Growth In Poplar Research Articles

A role for SPEECHLESS in the integration of leaf stomatal patterning with the growth vs disease trade-off in poplar.

Occurrence of stomata on both leaf surfaces (amphistomaty) promotes higher stomatal conductance and photosynthesis while simultaneously increasing exposure to potential disease agents in black cottonwood (Populus trichocarpa). A genome-wide association study (GWAS) with 2.2M single nucleotide polymorphisms generated through whole-genome sequencing found 280 loci associated with variation in adaxial stomatal traits, implicating genes regulating stomatal development and behavior. Strikingly, numerous loci regulating plant growth and response to biotic and abiotic stresses were also identified. The most significant locus was a poplar homologue of SPEECHLESS (PtSPCH1). Individuals possessing PtSPCH1 alleles associated with greater adaxial stomatal density originated primarily from environments with shorter growing seasons (e.g. northern latitudes, high elevations) or with less precipitation. PtSPCH1 was expressed in developing leaves but not developing stem xylem. In developing leaves, RNA sequencing showed patterns of coordinated expression between PtSPCH1 and other GWAS-identified genes. The breadth of our GWAS results suggests that the evolution of amphistomaty is part of a larger, complex response in plants. Suites of genes underpin this response, retrieved through genetic association to adaxial stomata, and show coordinated expression during development. We propose that the occurrence of amphistomaty in P.trichocarpa involves PtSPCH1 and reflects selection for supporting rapid growth over investment in immunity.

LHY2 Integrates Night-Length Information to Determine Timing of Poplar Photoperiodic Growth

Day length is a key indicator of seasonal information that determines major patterns of behavior in plants and animals. Photoperiodism has been described in plants for about 100 years, but the underlying molecular mechanisms of day length perception and signal transduction in many systems are not well understood. In trees, photoperiod perception plays a major role in growth cessation during the autumn as well as activating the resumption of shoot growth in the spring, both processes controlled by FLOWERING LOCUS T2 (FT2) expression levels and critical for the survival of perennial plants over winter [1-4]. It has been shown that the conserved role of poplar orthologs to Arabidopsis CONSTANS (CO) directly activates FT2 expression [1, 5]. Overexpression of poplar CO is, however, not sufficient to sustain FT2 expression under short days [5], pointing to the presence of an additional short-day-dependent FT2 repression pathway in poplar. We find that night length information is transmitted via the expression level of a poplar clock gene, LATE ELONGATED HYPOCOTYL 2 (LHY2), which controls FT2 expression. Repression of FT2 is a function of the night extension and LHY2 expression level. We show that LHY2 is necessary and sufficient to activate night length repressive signaling. We propose that the photoperiodic control of shoot growth in poplar involves a balance between FT2 activating and repressing pathways. Our results show that poplar relies on night length measurement to determine photoperiodism through interaction between light signaling pathways and the circadian clock.

Genome-wide identification of BOR genes in poplar and their roles in response to various environmental stimuli

Boron (B) is one of the microelements that is essential for plant growth and fitness. The uptake and distribution of B is facilitated by B transporters (BORs) that participate in many physiological and biochemical processes. In the current study, we identified 519 BOR proteins from 86 plant species utilizing their latest genomic database. Evolutionary analysis demonstrated that the BORs in plants can be clustered into three groups. Groups 1 and 2 represent the previously defined “BOR1-like” and “BOR4-like” proteins, respectively. Evidence from an evolutionary tree suggested that the BORs in flowering plants originated from two distinct ancestors: BOR1-like genes were from Bryophyta, whereas BOR4-like genes were produced after the segregation between gymnosperms and angiosperms. A total of ten PtrBOR genes were identified in the genome of Populus trichocarpa, with five members in Group 1 and Group 2, respectively. The identified PtrBOR genes were distributed across 9 different chromosomes and contained 10–12 introns. Each PtrBOR protein harbored three conserved HCO3− transporter domains and at least 10 transmembrane domains (TMDs). Gene duplication analysis indicated that segmental duplication played important roles in expanding this gene family, and these gene pairs have been undergoing purifying selection. In silico analysis of gene expression levels within tissues implicated BOR1-like genes and PtrBOR 6/9 in B uptake and reproductive organ formation, respectively. Coexpression analysis of PtrBOR genes uncovered their functions in nitrogen metabolism and response to drought, salt and heavy metal stresses, as well as pollen tube formation. In addition, the expression levels of the PtrBOR genes were verified by quantitative real-time PCR in roots and leaves from “NL895” (P. × euramericana) subjected to diverse abiotic stresses, including low B (LB), high B (HB), drought, cadmium (Cd), low nitrogen (LN) and salt conditions. Transcripts of BOR1-like genes were much more abundant than those of BOR4-like genes, whereas BOR4-like genes displayed much more sensitivity than BOR1-like genes in the roots of “NL895” in response to all tested abiotic stresses. In particular, PtrBOR6 from the BOR4-like group showed the largest variations among these PtrBOR genes and was induced under all the tested conditions in roots and leaves. These results will aid in the elucidation of the different roles played by the BOR1-like and BOR4-like genes in the growth, development, and stress response in poplar.

