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PubMed2003

Volatile compounds from leaf-buds of Populus nigra L (Salicaceae).

Volatile components from fresh and air-dried leaf-buds of Populus nigra L. (Salicaceae) were isolated by Likens-Nickerson apparatus and analyzed using GC/MS. Forty-eight components (ca. 95% of the total isolate) were identified among black poplar bud volatiles. Sesquiterpene alcohols beta-eudesmol and alpha-eudesmol represented 26.3-28.7% of the oil. Other major sesquiterpene compounds were gamma-selinene (7.6-8.8%), delta-cadinene (7.8-8.6%), alpha-elemene (3.3-5.2%) and gamma-cadinene (3.9-4.2%). Hemiterpenes were also identified (2.2-7.6%). Monoterpenes were present in low percentages (1.6-5.7%). Aliphatic and aromatic alcohols, carbonyl compounds and aliphatic acids were identified among non-terpene volatiles (9.8-13.5%). The fresh buds contained 0.27% and dried 0.12% essential oil. Air-drying moderately effected the volatiles qualitative and quantitative composition.

Gas Chromatography-Mass Spectrometry↗

PubMed2004

Recombinant expression and enzymatic characterization of PttCel9A, a KOR homologue from Populus tremula x tremuloides.

PttCel9A is a membrane-bound, family 9 glycosyl hydrolase from Populus tremula x tremuloides that is upregulated during secondary cell wall synthesis. The catalytic domain of PttCel9A, Delta(1-105)PttCel9A, was purified, and its activity was compared to TfCel9A and TfCel9B from Thermobifida fusca. Since aromatic amino acids involved in substrate binding at subsites -4, -3, and -2 are missing in PttCel9A, the activity of TfCel9A mutant enzymes W256S, W209A, and W313G was also investigated. Delta(1-105)PttCel9A hydrolyzed a comparatively narrow range of polymeric substrates, and the preferred substrate was (carboxymethyl)cellulose 4M. Moreover, Delta(1-105)PttCel9A did not hydrolyze oligosaccharides shorter than cellopentaose, whereas TfCel9A and TfCel9B hydrolyzed cellotetraose and cellotriose, respectively. These data suggest that the preferred substrates of PttCel9A are long, low-substituted, soluble cellulosic polymers. At 30 degrees C and pH 6.0, the kcat for cellohexaose of Delta(1-105)PttCel9A, TfCel9A, and TfCel9B were 0.023 +/- 0.001, 16.9 +/- 2.0, and 1.3 +/- 0.2, respectively. The catalytic efficiency (kcat/Km) of TfCel9B was 39% of that of TfCel9A, whereas the catalytic efficiency of Delta(1-105)PttCel9A was 0.04% of that of TfCel9A. Removing tryptophan residues at subsites -4, -3, and -2 decreased the efficiency of cellohexaose hydrolysis by TfCel9A. Mutation of W313 to G had the most drastic effect, producing a mutant enzyme with 1% of the catalytic efficiency of TfCel9A. The apparent narrow substrate range and catalytic efficiency of PttCel9A are correlated with a lack of aromatic amino acids in the substrate binding cleft and may be necessary to prevent excessive hydrolysis of cell wall polysaccharides during cell wall formation.

Amino Acid Sequence↗

PubMed2004

Metabolism and mineralization of hexahydro-1,3,5-trinitro-1,3,5-triazine inside poplar tissues (Populus deltoides x nigra DN-34).

Poplar tissue cultures and leaf crude extracts (Populus deltoides x nigra DN-34) were exposed to [U-14C]hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and incubated under light and in the dark. Poplar tissue cultures were able to partially reduce RDX to hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) and hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), regardless of the presence or absence of light. However, further transformation of RDX, MNX, and DNX required exposure to light and resulted in the formation of formaldehyde (CH2O), methanol (CH3OH), and carbon dioxide (CO2). Similarly, transformation of RDX by poplar leaf crude extracts required exposure to light. Neither reduction of RDX to MNX and DNX nor mineralization into CO2 were recorded in crude extracts, even when exposed to light, suggesting that both processes were light-independent and required intact plant cells. Control experiments without plant material showed that RDX was partially transformed abiotically, by the sole action of light, but to a lesser extent than in the presence of plant crude extracts, suggesting the intervention of plant subcellular structures through a light-mediated mechanism. Poplar tissue cultures were also shown to mineralize 14CH2O and 14CH3OH, regardless of the presence or absence of light. These results suggest that transformation of [U-14C]RDX by plant tissue cultures may occur through a three-step process, involving (i) a light-independent reduction of RDX to MNX and DNX by intact plant cells; (ii) a plant/light-mediated breakdown of the heterocyclic ring of RDX, MNX, or DNX into C1-labeled metabolites (CH2O and CH3OH); and (iii) a further light-independent mineralization of C1-labeled metabolites by intact plant cells. This is the first time that a significant mineralization of RDX into CO2 by light-exposed plant tissue cultures is reported.

