Axillary Bud Outgrowth Research Articles

Unveiling Signaling Events in Root Responses to Strigolactone

Strigolactones (SLs), present in a wide variety of plant species, have been recently recognized as plant hormones. In the shoot, they suppress the outgrowth of preformed axillary buds (reviewed by Brewer et al., 2013 ); they are negative regulators of adventitious root formation ( Rasmussen et al., 2012 ) and positive regulators of shoot secondary growth ( Agusti et al., 2011 ). In roots, SLs positively regulate root-hair length, suppress lateral root formation ( Kapulnik et al., 2011 ), and promote cell number in the primary root meristem ( Ruyter-Spira et al., 2011 ). However, little is known on the signal-transduction pathways associated with SL activities. Here, we focus on roots and on the recent advances in understanding their SL-related signal transduction.

EBE, an AP2/ERF Transcription Factor Highly Expressed in Proliferating Cells, Affects Shoot Architecture in Arabidopsis

We report about ERF BUD ENHANCER (EBE; At5g61890), a transcription factor that affects cell proliferation as well as axillary bud outgrowth and shoot branching in Arabidopsis (Arabidopsis thaliana). EBE encodes a member of the APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) transcription factor superfamily; the gene is strongly expressed in proliferating cells and is rapidly and transiently up-regulated in axillary meristems upon main stem decapitation. Overexpression of EBE promotes cell proliferation in growing calli, while the opposite is observed in EBE-RNAi lines. EBE overexpression also stimulates axillary bud formation and outgrowth, while repressing it results in inhibition of bud growth. Global transcriptome analysis of estradiol-inducible EBE overexpression lines revealed 48 EBE early-responsive genes, of which 14 were up-regulated and 34 were down-regulated. EBE activates several genes involved in cell cycle regulation and dormancy breaking, including D-type cyclin CYCD3;3, transcription regulator DPa, and BRCA1-ASSOCIATED RING DOMAIN1. Among the down-regulated genes were DORMANCY-ASSOCIATED PROTEIN1 (AtDRM1), AtDRM1 homolog, MEDIATOR OF ABA-REGULATED DORMANCY1, and ZINC FINGER HOMEODOMAIN5. Our data indicate that the effect of EBE on shoot branching likely results from an activation of genes involved in cell cycle regulation and dormancy breaking.

Effect of cytokinin type and concentration, and source of explant on shoot multiplication of pineapple plant (Ananas comosus ‘Queen’) in vitro

The current study was conducted to test cytokinin type and concentration, and source of explant on shoot multiplication of pineapple plant (Ananas comosus ‘Queen’) in vitro during 22/6/2009 to 1/5/2010. The results of the first experiment showed that axillary buds delayed to grow during 4-5 months of culturing in MS medium supplemented with different concentrations of BA (1.0, 1.5, 2.0, 2.5 or 3.0 mg l-1). The MS medium supplemented with 1.0 mg l-1 BA has led to the vegetative growth of axillary buds. The other concentrations of BA added to MS medium led to callus growth. The results showed that MS medium supplemented with 1.0 mg l-1 BA and 0.2 mg l-1 NAA gave adventitious shoots directly after two months from culturing of callus. The results of the second experiment showed that nodal segments cultured in MS medium supplemented with 1.0 mg l-1 kinetin was a significantly superior on other treatments studied (0.5 mg l-1 kinetin, 0.5 and 1.0 mg l-1 BA) in number of shoots/explant, leaf length, number of leaves/shoot and leaf area of formation shoot which reached 18.60 shoot/explant, 5.38 cm, 10.60 leaves/shoot and 3.64 cm2, respectively. The results showed that kinetin was a significant superior in all vegetative characteristics of shoots compared with BA.