Photoperiodic Regulation of Shoot Apical Growth in Poplar.

Woody perennials adapt their genetic traits to local climate conditions. Day length plays an essential role in the seasonal growth of poplar trees. When photoperiod falls below a given critical day length, poplars undergo growth cessation and bud set. A leaf-localized mechanism of photoperiod measurement triggers the transcriptional modulation of a long distance signaling molecule, FLOWERING LOCUS T (FT). This molecule targets meristem function giving rise to these seasonal responses. Studies over the past decade have identified conserved orthologous genes involved in photoperiodic flowering in Arabidopsis that regulate poplar vegetative growth. However, phenological and molecular examination of key photoperiod signaling molecules reveals functional differences between these two plant model systems suggesting alternative components and/or regulatory mechanisms operating during poplar vegetative growth. Here, we review current knowledge and provide new data regarding the molecular components of the photoperiod measuring mechanism that regulates annual growth in poplar focusing on main achievements and new perspectives.

Functional analysis of overexpressed PtDRS1 involved in abiotic stresses enhances growth in transgenic poplar

Drought and salinity are two main abiotic stressors that can disrupt plant growth and survival. Various biotechnological approaches have been used to alleviate the problem of drought stress by improving water stress resistance in forestry and agriculture. The drought sensitive 1 (DRS1) gene acts as a regulator of drought stress, identified in human, yeast and some model plants, such as Arabidopsis thaliana, but there have been no reports of DRS1 transformation in poplar plants to date. In this study, we transformed the DRS1 gene from Populus trichocarpa into Populus deltoides × Populus euramericana 'Nanlin895' using Agrobacterium tumefaciens-mediated transformation. We confirmed that the DRS1 gene was transformed into 'Nanlin895' poplar genomes using reverse transcription polymerase chain reaction (PCR), multiplex PCR, real-time PCR, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. All transformed and wild-type (WT) plants were then transferred into a greenhouse for complementary experiments. We analyzed the physiological and biochemical responses of transgenic plants under drought and salt stresses in the greenhouse, and the results were compared with control WT plants. Responses to abiotic stress were greater in transgenic plants compared with WT. Based on our results, introduction of the DRS1 gene into poplar 'Nanlin895' plants significantly enhanced the resistance of those plants to water deficit and high salinity, allowing higher growth rates of roots and shoots in those plants. Additionally, the clawed root rate increased in transformed poplars grown in culture media or in soil, and improved survival under drought and salt stress conditions.

Chilling-responsive DEMETER-LIKE DNA demethylase mediates in poplar bud break.

Annual dormancy-growth cycle is a developmental and physiological process essential for the survival of deciduous trees in temperate and boreal forests. Seasonal control of shoot growth in woody perennials requires specific genetic programmes responding to environmental signals. The environmental-controlled mechanisms that regulate the shift between winter dormancy and the growth-promoting genetic programmes are still unknown. Here, we show that dynamics in genomic DNA methylation levels are involved in the regulation of dormancy-growth cycle in poplar. The reactivation of growth in the apical shoot during bud break process in spring is preceded by a progressive reduction of genomic DNA methylation in apex tissue. The induction in apex tissue of a chilling-dependent poplar DEMETER-LIKE 10 (PtaDML10) DNA demethylase precedes shoot growth reactivation. Transgenic poplars showing downregulation of PtaDML8/10 caused delayed bud break. Genome-wide transcriptome and methylome analysis and data mining revealed that the gene targets of DEMETER-LIKE-dependent DNA demethylation are genetically associated with bud break. These data point to a chilling-dependent DEMETER-like DNA demethylase mechanisms being involved in the shift from winter dormancy to a condition that precedes shoot apical vegetative growth in poplar.

