A Tree Ortholog of SHORT VEGETATIVE PHASE Floral Repressor Mediates Photoperiodic Control of Bud Dormancy

A tree peony RING-H2 finger protein, PsATL33, plays an essential role in cold-induced bud dormancy release by regulating gibberellin content

Bud dormancy is crucial for woody perennial plants to resist low-temperature stress in winter. However, the molecular regulatory mechanisms underlying bud dormancy release are largely unclear. Here, a tree peony (Paeonia suffruticosa) transcript ARABIDOPSIS TOXICOS EN LEVADURA 33 (PsATL33), encoding a RING-H2 finger protein, was selected from previously generated RNA sequencing data of chilling-treated buds. The objective of this study is to investigate the role of PsATL33 in the regulation of cold-induced bud dormancy release. Subcellular localization assay revealed that PsATL33 was localized to the nucleus and plasma membrane. Reverse transcription-quantitative PCR analysis showed that PsATL33 was dramatically upregulated during cold-triggered bud dormancy release. Exogenous treatments with gibberellin (GA3) increased, but abscisic acid (ABA) inhibited the transcription of PsATL33. Ectopic transformation assay indicated that overexpression of PsATL33 in petunia promoted seed germination, plant growth, and axillary bud break. Silencing of PsATL33 in tree peony through virus-induced gene silencing assay delayed bud dormancy release. tobacco rattle virus (TRV)-PsATL33-infected buds exhibited reduced expression levels of dormancy break-related genes EARLY BUD-BREAK 1 (PsEBB1) and CARBOXYLESTERASE 15 (PsCXE15). Silencing of PsATL33 decreased the accumulation of bioactive GAs, GA1 and GA3, rather than ABA. Transcript levels of several genes involved in GA biosynthesis and signaling, including GA20-OXIDASE 1 (PsGA20ox1), GA3-OXIDASE 1 (PsGA3ox1), PsGA3ox3, GA2-OXIDASE 1 (PsGA2ox1), and GA-INSENSITIVE 1A (PsGAI1A), were changed by PsATL33 silencing. Taken together, our data suggest that PsATL33 functions as a positive regulator of cold-induced bud dormancy release by modulating GA production.

Multilayer regulatory landscape and new regulators identification for bud dormancy release and bud break in Populus

For trees originating from boreal and temperate regions, the dormancy-to-active transition, also known as bud dormancy release and bud break, are crucial processes that allow trees to reactive growth in the spring. The molecular mechanisms underlying these two processes remain poorly understood. Here, through integrative multiomics analysis of the transcriptome, DNA methylome, and proteome, we gained insights into the reprogrammed cellular processes associated with bud dormancy release and bud break. Our findings revealed multilayer regulatory landscapes governing bud dormancy release and bud break regulation, providing a valuable reference framework for future functional studies. Based on the multiomics analysis, we have determined a novel long intergenic noncoding RNA named Phenology Responsive Intergenic lncRNA 1 (PRIR1) plays a role in the activation of bud break. that the molecular mechanism of PRIR1 has been preliminary explored, and it may partially promote bud break by activating its neighbouring gene, EXORDIUM LIKE 5 (PtEXL5), which has also been genetically confirmed as an activator for bud break. This study has revealed a lncRNA-mediated regulatory mechanism for the control of bud break in Populus, operating independently of known regulatory pathways.

Integrated proteomic, transcriptomic, and metabolomic profiling reveals that the gibberellin-abscisic acid hub runs flower development in the Chinese orchid Cymbidium sinense