Global analysis of the sugarcane microtranscriptome reveals a unique composition of small RNAs associated with axillary bud outgrowth

Axillary bud outgrowth determines shoot architecture and is under the control of endogenous hormones and a fine-tuned gene-expression network, which probably includes small RNAs (sRNAs). Although it is well known that sRNAs act broadly in plant development, our understanding about their roles in vegetative bud outgrowth remains limited. Moreover, the expression profiles of microRNAs (miRNAs) and their targets within axillary buds are largely unknown. Here, we employed sRNA next-generation sequencing as well as computational and gene-expression analysis to identify and quantify sRNAs and their targets in vegetative axillary buds of the biofuel crop sugarcane (Saccharum spp.). Computational analysis allowed the identification of 26 conserved miRNA families and two putative novel miRNAs, as well as a number of trans-acting small interfering RNAs. sRNAs associated with transposable elements and protein-encoding genes were similarly represented in both inactive and developing bud libraries. Conversely, sequencing and quantitative reverse transcription-PCR results revealed that specific miRNAs were differentially expressed in developing buds, and some correlated negatively with the expression of their targets at specific stages of axillary bud development. For instance, the expression patterns of miR159 and its target GAMYB suggested that they may play roles in regulating abscisic acid-signalling pathways during sugarcane bud outgrowth. Our work reveals, for the first time, differences in the composition and expression profiles of diverse sRNAs and targets between inactive and developing vegetative buds that, together with the endogenous balance of specific hormones, may be important in regulating axillary bud outgrowth.

BRANCHED1 promotes axillary bud dormancy in response to shade in Arabidopsis.

Plants interpret a decrease in the red to far-red light ratio (R:FR) as a sign of impending shading by neighboring vegetation. This triggers a set of developmental responses known as shade avoidance syndrome. One of these responses is reduced branching through suppression of axillary bud outgrowth. The Arabidopsis thaliana gene BRANCHED1 (BRC1), expressed in axillary buds, is required for branch suppression in response to shade. Unlike wild-type plants, brc1 mutants develop several branches after a shade treatment. BRC1 transcription is positively regulated 4 h after exposure to low R:FR. Consistently, BRC1 is negatively regulated by phytochrome B. Transcriptional profiling of wild-type and brc1 buds of plants treated with simulated shade has revealed groups of genes whose mRNA levels are dependent on BRC1, among them a set of upregulated abscisic acid response genes and two networks of cell cycle- and ribosome-related downregulated genes. The downregulated genes have promoters enriched in TEOSINTE BRANCHED1, CYCLOIDEA, and PCF (TCP) binding sites, suggesting that they could be transcriptionally regulated by TCP factors. Some of these genes respond to BRC1 in seedlings and buds, supporting their close relationship with BRC1 activity. This response may allow the rapid adaptation of plants to fluctuations in the ratio of R:FR light.

The interaction between OsMADS57 and OsTB1 modulates rice tillering via DWARF14

Rice tillering is a multigenic trait that influences grain yield, but its regulation molecular module is poorly understood. Here we report that OsMADS57 interacts with OsTB1 (TEOSINTE BRANCHED1) and targets D14 (Dwarf14) to control the outgrowth of axillary buds in rice. An activation-tagged mutant osmads57-1 and OsMADS57-overexpression lines showed increased tillers, whereas OsMADS57 antisense lines had fewer tillers. OsMIR444a-overexpressing lines exhibited suppressed OsMADS57 expression and tillering. Furthermore, osmads57-1 was insensitive to strigolactone treatment to inhibit axillary bud outgrowth, and OsMADS57’s function in tillering was dependent on D14. D14 expression was downregulated in osmads57-1, but upregulated in antisense and OsMIR444a-overexpressing lines. OsMADS57 bound to the CArG motif [C(A/T)TTAAAAAG] in the promoter and directly suppressed D14 expression. Interaction of OsMADS57 with OsTB1 reduced OsMADS57 inhibition of D14 transcription. Therefore, OsMIR444a-regulated OsMADS57, together with OsTB1, target D14 to control tillering. This regulation mechanism could have important application in rice molecular breeding programs focused on high grain yield.