Overexpression of Populus trichocarpa CYP85A3 promotes growth and biomass production in transgenic trees.

SummaryBrassinosteroids (BRs) are essential hormones that play crucial roles in plant growth, reproduction and response to abiotic and biotic stress. In Arabidopsis, AtCYP85A2 works as a bifunctional cytochrome P450 monooxygenase to catalyse the conversion of castasterone to brassinolide, a final rate‐limiting step in the BR‐biosynthetic pathway. Here, we report the functional characterizations of PtCYP85A3, one of the three AtCYP85A2 homologous genes from Populus trichocarpa. PtCYP85A3 shares the highest similarity with AtCYP85A2 and can rescue the retarded‐growth phenotype of the Arabidopsis cyp85a2‐2 and tomato d x mutants. Constitutive expression of PtCYP85A3, driven by the cauliflower mosaic virus 35S promoter, increased the endogenous BR levels and significantly promoted the growth and biomass production in both transgenic tomato and poplar. Compared to the wild type, plant height, shoot fresh weight and fruit yield increased 50%, 56% and 43%, respectively, in transgenic tomato plants. Similarly, plant height and stem diameter increased 15% and 25%, respectively, in transgenic poplar plants. Further study revealed that overexpression of PtCYP85A3 enhanced xylem formation without affecting the composition of cellulose and lignin, as well as the cell wall thickness in transgenic poplar. Our finding suggests that PtCYP85A3 could be used as a potential candidate gene for engineering fast‐growing trees with improved wood production.

Down-regulation of GIGANTEA-like genes increases plant growth and salt stress tolerance in poplar.

SummaryThe flowering time regulator GIGANTEA (GI) connects networks involved in developmental stage transitions and environmental stress responses in Arabidopsis. However, little is known about the role of GI in growth, development and responses to environmental challenges in the perennial plant poplar. Here, we identified and functionally characterized three GI‐like genes (PagGIa, PagGIb and PagGIc) from poplar (Populus alba × Populus glandulosa). PagGIs are predominantly nuclear localized and their transcripts are rhythmically expressed, with a peak around zeitgeber time 12 under long‐day conditions. Overexpressing PagGIs in wild‐type (WT) Arabidopsis induced early flowering and salt sensitivity, while overexpressing PagGIs in the gi‐2 mutant completely or partially rescued its delayed flowering and enhanced salt tolerance phenotypes. Furthermore, the PagGIs‐PagSOS2 complexes inhibited PagSOS2‐regulated phosphorylation of PagSOS1 in the absence of stress, whereas these inhibitions were eliminated due to the degradation of PagGIs under salt stress. Down‐regulation of PagGIs by RNA interference led to vigorous growth, higher biomass and enhanced salt stress tolerance in transgenic poplar plants. Taken together, these results indicate that several functions of Arabidopsis GI are conserved in its poplar orthologues, and they lay the foundation for developing new approaches to producing salt‐tolerant trees for sustainable development on marginal lands worldwide.

Sucrose phosphate synthase and sucrose phosphate phosphatase interact in planta and promote plant growth and biomass accumulation.

Bioinformatic analysis indicates that sucrose phosphate synthase (SPS) contains a putative C-terminal sucrose phosphate phosphatase (SPP)-like domain that may facilitates the binding of SPP. If an SPS-SPP enzyme complex exists, it may provide sucrose biosynthesis with an additional level of regulation, forming a direct metabolic channel for sucrose-6-phosphate between these two enzymes. Herein, the formation of an enzyme complex between SPS and SPP was examined, and the results from yeast two-hybrid experiments suggest that there is indeed an association between these proteins. In addition, in planta bioluminescence resonance energy transfer (BRET) was observed in Arabidopsis seedlings, providing physical evidence for a protein interaction in live cells and in real time. Finally, bimolecular fluorescence complementation (BiFC) was employed in an attempt to detect SPS-SPP interactions visually. The findings clearly demonstrated that SPS interacts with SPP and that this interaction impacts soluble carbohydrate pools and affects carbon partitioning to starch. Moreover, a fusion construct between the two genes promotes plant growth in both transgenic Arabidopsis and hybrid poplar.

