Long and short photoperiod buds in hybrid aspen share structural development and expression patterns of marker genes

Light on perenniality: Para-dormancy is based on ABA-GA antagonism and endo-dormancy on the shutdown of GA biosynthesis

Perennial para- and endo-dormancy are seasonally separate phenomena. Whereas para-dormancy is the suppression of axillary buds (AXBs) by a growing shoot, endo-dormancy is the short-day elicited arrest of terminal and AXBs. In hybrid aspen (Populus tremula x P. tremuloides) compromising the apex releases para-dormancy, whereas endo-dormancy requires chilling. ABA and GA are implicated in both phenomena. To untangle their roles, we blocked ABA biosynthesis with fluridone (FD), which significantly reduced ABA levels, downregulated GA-deactivation genes, upregulated the major GA3ox-biosynthetic genes, and initiated branching. Comprehensive GA-metabolite analyses suggested that FD treatment shifted GA production to the non-13-hydroxylation pathway, enhancing GA₄ function. Applied ABA counteracted FD effects on GA metabolism and downregulated several GA_3/4 -inducible α- and γ-clade 1,3-β-glucanases that hydrolyze callose at plasmodesmata (PD), thereby enhancing PD-callose accumulation. Remarkably, ABA-deficient plants repressed GA₄ biosynthesis and established endo-dormancy like controls but showed increased stress sensitivity. Repression of GA₄ biosynthesis involved short-day induced DNA methylation events within the GA3ox2 promoter. In conclusion, the results cast new light on the roles of ABA and GA in dormancy cycling. In para-dormancy, PD-callose turnover is antagonized by ABA, whereas in short-day conditions, lack of GA₄ biosynthesis promotes callose deposition that is structurally persistent throughout endo-dormancy.

When to branch: seasonal control of shoot architecture in trees

Long-lived perennial plants optimize their shoot architecture by responding to seasonal cues. The main strategy used by plants of temperate and boreal regions with respect to surviving the extremely unfavourable conditions of winter comprises the protection of their apical and lateral meristematic tissues. This involves myriads of transcriptional, translational and metabolic changes in the plants because shoot architecture is controlled by multiple pathways that regulate processes such as bud formation and flowering, small RNAs, environmental factors (especially light quality, photoperiod and temperature), hormones, and sugars. Recent studies have begun to reveal how these pathways are recruited for the seasonal adaptation and regulation of shoot architecture in perennial plants, including the role of a regulatory module consisting of antagonistic players terminal flower 1 (TFL1) and like-ap1 (LAP1) in the hybrid aspen. Here, we review recent progress in our understanding of the genetic control of shoot architecture in perennials compared to in annuals.

Overexpression of SHORT-ROOT2 transcription factor enhances the outgrowth of mature axillary buds in poplar trees

SHORT-ROOT (SHR) transcription factors play important roles in asymmetric cell division and radial patterning of Arabidopsis roots. In hybrid poplar (P. tremula × P. alba clone INRA 717-1B4), PtaSHR2 was preferentially expressed in axillary buds (AXBs) and transcriptionally up-regulated during AXB maturation and activation. Overexpression of SHR2 (PtSHR2OE) induced an enhanced outgrowth of AXBs below the bud maturation point, with a simultaneous transition of an active shoot apex into an arrested terminal bud. The larger and more mature AXBs of PtSHR2OE trees revealed altered expression of genes involved in axillary meristem initiation and bud activation, as well as a higher ratio of cytokinin to auxin. To elucidate the underlying mechanism of PtSHR2OE-induced high branching, subsequent molecular and biochemical studies showed that compared with wild-type trees, decapitation induced a quicker bud outburst in PtSHR2OE trees, which could be fully inhibited by exogenous application of auxin or cytokinin biosynthesis inhibitor, but not by N-1-naphthylphthalamic acid. Our results indicated that overexpression of PtSHR2B disturbed the internal hormonal balance in AXBs by interfering with the basipetal transport of auxin, rather than causing auxin biosynthesis deficiency or auxin insensitivity, thereby releasing mature AXBs from apical dominance and promoting their outgrowth.

