genes by a direct candidate approach. Eight genes, two transposons related genes, three with no match to Databases sequences and three genes from siRNA pathways (PaDCL1 and 2, PaSGS3) showed differential expression in progeny from CE and WE correlated with the family phenotypic differences. These findings may contribute to our understanding of the epigenetic mechanisms underlying adaptive changes acquired during embryogenesis.

MicroRNAs, the epigenetic memory and climatic adaptation in Norway spruce

*Norway spruce expresses a temperature-dependent epigenetic memory from the time of embryo development, which thereafter influences the timing bud phenology. MicroRNAs (miRNAs)are endogenous small RNAs, exerting epigenetic gene regulatory impacts. We have tested for their presence and differential expression. *We prepared concatemerized small RNA libraries from seedlings of two full-sib families, originated from seeds developed in a cold and warm environment. One family expressed distinct epigenetic effects while the other not. We used available plant miRNA query sequences to search for conserved miRNAs and from the sequencing we found novel ones; the miRNAs were monitored using relative real time-PCR. *Sequencing identified 24 novel and four conserved miRNAs. Further screening of the conserved miRNAs confirmed the presence of 16 additional miRNAs. Most of the miRNAs were targeted to unknown genes. The expression of seven conserved and nine novel miRNAs showed significant differences in transcript levels in the full-sib family showing distinct epigenetic difference in bud set, but not in the nonresponding full-sib family. Putative miRNA targets were studied. *Norway spruce contains a set of conserved miRNAs as well as a large proportion of novel nonconserved miRNAs. The differentially expression of specific miRNAs indicate their putative participation in the epigenetic regulation.

Effect of bud burst forcing on transcript expression of selected genes in needles of Norway spruce during autumn

Expression of selected genes in needles of Norway spruce (Picea abies [L.] Karst) was investigated by following their transcription levels during late autumn. Transcription was assessed in mature needles which likely serve as sensor of environmental cues that enable trees in the temperate and boreal regions to change between stages of growth, frost tolerance and bud dormancy. Samples were collected from grafts kept under outdoor conditions and after bud burst forcing in greenhouse at 20 degrees C (12 h darkness) for one week. Transcription was assayed with real-time RT-PCR. During the sampling period, chilling requirement was partially fulfilled, and time to bud burst after forcing was decreased. Of the 27 transcripts studied, expression of 16 was significantly affected either by forcing, sampling time, or interaction between them. PaSAP, PaACP, PaSGS3, PaWRKY, PaDIR9, PaCCCH and dehydrin genes responded drastically to forcing temperatures at all sampling points, showing no correlation with readiness for bud burst. Expression patterns of some vernalization pathway gene homologs PaVIN3, and also of PaMDC, PaLOV1 and PaDAL3 had a clear opposite trends between forcing and outdoor conditions, which could imply their role in chilling accumulation and bud burst regulation/cold acclimation. These genes could constitute putative candidates for further detailed study, whose regulation in needles may be involved in preparation towards bud burst and chilling accumulation sensing.

Identification of PaCOL1 and PaCOL2, two CONSTANS-like genes showing decreased transcript levels preceding short day induced growth cessation in Norway spruce

In woody plants of the temperate zone short photoperiod (SD) leads to growth cessation. In angiosperms CONSTANS (CO) or CO-like genes play an important role in the photoperiodic control of flowering, tuberisation and shoot growth. To investigate the role of CO-like genes in photoperiodic control of shoot elongation in gymnosperms, PaCOL1 and PaCOL2 were isolated from Norway spruce. PaCOL1 encodes a 3.9kb gene with a predicted protein of 444 amino acids. PaCOL2 encodes a 1.2kb gene with a predicted protein of 385 amino acids. Both genes consist of two exons and have conserved domains found in other CO-like genes; two zinc finger domains, a CCT and a COOH domain. PaCOL1 and PaCOL2 fall into the group 1c clade of the CO-like genes, and are thus distinct from Arabidopsis CO that belongs to group 1a. Transcript levels of both PaCOL-genes appear to be light regulated, an increasing trend was observed upon transition from darkness to light, and a decreasing trend during darkness. The increasing trend at dawn was observed both in needles and shoot tips, whereas the decreasing trend in darkness was most prominent in shoot tips, and limited to the late part of the dark period in needles. The transcript levels of both genes decreased significantly in both tissues under SD prior to growth cessation and bud formation. This might suggest an involvement in photoperiodic control of shoot elongation or might be a consequence of regulation by light.