Bt-Cry3Aa transgene expression reduces insect damage and improves growth in field-grown hybrid poplar

The stability and value of transgenic pest resistance for promoting tree growth are poorly understood. These data are essential for determining if such trees could be beneficial to commercial growers in the face of substantial regulatory and marketing costs. We investigated growth and insect resistance in hybrid poplar expressing the cry3Aa transgene in two field trials. An initial screening of 502 trees comprising 51 transgenic gene insertion events in four clonal backgrounds (Populus trichocarpa × Populus deltoides, clones 24-305, 50-197, and 198-434; and P. deltoides × Populus nigra, clone OP-367) resulted in transgenic trees with greatly reduced insect damage. A large-scale study of 402 trees from nine insertion events in clone OP-367, conducted over two growing seasons, demonstrated reduced tree damage and significantly increased volume growth (mean 14%). Quantification of Cry3Aa protein indicated high levels of expression, which continued after 14 years of annual or biannual coppice in a clone bank. With integrated management, the cry3Aa gene appears to be a highly effective tool for protecting against leaf beetle damage and improving yields from poplar plantations.

Epigenetic control of heavy metal stress response in mycorrhizal versus non-mycorrhizal poplar plants

It was previously shown that arbuscular mycorrhizal fungi (AMF) exert a significant improvement of growth in a tolerant white poplar (Populus alba L.) clone (AL35) grown on Cu- and Zn-polluted soil via foliar alterations in the levels of defence/stress-related transcripts and molecules. However, nothing is known about the epigenetic changes which occur during tolerance acquisition in response to heavy metals (HMs) in the same mycorrhizal vs. non-mycorrhizal poplar plants. In order to analyse the epigenome in leaves of AL35 plants inoculated or not with AMF and grown in a greenhouse on multimetal polluted or unpolluted soil, the Methylation Sensitive Amplification Polymorphism (MSAP) approach was adopted to detect cytosine DNA methylation. Modest changes in cytosine methylation patterns were detected at first sampling (4 months from planting), whereas extensive alterations (hypomethylation) occurred at second sampling (after 6 months) in mycorrhizal plants grown in the presence of HMs. The sequencing of MSAP fragments led to the identification of genes belonging to several Gene Ontology categories. Seven MSAP fragments, selected on the basis of DNA methylation status in treated vs control AL35 leaves at the end of the experiment, were analysed for their transcript levels by means of qRT-PCR. Gene expression varied in treated samples relative to controls in response to HMs and/or AMF inoculation; in particular, transcripts of genes involved in RNA processing, cell wall and amino acid metabolism were upregulated in the presence of AMF with or without HMs.

Integrating genome annotation and QTL position to identify candidate genes for productivity, architecture and water-use efficiency in Populus spp

BackgroundHybrid poplars species are candidates for biomass production but breeding efforts are needed to combine productivity and water use efficiency in improved cultivars. The understanding of the genetic architecture of growth in poplar by a Quantitative Trait Loci (QTL) approach can help us to elucidate the molecular basis of such integrative traits but identifying candidate genes underlying these QTLs remains difficult. Nevertheless, the increase of genomic information together with the accessibility to a reference genome sequence (Populus trichocarpa Nisqually-1) allow to bridge QTL information on genetic maps and physical location of candidate genes on the genome. The objective of the study is to identify QTLs controlling productivity, architecture and leaf traits in a P. deltoides x P. trichocarpa F1 progeny and to identify candidate genes underlying QTLs based on the anchoring of genetic maps on the genome and the gene ontology information linked to genome annotation. The strategy to explore genome annotation was to use Gene Ontology enrichment tools to test if some functional categories are statistically over-represented in QTL regions.ResultsFour leaf traits and 7 growth traits were measured on 330 F1 P. deltoides x P. trichocarpa progeny. A total of 77 QTLs controlling 11 traits were identified explaining from 1.8 to 17.2% of the variation of traits. For 58 QTLs, confidence intervals could be projected on the genome. An extended functional annotation was built based on data retrieved from the plant genome database Phytozome and from an inference of function using homology between Populus and the model plant Arabidopsis. Genes located within QTL confidence intervals were retrieved and enrichments in gene ontology (GO) terms were determined using different methods. Significant enrichments were found for all traits. Particularly relevant biological processes GO terms were identified for QTLs controlling number of sylleptic branches: intervals were enriched in GO terms of biological process like ‘ripening’ and ‘adventitious roots development’.ConclusionBeyond the simple identification of QTLs, this study is the first to use a global approach of GO terms enrichment analysis to fully explore gene function under QTLs confidence intervals in plants. This global approach may lead to identification of new candidate genes for traits of interest.