The interaction between strigolactones and other plant hormones in the regulation of plant development

Plant hormones are small molecules derived from various metabolic pathways and are important regulators of plant development. The most recently discovered phytohormone class comprises the carotenoid-derived strigolactones (SLs). For a long time these compounds were only known to be secreted into the rhizosphere where they act as signaling compounds, but now we know they are also active as endogenous plant hormones and they have been in the spotlight ever since. The initial discovery that SLs are involved in the inhibition of axillary bud outgrowth, initiated a multitude of other studies showing that SLs also play a role in defining root architecture, secondary growth, hypocotyl elongation, and seed germination, mostly in interaction with other hormones. Their coordinated action enables the plant to respond in an appropriate manner to environmental factors such as temperature, shading, day length, and nutrient availability. Here, we will review the current knowledge on the crosstalk between SLs and other plant hormones—such as auxin, cytokinin, abscisic acid (ABA), ethylene (ET), and gibberellins (GA)—during different physiological processes. We will furthermore take a bird's eye view of how this hormonal crosstalk enables plants to respond to their ever changing environments.

Mutation of the cytosolic ribosomal protein-encoding RPS10Bgene affects shoot meristematic function in Arabidopsis

BackgroundPlant cytosolic ribosomal proteins are encoded by small gene families. Mutants affecting these genes are often viable, but show growth and developmental defects, suggesting incomplete functional redundancy within the families. Dormancy to growth transitions, such as the activation of axillary buds in the shoot, are characterised by co-ordinated upregulation of ribosomal protein genes.ResultsA recessive mutation in RPS10B, one of three Arabidopsis genes encoding the eukaryote-specific cytoplasmic ribosomal protein S10e, was found to suppress the excessive shoot branching mutant max2-1. rps10b-1 mildly affects the formation and separation of shoot lateral organs, including the shoot axillary meristems. Axillary meristem defects are enhanced when rps10b-1 is combined with mutations in REVOLUTA, AUXIN-RESISTANT1, PINOID or another suppressor of max2-1, FAR-RED ELONGATED HYPOCOTYL3. In some of these double mutants, the maintenance of the primary shoot meristem is also affected. In contrast, mutation of ALTERED MERISTEM PROGRAMME1 suppresses the rps10b-1axillary shoot defect. Defects in both axillary shoot formation and organ separation were enhanced by combining rps10b-1 with cuc3, a mutation affecting one of three Arabidopsis NAC transcription factor genes with partially redundant roles in these processes. To assess the effect of rps10b-1 on bud activation independently from bud formation, axillary bud outgrowth on excised cauline nodes was analysed. The outgrowth rate of untreated buds was reduced only slightly by rps10b-1 in both wild-type and max2-1 backgrounds. However, rps10b-1 strongly suppressed the auxin resistant outgrowth of max2-1 buds. A developmental phenotype of rps10b-1, reduced stamen number, was complemented by the cDNA of another family member, RPS10C, under the RPS10B promoter.ConclusionsRPS10B promotes shoot branching mainly by promoting axillary shoot development. It contributes to organ boundary formation and leaf polarity, and sustains max2-1 bud outgrowth in the presence of auxin. These processes require the auxin response machinery and precise spatial distribution of auxin. The correct dosage of protein(s) involved in auxin-mediated patterning may be RPS10B-dependent. Inability of other RPS10 gene family members to maintain fully S10e levels might cause the rps10b-1 phenotype, as we found no evidence for unique functional specialisation of either RPS10B promoter or RPS10B protein.

DAD2 Is an α/β Hydrolase Likely to Be Involved in the Perception of the Plant Branching Hormone, Strigolactone

Strigolactones are a recently discovered class of plant hormone involved in branching, leaf senescence, root development, and plant-microbe interactions. They are carotenoid-derived lactones, synthesized in the roots and transported acropetally to modulate axillary bud outgrowth (i.e., branching). However, a receptor for strigolactones has not been identified. We have identified the DAD2 gene from petunia, an ortholog of the rice and Arabidopsis D14 genes, and present evidence for its roles in strigolactone perception and signaling. DAD2 acts in the strigolactone pathway, and the dad2 mutant is insensitive to the strigolactone analog GR24. The crystal structure of DAD2 reveals an α/β hydrolase fold containing a canonical catalytic triad with a large internal cavity capable of accommodating strigolactones. In the presence of GR24 DAD2 interacts with PhMAX2A, a central component of strigolactone signaling, in a GR24 concentration-dependent manner. DAD2 can hydrolyze GR24, with mutants of the catalytic triad abolishing both this activity and the ability of DAD2 to interact with PhMAX2A. The hydrolysis products can neither stimulate the protein-protein interaction nor modulate branching. These observations suggest that DAD2 acts to bind the mobile strigolactone signal and then interacts with PhMAX2A during catalysis to initiate an SCF-mediated signal transduction pathway.