Biodegradation, Environmental↗

PubMed2006

Allergy-preventive phenolic glycosides from Populus sieboldii.

Allergy-preventive activity was demonstrated for an extract of the bark of Populus sieboldii in a continuing search for allergy-preventive substances from natural sources. By bioassay-directed fractionation of this plant bark, two new phenolic glycosides, siebolside A {2-hydroxy-5-[(benzoyloxy)methyl]phenyl (6'-O-acetyl) beta-D-glucopyranoside} (1) and siebolside B {2-hydroxy-5-[(benzoyloxy)methyl]phenyl beta-D-glucopyranoside} (2), were isolated, together with three known compounds, salicin (3), sakuranin (4), and neosakuranin (5). The structures of 1 and 2 were elucidated by spectroscopic methods. Compounds 1-5 all showed allergy-preventive effects.

Animals↗

PubMed2006

Phenolic glycosides with antioxidant activity from the stem bark of Populus davidiana.

Phytochemical study on the EtOAc-soluble fraction of the stem bark of Populus davidiana resulted in the isolation of 10 phenolic glycosides (1-10), which were identified on the basis of physicochemical and spectroscopic analyses. Among these, three new compounds, populosides A-C (1-3), were determined to be 2-coumaroylmethyl-4-hydroxyphenyl-beta-D-glucopyranoside, 2-coumaroylmethylphenyl-beta-D-glucopyranoside, and 2-feruoylmethylphenyl-beta-D-glucopyranoside, respectively. Compounds 1-10 were tested for their radical scavenging activity against an azo radical, ABTS*+. Of these, populosides A-C (1-3), populoside (4), grandidentatin (8), salireposide (9), and coumaroyl-beta-D-glucoside (10) exhibited antioxidant activity in this assay.

Antioxidants↗

PubMed2005

Enzymatic properties of native and deglycosylated hybrid aspen (Populus tremulaxtremuloides) xyloglucan endotransglycosylase 16A expressed in Pichia pastoris.

The cDNA encoding a xyloglucan endotransglycosylase, PttXET16A, from hybrid aspen (Populus tremulaxtremuloides) has been isolated from an expressed sequence tag library and expressed in the methylotrophic yeast Pichia pastoris. Sequence analysis indicated a high degree of similarity with other proteins in the XTH (xyloglucan transglycosylase/hydrolase) gene subfamily of GH16 (glycoside hydrolase family 16). In addition to the conserved GH16 catalytic sequence motif, PttXET16A contains a conserved N-glycosylation site situated proximal to the predicted catalytic residues. MS analysis indicated that the recombinant PttXET16A expressed in P. pastoris is heterogeneous due to the presence of variable N-glycosylation and incomplete cleavage of the alpha-factor secretion signal peptide. Removal of the N-glycan by endoglycosidase H treatment did not influence the catalytic activity significantly. Similarly, site-directed mutagenesis of Asn93 to serine to remove the N-glycosylation site resulted in an enzyme which was comparable with the wild-type enzyme in specific activity and thermal stability but had clearly reduced solubility. Hydrolytic activity was detected neither in wild-type PttXET16A before or after enzymatic deglycosylation nor in PttXET16A N93S (Asn93-->Ser) mutant.

Amino Acid Sequence↗

PubMed2006

Kinetic analysis using low-molecular mass xyloglucan oligosaccharides defines the catalytic mechanism of a Populus xyloglucan endotransglycosylase.