PHOTOPERIOD RESPONSE 1 (PHOR1)-like Genes Regulate Shoot/root Growth, Starch Accumulation, and Wood Formation in Populus

This study describes functional characterization of two putative poplar PHOTOPERIOD RESPONSE 1 (PHOR1) orthologues. The expression and sequence analyses indicate that the two poplar genes diverged, at least partially, in function. PtPHOR1_1 is most highly expressed in roots and induced by short days, while PtPHOR1_2 is more uniformly expressed throughout plant tissues and is not responsive to short days. The two PHOR1 genes also had distinct effects on shoot and root growth when their expression was up- and downregulated transgenically. PtPHOR1_1 effects were restricted to roots while PtPHOR1_2 had similar effects on aerial and below-ground development. Nevertheless, both genes seemed to be upregulated in transgenic poplars that are gibberellin-deficient and gibberellin-insensitive, suggesting interplay with gibberellin signalling. PHOR1 suppression led to increased starch accumulation in both roots and stems. The effect of PHOR1 suppression on starch accumulation was coupled with growth-inhibiting effects in both roots and shoots, suggesting that PHOR1 is part of a mechanism that regulates the allocation of carbohydrate to growth or storage in poplar. PHOR1 downregulation led to significant reduction of xylem formation caused by smaller fibres and vessels suggesting that PHOR1 likely plays a role in the growth of xylem cells.

Reduced Expression of the SHORT-ROOT Gene Increases the Rates of Growth and Development in Hybrid Poplar and Arabidopsis

SHORT-ROOT (SHR) is a well characterized regulator of cell division and cell fate determination in the Arabidopsis primary root. However, much less is known about the functions of SHR in the aerial parts of the plant. In this work, we cloned SHR gene from Populus trichocarpa (PtSHR1) as an AtSHR ortholog and down-regulated its expression in hybrid poplar (Populus tremula×P. tremuloides Michx-clone T89) in order to determine its physiological functions in shoot development. Sharing a 90% similarity to AtSHR at amino acid level, PtSHR1 was able to complement the Arabidopsis shr mutant. Down regulation of PtSHR1 led to a strong enhancement of primary (height) and secondary (girth) growth rates in the transgenic poplars. A similar approach in Arabidopsis showed a comparable accelerated growth and development phenotype. Our results suggest that the response to SHR could be dose-dependent and that a partial down-regulation of SHR could lead to enhanced meristem activity and a coordinated acceleration of plant growth in woody species. Therefore, SHR functions in plant growth and development as a regulator of cell division and meristem activity not only in the roots but also in the shoots. Reducing SHR expression in transgenic poplar was shown to lead to significant increases in primary and secondary growth rates. Given the current interest in bioenergy crops, SHR has a broader role as a key regulator of whole plant growth and development and SHR suppression has considerable potential for accelerating biomass accumulation in a variety of species.