FHY3 promotes shoot branching and stress tolerance in Arabidopsis in an AXR1‐dependent manner

The transposase-related transcription factor FAR-RED ELONGATED HYPOCOTYL3 (FHY3) promotes seedling de-etiolation in far-red light, which is perceived by phytochrome A (phyA). In this role, FHY3 indirectly mediates the nuclear import of light-activated phyA, which triggers downstream transcriptional responses. Here, we present genetic evidence for additional roles of FHY3 in plant development and growth. New fhy3 alleles were isolated as suppressors of max2-1 (more axillary branching2-1), a strigolactone-insensitive mutant characterised by highly branched shoots. Branching suppression by fhy3, in both wild-type and max2-1 backgrounds, resulted from inhibition of axillary bud outgrowth. Additional roles in axillary meristem initiation were revealed in the revoluta (rev) fhy3 double mutant, with fhy3 enhancing rev mutant defects in axillary shoot meristem formation, as well as in floral meristem maintenance. fhy3 also affected embryonic and floral patterning with low penetrance, and displayed oxidative stress-related phenotypes of retarded leaf growth and of cell death. The fhy3 phenotypes of axillary bud outgrowth suppression and of stress-induced leaf growth retardation both required the AUXIN-RESISTANT1 gene, and are independent of phyA. Consistent with the recent discovery that FHY3 regulates many Arabidopsis promoters, our results suggest much wider roles for FHY3 in growth and development, either in concert with, or beyond, light signalling.

Towards an integrated view of monocarpic plant senescence

After the flowering of an annual plant, the whole plant will senesce and die. For the process to go to completion, this monocarpic senescence must include three coordinated processes, which have not previously been considered as a total syndrome: (1) the arrest of growth and senescence of the shoot apical meristem; (2) senescence of the leaves; and (3) the suppression of axillary bud growth. Concurrently there is a shift in resource allocation from continued vegetative growth to reproductive growth, combined with a withdrawal of nutrients, especially nitrogen compounds, from the leaves and the transfer of these nutrients to the developing seeds. The start of the senescence process is caused by a shift, almost certainly in gene expression, very early in the reproductive phase. Continuation of the resource transfer and senescence of the vegetative plant involves hormonal regulation and continued changes in gene expression. Each of these processes is examined, especially with reference to the transfer of resources from vegetative to reproductive growth.

Moderate abiotic stresses increase rhizome growth and outgrowth of axillary buds in Alstroemeria cultured in vitro

Alstroemeria is multiplied in vitro by forced outgrowth of lateral rhizomes from rhizome explants. The multiplication rate is very low because of strong apical dominance and poor rhizome growth. We report here that moderate abiotic stresses stimulate both rhizome growth and outgrowth of lateral rhizomes, and accordingly increase multiplication. Rhizome explants were exposed to heat by a hot-water treatment (HWT) or by a hot-air treatment. Both increased rhizome growth when applied for 1 or 2 h in the range of 30–40 °C. The maximal enhancement was 75 %. Other abiotic stresses were also examined. Cold (0 °C) and partial anaerobiosis increased rhizome growth significantly. The increases brought about by drought and salinity were not statistically significant. Because underground storage tissues like rhizomes are adaptations to survive climatic stresses, we presume that the increased sink-strength of rhizomes induced by moderate stress is related to stress adaptation. Moderate heat stress (38 °C HWT, 1 h) also resulted in protection of Alstroemeria plantlets from severe heat stress (45 °C HWT, 1–2 h) a few hours after the moderate stress. All abiotic stresses also increased the outgrowth of lateral rhizomes.

