Explore the vast range of titles available.

SHORT-ROOT (SHR) is a GRAS transcription factor first characterized for its role in the specification of the stem cell niche and radial patterning in Arabidopsis thaliana (At) roots. Three SHR-like genes have been identified in Populus trichocarpa (Pt). PtSHR1 shares high similarity with AtSHR over the entire length of the coding sequence. The two other Populus SHR-like genes, PtSHR2A and PtSHR2B, are shorter in their 5' ends when compared with AtSHR. Unlike PtSHR1, that is expressed throughout the cambial zone of greenhouse-grown Populus trees, PtSHR2Bprom:uidA expression was detected in the phellogen. Additionally, PtSHR1 and PtSHR2B expression patterns markedly differ in the shoot apex and roots of in vitro plants. Transgenic hybrid aspen expressing PtSHR2B under the 35S constitutive promoter showed overall reduced tree growth while the proportion of bark increased relative to the wood. Reverse transcription–quantitative PCR (RT–qPCR) revealed increased transcript levels of cytokinin metabolism and response-related genes in the transgenic plants consistent with an increase of total cytokinin levels. This was confirmed by cytokinin quantification by LC-MS/MS. Our results indicate that PtSHR2B appears to function in the phellogen and therefore in the regulation of phellem and periderm formation, possibly acting through modulation of cytokinin homeostasis. Furthermore, this work points to a functional diversification of SHR after the divergence of the Populus and Arabidopsis lineages. This finding may contribute to selection and breeding strategies of cork oak in which, unlike Populus, the phellogen is active throughout the entire tree lifespan, being at the basis of a highly profitable cork industry.

A novel method was tested for improving tree breeding strategies that integrate quantitative and population genetics based on range‐wide reciprocal transplant experiments. Five reciprocal common garden tests of Populus tremuloides were investigated including 6450 trees across western Canada focusing on adaptation traits and growth. Both genetic parameters and home‐site transplant models were evaluated. We found a genetic trade‐off between growth and early spring leaf flush and late fall senescence. Coefficients of phenotypic variation (CVp) of cell lysis (CL), a measure of freezing injury, shrank from 0.28 to 0.10 during acclimation in the fall, and the CVp slope versus the freezing temperature was significantly different from zero (R2 = 0.33, p = .02). There was more between‐population genetic variation in fall phenology than in spring leaf phenology. We suggest that P. tremuloides demonstrated a discrepancy between the ecological optimum and the physiological optimum minimum winter temperature. The sub‐optimal growing condition of P. tremuloides is potentially caused by the warmer ecological optimum than the physiological optimum. Assisted migration and breeding of fast growers to reforest cooler plantation sites can improve productivity. Transferring the study populations to less than 4°C of extreme minimum temperature appears safe for reforestation aligning with the historical recolonization direction of the species. This is equivalent to a 5–10° latitudinal northward movement. Fall frost hardiness is an effective criterion for family selection in the range tested in this study. The sub‐optimality of local Populus tremuloides demonstrated discrepancies between the ecological optimum and physiological optimum and the tree species tended to express a colder ecological optimum than the physiological optimum. Transferring the study populations (<‐4°C of extreme minimum temperature, EMT) appears safe for reforestation, equivalent to 5‐10° latitudinal northward movement.

• Costs of defense are thought to maintain genetic variations in the expression of defense within plant populations. As with many plant species, aspen exhibits considerable variation in allocation to secondary metabolites. This study examined the independent and interactive effects of genotype, soil fertility and belowground competition on defensive chemistry and growth in trembling aspen (Populus tremuloides). • Four aspen genotypes were grown with high and low soil fertility, and with and without root competition. Physiological, morphological and allocational determinants of growth were measured to identify growth-defense tradeoffs. • Nutrient limitation and competition decreased growth, leaf mass ratio, leaf nitrogen concentration and photosynthesis, and increased root : shoot ratio and leaf condensed tannin concentrations. The competition treatment also resulted in increased leaf phenolic glycoside (PG) concentrations. • Aspen growth was negatively correlated with PG concentrations under low fertility with competition. The relationship between growth and its major determinants was also negatively related to foliar condensed tannins expressed as a proportion of tree mass, indicating an additional indirect cost of allocation to secondary metabolites.