Expression Analysis of Two NAC Transcription Factors PtNAC068 and PtNAC154 from Poplar

NAM/ATAF/CUC (NAC) family genes comprise one of the largest families of transcription factors in plant genomes and are widely expressed in developing woody tissues. In the present study, we constructed plant transformation vectors using the β-glucuronidase (GUS) reporter gene system and detected the promoter expression patterns derived from the PtNAC068 and PtNAC154 genes of Chinese white poplar (Populus tomentosa Carr.) in transgenic poplars (Populus alba × Populus glandulosa). The results showed that the GUS expression driven by PtNAC068 and PtNAC154 promoters may be more complex in poplar than they are in Arabidopsis. Histochemical GUS assays showed that GUS activity driven by PtNAC068 promoter was mainly in vascular tissues of stems, leaves, petioles, and roots, while that driven by PtNAC154 promoter was confined to the developing secondary xylem of stems and veins of leaves. The transcript level of both PtNAC068 and PtNAC154 in successive internodes below the apex was found to be much higher in IN5-10 compared to that in IN2-4 as measured by real-time RT-PCR, suggesting that PtNAC068 and PtNAC154 upregulation is related to secondary growth in poplar. GUS expression in internodes 3–8 of ProNAC068::GUS transgenic plants was 30-fold higher than that in ProNAC154::GUS transgenic plants. The differences in the expression pattern and transcript level of mRNA accumulation indicate that PtNAC068 and PtNAC154 may be involved in two distinct aspects of vascular tissue development.

Transgenic poplar expressing Arabidopsis NDPK2 enhances growth as well as oxidative stress tolerance

Nucleoside diphosphate kinase 2 (NDPK2) is known to regulate the expression of antioxidant genes in plants. Previously, we reported that overexpression of Arabidopsis NDPK2 (AtNDPK2) under the control of an oxidative stress-inducible SWPA2 promoter in transgenic potato and sweetpotato plants enhanced tolerance to various abiotic stresses. In this study, transgenic poplar (Populus alba × Poplus glandulosa) expressing the AtNDPK2 gene under the control of a SWPA2 promoter (referred to as SN) was generated to develop plants with enhanced tolerance to oxidative stress. The level of AtNDPK2 expression and NDPK activity in SN plants following methyl viologen (MV) treatment was positively correlated with the plant's tolerance to MV-mediated oxidative stress. We also observed that antioxidant enzyme activities such as ascorbate peroxidase, catalase and peroxidase were increased in MV-treated leaf discs of SN plants. The growth of SN plants was substantially increased under field conditions including increased branch number and stem diameter. SN plants exhibited higher transcript levels of the auxin-response genes IAA2 and IAA5. These results suggest that enhanced AtNDPK2 expression affects oxidative stress tolerance leading to improved plant growth in transgenic poplar.

Members of the LATERAL ORGAN BOUNDARIES DOMAIN transcription factor family are involved in the regulation of secondary growth in Populus.

Regulation of secondary (woody) growth is of substantial economic and environmental interest but is poorly understood. We identified and subsequently characterized an activation-tagged poplar (Populus tremula × Populus alba) mutant with enhanced woody growth and changes in bark texture caused primarily by increased secondary phloem production. Molecular characterization of the mutation through positioning of the tag and retransformation experiments shows that the phenotype is conditioned by activation of an uncharacterized gene that encodes a novel member of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors. Homology analysis showed highest similarity to an uncharacterized LBD1 gene from Arabidopsis thaliana, and we consequently named it Populus tremula × Populus alba (Pta) LBD1. Dominant-negative suppression of Pta LBD1 via translational fusion with the repressor SRDX domain caused decreased diameter growth and suppressed and highly irregular phloem development. In wild-type plants, LBD1 was most highly expressed in the phloem and cambial zone. Two key Class I KNOTTED1-like homeobox genes that promote meristem identity in the cambium were downregulated, while an Altered Phloem Development gene that is known to promote phloem differentiation was upregulated in the mutant. A set of four LBD genes, including the LBD1 gene, was predominantly expressed in wood-forming tissues, suggesting a broader regulatory role of these transcription factors during secondary woody growth in poplar.

Phytohormones and shoot growth in a three-generation hybrid poplar family.