The seasonal flowering Chinese Cymbidium produce an axillary floral meristem and require a dormancy period during cold conditions for flower development. However, the bud activation mechanism remains elusive. This study evaluates the multi-omics across six stages of flower development, along with functional analysis of core genes to decipher the innate mechanism of floral bud initiation and outgrowth in the Chinese orchid Cymbidium sinense. Transcriptome and proteome analyses identified 10 modules with essential roles in floral bud dormancy and activation. Gene clusters in the early stages of flower development were mainly related to flowering time regulation and meristem determination, while the late stages were correlated with hormone signaling pathways. The metabolome identified 69 potential hormones in which gibberellin (GA) and abscisic acid (ABA) were the main regulatory hubs, and GA4 and GA53 exhibited a reciprocal loop. Extraneous GA application caused rapid elongation of flower buds and promoted the expression of flower development genes. Contrarily, exogenous ABA application extended the dormancy process and ABA inhibitors induced dormancy release. Moreover, CsAPETALA1 (CsAP1) was identified as the potential target of ABA for floral bud activation. Transformation of CsAP1 in Arabidopsis and its transient overexpression in C. sinense protoplasts not only affected flowering time and floral organ morphogenesis in Arabidopsis but also orchestrated the expression of flowering and hormone regulatory genes. The presence of ABA response elements in the CsAP1 promoter, rapid downregulation of CsAP1 after exogenous ABA application, and the activation of the floral bud after ABA inhibitor treatment suggest that ABA can control bud outgrowth through CsAP1.

PsmiR159b-PsMYB65 module functions in the resumption of bud growth after endodormancy by affecting the cell cycle in tree peony

Bud endodormancy in perennial plants is a sophisticated system that adapts to seasonal climatic changes. Growth-promoting signals such as low temperature and gibberellins (GAs) are crucial for facilitating budbreak following endodormancy release (EDR). However, the regulatory mechanisms underlying GA-mediated budbreak in tree peony (Paeonia suffruticosa) remain unclear. In tree peony, the expression of PsmiR159b among three differentially expressed miR159 members was inhibited with the prolonged chilling, and overexpression of PsMIR159b delayed budbreak, whereas silencing PsmiR159b promoted budbreak after dormancy. PsMYB65, a downstream transcription factor in the GA pathway, was induced by prolonged chilling and exogenous GA3 treatments. PsMYB65 was identified as a target of PsmiR159b, and promoted budbreak in tree peony. RNA-seq of PsMYB65-slienced buds revealed significant enrichment in the GO terms regulation of 'cell cycle' and 'DNA replication' among differentially expressed genes. Yeast one-hybrid and electrophoretic mobility shift assays demonstrated that PsMYB65 directly bound to the promoter of the type-D cyclin gene PsCYCD3;1. Dual-luciferase reporter assay indicated that PsMYB65 positively regulate PsCYCD3;1 expression, suggesting that miR159b-PsMYB65 module contributes to budbreak by influencing the cell cycle. Our findings revealed that the PsmiR159b-PsMYB65 module functioned in budbreak after dormancy by regulating cell proliferation, providing valuable insights into the endodormancy release regulation mechanism.

IAA-miR164a-NAC100L1 module mediates symbiotic incompatibility of cucumber/pumpkin grafted seedlings through regulating callose deposition

Grafting is one of the key technologies to overcome the obstacles of continuous cropping, and improve crop yield and quality. However, the symbiotic incompatibility between rootstock and scion affects the normal growth and development of grafted seedlings after survival. The specific molecular regulation mechanism of graft incompatibility is still largely unclear. In this study, we found that the IAA-miR164a-NAC100L1 module induced callose deposition to mediate the symbiotic incompatibility of cucumber/pumpkin grafted seedlings. The incompatible combination (IG) grafting interface accumulated more callose, and the activity of callose synthase (CmCalS1) and IAA content were significantly higher than in the compatible combination (CG). Treatment with IAA polar transport inhibitor in the root of the IG plants decreased CmCalS activity and callose content. Furthermore, IAA negatively regulated the expression of Cm-miR164a, which directly targeted cleavage of CmNAC100L1. Interestingly, CmNAC100L1 interacted with CmCalS1 to regulate its activity. Further analysis showed that the interaction between CmNAC100L1 and CmCalS1 increased the activity of CmCalS1 in the IG plants but decreased it in the CG plants. Point mutation analysis revealed that threonine at the 57th position of CmCalS1 protein played a critical role to maintain its enzyme activity in the incompatible rootstock. Thus, IAA inhibited the expression of Cm-miR164a to elevate the expression of CmNAC100L1, which promoted CmNAC100L1 interaction with CmCalS1 to enhance CmCalS1 activity, resulting in callose deposition and symbiotic incompatibility of cucumber/pumpkin grafted seedlings.