Plant XETs [XG (xyloglucan) endotransglycosylases] catalyse the transglycosylation from a XG donor to a XG or low-molecular-mass XG fragment as the acceptor, and are thought to be important enzymes in the formation and remodelling of the cellulose-XG three-dimensional network in the primary plant cell wall. Current methods to assay XET activity use the XG polysaccharide as the donor substrate, and present limitations for kinetic and mechanistic studies of XET action due to the polymeric and polydisperse nature of the substrate. A novel activity assay based on HPCE (high performance capillary electrophoresis), in conjunction with a defined low-molecular-mass XGO {XG oligosaccharide; (XXXGXXXG, where G=Glcbeta1,4- and X=[Xylalpha1,6]Glcbeta1,4-)} as the glycosyl donor and a heptasaccharide derivatized with ANTS [8-aminonaphthalene-1,3,6-trisulphonic acid; (XXXG-ANTS)] as the acceptor substrate was developed and validated. The recombinant enzyme PttXET16A from Populus tremula x tremuloides (hybrid aspen) was characterized using the donor/acceptor pair indicated above, for which preparative scale syntheses have been optimized. The low-molecular-mass donor underwent a single transglycosylation reaction to the acceptor substrate under initial-rate conditions, with a pH optimum at 5.0 and maximal activity between 30 and 40 degrees C. Kinetic data are best explained by a ping-pong bi-bi mechanism with substrate inhibition by both donor and acceptor. This is the first assay for XETs using a donor substrate other than polymeric XG, enabling quantitative kinetic analysis of different XGO donors for specificity, and subsite mapping studies of XET enzymes.

Catalysis↗

PubMed2002

Molecular analysis of herbivore-induced condensed tannin synthesis: cloning and expression of dihydroflavonol reductase from trembling aspen (Populus tremuloides).

In order to study condensed tannin synthesis and its induction by herbivory, a dihydroflavonol reductase (DFR) cDNA was isolated from trembling aspen (Populus tremuloides). Bacterial overexpression demonstrated that this cDNA encodes a functional DFR enzyme, and Southern analysis revealed that DFR likely is a single-copy gene in the aspen genome. Aspen plants that were mechanically wounded showed a dramatic increase in DFR expression after 24 h in both wounded leaves and unwounded leaves on wounded trees. Feeding by forest tent caterpillar (Malacosoma disstria) and satin moth (Leucoma salicis) larvae, and treatment with methyl jasmonate, all strongly induced DFR expression. DFR enzyme activity was also induced in wounded aspen leaves, and phytochemical assays revealed that condensed tannin concentrations significantly increased in wounded and systemic leaves. The expression of other genes involved in the phenylpropanoid pathway were also induced by wounding. Our findings suggest that the induction of condensed tannins, compounds known to be important for defense against herbivores, is mediated by increased expression of DFR and other phenylpropanoid genes.

Alcohol Oxidoreductases↗

PubMedDate not supplied

Polyamine, carbohydrate, and proline content changes during salt stress exposure of aspen (Populus tremula L.): involvement of oxidation and osmoregulation metabolism.

Excess salt in the soil solution affects the plant either through osmotic or ionic effects. Poplar trees, as fast growing pioneer tree species, are thought to be potential suitable candidates for afforestation on saline soils. Osmotic and oxidative stress induced by salinity could be reduced by the production and accumulation of compatible solutes and osmoprotectants in the plant. In this respect, metabolites of this type could be interesting markers for the improvement of salt stress tolerance in breeding programmes. Results have shown that Populus tremula was able to cope with up to 150 mM NaCl without any effect on plant survival. During stress application, the endogenous level of malondialdehyde did not vary significantly between the treatments, indicating that the level of lipid peroxidation was similar in the control and in the stressed plants. However, proline, spermine, sucrose, mannitol, and raffinose levels increased temporarily or throughout the salt treatment. All these molecules are more or less closely related to antioxidant or osmoprotectant mechanisms during stress, suggesting a key role for these compatible solutes, osmoprotectants, and their metabolism for salt stress resistance. The accumulation of free proline, sucrose and mannitol, and the transitory increase in spermine level observable during low and high NaCl application must be considered as general salt stress reaction markers.

Carbohydrate Metabolism↗

PubMedDate not supplied

Compatible and incompetent Paxillus involutus isolates for ectomycorrhiza formation in vitro with poplar (Populus x canescens) differ in H2O2 production.

Isolates of Paxillus involutus (Batsch) Fr. collected from different hosts and environmental conditions were screened for their ability to form ectomycorrhizal symbiosis with hybrid poplar P. x canescens (= Populus tremula L. x P. alba) in vitro. The ability to form ectomycorrhiza varied between the fungal isolates and was not correlated with the growth rate of the fungi on agar-based medium. The isolate MAJ, which was capable of mycorrhiza synthesis under axenic conditions, and the incompetent isolate NAU were characterized morphologically and anatomically. MAJ formed a typical hyphal mantle and a Hartig net, whereas NAU was not able to penetrate the host cell walls and caused thickenings of the outer cell walls of the host. MAJ, but not NAU, displayed strong H2O2 accumulation in the outer hyphal mantle. Increases in H2O2 in the outer epidermal walls and adjacent hyphae of the incompetent isolate were moderate. No increases of H2O2 in response to the mycobionts were found inside roots. Suggested functions of H2O2 production in the outer hyphal mantle of the compatible interaction are: growth regulation of the host's roots, defence against other invading microbes, or increasing plant-innate immunity. The system established here for P. x canescens compatible and incompetent fungal associations will be useful to take advantage of genomic information now available for poplar to study tree-fungal interactions at the molecular and physiological level.