Growth and photosynthetic traits differ between shoots originated from axillary buds or from adventitious buds in Populus balsamifera L. cuttings

Bud development influences shoot branching and the plasticity and adaptability of plants. To explore the differences of post-embryonic development of different types of buds, shoots originated from adventitious buds and axillary buds of cuttings in two populations of balsam poplar (Populus balsamifera L.) were investigated for differences in leaf morphology, photosynthetic and growth characteristics, and the effects of a carbonic anhydrase (CA) inhibitor on CA activity, photosynthesis and mesophyll conductance (g_m ). The results showed that axillary buds produced ovate first few leaves and longer shoots while adventitious buds produced lanceolate first few leaves with higher specific leaf area (SLA). There were no significant differences in leaf area-based photosynthetic rate (A_n ), maximum carboxylation rate (V_cmax ), and maximum electron transport rate (J_max ) between shoots originated from the two bud types. Based on the principal component analysis, shoots of adventitious bud origin grouped on daytime respiration and SLA, while cuttings from axillary buds clustered toward the opposite direction of quantum yield and light saturation point. Shoots originated from different types of buds had different growth rates and biomass, but the direction of the differences varied with the population of the mother tree. The two populations differed in A_n , g_m , and relationships between CA, A_n , and g_m . There were differences in post-embryonic growth traits of shoots from axillary buds and those from adventitious buds, which may be an adaptive strategy for regeneration under different light conditions.

Lipid Body Dynamics in Shoot Meristems: Production, Enlargement, and Putative Organellar Interactions and Plasmodesmal Targeting

Post-embryonic cells contain minute lipid bodies (LBs) that are transient, mobile, engage in organellar interactions, and target plasmodesmata (PD). While LBs can deliver γ-clade 1,3-β-glucanases to PD, the nature of other cargo is elusive. To gain insight into the poorly understood role of LBs in meristems, we investigated their dynamics by microscopy, gene expression analyzes, and proteomics. In developing buds, meristems accumulated LBs, upregulated several LB-specific OLEOSIN genes and produced OLEOSINs. During bud maturation, the major gene OLE6 was strongly downregulated, OLEOSINs disappeared from bud extracts, whereas lipid biosynthesis genes were upregulated, and LBs were enlarged. Proteomic analyses of the LB fraction of dormant buds confirmed that OLEOSINs were no longer present. Instead, we identified the LB-associated proteins CALEOSIN (CLO1), Oil Body Lipase 1 (OBL1), Lipid Droplet Interacting Protein (LDIP), Lipid Droplet Associated Protein1a/b (LDAP1a/b) and LDAP3a/b, and crucial components of the OLEOSIN-deubiquitinating and degradation machinery, such as PUX10 and CDC48A. All mRFP-tagged LDAPs localized to LBs when transiently expressed in Nicotiana benthamiana. Together with gene expression analyzes, this suggests that during bud maturation, OLEOSINs were replaced by LDIP/LDAPs at enlarging LBs. The LB fraction contained the meristem-related actin7 (ACT7), "myosin XI tail-binding" RAB GTPase C2A, an LB/PD-associated γ-clade 1,3-β-glucanase, and various organelle- and/or PD-localized proteins. The results are congruent with a model in which LBs, motorized by myosin XI-k/1/2, traffic on F-actin, transiently interact with other organelles, and deliver a diverse cargo to PD.

Plant Lipid Bodies Traffic on Actin to Plasmodesmata Motorized by Myosin XIs

Late 19th-century cytologists observed tiny oil drops in shoot parenchyma and seeds, but it was discovered only in 1972 that they were bound by a half unit-membrane. Later, it was found that lipid bodies (LBs) arise from the endoplasmic reticulum. Seeds are known to be packed with static LBs, coated with the LB-specific protein OLEOSIN. As shown here, apices of Populustremula x P. tremuloides also express OLEOSIN genes and produce potentially mobile LBs. In developing buds, PtOLEOSIN (PtOLE) genes were upregulated, especially PtOLE6, concomitant with LB accumulation. To investigate LB mobility and destinations, we transformed Arabidopsis with PtOLE6-eGFP. We found that PtOLE6-eGFP fusion protein co-localized with Nile Red-stained LBs in all cell types. Moreover, PtOLE6-eGFP-tagged LBs targeted plasmodesmata, identified by the callose marker aniline blue. Pharmacological experiments with brefeldin, cytochalasin D, and oryzalin showed that LB-trafficking requires F-actin, implying involvement of myosin motors. In a triple myosin-XI knockout (xi-k/1/2), transformed with PtOLE6-eGFP, trafficking of PtOLE6-eGFP-tagged LBs was severely impaired, confirming that they move on F-actin, motorized by myosin XIs. The data reveal that LBs and OLEOSINs both function in proliferating apices and buds, and that directional trafficking of LBs to plasmodesmata requires the actomyosin system.