Molecular Mechanisms of Seasonal Gene Expression in Trees

In trees, the annual cycling of active and dormant states in buds is closely regulated by environmental factors, which are of primary significance to their productivity and survival. It has been found that the parallel or convergent evolution of molecular pathways that respond to day length or temperature can lead to the establishment of conserved periodic gene expression patterns. In recent years, it has been shown in many woody plants that change in annual rhythmic patterns of gene expression may underpin the adaptive evolution in forest trees. In this review, we summarize the progress on the molecular mechanisms of seasonal regulation on the processes of shoot growth, bud dormancy, and bud break in response to day length and temperature factors. We focus on seasonal expression patterns of genes involved in dormancy and their associated epigenetic modifications; the seasonal changes in the extent of modifications, such as DNA methylation, histone acetylation, and histone methylation, at dormancy-associated loci have been revealed for their actions on gene regulation. In addition, we provide an outlook on the direction of research on the annual cycle of tree growth under climate change.

Advancing tree genomics to future proof next generation orchard production

The challenges facing tree orchard production in the coming years will be largely driven by changes in the climate affecting the sustainability of farming practices in specific geographical regions. Identifying key traits that enable tree crops to modify their growth to varying environmental conditions and taking advantage of new crop improvement opportunities and technologies will ensure the tree crop industry remains viable and profitable into the future. In this review article we 1) outline climate and sustainability challenges relevant to horticultural tree crop industries, 2) describe key tree crop traits targeted for improvement in agroecosystem productivity and resilience to environmental change, and 3) discuss existing and emerging genomic technologies that provide opportunities for industries to future proof the next generation of orchards.

Perenniality: From model plants to applications in agriculture

To compensate for their sessile nature, plants have evolved sophisticated mechanisms enabling them to adapt to ever-changing environments. One such prominent feature is the evolution of diverse life history strategies, particularly such that annuals reproduce once followed by seasonal death, while perennials live longer by cycling growth seasonally. This intrinsic phenology is primarily genetic and can be altered by environmental factors. Although evolutionary transitions between annual and perennial life history strategies are common, perennials account for most species in nature because they survive well under year-round stresses. This proportion, however, is reversed in agriculture. Hence, perennial crops promise to likewise protect and enhance the resilience of agricultural ecosystems in response to climate change. Despite significant endeavors that have been made to generate perennial crops, progress is slow because of barriers in studying perennials, and many developed species await further improvement. Recent findings in model species have illustrated that simply rewiring existing genetic networks can lead to lifestyle variation. This implies that engineering plant life history strategy can be achieved by manipulating only a few key genes. In this review, we summarize our current understanding of genetic basis of perenniality and discuss major questions and challenges that remain to be addressed.

Genome-Wide Identification of Callose Synthase Family Genes and Their Expression Analysis in Floral Bud Development and Hormonal Responses in Prunus mume

Callose is an important polysaccharide composed of beta-1,3-glucans and is widely implicated in plant development and defense responses. Callose synthesis is mainly catalyzed by a family of callose synthases, also known as glucan synthase-like (GSL) enzymes. Despite the fact that GSL family genes were studied in a few plant species, their functional roles have not been fully understood in woody perennials. In this study, we identified total of 84 GSL genes in seven plant species and classified them into six phylogenetic clades. An evolutionary analysis revealed different modes of duplication driving the expansion of GSL family genes in monocot and dicot species, with strong purifying selection constraining the protein evolution. We further examined the gene structure, protein sequences, and physiochemical properties of 11 GSL enzymes in Prunus mume and observed strong sequence conservation within the functional domain of PmGSL proteins. However, the exon-intron distribution and protein motif composition are less conservative among PmGSL genes. With a promoter analysis, we detected abundant hormonal responsive cis-acting elements and we inferred the putative transcription factors regulating PmGSLs. To further understand the function of GSL family genes, we analyzed their expression patterns across different tissues, and during the process of floral bud development, pathogen infection, and hormonal responses in Prunus species and identified multiple GSL gene members possibly implicated in the callose deposition associated with bud dormancy cycling, pathogen infection, and hormone signaling. In summary, our study provides a comprehensive understanding of GSL family genes in Prunus species and has laid the foundation for future functional research of callose synthase genes in perennial trees.