Basidiomycota↗

PubMed2004

The structural basis of the thermostability of SP1, a novel plant (Populus tremula) boiling stable protein.

We previously reported on a new boiling stable protein isolated from aspen plants (Populus tremula), which we named SP1. SP1 is a stress-related protein with no significant sequence homology to other stress-related proteins. It is a 108-amino-acid hydrophilic polypeptide with a molecular mass of 12.4 kDa (Wang, W. X., Pelah, D., Alergand, T., Shoseyov, O., and Altman, A. (2002) Plant Physiol. 130, 865-875) and is found in an oligomeric form. Preliminary electron microscopy studies and matrix-assisted laser desorption ionization time-of-flight mass spectrometry experiments showed that SP1 is a dodecamer composed of two stacking hexamers. We performed a SDS-PAGE analysis, a differential scanning calorimetric study, and crystal structure determination to further characterize SP1. SDS-PAGE indicated a spontaneous assembly of SP1 to one stable oligomeric form, a dodecamer. Differential scanning calorimetric showed that SP1 has high thermostability i.e. Tm of 107 degrees C (at pH 7.8). The crystal structure of SP1 was initially determined to 2.4 A resolution by multi-wavelength anomalous dispersion method from a crystal belonging to the space group I422. The phases were extended to 1.8 A resolution using data from a different crystal form (P21). The final refined molecule includes 106 of the 108 residues and 132 water molecules (on average for each chain). The R-free is 20.1%. The crystal structure indicated that the SP1 molecule has a ferredoxin-like fold. Strong interactions between each two molecules create a stable dimer. Six dimers associate to form a ring-like-shaped dodecamer strongly resembling the particle visualized in the electron microscopy studies. No structural similarity was found between the crystal structure of SP1 and the crystal structure of other stress-related proteins such as small heat shock proteins, whose structure has been already determined. This structural study further supports our previous report that SP1 may represent a new family of stress-related proteins with high thermostability and oligomerization.

Amino Acid Sequence↗

PubMed2005

Topochemical studies on modified lignin distribution in the xylem of Poplar (Populus spp.) after wounding.

BACKGROUND AND AIMS: Information on the influence of wounding on lignin synthesis and distribution in differentiating xylem tissue is still scarce. The present paper provides information on cell modifications with regard to wall ultrastructure and lignin distribution on cellular and subcellular levels in poplar after wounding. METHODS: Xylem of Populus spp. close to a wound was collected and processed for light microscopy, transmission electron microscopy and cellular UV microspectrophotometry. Cell wall modification with respect to lignin distribution was examined at different stages of wound tissue development. Scanning UV microspectrophotometry and point measurements were used to determine the lignin distribution. KEY RESULTS: Xylem fibres within a transition zone between differentiated xylem laid down prior to wounding and the tissues formed after wounding developed distinctively thickened secondary cell walls. Those modified walls and cell corners showed, on average, a higher lignin content and an inhomogeneous lignin distribution within the individual wall layers. CONCLUSIONS: The work presented shows that wounding of the xylem may induce a modified wall architecture and lignin distribution in tissues differentiating at the time of wounding. An increasing lignin content and distinctively thickened walls can contribute to improved resistance as part of the compartmentalization process.

Cell Differentiation↗

PubMed2006

Proteome profiling of Populus euphratica Oliv. upon heat stress.