Dual Role of Gibberellin in Perennial Shoot Branching: Inhibition and Activation

Shoot branching from axillary buds (AXBs) is regulated by a network of inhibitory and promotive forces, which includes hormones. In perennials, the dwarfed stature of the embryonic shoot inside AXBs is indicative of gibberellin (GA) deficiency, suggesting that AXB activation and outgrowth require GA. Nonetheless, the role of GA in branching has remained obscure. We here carried out comprehensive GA transcript and metabolite analyses in hybrid aspen, a perennial branching model. The results indicate that GA has an inhibitory as well as promotive role in branching. The latter is executed in two phases. While the expression level of GA2ox is high in quiescent AXBs, decapitation rapidly downregulated it, implying increased GA signaling. In the second phase, GA3ox2-mediated de novo GA-biosynthesis is initiated between 12 and 24 h, prior to AXB elongation. Metabolite analyzes showed that GA_1/4 levels were typically high in proliferating apices and low in the developmentally inactive, quiescent AXBs, whereas the reverse was true for GA_3/6. To investigate if AXBs are differently affected by GA₃, GA₄, and GR24, an analog of the branch-inhibitor hormone strigolactone, they were fed into AXBs of single-node cuttings. GA₃ and GA₄ had similar effects on GA and SL pathway genes, but crucially GA₃ induced AXB abscission whereas GA₄ promoted outgrowth. Both GA₃ and GA₄ strongly upregulated GA2ox genes, which deactivate GA_1/4 but not GA_3/6. Thus, the observed production of GA_3/6 in quiescent AXBs targets GA_1/4 for GA2ox-mediated deactivation. AXB quiescence can therefore be maintained by GA_3/6, in combination with strigolactone. Our discovery of the distinct tasks of GA₃ and GA₄ in AXB activation might explain why the role of GA in branching has been difficult to decipher. Together, the results support a novel paradigm in which GA_3/6 maintains high levels of GA2ox expression and low levels of GA₄ in quiescent AXBs, whereas activation and outgrowth require increased GA_1/4 signaling through the rapid reduction of GA deactivation and subsequent GA biosynthesis.

Strigolactone-Based Node-to-Bud Signaling May Restrain Shoot Branching in Hybrid Aspen

The biosynthesis and roles of strigolactones (SLs) have been investigated in herbaceous plants, but so far, their role in trees has received little attention. In this study, we analyzed the presence, spatial/temporal expression and role of SL pathway genes in Populus tremula � Populus tremuloides. In this proleptic species, axillary buds (AXBs) become para-dormant at the bud maturation point, providing an unambiguous starting point to study AXB activation. We identified previously undescribed Populus homologs of DWARF27 (D27), LATERAL BRANCHING OXIDOREDUCTASE (LBO) and DWARF53-like (D53-like) and analyzed the relative expression of all SL pathway genes in root tips and shoot tissues. We found that, although AXBs expressed MORE AXILLARY GROWTH1 (MAX1) and LBO, they did not express MAX3 and MAX4, whereas nodal bark expressed high levels of all SL biosynthesis genes. By contrast, expression of the SL perception and signaling genes MAX2, D14 and D53 was high in AXBs relative to nodal bark and roots. This suggests that AXBs are reliant on the associated nodes for the import of SLs and SL precursors. Activation of AXBs was initiated by decapitation and single-node isolation. This rapidly downregulated SL pathway genes downstream of MAX4, although later these genes were upregulated coincidently with primordia formation. GR24-feeding counteracted all activation-related changes in SL gene expression but did not prevent AXB outgrowth showing that SL is ineffective once AXBs are activated. The results indicate that nodes rather than roots supply SLs and its precursors to AXBs, and that SLs may restrain embryonic shoot elongation during AXB formation and para-dormancy in intact plants.

Decoupling photo- and thermoperiod by projected climate change perturbs bud development, dormancy establishment and vernalization in the model tree Populus

BACKGROUND

The performance and survival of deciduous trees depends on their innate ability to anticipate seasonal change. A key event is the timely production of short photoperiod-induced terminal and axillary buds that are dormant and freezing-tolerant. Some observations suggest that low temperature contributes to terminal bud initiation and dormancy. This is puzzling because low temperatures in the chilling range universally release dormancy. It also raises the broader question if the projected climate instabilities, as well as the northward migration of trees, will affect winter preparations and survival of trees.