Interaction of Phytohormones and External Environmental Factors in the Regulation of the Bud Dormancy in Woody Plants

Bud dormancy and release are essential phenomena that greatly assist in adapting to adverse growing conditions and promoting the holistic growth and development of perennial plants. The dormancy and release process of buds in temperate perennial trees involves complex interactions between physiological and biochemical processes influenced by various environmental factors, representing a meticulously orchestrated life cycle. In this review, we summarize the role of phytohormones and their crosstalk in the establishment and release of bud dormancy. External environmental factors, such as light and temperature, play a crucial role in regulating bud germination. We also highlight the mechanisms of how light and temperature are involved in the regulation of bud dormancy by modulating phytohormones. Moreover, the role of nutrient factors, including sugar, in regulating bud dormancy is also discussed. This review provides a foundation for enhancing our understanding of plant growth and development patterns, fostering agricultural production, and exploring plant adaptive responses to adversity.

Age-dependent seasonal growth cessation in Populus

In temperate and boreal regions, perennial plants adapt their annual growth cycle to the change of seasons. In natural forests, juvenile seedlings usually display longer growth seasons compared to adult trees to ensure their establishment and survival under canopy shade. However, how trees adjust their annual growth according to their age is not known. In this study, we show that age-dependent seasonal growth cessation is genetically controlled and found that the miR156-SPL3/5 module, a key regulon of vegetative phase change (VPC), also triggers age-dependent growth cessation in Populus trees. We show that miR156 promotes shoot elongation during vegetative growth, and its targets SPL3/5s function in the same pathway but as repressors. We find that the miR156-SPL3/5s regulon controls growth cessation in both leaves and shoot apices and through multiple pathways, but with a different mechanism compared to how the miR156-SPL regulon controls VPC in annual plants. Taken together, our results reveal an age-dependent genetic network in mediating seasonal growth cessation, a key phenological process in the climate adaptation of perennial trees.

Transport capacity is uncoupled with endodormancy breaking in sweet cherry buds: physiological and molecular insights

Introduction

To avoid the negative impacts of winter unfavorable conditions for plant development, temperate trees enter a rest period called dormancy. Winter dormancy is a complex process that involves multiple signaling pathways and previous studies have suggested that transport capacity between cells and between the buds and the twig may regulate the progression throughout dormancy stages. However, the dynamics and molecular actors involved in this regulation are still poorly described in fruit trees.

Methods

Here, in order to validate the hypothesis that transport capacity regulates dormancy progression in fruit trees, we combined physiological, imaging and transcriptomic approaches to characterize molecular pathways and transport capacity during dormancy in sweet cherry (Prunus avium L.) flower buds.

Results

Our results show that transport capacity is reduced during dormancy and could be regulated by environmental signals. Moreover, we demonstrate that dormancy release is not synchronized with the transport capacity resumption but occurs when the bud is capable of growth under the influence of warmer temperatures. We highlight key genes involved in transport capacity during dormancy.

Discussion

Based on long-term observations conducted during six winter seasons, we propose hypotheses on the environmental and molecular regulation of transport capacity, in relation to dormancy and growth resumption in sweet cherry.

Natural variation in the prolyl 4-hydroxylase gene PtoP4H9 contributes to perennial stem growth in Populus

