Molecular Aspects of Bud Dormancy in Trees

FLOWERING LOCUS T2 Promotes Shoot Apex Development and Restricts Internode Elongation via the 13-Hydroxylation Gibberellin Biosynthesis Pathway in Poplar

The adaptation and survival of boreal and temperate perennials relies on the precise demarcation of the growing season. Seasonal growth and development are defined by day length and temperature signals. Under long-day conditions in spring, poplar FLOWERING LOCUS T2 (FT2) systemically induces shoot growth. In contrast, FT2 downregulation induced by autumnal short days triggers growth cessation and bud set. However, the molecular role of FT2 in local and long-range signaling is not entirely understood. In this study, the CRISPR/Cas9 editing tool was used to generate FT2 loss of function lines of hybrid poplar. Results indicate that FT2 is essential to promote shoot apex development and restrict internode elongation under conditions of long days. The application of bioactive gibberellins (GAs) to apical buds in FT2 loss of function lines was able to rescue bud set. Expression analysis of GA sensing and metabolic genes and hormone quantification revealed that FT2 boosts the 13-hydroxylation branch of the GA biosynthesis pathway in the shoot apex. Paclobutrazol treatment of WT leaves led to limited internode growth in the stem elongation zone. In mature leaves, FT2 was found to control the GA 13-hydroxylation pathway by increasing GA2ox1 and reducing GA3ox2 expression, causing reduced GA1 levels. We here show that in poplar, the FT2 signal promotes shoot apex development and restricts internode elongation through the GA 13-hydroxylation pathway.

Unisexual flower initiation in the monoecious Quercus suber L.: a molecular approach

Several plant species display a temporal separation of the male and female flower organ development to enhance outbreeding; however, little is known regarding the genetic mechanisms controlling this temporal separation. Quercus suber is a monoecious oak tree with accentuated protandry: in late winter, unisexual male flowers emerge adjacent to the swollen buds, whereas unisexual female flowers emerge in the axils of newly formed leaves formed during spring (4-8 weeks after male flowering). Here, a phylogenetic profiling has led to the identification of cork oak homologs of key floral regulatory genes. The role of these cork oak homologs during flower development was identified with functional studies in Arabidopsis thaliana. The expression profile throughout the year of flower regulators (inducers and repressors), in leaves and buds, suggests that the development of male and female flowers may be preceded by separated induction events. Female flowers are most likely induced during the vegetative flush occurring in spring, whereas male flowers may be induced in early summer. Male flowers stay enclosed within the pre-dormant buds, but complete their development before the vegetative flush of the following year, displaying a long period of anthesis that spans the dormant period. Our results portray a genetic mechanism that may explain similar reproductive habits in other monoecious tree species.

Identification and characterization of microRNAs in tree peony during chilling induced dormancy release by high-throughput sequencing

Tree peony, one of the most valuable horticultural and medicinal plants in the world, has to go through winter to break dormancy. Growing studies from molecular aspects on dormancy release process have been reported, but inadequate study has been done on miRNA-guided regulation in tree peony. In this study, high-throughput sequencing was employed to identify and characterize miRNAs in three libraries (6 d, 18 d and 24 d chilling treatments). There were 7,122, 10,076 and 9,097 unique miRNA sequences belonging to 52, 87 and 68 miRNA families, respectively. A total of 32 conserved miRNAs and 17 putative novel miRNAs were identified during dormancy release. There were 771 unigenes as potential targets of 62 miRNA families. Total 112 known miRNAs were differentially expressed, of which 55 miRNAs were shared among three libraries and 28 miRNAs were only found in 18 d chilling duration library. The expression patterns of 15 conserved miRNAs were validated and classified into four types by RT-qPCR. Combining with our microarray data under same treatments, five miRNAs (miR156k, miR159a, miR167a, miR169a and miR172a) were inversely correlated to those of their target genes. Our results would provide new molecular basis about dormancy release in tree peony.