Hybrid vigor for secondary growth in poplar has been linked with increased gibberellin (GA) concentration in cambial tissue, but the relationship between concentrations of GAs and hybrid vigor of primary growth has not been investigated. We explored associations between concentrations of GAs, abscisic acid (ABA) and indoleacetic acid (IAA) and shoot extension in a hybrid family of Populus deltoides Bartr. ex Marsh. (Clone ILL-129) and P. trichocarpa Torr. & Gray (Clone 93-968) and two F1 and 67 F2 genotypes. Rapidly elongating subapical internodes from shoots of 4-year-old clonal saplings were selected for hormone analysis. The F1 hybrids displayed heterosis (hybrid vigor) for primary growth as a result of the complementation of dominance for increased internode length from the P. trichocarpa parent and dominance for increased diameter from the P. deltoides parent. Internodes from the faster-elongating shoots of the P. trichocarpa parent had a fourfold higher concentration of bioactive GA1 and higher concentrations of GA20, GA44, GA29 and GA8 than the P. deltoides parent. However, the two fast-growing F1 hybrids had low concentrations of all five GAs, with concentrations similar to those of the slower-elongating P. deltoides parent. Concentrations of ABA and IAA were correlated with GA concentrations and there was thus no evidence of a consistent promoting or inhibiting effect of ABA or IAA within the F1 family. These results indicate that heterosis for internode growth was not primarily regulated by hormone concentration. The segregating population of F2 hybrids was analyzed to assess the possible utility of hormone screening as a selection tool. The internodes of most of the F2 hybrids were smaller than those of their F1 parents and the larger P. trichocarpa parent. Among the F2 hybrids, mature internode length varied fourfold, and was correlated with lengths of young, subapical internodes from which hormones were analyzed. In these internodes, GA1 concentrations were negatively correlated with length (r = 0.41), diameter (0.33) and mass (0.50). Indoleacetic acid concentration was negatively correlated only with diameter (r = 0.37) and positively correlated with GA1 (r = 0.46), whereas ABA concentration was not correlated with any parameter. Thus, in the F2 population, variation in hormone concentration was not primarily responsible for the variation in shoot growth, indicating that selection for individuals with high GA concentration may not be an effective strategy for identifying vigorous hybrid genotypes.

Selection for Both Growth and Wood Properties in Hybrid Poplar Clones

Abstract To better understand the genetic control of growth and wood properties in hybrid poplar (Populus spp.), genetic relationships of selected wood properties with growth traits were examined in a 3-yr-old clonal trial located in Windsor and St-Ours, southern Quebec, Canada. In total, 371 trees from 21 hybrid poplar clones were sampled at the two locations. Clonal effects are stronger than growth traits effects on wood density and initial moisture content. The estimated clonal repeatability for wood density (0.92) and initial moisture content (0.80) across the two sites are thus considerably higher than for dbh (0.64) and tree height (0.72). Neither wood density nor mean fiber length were found to significantly correlate with growth traits. This suggests that in the hybrid poplar clones selection for growth traits will not result in a significant reduction in the wood quality traits. Selection for stem dry fiber weight appears to be an optimal selection strategy, as it will lead to the highest genetic gain in stem dry fiber yield, or maximum fiber production while wood density is improved.

The role of PopCel1 and PopCel2 in poplar leaf growth and cellulose biosynthesis.

Poplar calli transcribed two cellulase (endo-1,4-beta-glucanase) genes, PopCel1 and PopCel2, whose mRNAs were differentially located in the growing leaves of poplar during cell wall synthesis. Histochemical and RT-PCR analyses of promoter-GUS fusion gene activities in transgenic poplar demonstrated that PopCel1 promoter-derived GUS activity was localized in the petiole and leaf veins, whereas PopCel2 was confined to mesophyll cells and disappeared from the tip during the development of leaves. Autoradiography of the leaf showed that the radioactivity of [14C]sucrose incorporated into cellulose corresponded to the combination of the sucrose-induced tissue-specific patterns of PopCel1 and PopCel2. Interestingly, 2,6-dichlorobenzonitrile (DCB) not only inhibited the incorporation of the radioactivity into cellulose, but also repressed the induction of both cellulase genes. Suppression of cellulases by expression of PopCel1 antisense cDNA or co-suppression of PopCel1 mRNA by overexpression of PopCel1 sense cDNA reduced leaf growth. Therefore, we came to the conclusion that PopCel1 and PopCel2 probably function to promote leaf growth in poplar by the endohydrolysis of 1,4-beta-glucan.