Dehydrins expression related to timing of bud burst in Norway spruce

Cold deacclimation and preparation to flushing likely requires rehydration of meristems. Therefore, water stress related genes, such as dehydrins (DHN), might play an important role in providing protection during winter dormancy, deacclimation and bud burst timing processes. Here we report the sequence analysis of several Norway spruce DHN identified in late and early flushing suppressive subtraction hybridization cDNA libraries and in our Norway spruce EST database. We obtained 15 cDNAs, representing eight genes from three distinct types of DHN, and studied differential expression of these genes before and during bud burst in spring, using qRT-PCR. We found the visible reduction in transcript level of most DHN towards the bud burst, supported by a significant down-regulation of the DHN in needles during experimental induction of bud burst applied at three time points during autumn in Norway spruce grafts. For most of the DHN transcripts, their expression levels in late-flushing spruces were significantly higher than in the early flushing ones at the same calendar dates but were remarkably similar at the same bud developmental stage. From our results we may conclude that the difference between the early and the late families is in timing of the molecular processes leading to bud burst due to differences in their response to the increasing temperature in the spring. They are induced much earlier in the early flushing families.

Real-time PCR-based monitoring of DNA pools in the tri-trophic interaction between Norway spruce, the rust Thekopsora areolata, and an opportunistic ascomycetous Phomopsis sp

The difficulty in subculturing biotrophic fungi complicates etiological studies related to the associated plant diseases. By employing internal transcribed spacer rDNA-targeted quantitative real-time polymerase chain reaction, we now show that the heteroecious rust Thekopsora areolata, commonly associated in natural conditions to sapling shoots and cones of Norway spruce and leaves of wild bird cherry, frequently infects nursery-grown seedlings of the conifer. A spatial sampling scheme was used to investigate seedlings and saplings of Norway spruce showing phloem necrosis: the highest concentration of DNA of T. areolata was recorded in the area with necrotic phloem. The separate analysis of bark and wood tissues suggested that the initial spread of the rust to healthy tissues neighboring the infection site takes place in the bark. A Phomopsis species found to coexist with T. areolata in several seedlings showed very high DNA levels in the upper part of the lesion, and even in the visually healthy proximal tissues above the lesions, which indicates that the ascomycete, most probably a secondary invader following primary infection by T. areolata, has a latent stage during early host colonization. We hypothesize that this hemibiotrophic mode of infection contributes to the successful coexistence of Phomopsis with a biotrophic rust.

Identification and analysis of differentially expressed Heterobasidion parviporum genes during natural colonization of Norway spruce stems

To identify differentially expressed genes of the white-rot fungus Heterobasidion parviporum, two cDNA libraries were constructed using suppressive subtraction hybridization (SSH) technique with RNA extracted from an advanced stage of decay area and from colonization front next to the reaction zone of the stem of a mature Norway spruce naturally colonized by the fungus. Besides several cytochrome P450s and hypothetical proteins with unknown function, the SSH libraries constructed contained, among others, genes involved in basic cellular processes, and lignin and cellulose degradation. To examine the role of selected candidate genes for each functional group, three trees, each colonized by a different genotype of the pathogen and showing a variable degree of wood decay, were used for real-time RT-PCR profiling of candidate genes. In the decay transition areas the study revealed activity centers that showed remarkable similarity in the transcript profiles of the monitored genes.