The world's forests are currently exposed to increasing concentrations of carbon dioxide (CO sub(2)) and ozone (O sub(3)). Both pollutants can potentially exert a selective effect on plant populations. This, in turn, may lead to changes in ecosystem properties, such as carbon sequestration. Here, we report how elevated CO sub(2) and O sub(3) affect the genetic composition of a woody plant population via altered survival.Using data from the Aspen free-air CO sub(2) enrichment (FACE) experiment (in which aspen clones were grown in factorial combinations of CO sub(2) and O sub(3)), we develop a hierarchical Bayesian model of survival. We also examine how survival differences between clones could affect pollutant responses in the next generation.Our model predicts that the relative abundance of the tested clones, given equal initial abundance, would shift under either elevated CO sub(2) or O sub(3) as a result of changing survival rates. Survival was strongly affected by between-clone differences in growth responses. Selection could noticeably decrease O sub(3) sensitivity in the next generation, depending on the heritability of growth responses and the distribution of seed production. The response to selection by CO sub(2), however, is likely to be small.Our results suggest that the changing atmospheric composition could shift the genotypic composition and average pollutant responses of tree populations over moderate timescales.

The concentration of CO sub(2) in the Earth's atmosphere has increased over the last century. Although this increase is unlikely to have direct effects on soil microbial communities, increased atmospheric CO sub(2) may impact soil ecosystems indirectly through plant responses. This study tested the hypothesis that exposure of plants to elevated CO sub(2) would impact soil microorganisms responsible for key nitrogen cycling processes, specifically denitrification and nitrification. We grew trembling aspen (Populus tremuloides) trees in outdoor chambers under ambient (360 ppm) or elevated (720 ppm) levels of CO sub(2) for 5 years and analyzed the microbial communities in the soils below the trees using quantitative polymerase chain reaction and clone library sequencing targeting the nitrite reductase (nirK) and ammonia monooxygenase (amoA) genes. We observed a more than twofold increase in copy numbers of nirK and a decrease in nirK diversity with CO sub(2) enrichment, with an increased predominance of Bradyrhizobia-like nirK sequences. We suggest that this dramatic increase in nirK-containing bacteria may have contributed to the significant loss of soil N in the CO sub(2)-treated chambers. Elevated CO sub(2) also resulted in a significant decrease in copy numbers of bacterial amoA, but no change in archaeal amoA copy numbers. The decrease in abundance of bacterial amoA was likely a result of the loss of soil N in the CO sub(2)-treated chambers, while the lack of response for archaeal amoA supports the hypothesis that physiological differences in these two groups of ammonia oxidizers may enable them to occupy distinct ecological niches and respond differently to environmental change.

Summary •  Saplings of three aspen (Populus tremuloides) genotypes and seedlings of paper birch (Betula papyrifera) were exposed to elevated ozone (1.5× ambient) and 560 p.p.m. CO2, singly and in combination, from 1998 at the Aspen‐FACE (free‐air CO2 enrichment) site (Rhinelander, USA). •  The plants were studied for H2O2 accumulation within the leaf mesophyll, number of peroxisomes, level of gene expression for catalase (Cat), and changes in ultrastructure. •  In tolerant clones, ozone‐elicited excess H2O2 production was restricted to the apoplast, without any ultrastructural injuries. This was associated with ozone‐induced proliferation of peroxisomes and increased transcript levels of Cat. In sensitive plants, ozone‐induced H2O2 accumulation continued from the cell wall to the plasma membrane, cytosol and chloroplasts, particularly in older leaves. However, chloroplastic precipitation was absent in the presence of elevated CO2. In the most sensitive aspen clone, H2O2 accumulation was found in conjunction with chloroplast injuries, low number of peroxisomes and low cell wall volume, whereas in birch a simultaneous increase in cell wall thickness indicated defence activation. •  Our results indicate that oxidative stress manifests as H2O2 effects on leaf ultrastructure in sensitive trees exposed to elevated ozone. However, CO2 enrichment appears to alleviate chloroplastic oxidative stress.