Differential expression proteins associated with bud dormancy release during chilling treatment of tree peony (Paeonia suffruticosa)

Endo-dormant flower buds of tree peony must have sufficient chilling duration to reinitiate growth, which is a major obstacle to the forcing culture of tree peony in winter. We used a combination of two-dimensional gel electrophoresis (2-DE) and matrix-assisted laser desorption/ionisation time of flight/time of flight mass spectrometry (MALDI-TOF/TOF MS) to identify the differentially expressed proteins of tree peony after three different chilling treatments: endo-dormancy, endo-dormancy release and eco-dormancy stages. More than 200 highly reproducible protein spots were detected, and 31 differentially expressed spots (P < 0.05) were selected for further analysis. Finally, 20 protein spots were confidently identified from databases, which were annotated and classified into seven functional categories: response to abiotic or biotic stimulus (four), metabolic processes (four), other binding (three), transcription or transcription regulation (two), biological processes (one), cell biogenesis (one) and unclassified (five). The results of qPCR of five genes were mainly consistent with that of the protein accumulation analysis as determined by 2-DE. This indicated that most of these genes were mainly regulated at transcriptional level. The activity of nitrate reductase and pyruvate dehydrogenase E1 was consistent with the 2-DE results. The proteomic profiles indicated activation of citrate cycle, amino acid metabolism, lipid metabolism, energy production, calcium signalling and cell growth processes by chilling fulfilment to facilitate dormancy release in tree peony. Analysis of functions of identified proteins will increase our knowledge of endo-dormancy release in tree peony.

EARLY BUD-BREAK 1 (EBB1) is a regulator of release from seasonal dormancy in poplar trees

Trees from temperate latitudes transition between growth and dormancy to survive dehydration and freezing stress during winter months. We used activation tagging to isolate a dominant mutation affecting release from dormancy and identified the corresponding gene EARLY BUD-BREAK 1 (EBB1). We demonstrate through positioning of the tag, expression analysis, and retransformation experiments that EBB1 encodes a putative APETALA2/Ethylene responsive factor transcription factor. Transgenic up-regulation of the gene caused early bud-flush, whereas down-regulation delayed bud-break. Native EBB1 expression was highest in actively growing apices, undetectable during the dormancy period, but rapidly increased before bud-break. The EBB1 transcript was localized in the L1/L2 layers of the shoot meristem and leaf primordia. EBB1-overexpressing transgenic plants displayed enlarged shoot meristems, open and poorly differentiated buds, and a higher rate of cell division in the apex. Transcriptome analyses of the EBB1 transgenics identified 971 differentially expressed genes whose expression correlated with the EBB1 expression changes in the transgenic plants. Promoter analysis among the differentially expressed genes for the presence of a canonical EBB1-binding site identified 65 putative target genes, indicative of a broad regulatory context of EBB1 function. Our results suggest that EBB1 has a major and integrative role in reactivation of meristem activity after winter dormancy.

Transcript profiling of Paoenia ostii during artificial chilling induced dormancy release identifies activation of GA pathway and carbohydrate metabolism

Endo-dormant flower buds must pass through a period of chilling to reinitiate growth and subsequent flowering, which is a major obstacle to the forcing culture of tree peony in winter. Customized cDNA microarray (8×15 K element) was used to investigate gene expression profiling in tree peony 'Feng Dan Bai' buds during 24 d chilling treatment at 0-4°C. According to the morphological changes after the whole plants were transferred to green house, endo-dormancy was released after 18 d chilling treatment, and prolonged chilling treatment increased bud break rate. Pearson correlation hierarchical clustering of sample groups was highly consistent with the dormancy transitions revealed by morphological changes. Totally 3,174 significantly differentially-expressed genes (P<0.05) were observed through endo-dormancy release process, of which the number of up-regulated (1,611) and that of down-regulated (1,563) was almost the same. Functional annotation of differentially-expressed genes revealed that cellular process, metabolic process, response to stimulus, regulation of biological process and development process were well-represented. Hierarchical clustering indicated that activation of genes involved in carbohydrate metabolism (Glycolysis, Citrate cycle and Pentose phosphate pathway), energy metabolism and cell growth. Based on the results of GO analysis, totally 51 probes presented in the microarray were associated with GA response and GA signaling pathway, and 22 of them were differently expressed. The expression profiles also revealed that the genes of GA biosynthesis, signaling and response involved in endo-dormancy release. We hypothesized that activation of GA pathway played a central role in the regulation of dormancy release in tree peony.