Models of long-distance transport: how is carrier-dependent auxin transport regulated in the stem?

• This paper presents two models of carrier-dependent long-distance auxin transport in stems that represent the process at different scales. • A simple compartment model using a single constant auxin transfer rate produced similar data to those observed in biological experiments. The effects of different underlying biological assumptions were tested in a more detailed model representing cellular and intracellular processes that enabled discussion of different patterns of carrier-dependent auxin transport and signalling. • The output that best fits the biological data is produced by a model where polar auxin transport is not limited by the number of transporters/carriers and hence supports biological data showing that stems have considerable excess capacity to transport auxin. • All results support the conclusion that auxin depletion following apical decapitation in pea (Pisum sativum) occurs too slowly to be the initial cause of bud outgrowth. Consequently, changes in auxin content in the main stem and changes in polar auxin transport/carrier abundance in the main stem are not correlated with axillary bud outgrowth.

Sucrose Mobilisation in Sugarcane Stalk Induced by Heterotrophic Axillary Bud Growth

A theoretical high-yield sugarcane biofactory can be idealised as containing culm tissue that functions as a secondary source tissue rather than a sink. To investigate this potential process, heterotrophic axillary bud outgrowth from sugarcane (Saccharum spp. hybrids) setts was used as a model system to demonstrate that sucrose is a mobilisable carbon source. The outgrowth and subsequent biomass accumulation of axillary buds from two-eye setts of mature sugarcane stalks grown in the dark was used to measure carbon mobilisation from sett internode pith tissue. After 42 days growth 99.0 ± 0.72% of sett internode pith sucrose was depleted and 2.66 ± 0.16 g of new tissue accumulated. Comparison with a control treatment in which axillary buds were excised at day zero demonstrated that carbon mobilisation was driven by the accumulation of new biomass. Profiling of soluble carbohydrates (viz. sucrose, glucose and fructose), starch, total soluble protein, total amino nitrogen, free amino acids and total insoluble material showed that the sucrose stored in the sett internode pith was the only available carbon source of sufficient size at day zero for the observed biomass accumulation. Other metabolites mobilised were glucose, fructose and some amino acids, notably isoleucine and leucine that were depleted in shoot treatment setts at day 42.

A petunia ABC protein controls strigolactone-dependent symbiotic signalling and branching

Strigolactones were originally identified as stimulators of the germination of root-parasitic weeds that pose a serious threat to resource-limited agriculture. They are mostly exuded from roots and function as signalling compounds in the initiation of arbuscular mycorrhizae, which are plant-fungus symbionts with a global effect on carbon and phosphate cycling. Recently, strigolactones were established to be phytohormones that regulate plant shoot architecture by inhibiting the outgrowth of axillary buds. Despite their importance, it is not known how strigolactones are transported. ATP-binding cassette (ABC) transporters, however, are known to have functions in phytohormone translocation. Here we show that the Petunia hybrida ABC transporter PDR1 has a key role in regulating the development of arbuscular mycorrhizae and axillary branches, by functioning as a cellular strigolactone exporter. P. hybrida pdr1 mutants are defective in strigolactone exudation from their roots, resulting in reduced symbiotic interactions. Above ground, pdr1 mutants have an enhanced branching phenotype, which is indicative of impaired strigolactone allocation. Overexpression of Petunia axillaris PDR1 in Arabidopsis thaliana results in increased tolerance to high concentrations of a synthetic strigolactone, consistent with increased export of strigolactones from the roots. PDR1 is the first known component in strigolactone transport, providing new opportunities for investigating and manipulating strigolactone-dependent processes.