In documenting biological responses to climate change, the Intergovernmental Panel on Climate Change has used phenology studies from many parts of the world, hut data from the high latitudes of North America are missing. In the present article, we evaluate climate trends and the corresponding changes in sequential bloom times for seven plant species in the central parklands of Alberta, Canada (latitude 52 degree degree --57 degree degree north). For the study period of 71 years (1936--2006), we found a substantial warming signal, which ranged from an increase of 5.3 degrees Celsius CC) in the mean monthly temperatures for February to an increase of 1.5 degree degree C in those for May. The earliest-blooming species' (Populus tremuloides and Anemone patens) bloom dates advanced by two weeks during the seven decades, whereas the later-blooming species' bloom dates advanced between zero and six days. The early-blooming species' bloom dates advanced faster than was predicted by thermal time models, which we attribute to decreased diurnal temperature fluctuations. This unexpectedly sensitive response results in an increased exposure to late-spring frosts.

  Trembling aspen (Populus tremuloides) is widely distributed in North American forests. Increased stand productivity with resource availability has been reported, but the relationship between soil microbial community structure and stand productivity remains unclear. To examine soil microbial composition of 4 aspen stand productivity classes, we assessed soil properties, microbial biomass and respiration, and bacterial and ectomycorrhizal diversity. Most variables showed no significant differences between productivity classes. However, mean values for basal respiration (0.05 to 27.99 µg CO^sub 2^-C·g^sup -1^ soil·h^sup -1^), bacterial biomass, and metabolic quotient (0.08 to 5.22 CO^sub 2^-C·mg^sub -1^ C^sub mic^·h^sup -1^) were lowest in low productivity (Class 1) sites. Bacteria to fungi ratios were significantly lower (P = 0.05) in Class 1 compared to other classes. Microbial biomass ranged from 1.39 to 8.11 mg C^sub mic^·g^sup -1^ soil. Thirty-seven distinct aspen ectomycorrhizas (ECM) were characterized, 21 were considered rare (from ≤3 trees). ECM richness did not differ significantly between classes, although relative abundance for some types did. Canonical correspondence analysis showed productivity class explained most microbial community variation, e.g., ECM fungi (80% explained) and soil bacteria (46%). Despite some differences, we could not identify statistically significant bacterial or ECM assemblages linked to stand productivity. Results may reflect a strong association between microbial processes and the dominant host, aspen. Aspen associated with widely distributed fungi common to all classes, possibly facilitating its survival and growth, including on sites exhibiting low pH and low soil fertility. [PUBLICATION ABSTRACT]

The Populus genus is one of the major plant model systems, but genomic resources have thus far primarily been available for poplar species, and primarily Populus trichocarpa (Torr. & Gray), which was the first tree with a whole-genome assembly. To further advance evolutionary and functional genomic analyses in Populus, we produced genome assemblies and population genetics resources of two aspen species, Populus tremula L. and Populus tremuloides Michx. The two aspen species have distributions spanning the Northern Hemisphere, where they are keystone species supporting a wide variety of dependent communities and produce a diverse array of secondary metabolites. Our analyses show that the two aspens share a similar genome structure and a highly conserved gene content with P. trichocarpa but display substantially higher levels of heterozygosity. Based on population resequencing data, we observed widespread positive and negative selection acting on both coding and noncoding regions. Furthermore, patterns of genetic diversity and molecular evolution in aspen are influenced by a number of features, such as expression level, coexpression network connectivity, and regulatory variation. To maximize the community utility of these resources, we have integrated all presented data within the PopGenIE web resource (PopGenIE.org).

$\\bullet$ To develop a robust basis for inferences about population genetics and evolution, this work assayed 192 aspens (Populus tremuloides) from 11 sites in Wisconsin, USA, for allelic and population variation at 16 microsatellite loci distributed across the Populus genome. $\\bullet$ Frequency distributions of fluorochrome-labeled alleles resolved by capillary electrophoresis were analyzed for relationships to repeat size and number. Population-level statistics were compared with those of other studies, especially in Populus. $\\bullet$ All loci were polymorphic, varying widely in the number of alleles per locus (mean = 8.25, range 2-20). Expected and observed heterozygosities were high (0.45 and 0.41, respectively), with little differentiation among populations ($F_{ST} = 0.006-0.045$) and a moderate level of inbreeding ($F_{IS} = 0.09$), intermediate among levels reported in studies based on isozymes. $\\bullet$ Contrary to several other reports, allele frequencies clustered tightly around the modal frequency, and the genetic diversity (measured as alleles per locus or as expected heterozygosity) was not related to either the repeat unit size or to the number of repeats.