Differential expression of proteins associated with seasonal bud dormancy at four critical stages in Japanese apricot

Dormancy is of great significance in the growth and development of deciduous fruit trees. We used a combination of two-dimensional gel electrophoresis (2-DE) and matrix-assisted laser desorption/ionisation time of flight/time of flight mass spectrometry (MALDI-TOF/TOF MS) to identify the differentially expressed proteome of Japanese apricot flower buds at four critical stages, from paradormancy before leaf fall to dormancy release. More than 400 highly reproducible protein spots (P < 0.05) were detected: 34 protein spots showed a greater than twofold difference in expression values, of which 32 protein spots were confidently identified from databases. Identified proteins were classified into six functional categories: stress response and defence (11), energy metabolism (ten), protein metabolism (five), cell structure (three), transcription (one) and unclassified (two). The glyoxalase I homologue could help Japanese apricot survival under various abiotic and biotic stresses, greatly contributing to its dormancy. Enolase, thioredoxin family proteins and triose phosphate isomerase provide adequate energy to complete consecutive dormancy release and bud break in Japanese apricot. Cinnamyl alcohol dehydrogenase 9 and arginase enhance the resilience of plants, enabling them to complete dormancy safely. Analysis of functions of identified proteins and related metabolic pathways will increase our knowledge of dormancy in woody plants.

The dynamic nature of bud dormancy in trees: environmental control and molecular mechanisms

In tree species native to temperate and boreal regions, the activity-dormancy cycle is an important adaptive trait both for survival and growth. We discuss recent research on mechanisms controlling the overlapping developmental processes that define the activity-dormancy cycle, including cessation of apical growth, bud development, induction, maintenance and release of dormancy, and bud burst. The cycle involves an extensive reconfiguration of metabolism. Environmental control of the activity-dormancy cycle is based on perception of photoperiodic and temperature signals, reflecting adaptation to prevailing climatic conditions. Several molecular actors for control of growth cessation have been identified, with the CO/FT regulatory network and circadian clock having important coordinating roles in control of growth and dormancy. Other candidate regulators of bud set, dormancy and bud burst have been identified, such as dormancy-associated MADS-box factors, but their exact roles remain to be discovered. Epigenetic mechanisms also appear to factor in control of the activity-dormancy cycle. Despite evidence for gibberellins as negative regulators in growth cessation, and ABA and ethylene in bud formation, understanding of the roles that plant growth regulators play in controlling the activity-dormancy cycle is still very fragmentary. Finally, some of the challenges for further research in bud dormancy are discussed.

Phenotypic plasticity, QTL mapping and genomic characterization of bud set in black poplar

BACKGROUND

The genetic control of important adaptive traits, such as bud set, is still poorly understood in most forest trees species. Poplar is an ideal model tree to study bud set because of its indeterminate shoot growth. Thus, a full-sib family derived from an intraspecific cross of P. nigra with 162 clonally replicated progeny was used to assess the phenotypic plasticity and genetic variation of bud set in two sites of contrasting environmental conditions.

RESULTS

Six crucial phenological stages of bud set were scored. Night length appeared to be the most important signal triggering the onset of growth cessation. Nevertheless, the effect of other environmental factors, such as temperature, increased during the process. Moreover, a considerable role of genotype × environment (G × E) interaction was found in all phenological stages with the lowest temperature appearing to influence the sensitivity of the most plastic genotypes.Descriptors of growth cessation and bud onset explained the largest part of phenotypic variation of the entire process. Quantitative trait loci (QTL) for these traits were detected. For the four selected traits (the onset of growth cessation (date2.5), the transition from shoot to bud (date1.5), the duration of bud formation (subproc1) and bud maturation (subproc2)) eight and sixteen QTL were mapped on the maternal and paternal map, respectively. The identified QTL, each one characterized by small or modest effect, highlighted the complex nature of traits involved in bud set process. Comparison between map location of QTL and P. trichocarpa genome sequence allowed the identification of 13 gene models, 67 bud set-related expressional and six functional candidate genes (CGs). These CGs are functionally related to relevant biological processes, environmental sensing, signaling, and cell growth and development. Some strong QTL had no obvious CGs, and hold great promise to identify unknown genes that affect bud set.

CONCLUSIONS

This study provides a better understanding of the physiological and genetic dissection of bud set in poplar. The putative QTL identified will be tested for associations in P. nigra natural populations. The identified QTL and CGs will also serve as useful targets for poplar breeding.