ABERRANT SPIKELET AND PANICLE1, encoding a TOPLESS‐related transcriptional co‐repressor, is involved in the regulation of meristem fate in rice

Post-embryonic development depends on the activity of meristems in plants, and thus control of cell fate in the meristem is crucial to plant development and its architecture. In grasses such as rice and maize, the fate of reproductive meristems changes from indeterminate meristems, such as inflorescence and branch meristems, to determinate meristems, such as the spikelet meristem. Here we analyzed a recessive mutant of rice, aberrant spikelet and panicle1 (asp1), that showed pleiotropic phenotypes such as a disorganized branching pattern, aberrant spikelet morphology, and disarrangement of phyllotaxy. Close examination revealed that regulation of meristem fate was compromised in asp1: degeneration of the inflorescence meristem was delayed, transition from the branch meristem to the spikelet meristem was accelerated, and stem cell maintenance in both the branch meristem and the spikelet meristem was compromised. The genetic program was also disturbed in terms of spikelet development. Gene isolation revealed that ASP1 encodes a transcriptional co-repressor that is related to TOPLESS (TPL) in Arabidopsis and RAMOSA ENHANCER LOCUS2 (REL2) in maize. It is likely that the pleiotropic defects are associated with de-repression of multiple genes related to meristem function in the asp1 mutant. The asp1 mutant also showed de-repression of axillary bud growth and disturbed phyllotaxy in the vegetative phase, suggesting that the function of this gene is closely associated with auxin action. Consistent with these observations and the molecular function of Arabidopsis TPL, auxin signaling was also compromised in the rice asp1 mutant. Taken together, these results indicate that ASP1 regulates various aspects of developmental processes and physiological responses as a transcriptional co-repressor in rice.

Functional analysis of the isopentenyltransferase gene MdIPT3a from apple (Malus pumila Mill.)

SummaryPlant isopentenyltransferases (IPTs) catalyse the first and rate-limiting step in the synthesis of cytokinin phytohormones. To understand the function of IPTs in apple (Malus pumila Mill.), a gene encoding an IPT (MdIPT3a; GenBank Accession No. HQ585947) was cloned from shoot tips of ‘Fuji’ apple using the reverse transcriptionpolymerase chain reaction (RT-PCR) method. Semi-quantitative RT-PCR showed that expression of MdIPT3a was detected in shoot-tips, young leaves, bark tissue, leaf buds, floral buds, flowers, and young fruit. Transformation of tobacco plants with the MdIPT3a gene, driven by the cauliflower mosaic virus (CaMV) 35S promoter, showed that over-expression of MdIPT3a led to the inhibition of root development, enhanced outgrowth of axillary buds, and a delay in flowering. Cytokinin-specific organogenesis of shoots in transformed tobacco was observed by in vitro culture of MdIPT3a-transformed leaf explants that over-expressed MdIPT3a and generated adventitious shoots on plant growth regulator (PGR)-free Murashige and Skoog (MS) medium. The endogenous concentrations of isopentenyl adenosine, zeatin riboside, and dihydrozeatin were significantly higher in MdIPT3a–transformed T1 tobacco plants with more axillary buds than in non-transformed control plants. These results indicate that expression of MdIPT3a is involved in plant growth and development. Expression of MdIPT3a from the CaMV 35S promoter appears to enhance the morphogenic potential of transformed plants by increasing endogenous cytokinin levels.

Characterization of Shade Avoidance Responses inLotus japonicus

Sessile plants must continuously adjust their growth and development to optimize photosynthetic activity under ever-fluctuating light conditions. Among such light responses in plants, one of the best-characterized events is the so-called shade avoidance, for which a low ratio of the red (R):far-red (FR) light intensities is the most prominent stimulus. Such shade avoidance responses enable plants to overtop their neighbors, thereby enhancing fitness and competitiveness in their natural habitat. Considerable progress has been achieved during the last decade in understanding the molecular mechanisms underlying the shade avoidance responses in the model rosette plant, Arabidopsis thaliana. We characterize here the fundamental aspects of the shade avoidance responses in the model legume, Lotus japonicus, based on the fact that its phyllotaxis (or morphological architecture) is quite different from that of A. thaliana. It was found that L. japonicus displays the characteristic shade avoidance syndrome (SAS) under defined laboratory conditions (a low R:FR ratio, low light intensity, and low blue light intensity) that mimic the natural canopy. In particular, the outgrowth of axillary buds (i.e., both aerial and cotyledonary shoot branching) was severely inhibited in L. japonicus grown in the shade. These results are discussed with special emphasis on the unique aspects of SAS observed with this legume.