Perennial trees must maintain stem growth throughout their entire lifespan to progressively increase in size as they age. The overarching question of the molecular mechanisms that govern stem perennial growth in trees remains largely unanswered. Here we deciphered the genetic architecture that underlies perennial growth trajectories using genome-wide association studies (GWAS) for measures of growth traits across years in a natural population of Populus tomentosa. By analyzing the stem growth trajectory, we identified PtoP4H9, encoding prolyl 4-hydroxylase 9, which is responsible for the natural variation in the growth rate of diameter at breast height (DBH) across years. Quantifying the dynamic genetic contribution of PtoP4H9 loci to stem growth showed that PtoP4H9 played a pivotal role in stem growth regulation. Spatiotemporal expression analysis showed that PtoP4H9 was highly expressed in cambium tissues of poplars of various ages. Overexpression and knockdown of PtoP4H9 revealed that it altered cell expansion to regulate cell wall modification and mechanical characteristics, thereby promoting stem growth in Populus. We showed that natural variation in PtoP4H9 occurred in a BASIC PENTACYSTEINE transcription factor PtoBPC1-binding promoter element controlling PtoP4H9 expression. The geographic distribution of PtoP4H9 allelic variation was consistent with the modes of selection among populations. Altogether, our study provides important genetic insights into dynamic stem growth in Populus, and we confirmed PtoP4H9 as a potential useful marker for breeding or genetic engineering of poplars.

Comparative transcriptome analysis of genes involved in paradormant bud release response in 'Summer Black' grape

Grapevines possess a hierarchy of buds, and the fruitful winter bud forms the foundation of the two-crop-a-year cultivation system, yielding biannual harvests. Throughout its developmental stages, the winter bud sequentially undergoes paradormancy, endodormancy, and ecodormancy to ensure survival in challenging environmental conditions. Releasing the endodormancy of winter bud results in the first crop yield, while breaking the paradormancy of winter bud allows for the second crop harvest. Hydrogen cyanamide serves as an agent to break endodormancy, which counteracting the inhibitory effects of ABA, while H2O2 and ethylene function as signaling molecules in the process of endodormancy release. In the context of breaking paradormancy, common agronomic practices include short pruning and hydrogen cyanamide treatment. However, the mechanism of hydrogen cyanamide contributes to this process remains unknown. This study confirms that hydrogen cyanamide treatment significantly improved both the speed and uniformity of bud sprouting, while short pruning proved to be an effective method for releasing paradormancy until August. This observation highlights the role of apical dominance as a primary inhibitory factor in suppressing the sprouting of paradormant winter bud. Comparative transcriptome analysis revealed that the sixth node winter bud convert to apical tissue following short pruning and established a polar auxin transport canal through the upregulated expression of VvPIN3 and VvTIR1. Moreover, short pruning induced the generation of reactive oxygen species, and wounding, ethylene, and H2O2 collectively acted as stimulating signals and amplified effects through the MAPK cascade. In contrast, hydrogen cyanamide treatment directly disrupted mitochondrial function, resulting in ROS production and an extended efficacy of the growth hormone signaling pathway induction.

Epigenetic silencing of callose synthase by VIL1 promotes bud-growth transition in lily bulbs

In plants, restoring intercellular communication is required for cell activity in buds during the growth transition from slow to fast growth after dormancy release. However, the epigenetic regulation of this phenomenon is far from understood. Here we demonstrate that lily VERNALIZATION INSENSITIVE 3-LIKE 1 (LoVIL1) confers growth transition by mediating plasmodesmata opening via epigenetic repression of CALLOSE SYNTHASE 3 (LoCALS3). Moreover, we found that a novel transcription factor, NUCLEAR FACTOR Y, SUBUNIT A7 (LoNFYA7), is capable of recruiting the LoVIL1-Polycomb Repressive Complex 2 (PRC2) and enhancing H3K27me3 at the LoCALS3 locus by recognizing the CCAAT cis-element (Cce) of its promoter. The LoNFYA7-LoVIL1 module serves as a key player in orchestrating the phase transition from slow to fast growth in lily bulbs. These studies also indicate that LoVIL1 is a suitable marker for the bud-growth-transition trait following dormancy release in lily cultivars.