BACKGROUND AND AIMS: Populus euphratica is a light-demanding species ecologically characterized as a pioneer. It grows in shelter belts along riversides, being part of the natural desert forest ecosystems in China and Middle Eastern countries. It is able to survive extreme temperatures, drought and salt stress, marking itself out as an important plant species to study the mechanisms responsible for survival of woody plants under heat stress. METHODS: Heat effects were evaluated through electrolyte leakage on leaf discs, and LT(50) was determined to occur above 50 degrees C. Protein accumulation profiles of leaves from young plants submitted to 42/37 degrees C for 3 d in a phytotron were determined through 2D-PAGE, and a total of 45 % of up- and downregulated proteins were detected. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF)/TOF analysis, combined with searches in different databases, enabled the identification of 82 % of the selected spots. KEY RESULTS: Short-term upregulated proteins are related to membrane destabilization and cytoskeleton restructuring, sulfur assimilation, thiamine and hydrophobic amino acid biosynthesis, and protein stability. Long-term upregulated proteins are involved in redox homeostasis and photosynthesis. Late downregulated proteins are involved mainly in carbon metabolism. CONCLUSIONS: Moderate heat response involves proteins related to lipid biogenesis, cytoskeleton structure, sulfate assimilation, thiamine and hydrophobic amino acid biosynthesis, and nuclear transport. Photostasis is achieved through carbon metabolism adjustment, a decrease of photosystem II (PSII) abundance and an increase of PSI contribution to photosynthetic linear electron flow. Thioredoxin h may have a special role in this process in P. euphratica upon moderate heat exposure.

Cell Membrane↗

PubMed2004

Regulation of sulphate assimilation by glutathione in poplars (Populus tremula x P. alba) of wild type and overexpressing gamma-glutamylcysteine synthetase in the cytosol.

Glutathione (GSH) is the major low molecular weight thiol in plants with different functions in stress defence and the transport and storage of sulphur. Its synthesis is dependent on the supply of its constituent amino acids cysteine, glutamate, and glycine. GSH is a feedback inhibitor of the sulphate assimilation pathway, the primary source of cysteine synthesis. Sulphate assimilation has been analysed in transgenic poplars (Populus tremula x P. alba) overexpressing gamma-glutamylcysteine synthetase, the key enzyme of GSH synthesis, and the results compared with the effects of exogenously added GSH. Although foliar GSH levels were 3-4-fold increased in the transgenic plants, the activities of enzymes of sulphate assimilation, namely ATP sulphurylase, adenosine 5'-phosphosulphate reductase (APR), sulphite reductase, serine acetyltransferase, and O-acetylserine (thiol)lyase were not affected in three transgenic lines compared with the wild type. Also the mRNA levels of these enzymes were not altered by the increased GSH levels. By contrast, an increase in GSH content due to exogenously supplied GSH resulted in a strong reduction in APR activity and mRNA accumulation. This feedback regulation was reverted by simultaneous addition of O-acetylserine (OAS). However, OAS measurements revealed that OAS cannot be the only signal responsible for the lack of feedback regulation of APR by GSH in the transgenic poplars.

Cloning, Molecular↗

PubMed2005

Cavitation vulnerability in roots and shoots: does Populus euphratica Oliv., a poplar from arid areas of Central Asia, differ from other poplar species?

Populus euphratica is a poplar species growing in arid regions of Central Asia, where its distribution remains nevertheless restricted to river-banks or to areas with an access to deep water tables. To test whether the hydraulic architecture of this species differs from that of other poplars with respect to this ecological distribution, the vulnerability to cavitation of P. euphratica was compared with that of P. alba and of P. trichocarpa x koreana. The occurrence of a potential hydraulic segmentation through cavitation was also investigated by assessing the vulnerability of roots, stems, and leaf mid-rib veins. Cryo-scanning electron microscopy (cryo-SEM) was used to assess the level of embolism in fine roots and leaf mid-ribs and a low pressure flowmeter (LPFM) was used for stems and main roots. The cryo-SEM technique was validated against LPFM measurements on paired samples. In P. alba and P. trichocarpa x koreana, leaf mid-ribs were more vulnerable to cavitation than stems and roots. In P. euphratica, leaf mid-ribs and stems were equally vulnerable and, contrary to what has been observed in other species, roots were significantly less vulnerable than shoots. P. euphratica was by far the most vulnerable. The water potential inducing 50% loss of conductivity in stems was close to -0.7 MPa, against approximately -1.45 MPa for the two others species. Such a large vulnerability was confirmed by recording losses of conductivity during a gradual drought. Moreover, significant stem embolism was recorded before stomatal closure, indicating the lack of an efficient safety margin for hydraulic functions in this species. Embolism was not reversed by rewatering. These observations are discussed with respect to the ecology of P. euphratica.

Asia↗

PubMed2005

The potential of metabolite profiling as a selection tool for genotype discrimination in Populus.