Hormonal control of the outgrowth of axillary buds in Alstroemeria cultured in vitro

We study apical dominance in Alstroemeria, a plant with an architecture very different from the model species used in research on apical dominance. The standard explant was a rhizome with a tip and two vertically growing shoots from which the larger part had been excised leaving ca. 1 cm stem. The axillary buds that resumed growth were located at this 1-cm stem just above the rhizome. They were released by removal of the rhizome tip and the shoot tips. Replacement of excised tips by lanolin with indole-3-butyric acid (IBA) restored apical dominance. The auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) and N-1-napthylphthalamic acid (NPA) reduced apical dominance. 6-Benzylaminopurine (BAP) enhanced axillary bud outgrowth but the highest concentrations (> 9 μM) caused fasciation. Thidiazuron (TDZ) did not show improvement relative to BAP. Even though the architecture of Alstroemeria and the model species are very different, their hormonal mechanisms in apical dominance are for the greater part very similar.

Regulation of shoot-system development in Populus

Shoot phenology interacts with crown morphology to determine leaf production and duration over the growing season and throughout a tree’s life span; and thus, this interaction has a major role in determining whole-tree photosynthesis and biomass yield. For several years, the pattern of shoot meristem activity determines Populus crown architecture, but after the first onset of flowering the specification of meristem identity (inflorescence vs. vegetative) also has a major influence on crown form. To study how shoot meristem activity and identity are regulated in Populus, we are focusing on two pathways that have been extensively characterized in annual plants: 1) the MORE AXILLARY BRANCHING (MAX) genes that are involved in the synthesis and perception of upwardly mobile strigolactones that inhibit axillary bud outgrowth, and 2) members of the TERMINAL FLOWER1 (TFL1)/FLOWERING LOCUS T (FT) and APETALA1(AP1)/FRUITFULL(FUL) gene families that regulate flowering. We are taking a multi-pronged approach that includes field and controlled-environment study of poplar transgenics, transcriptome analyses, cell-specific expression studies, and QTL and association mapping. QTL mapping in a hybrid poplar pedigree localized three of seven poplar MAX genes in QTL for bud set and two MAX genes in sylleptic branching QTL. All MAX transgenics were established in a field trial and phenotyping of phenology, crown morphology, and growth is in progress. Greenhouse studies of poplar transgenics with the seven different poplar MAX gene promoters driving the reporter gene GUS show that most of the MAX genes are expressed in vascular tissues. However, vascular expression patterns are different, especially in stems undergoing secondary growth. Surprisingly, none of the RNAi transgenics showed differences in branching in the greenhouse under standard growing conditions. High nitrogen levels can induce sylleptic branching in poplar and greenhouse experiments with MAX transgenics indicate a possible role for one MAX gene in nitrogen-induced sylleptic branching. Ongoing studies of the effects of decapitation of the shoot apex with and without subsequent auxin application to the cut stem indicate that, as predicted based on studies in annual plants, the poplar MAX RNAi transgenics show a reduced response to apically-applied auxin. Recent work has revealed that poplar FT1 and FT2 have diverged in regulation and function with FT1 promoting the transition to flowering in response to winter temperatures, whereas warm temperatures and long days promote FT2 expression and shoot growth [1]. In Arabidopsis, FT activates AP1, a key regulator of flower initiation. Expression and transgenic studies in poplar indicate that similar to the two poplar FTs, poplar AP1 subfamily members have roles in both flowering and bud set in response to short-days and abiotic stress. Ongoing work is aimed at defining the roles of FT and AP1 modules in these processes and to delineate the roles of the five different poplar AP1/FUL genes. Finally, TFL1 acts opposite to FT to promote vegetative meristem identity in Arabidopsis, and AP1 and TFL1 repress each other’s expression. The poplar TFL1 homologs, CEN1 and/or CEN2 have roles in maintaining vegetative identity and in the dormancy-growth transition [2] and we are currently working to define the individual roles of the poplar CENs and their relationships to FT and AP1 activities.