MADS-box protein PpDAM6 regulates chilling requirement-mediated dormancy and bud break in peach

Bud dormancy is crucial for winter survival and is characterized by the inability of the bud meristem to respond to growth-promotive signals before the chilling requirement (CR) is met. However, our understanding of the genetic mechanism regulating CR and bud dormancy remains limited. This study identified PpDAM6 (DORMANCY-ASSOCIATED MADS-box) as a key gene for CR using a genome-wide association study analysis based on structural variations in 345 peach (Prunus persica (L.) Batsch) accessions. The function of PpDAM6 in CR regulation was demonstrated by transiently silencing the gene in peach buds and stably overexpressing the gene in transgenic apple (Malus × domestica) plants. The results showed an evolutionarily conserved function of PpDAM6 in regulating bud dormancy release, followed by vegetative growth and flowering, in peach and apple. The 30-bp deletion in the PpDAM6 promoter was substantially associated with reducing PpDAM6 expression in low-CR accessions. A PCR marker based on the 30-bp indel was developed to distinguish peach plants with non-low and low CR. Modification of the H3K27me3 marker at the PpDAM6 locus showed no apparent change across the dormancy process in low- and non-low- CR cultivars. Additionally, H3K27me3 modification occurred earlier in low-CR cultivars on a genome-wide scale. PpDAM6 could mediate cell-cell communication by inducing the expression of the downstream genes PpNCED1 (9-cis-epoxycarotenoid dioxygenase 1), encoding a key enzyme for ABA biosynthesis, and CALS (CALLOSE SYNTHASE), encoding callose synthase. We shed light on a gene regulatory network formed by PpDAM6-containing complexes that mediate CR underlying dormancy and bud break in peach. A better understanding of the genetic basis for natural variations of CR can help breeders develop cultivars with different CR for growing in different geographical regions.

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.

The multifarious role of callose and callose synthase in plant development and environment interactions

Callose is an important linear form of polysaccharide synthesized in plant cell walls. It is mainly composed of β-1,3-linked glucose residues with rare amount of β-1,6-linked branches. Callose can be detected in almost all plant tissues and are widely involved in various stages of plant growth and development. Callose is accumulated on plant cell plates, microspores, sieve plates, and plasmodesmata in cell walls and is inducible upon heavy metal treatment, pathogen invasion, and mechanical wounding. Callose in plant cells is synthesized by callose synthases located on the cell membrane. The chemical composition of callose and the components of callose synthases were once controversial until the application of molecular biology and genetics in the model plant Arabidopsis thaliana that led to the cloning of genes encoding synthases responsible for callose biosynthesis. This minireview summarizes the research progress of plant callose and its synthetizing enzymes in recent years to illustrate the important and versatile role of callose in plant life activities.

Disruption of BG14 results in enhanced callose deposition in developing seeds and decreases seed longevity and seed dormancy in Arabidopsis

Seed longevity is an important trait for agriculture and the conservation of genetic resources. β-1,3-Glucanases were first recognized as pathogenesis-related proteins involved in plant defense, but their roles in seeds are largely unknown. Here, we report a glycosylphosphatidylinositol-anchored β-1,3-glucanase, BG14, that degrades callose in seed embryos and functions in seed longevity and dormancy in Arabidopsis. The loss of function of BG14 significantly decreased seed longevity, whereas functional reversion (RE) and overexpression (OE) lines reversed and increased the impaired phenotype, respectively. The loss of function of BG14 enhanced callose deposition in the embryos of mature seeds, confirmed by quantitative determination and the decreased callose degrading ability in bg14. The drop-and-see (DANS) assay revealed that the fluorescence signal in bg14 was significantly lower than that observed in the other three genotypes. BG14 is located on the periphery of the cell wall and can completely merge with callose at the plasmodesmata of epidermal cells. BG14 was highly expressed in developing seeds and was induced by aging and abscisic acid (ABA). The loss of function of BG14 led to a variety of phenotypes related to ABA, including reduced seed dormancy and reduced responses to treatment with ABA or pacolblltrazol, whereas OE lines showed the opposite phenotype. The reduced ABA response is because of the decreased level of ABA and the lowered expression of ABA synthesis genes in bg14. Taken together, this study demonstrated that BG14 is a bona fide BG that mediates callose degradation in the plasmodesmata of embryo cells, transcriptionally influences ABA synthesis genes in developing seeds, and positively affects seed longevity and dormancy in Arabidopsis.