Integrated transcriptomic and proteomic profiling of white spruce stems during the transition from active growth to dormancy

In the autumn, stems of woody perennials such as forest trees undergo a transition from active growth to dormancy. We used microarray transcriptomic profiling in combination with a proteomics analysis to elucidate processes that occur during this growth-to-dormancy transition in a conifer, white spruce (Picea glauca[Moench] Voss). Several differentially expressed genes were likely associated with the developmental transition that occurs during growth cessation in the cambial zone and the concomitant completion of cell maturation in vascular tissues. Genes encoding for cell wall and membrane biosynthetic enzymes showed transcript abundance patterns consistent with completion of cell maturation, and also of cell wall and membrane modifications potentially enabling cells to withstand the harsh conditions of winter. Several differentially expressed genes were identified that encoded putative regulators of cambial activity, cell development and of the photoperiodic pathway. Reconfiguration of carbon allocation figured centrally in the tree's overwintering preparations. For example, genes associated with carbon-based defences such as terpenoids were down-regulated, while many genes associated with protein-based defences and other stress mitigation mechanisms were up-regulated. Several of these correspond to proteins that were accumulated during the growth-to-dormancy transition, emphasizing the importance of stress protection in the tree's adaptive response to overwintering.

Adaptive responses for seed and leaf phenology in natural populations of sessile oak along an altitudinal gradient

We assessed the adaptive potential of seed and leaf phenology in 10 natural populations of sessile oak (Quercus petraea) sampled along two altitudinal transects using common garden experiments. Population differentiation for both phenological traits was observed with high-altitude populations germinating and flushing later than low altitude ones. However, high genetic variation and heritability values were also maintained within populations, despite slightly decreasing for dates of leaf unfolding with increasing altitude. We suggest that biotic and abiotic fluctuating selection pressures within populations and high gene flow are the main mechanisms maintaining high genetic variation for these fitness related traits. Moreover, changes in selection intensity and/or selection pressures along the altitudinal gradient can explain the reduction in genetic variation observed for leaf phenology. We anticipate that the maintenance of high genetic variation will be a valuable resource for future adaptation of sessile oak populations undergoing an upslope shift caused by climate change.

Molecular events of apical bud formation in white spruce, Picea glauca

Bud formation is an adaptive trait that temperate forest trees have acquired to facilitate seasonal synchronization. We have characterized transcriptome-level changes that occur during bud formation of white spruce [Picea glauca (Moench) Voss], a primarily determinate species in which preformed stem units contained within the apical bud constitute most of next season's growth. Microarray analysis identified 4460 differentially expressed sequences in shoot tips during short day-induced bud formation. Cluster analysis revealed distinct temporal patterns of expression, and functional classification of genes in these clusters implied molecular processes that coincide with anatomical changes occurring in the developing bud. Comparing expression profiles in developing buds under long day and short day conditions identified possible photoperiod-responsive genes that may not be essential for bud development. Several genes putatively associated with hormone signalling were identified, and hormone quantification revealed distinct profiles for abscisic acid (ABA), cytokinins, auxin and their metabolites that can be related to morphological changes to the bud. Comparison of gene expression profiles during bud formation in different tissues revealed 108 genes that are differentially expressed only in developing buds and show greater transcript abundance in developing buds than other tissues. These findings provide a temporal roadmap of bud formation in white spruce.

Gene expression of DAM5 and DAM6 is suppressed by chilling temperatures and inversely correlated with bud break rate

We previously identified a cluster of d ormancy-a ssociated M ADS-box transcription factors (DAM genes) in peach [Prunus persica (L.) Batsch] as potential candidates for control of the non-dormant phenotype observed in the evg mutant. Of these genes, DAM3, DAM5 and DAM6 were winter expressed, suggesting a role for these genes during endodormancy. We used peach cultivars with contrasting chilling requirements (CR) for bud break to observe the expression of DAM3, DAM5 and DAM6 in response to chilling accumulation in the field and controlled environments. Vegetative terminal and floral buds were sampled weekly from field grown 'Contender' (1050 h CR), 'Rubyprince' (850 h CR) and 'Springprince' (650 h CR) peach cultivars through winter 2008-2009. Flower and vegetative terminal bud break potential was evaluated at each sampling by forcing cuttings in a growth-permissive environment. We also measured vegetative terminal bud break and DAM gene expression in potted 'Contender' and 'Peen-To' (450 h CR) trees under controlled-environment cold exposure. DAM3, DAM5 and DAM6 are all suppressed by exposure to chilling temperatures in the field and in controlled conditions. Expression of DAM5 and DAM6 are higher in high chill cultivars prior to chilling accumulation and their expression level reaches a minimum in each cultivar coincident with acquisition of bud break competence. Expression levels of DAM5 and DAM6 in vegetative tips in controlled environment conditions were negatively correlated with the time required for bud break in forcing conditions. The expression patterns of DAM5 and DAM6 are consistent with a role as quantitative repressors of bud break.