Differences between wild-type Populus tremulaxalba and two transgenic lines with modified lignin monomer composition, were interrogated using metabolic profiling. Analysis of metabolite abundance data by GC-MS, coupled with principal components analysis (PCA), successfully differentiated between lines that had distinct phenotypes, whether samples were taken from the cambial zone or non-lignifying suspension tissue cultures. Interestingly, the GC-MS analysis detected relatively few phenolic metabolites in cambial extracts, although a single metabolite associated with the differentiation between lines was directly related to the phenylpropanoid pathway or other down-stream aspects of lignin biosynthesis. In fact, carbohydrates, which have only an indirect relationship with the modified lignin monomer composition, featured strongly in the line-differentiating aspects of the statistical analysis. Traditional HPLC analysis was employed to verify the GC-MS data. These findings demonstrate that metabolic traits can be dissected reliably and accurately by metabolomic analyses, enabling the discrimination of individual genotypes of the same tree species that exhibit marked differences in industrially relevant wood traits. Furthermore, this validates the potential of using metabolite profiling techniques for marker generation in the context of plant/tree breeding for industrial applications.

Chromatography, High Pressure Liquid↗

PubMed2005

Stomatal development in new leaves is related to the stomatal conductance of mature leaves in poplar (Populus trichocarpaxP. deltoides).

In general, stomatal density (SD) decreases when plants are grown at high CO2 concentrations. Recent studies suggest that signals produced from mature leaves regulate the SD of expanding leaves. To determine the underlying driver of these signals in poplar (Populus trichocarpaxP. deltoides) saplings, a cuvette system was used whereby the environment around mature (lower) leaves could be controlled independently of that around developing (upper) leaves. A series of experiments were performed in which the CO2 concentration, vapour pressure deficit (D), and irradiance (Q) around the lower leaves were varied while the (ambient) conditions around the upper leaves were unchanged. The overall objective was to break the nexus between leaf stomatal conductance and transpiration and photosynthesis rates of lower leaves and determine which, if any, of these parameters regulate stomatal development in the upper expanding leaves. SD, stomatal index (SI), and epidermal cell density (ED) were measured on the adaxial and abaxial surfaces of fully expanded upper leaves. SD and SI decreased with increasing lower leaf CO2 concentration (150-780 ppm) at both ambient (1.3-1.6 kPa) and low (0.7-1.0 kPa) D. SD and SI at low D were generally higher than at ambient D. By contrast, ED was relatively insensitive to both vapour pressure and CO2 concentration. When lower leaves were shaded, upper leaf SD, SI, and ED decreased but did not change with varying CO2 concentration. These results suggest that epidermal cell development and stomatal development are regulated by different physiological mechanisms. SI of the upper leaves was positively and highly correlated (r2>0.84) with the stomatal conductance of the lower leaves independent of their net photosynthesis and transpiration rates, suggesting that the stomatal conductance of mature leaves has a regulatory effect on the stomatal development of expanding leaves.

Carbon Dioxide↗

PubMed2005

Molecular population genetics of herbivore-induced protease inhibitor genes in European aspen (Populus tremula L., Salicaceae).

Plants defend themselves against the attack of natural enemies by using an array of both constitutively expressed and induced defenses. Long-lived woody perennials are overrepresented among plant species that show strong induced defense responses, whereas annual plants and crop species are underrepresented. However, most studies of plant defense genes have been performed on annual or short-lived perennial weeds or crop species. Here I use molecular population genetic methods to survey six wound-inducible protease inhibitors (PIs) in a long-lived woody, perennial plant species, the European aspen (Populus tremula), to evaluate the likelihood of either recurrent selective sweeps or balancing selection maintaining amino acid polymorphisms in these genes. The results show that none of the six PI genes have reduced diversities at synonymous sites, as would be expected in the presence of recurrent selective sweeps. However, several genes show some evidence of nonneutral evolution such as enhanced linkage disequilibrium and a large number of high-frequency-derived mutations. A group of at least four Kunitz trypsin inhibitor genes appear to have experienced elevated levels of nonsynonymous substitutions, indicating allelic turnover on an evolutionary timescale. One gene, TI1, has enhanced levels of intraspecific polymorphism at nonsynonymous sites and also has an unusual haplotype structure characterized by two divergent haplotypes occurring at roughly equal frequencies in the sample. One haplotype has very low levels of intraallelic nucleotide diversity, whereas the other haplotype has levels of diversity comparable to other genes in P. tremula. Patterns of sequence diversity at TI1 do not fit a simple model of either balancing selection or recurrent selective sweeps. This suggests that selection at TI1 is more complex, possibly involving allelic cycling.

Base Sequence↗