Etiology and real-time polymerase chain reaction-based detection of gremmeniella- and phomopsis-associated disease in norway spruce seedlings

ABSTRACT In spring 2002, an unusual disease outburst was recorded on Norway spruce seedlings in southeast Norway. Extensive damage was recorded on 1- and 2-year-old Norway spruce seedlings that either had wintered in nursery cold storage or had been planted out in autumn 2001. The damage was characterized by leader shoot dieback and stem necroses on the upper or lower part of the shoot from 2001. Gremmeniella abietina and a Phomopsis sp. frequently were isolated from the diseased seedlings. Internal transcribed spacer (ITS) ribosomal (r)DNA sequence analysis and random amplified microsatellites profiling indicated that the G. abietina strains associated with diseased nursery seedlings belonged to the large-tree type (LTT) ecotype of the European race of G. abietina var. abietina, and inoculation tests confirmed their pathogenicity on Norway spruce. Based on ITS rDNA sequence analysis, the Phomopsis strains associated with diseased seedlings did not represent any characterized Phomopsis spp. associated with conifers. The Phomopsis sp. was not pathogenic in inoculation tests, indicating that it may be a secondary colonizer. ITS-based real-time polymerase chain reaction assays were developed in order to detect and quantify G. abietina and Phomopsis in the nursery stock. We describe here the G. abietina-associated shoot dieback symptoms on Norway spruce seedlings and conclude that the unusual disease outburst likely was related to the G. abietina var. abietina epidemic caused by the LTT on large Scots pines in 2001.

Changes in host chitinase isoforms in relation to wounding and colonization by Heterobasidion annosum: early and strong defense response in 33-year-old resistant Norway spruce clone

We studied the defense reactions of 33-year-old susceptible and resistant clones of Norway spruce (Picea abies (L.) Karst.) to the major root-rot fungus Heterobasidion annosum (Fr.) Bref. and determined if tissue cultures can be used as a model system for studying defense responses of mature trees at the molecular level. Quantitative PCR analysis of genomic DNA obtained from samples taken at different times along the lesion length in living bark indicated that the fungus was present in higher amounts and extended further into the host tissue in the susceptible clone than in the resistant clone. In protein extracts from the same lesion samples, there were differences in temporal and spatial changes in host chitinase isoform profiles between the resistant and susceptible clones. Host chitinase isoforms with pI values approximately 4.8, 4.4 and 3.7 increased more during the first 7 days after wounding and inoculation and extended further along the lesion length in the resistant clone than in the susceptible clone. These results suggest that the time from wounding and infection to induction of defense-related expression is shorter in the resistant clone indicating a more efficient host defense response than in the susceptible clone. Tissue cultures from the same clones were not resistant to H. annosum and showed no difference in the timing of the increase in chitinase isoforms in response to the pathogen. However, tissue cultures from both clones showed an increase in chitinase isoforms within 6 to 24 h past inoculation, indicating that increased chitinase expression in response to the pathogen is part of a general defense response common to both mature clones and tissue cultures.

Cytological and enzymatic responses to aluminium stress in root tips of Norway spruce seedlings

•  Aluminium (Al) stress reduces plant growth. However, some species such as Norway spruce (Picea abies) seem to tolerate high Al concentrations. The aim of this study was to investigate characteristics possibly involved in Al tolerance in Norway spruce seedlings. •  Seedlings (10-d-old) were exposed to Al³⁺ concentrations of 0.5 and 5 mm for up to 168 h. The effect of Al stress on root growth, cell morphology and Al distribution, callose production, and peroxidase and chitinase activity was analysed. •  Root growth decreased after 1 d and 2 d with 5 and 0.5 mm Al, respectively. Callose concentration increased strongly after 6 h treatment with 5 mm Al. The activity of many peroxidase and chitinase isoforms decreased after 1-24 h exposure of both treatments. Several isoforms increased after 48-168 h exposure to 5 mm Al. •  We postulate that, with external Al concentrations 0.5 mm or lower, an increased production above constitutive levels of peroxidase or chitinase is not required for Al tolerance in young Norway spruce seedlings. High constitutive levels of peroxidase and chitinase in this species may be part of this Al tolerance.

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.