Plant dormancy in the perennial context

A key feature of the perennial life style in plants is the ability to cease meristem activity and to establish a dormant state in which the meristem is rendered insensitive to growth-promoting signals for some time before it is released and can resume growth. The seasonal cycling between growth and dormancy has received little attention despite its importance for perennial behaviour. In this review, we reconsider seasonal cycles of growth and dormancy in view of a new definition of dormancy as a state within the meristem, together with recent exciting developments in the study of perennials, particularly the identification of common signalling intermediates between flowering time and growth cessation in trees.

Populus: a model system for plant biology

With the completion of the Populus trichocarpa genome sequence and the development of various genetic, genomic, and biochemical tools, Populus now offers many possibilities to study questions that cannot be as easily addressed in Arabidopsis and rice, the two prime model systems of plant biology and genomics. Tree-specific traits such as wood formation, long-term perennial growth, and seasonality are obvious areas of research, but research in other areas such as control of flowering, biotic interactions, and evolution of adaptive traits is enriched by adding a tree to the suite of model systems. Furthermore, the reproductive biology of Populus (a dioeceous wind-pollinated long-lived tree) offers both new possibilities and challenges in the study and analysis of natural genetic and phenotypic variation. The relatively close phylogenetic relationship of Populus to Arabidopsis in the Eurosid clade of Eudicotyledonous plants aids in comparative functional studies and comparative genomics, and has the potential to greatly facilitate studies on genome and gene family evolution in eudicots.

Gene expression during the induction, maintenance, and release of dormancy in apical buds of poplar

The perennial lifestyle of trees is characterized by seasonal cycles of growth and dormancy. The recurrent transitions into and out of dormancy represent an adaptation mechanism that largely determines survival and, hence, the geographical distribution of tree species. To understand better the molecular basis of bud dormancy, cDNA-amplified fragment length polymorphism (AFLP) transcript profiling was used to map differential gene expression during dormancy induction, dormancy, dormancy release by chilling, and subsequent bud break in apical buds of poplar (Populus tremulaxP. alba). Unexpectedly, besides poplar transcript sequences, the cDNA-AFLP profiles revealed sequence signatures originating from a complex bacterial community, which was more pronounced during dormancy and displayed temporal dynamics in composition and complexity. Based on poplar gene expression dynamics, processes and potential regulators during different phases of dormancy are described. Novel genes were linked to a crucial transitory step in dormancy induction, and to dormancy release through chilling, a molecularly unresolved phenomenon. One WRKY- and two ERF-related transcription factors were similarly expressed during the transition to dormancy in apical and axillary buds. These regulatory genes could be involved in the differentiation of stipule-like leaf organs protecting the bud, or act during the growth-dormancy transition in the meristem, revealing commonalities between para- and endodormancy.

Transitions in the functioning of the shoot apical meristem in birch (Betula pendula) involve ethylene

In many trees, a short photoperiod (SD) triggers substantial physiological adjustments necessary for over-wintering. We have used transgenic ethylene-insensitive birches (Betula pendula), which express the Arabidopsis ethylene receptor gene ETR1 carrying the dominant mutation etr1-1, to investigate the role of ethylene in SD-induced responses in the shoot apical meristem (SAM). Under SD, the ethylene-insensitive trees ceased elongation growth comparably to the wild-type. In contrast, the formation of terminal buds, which in trees is typically induced by SD, was abolished. However, although delayed, endo-dormancy did eventually develop in the ethylene-insensitive trees. This, together with the rapid resumption of growth in the ethylene-insensitive trees after transfer from non-permissive to permissive conditions suggests that ethylene facilitates the SD-induced terminal bud formation, as well as growth arrest. In addition, apical buds of the ethylene-insensitive birch did not accumulate abscisic acid (ABA) under SD, suggesting interaction between ethylene and ABA signalling in the bud. Alterations in SAM functioning were further exemplified by reduced apical dominance and early flowering in ethylene-insensitive birches. Gene expression analysis of shoot apices revealed that the ethylene-insensitive birch lacked the marked increase in expression of a beta-xylosidase gene typical to the SD-exposed wild-type. The ethylene-dependent beta-xylosidase gene expression is hypothesized to relate to modification of cell walls in terminal buds during SD-induced growth cessation. Our results suggest that ethylene is involved in terminal bud formation and in the timely suppression of SAM activity, not only in the shoot apex, but also in axillary and reproductive meristems.