RESULTS

To gauge the response capacity of trees, we exposed juvenile hybrid aspens to a 10-h short photoperiod in combination with different day/night temperature regimes: high (24/24 °C), moderate (18/18 °C), moderate-low (18/12 °C) and low (12/12 °C), and analysed bud development, dormancy establishment, and marker gene expression. We found that low temperature during the bud formation period (pre-dormancy) upregulated dormancy-release genes of the gibberellin (GA) pathway, including the key GA biosynthesis genes GA20oxidase and GA3oxidase, the GA-receptor gene GID1, as well as GA-inducible enzymes of the 1,3-β-glucanase family that degrade callose at plasmodesmal Dormancy Sphincter Complexes. Simultaneously, this pre-dormancy low temperature perturbed the expression of flowering pathway genes, including CO, FT, CENL1, AGL14, LFY and AP1. In brief, pre-dormancy low temperature compromised bud development, dormancy establishment, and potentially vernalization. On the other hand, a high pre-dormancy temperature prevented dormancy establishment and resulted in flushing.

CONCLUSIONS

The results show that pre-dormancy low temperature represents a form of chilling that antagonizes dormancy establishment. Combined with available field data, this indicates that natural Populus ecotypes have evolved to avoid the adverse effects of high and low temperatures by initiating and completing dormant buds within an approximate temperature-window of 24-12 °C. Global warming and erratic temperature patterns outside this range can therefore endanger the successful propagation of deciduous perennials.

Sprouting of paradormant and endodormant grapevine buds under conditions of forced growth: similarities and differences

Bud-break assays under forced growth conditions suggest that a drop in ABA content and an increase in sugars are common features in the sprouting of paradormant (PD) and endodormant (ED) grapevine buds. However, increases in cell division and in respiration are unique characteristics of the ED budding. In tropical and subtropical regions where the variations in day length and temperatures are minor throughout the year, the rupture of grapevine buds can be achieved during the current growing season given rise to a double-cropping system annually. However, it is unknown whether the breaking buds are in the paradormancy (PD) or endodormancy (ED) stage. In this study, we compared the breakage of PD and ED buds under conditions of forced growth. To do this, the expression of genes related to the metabolism of phytohormones and sugars, and of relevant physiological functions such as respiration and cell division was analyzed temporally throughout the incubation period in both types of buds. An early fall in the expression of the ABA biosynthesis gene (VvNCED1) and increases in genes related to sugar metabolism and transports were observed during the incubation period in both types of buds. However, while in the PD buds, the genes related to respiration and the cell cycle did not undergo significant changes in their expression during the incubation period, in the ED buds, the expression of these genes together with those related to auxin and cytokinin biosynthesis experienced a large increase. The results suggest that a drop in ABA content and an increase in sugars are early signals for the onset of bud break in both PD and ED vines, while the increase in respiration and cell division are unique characteristics of the ED buds, which reflect its transition from a resting state to a state of active growth.

Comparative transcriptome analysis of axillary buds in response to the shoot branching regulators gibberellin A3 and 6-benzyladenine in Jatropha curcas

Cytokinin (CK) is the primary hormone that positively regulates axillary bud outgrowth. However, in many woody plants, such as Jatropha curcas, gibberellin (GA) also promotes shoot branching. The molecular mechanisms underlying GA and CK interaction in the regulation of bud outgrowth in Jatropha remain unclear. To determine how young axillary buds respond to GA₃ and 6-benzyladenine (BA), we performed a comparative transcriptome analysis of the young axillary buds of Jatropha seedlings treated with GA₃ or BA. Two hundred and fifty genes were identified to be co-regulated in response to GA₃ or BA. Seven NAC family members were down-regulated after treatment with both GA₃ and BA, whereas these genes were up-regulated after treatment with the shoot branching inhibitor strigolactone. The expressions of the cell cycle genes CDC6, CDC45 and GRF5 were up-regulated after treatment with both GA₃ and BA, suggesting they may promote bud outgrowth via regulation of the cell cycle machinery. In the axillary buds, BA significantly increased the expression of GA biosynthesis genes JcGA20oxs and JcGA3ox1, and down-regulated the expression of GA degradation genes JcGA2oxs. Overall, the comprehensive transcriptome data set provides novel insight into the responses of young axillary buds to GA and CK.