Effects of Rhizoctonia infection and drought on peroxidase and chitinase activity in Norway spruce (Picea abies)

Seedlings of Norway spruce were exposed to fungal infection and drought in order to investigate differences in their stress responses on the enzymatic level. Six-week-old seedlings were infected with the root rot fungus Rhizoctonia, or subjected to drought, respectively. Changes at the enzymatic level were more rapid and significantly higher in infected plants in comparison with drought-stressed spruce plants. Rhizoctonia infection resulted in early local and systemic increase in peroxidase and chitinase activity. The most prominent isoforms responding were highly basic peroxidases and chitinases (pI 9-9.5) and several acidic chitinases (pI3-4). An increased intensity of similar peroxidase isoforms was found in drought-affected plants. Two peroxidase isoforms (with pI < 9) accumulated exclusively in response to drought. These results suggest that at an early stage of infection and drought stress, the two stresses can be distinguished by the temporal appearance and isoform profile of peroxidases and chitinases. Changes in enzyme activity appeared before changes in physiological parameters, thus these isoform profiles could be used as early markers of stress conditions in spruce.

The putative gymnosperm plant defensin polypeptide (SPI1) accumulates after seed germination, is not readily released, and the SPI1 levels are reduced in Pythium dimorphum-infected spruce roots

The putative plant defensin SPI1 cDNA from the conifer Norway spruce (Picea abies) is the only known plant defensin-like sequence from a gymnosperm. The predicted translational product SPI1 was not detected in the embryo or other parts of the seed by means of antibodies, but it accumulated in the root cortex after germination. In roots of seedlings infected with the root pathogenic oomycete Pythium dimorphum and the blue stain fungus Ceratocystis polonica, variable levels of SPI1 was detected during the first day as a response to the infection, however a significant increase was seen as an initial response to the root-rot fungus Heterobasidion annosum. After the first day of infection, the amount of SPI1 polypeptide was dramatically reduced in response to either of the pathogens, but not in response to the ectomycorrhizal fungus Laccaria bicolor. During the same time of infection, extensive damage to cortical root cells resulted from the infecting pathogens, but not from the mycorrhiza. These results indicate that pathogens may reduce the level of SPI1 by suppressing its expression, but may also reduce the SPI1 level by invading and disrupting the root cortical cells or by a combination of these mechanisms.

Isolation of the first putative peroxidase cDNA from a conifer and the local and systemic accumulation of related proteins upon pathogen infection

Peroxidases are associated with the active defence reactions in higher plants in response to foreign organisms. They are involved in the oxidation of phenolic compounds in cell walls, polymerization of lignin and suberin, and in several other oxidation processes but the exact function of individual peroxidases is not known. We have isolated a cDNA encoding the putative defence-related and basic plant peroxidase SPI2 (spruce pathogen-induced 2), with an estimated molecular mass of 34 kDa, from roots of Norway spruce (Picea abies) seedlings. This is the first description of the isolation of a complete cDNA encoding a putative peroxidase from a gymnosperm. The transcript was present in the roots of healthy seedlings, and during infection with the pathogen Pythium dimorphum there was a rapid initial increase followed by a dramatic reduction of the transcript. The 34 kDa mature SPI2 protein was detected in both the developing root and shoot of healthy seedlings and increased amounts of SPI2 and increased accumulation of highly basic peroxidase isoforms was observed in roots after infection. In addition, two SPI2-related proteins with apparent molecular masses of 38 and 39 kDa, were also detected. Both these proteins accumulated in roots only after infection, and the 39 kDa protein was in addition detected in shoots of root-infected seedlings. Thus, both SPI2 and the SPI2-related proteins accumulate as a local response, in roots, and as a systemic response to infection the 39 kDa protein accumulates in the shoot.

Bacteriophage P4 capsid-size determination and its relationship to P2 helper interference

The sid (size determination) gene product of phage P4 is known to be involved in capsid-size determination. Moreover, the capsid-size determination function interferes with the lytic development of its helper P2, presumably because the helper genome is too large to be packaged into P4-size capsids. In order to study P4-specified helper interference, we cloned the sid gene for expression during phage infection. Even though gpSid restores the capsid-size determination function of a sid defective P4 mutant, we find that gpSid alone is not sufficient to establish full interference of helper P2 phage production. Complete helper interference requires some P4 function in addition to gpSid. Complementation tests show that none of the known P4 genes display this property. We propose that P4 encodes a yet-unidentified function that in concert with gpSid establishes full P2 helper interference at the level of capsid-size determination.