Transcriptome analysis of bud burst in sessile oak (Quercus petraea)

Expression patterns of hundreds of transcripts in apical buds were monitored during bud flushing in sessile oak (Quercus petraea), in order to identify genes differentially expressed between the quiescent and active stage of bud development. Different transcriptomic techniques combining the construction of suppression subtractive hybridization (SSH) libraries and the monitoring of gene expression using macroarray and real-time reverse transcriptase polymerase chain reaction (RT-PCR) were performed to dissect bud burst, with a special emphasis on the onset of the process. We generated 801 expressed sequence tags (ESTs) derived from six developmental stages of bud burst. Macroarray experiment revealed a total of 233 unique transcripts exhibiting differential expression during the process, and a putative function was assigned to 65% of them. Cell rescue/defense-, metabolism-, protein synthesis-, cell cycle- and transcription-related transcripts were among the most regulated genes. Macroarray and real-time RT-PCR showed that several genes exhibited contrasted expressions between quiescent and swelling buds, such as a putative homologue of the transcription factor DAG2 (Dof Affecting Germination 2), previously reported to be involved in the control of seed germination in Arabidopsis thaliana. These differentially expressed genes constitute relevant candidates for signaling pathway of bud burst in trees.

Revisiting tree maturation and floral initiation in the poplar functional genomics era

The recent release of the Populus trichocarpa genome sequence will dramatically enhance the efficiency of functional and comparative genomics research in trees. This provides researchers studying various developmental processes related to the perennial and tree life strategies with a completely new set of tools. Intimately associated with the life strategy of trees are their abilities to maintain juvenile or nonflowering phases for years to decades, and once reproductively competent, to alternate between the production of vegetative and reproductive shoots. Most of what we know about the regulation of the floral transition comes from research on Arabidopsis thaliana, a small, herbaceous, rapid-cycling, annual plant. In this review, we discuss the similarities and differences between Arabidopsis and tree flowering, and how recent findings in Arabidopsis, coupled to comparative and functional genomics in poplars, will help answer the question of how tree maturation and floral initiation is regulated.

Cambial meristem dormancy in trees involves extensive remodelling of the transcriptome

The establishment of the dormant state in meristems involves considerable physiological and metabolic alterations necessary for surviving unfavourable growth conditions. However, a global molecular analysis of dormancy in meristems has been hampered by the difficulty in isolating meristem cells. We used cryosectioning to isolate purified cambial meristem cells from the woody plant Populus tremula during active growth and dormancy. These samples were used to generate meristem-specific cDNA libraries and for cDNA microarray experiments to define the global transcriptional changes underlying cambial dormancy. The results indicate a significant reduction in the complexity of the cambial transcriptome in the dormant state. Although cell division is terminated in the dormant cambium, the cell cycle machinery appears to be maintained in a skeletal state as suggested by the continued presence of transcripts for several cell cycle regulators. The downregulation of PttPIN1 and PttPIN2 transcripts explains the reduced basipetal polar auxin transport during dormancy. The induction of a member of the SINA family of ubiquitin ligases implicated in auxin signalling indicates a potential mechanism for modulation of auxin sensitivity during cambial dormancy. The metabolic alterations during dormancy are mirrored in the induction of genes involved in starch breakdown and the glyoxysomal cycle. Interestingly, the induction of RGA1 like gene suggests modification of gibberellin signalling in cambial dormancy. The induction of genes such as poplar orthologues of FIE and HAP2 indicates a potential role for these global regulators of transcription in orchestrating extensive changes in gene expression during dormancy.