Axillary buds are dwarfed shoots that tightly regulate GA pathway and GA-inducible 1,3-β-glucanase genes during branching in hybrid aspen

Axillary buds (AXBs) of hybrid aspen (Populus tremula×P. tremuloides) contain a developing dwarfed shoot that becomes para-dormant at the bud maturation point. Para-dormant AXBs can grow out after stem decapitation, while dormant AXBs pre-require long-term chilling to release them from dormancy. The latter is mediated by gibberellin (GA)-regulated 1,3-β-glucanases, but it is unknown if GA is also important in the development, activation, and outgrowth of para-dormant AXBs. The present data show that para-dormant AXBs up-regulate GA receptor genes during their maturation, but curtail GA biosynthesis by down-regulating the rate-limiting GIBBERELLIN 3-OXIDASE2 (GA3ox2), which is characteristically expressed in the growing apex. However, decapitation significantly up-regulated GA3ox2 and GA₄-responsive 1,3-β-glucanases (GH17-family; α-clade). In contrast, decapitation down-regulated γ-clade 1,3-β-glucanases, which were strongly up-regulated in maturing AXBs concomitant with lipid body accumulation. Overexpression of selected GH17 members in hybrid aspen resulted in characteristic branching patterns. The α-clade member induced an acropetal branching pattern, whereas the γ-clade member activated AXBs in recurrent flushes during transient cessation of apex proliferation. The results support a model in which curtailing the final step in GA biosynthesis dwarfs the embryonic shoot, while high levels of GA precursors and GA receptors keep AXBs poised for growth. GA signaling, induced by decapitation, reinvigorates symplasmic supply routes through GA-inducible 1,3-β-glucanases that hydrolyze callose at sieve plates and plasmodesmata.

On the language and physiology of dormancy and quiescence in plants

The language of dormancy is rich and poetic, as researchers spanning disciplines and decades have attempted to understand the spell that entranced 'Sleeping Beauty', and how she was gently awoken. The misleading use of 'dormancy', applied to annual axillary buds, for example, has confounded progress. Language is increasingly important as genetic and genomic approaches become more accessible to species of agricultural and ecological importance. Here we examine how terminology has been applied to different eco-physiological states in plants, and with pertinent reference to quiescent states described in other domains of life, in order to place plant quiescence and dormancy in a more complete context than previously described. The physiological consensus defines latency or quiescence as opportunistic avoidance states, where growth resumes in favourable conditions. In contrast, the dormant state in higher plants is entrained in the life history of the organism. Competence to resume growth requires quantitative and specific conditioning. This definition applies only to the embryo of seeds and specialized meristems in higher plants; however, mechanistic control of dormancy extends to mobile signals from peripheral tissues and organs, such as the endosperm of seed or subtending leaf of buds. The distinction between dormancy, quiescence, and stress-hardiness remains poorly delineated, most particularly in buds of winter perennials, which comprise multiple meristems of differing organogenic states. Studies in seeds have shown that dormancy is not a monogenic trait, and limited study has thus far failed to canalize dormancy as seen in seeds and buds. We argue that a common language, based on physiology, is central to enable further dissection of the quiescent and dormant states in plants. We direct the topic largely to woody species showing a single cycle of growth and reproduction per year, as these bear the majority of global timber, fruit, and nut production, as well being of great ecological value. However, for context and hypotheses, we draw on knowledge from annuals and other specialized plant conditions, from a perspective of the major physical, metabolic, and molecular cues that regulate cellular activity.

Transcriptomic Analysis Using Olive Varieties and Breeding Progenies Identifies Candidate Genes Involved in Plant Architecture

Plant architecture is a critical trait in fruit crops that can significantly influence yield, pruning, planting density and harvesting. Little is known about how plant architecture is genetically determined in olive, were most of the existing varieties are traditional with an architecture poorly suited for modern growing and harvesting systems. In the present study, we have carried out microarray analysis of meristematic tissue to compare expression profiles of olive varieties displaying differences in architecture, as well as seedlings from their cross pooled on the basis of their sharing architecture-related phenotypes. The microarray used, previously developed by our group has already been applied to identify candidates genes involved in regulating juvenile to adult transition in the shoot apex of seedlings. Varieties with distinct architecture phenotypes and individuals from segregating progenies displaying opposite architecture features were used to link phenotype to expression. Here, we identify 2252 differentially expressed genes (DEGs) associated to differences in plant architecture. Microarray results were validated by quantitative RT-PCR carried out on genes with functional annotation likely related to plant architecture. Twelve of these genes were further analyzed in individual seedlings of the corresponding pool. We also examined Arabidopsis mutants in putative orthologs of these targeted candidate genes, finding altered architecture for most of them. This supports a functional conservation between species and potential biological relevance of the candidate genes identified. This study is the first to identify genes associated to plant architecture in olive, and the results obtained could be of great help in future programs aimed at selecting phenotypes adapted to modern cultivation practices in this species.