The poplar K+ channel KPT1 is associated with K+ uptake during stomatal opening and bud development

To gain insights into the performance of poplar guard cells, we have measured stomatal conductance and aperture, guard cell K+ content and K+-channel activity of the guard cell plasma membrane in intact poplar leaves. In contrast to Arabidopsis, broad bean and tobacco grown under same conditions, poplar stomata operated just in the dynamic range - any change in conductance altered the rate of photosynthesis. In response to light, CO2 and abscisic acid (ABA), the stomatal opening velocity was two to five times faster than that measured for Arabidopsis thaliana, Nicotiana tabacum and Vicia faba. When stomata opened, the K+ content of guard cells increased almost twofold, indicating that the very fast stomatal opening in this species is mediated via potassium uptake. Following impalement of single guard cells embedded in their natural environment of intact leaves with triple-barrelled microelectrodes, time-dependent inward and outward-rectifying K+-channel-mediated currents of large amplitude were recorded. To analyse the molecular nature of genes encoding guard cell K+-uptake channels, we cloned K+-transporter Populustremula (KPT)1 and functionally expressed this potassium channel in a K+-uptake-deficient Escherichia coli mutant. In addition to guard cells, this K+-transporter gene was expressed in buds, where the KPT1 gene activity strongly correlated with bud break. Thus, KPT1 represents one of only few poplar genes associated with bud flush.

Independent activation of cold acclimation by low temperature and short photoperiod in hybrid aspen

Temperate zone woody plants cold acclimate in response to both short daylength (SD) and low temperature (LT). We were able to show that these two environmental cues induce cold acclimation independently by comparing the wild type (WT) and the transgenic hybrid aspen (Populus tremula x Populus tremuloides Michx.) line 22 overexpressing the oat (Avena sativa) PHYTOCHROME A gene. Line 22 was not able to detect the SD and, consequently, did not stop growing in SD conditions. This resulted in an impaired freezing tolerance development under SD. In contrast, exposure to LT resulted in cold acclimation of line 22 to a degree comparable with the WT. In contrast to the WT, line 22 could not dehydrate the overwintering tissues or induce the production of dehydrins (DHN) under SD conditions. Furthermore, abscisic acid (ABA) content of the buds of line 22 were the same under SD and long daylength, whereas prolonged SD exposure decreased the ABA level in the WT. LT exposure resulted in a rapid accumulation of DHN in both the WT and line 22. Similarly, ABA content increased transiently in both the WT and line 22. Our results indicate that phytochrome A is involved in photoperiodic regulation of ABA and DHN levels, but at LT they are regulated by a different mechanism. Although SD and LT induce cold acclimation independently, ABA and DHN may play important roles in both modes of acclimation.

Molecular analysis of signals controlling dormancy and growth in underground adventitious buds of leafy spurge

Dormancy and subsequent regrowth of adventitious buds is a critical physiological process for many perennial plants. We have used the expression of hormone and cell cycle-responsive genes as markers to follow this process in leafy spurge (Euphorbia esula). In conjunction with earlier studies, we show that loss of mature leaves results in decreased sugar levels and increased gibberellin perception in underground adventitious buds. Gibberellin is sufficient for induction of S phase-specific but not M phase-specific gene expression. Loss of both apical and axillary buds or inhibition of polar auxin transport did not result in induction of S phase- or M phase-specific gene expression. Loss of polar auxin transport was necessary for continuation of the cell cycle and further bud development if the S phase was previously initiated.

Requirement for far-red light to maintain secondary needle extension growth in northern but not southern populations of Pinus sylvestris (Scots pine)

Extension growth of secondary needles is under photoperiodic control in Pinus sylvestris. To test for the effects of far-red light on maintaining this extension growth, seedlings of six populations originating from latitudes between 57 degrees and 67 degrees N were raised for 11 weeks in continuous incandescent (metal halogen) light at 300 &mgr;mol m-2 s-1 and 20 degrees C and then transferred at the same temperature to a daily regime of 8 h incandescent light (230 &mgr;mol m-2 s-1) followed by a 16 h day extension with cool white fluorescent light (40 &mgr;mol m-2 s-1, R/FR ratio 7.5) or with incandescent lamps (20 &mgr;mol m-2 s-1, R/FR ratio 2.0). For the seedlings from the three populations north of 64 degrees, needle extension growth over 42 days in the FR-poor day extension treatment was lower by up to 40% than in the FR-rich day extension treatment, whereas for the seedlings from the three southern populations the needle extension growth was similar in both day extension treatments. The requirement for FR in day extensions is characteristic of 'light-dominant' photoperiodic control mechanisms. It appears that P. sylvestris changes from dark-dominant night timekeeping to light-dominant day timekeeping with increasing latitude, as with the photoperiodic control of budset in Picea abies.