Cold stress induces malformed tomato fruits by breaking the feedback loops of stem cell regulation in floral meristem

Fruit malformation is a major constrain in fruit production worldwide resulting in substantial economic losses. The farmers for decades noticed that the chilling temperature before blooming often caused malformed fruits. However, the molecular mechanism underlying this phenomenon is unclear. Here we examined the fruit development in response to cold stress in tomato, and demonstrated that short-term cold stress increased the callose accumulation in both shoot apical and floral meristems, resulting in the symplastic isolation and altered intercellular movement of WUS. In contrast to the rapidly restored SlWUS transcription during the recovery from cold stress, the callose removal was delayed due to obstructed plasmodesmata. The delayed reinstatement of cell-to-cell transport of SlWUS prevented the activation of SlCLV3 and TAG1, causing the interrupted feedback inhibition of SlWUS expression, leading to the expanded stem cell population and malformed fruits. We further showed that the callose dynamics in response to short-term cold stress presumably exploits the mechanism of bud dormancy during the seasonal growth, involving two antagonistic hormones, abscisic acid and gibberellin. Our results provide a novel insight into the cold stress regulated malformation of fruit.

Callose metabolism and the regulation of cell walls and plasmodesmata during plant mutualistic and pathogenic interactions

Cell walls are essential for plant growth and development, providing support and protection from external environments. Callose is a glucan that accumulates in specialized cell wall microdomains including around intercellular pores called plasmodesmata. Despite representing a small percentage of the cell wall (~0.3% in the model plant Arabidopsis thaliana), callose accumulation regulates important biological processes such as phloem and pollen development, cell division, organ formation, responses to pathogenic invasion and to changes in nutrients and toxic metals in the soil. Callose accumulation modifies cell wall properties and restricts plasmodesmata aperture, affecting the transport of signaling proteins and RNA molecules that regulate plant developmental and environmental responses. Although the importance of callose, at and outside plasmodesmata cell walls, is widely recognized, the underlying mechanisms controlling changes in its synthesis and degradation are still unresolved. In this review, we explore the most recent literature addressing callose metabolism with a focus on the molecular factors affecting callose accumulation in response to mutualistic symbionts and pathogenic elicitors. We discuss commonalities in the signaling pathways, identify research gaps and highlight opportunities to target callose in the improvement of plant responses to beneficial versus pathogenic microbes.

The Integrated mRNA and miRNA Approach Reveals Potential Regulators of Flowering Time in Arundina graminifolia

Orchids are among the most precious flowers in the world. Regulation of flowering time is one of the most important targets to enhance their ornamental value. The beauty of Arundina graminifolia is its year-round flowering, although the molecular mechanism of this flowering ability remains masked. Therefore, we performed a comprehensive assessment to integrate transcriptome and miRNA sequencing to disentangle the genetic regulation of flowering in this valuable species. Clustering analyses provided a set of molecular regulators of floral transition and floral morphogenesis. We mined candidate floral homeotic genes, including FCA, FPA, GI, FT, FLC, AP2, SOC1, SVP, GI, TCP, and CO, which were targeted by a variety of miRNAs. MiR11091 targeted the highest number of genes, including candidate regulators of phase transition and hormonal control. The conserved miR156-miR172 pathway of floral time regulation was evident in our data, and we found important targets of these miRNAs in the transcriptome. Moreover, endogenous hormone levels were determined to decipher the hormonal control of floral buds in A. graminifolia. The qRT-PCR analysis of floral and hormonal integrators validated the transcriptome expression. Therefore, miRNA-mediated mining of candidate genes with hormonal regulation forms the basis for comprehending the complex regulatory network of perpetual flowering in precious orchids. The findings of this study can do a great deal to broaden the breeding programs for flowering time manipulation of orchids.

Overexpression of <italic>PvSVP1</italic>, an <italic>SVP</italic>-like gene of bamboo, causes early flowering and abnormal floral organs in <italic>Arabidopsis</italic> and rice

<p indent="0mm">Bamboo is a nontimber woody plant featuring a long vegetative stage and uncertain flowering time. Therefore, the genes belonging to flowering repressors might be essential in regulating the transition from the vegetative to reproductive stage in bamboo. The <italic>Short Vegetative Phase</italic> ( <italic>SVP</italic>) gene plays a pivotal role in floral transition and development. However, little is known about the bamboo <italic>SVP</italic> homologues. In this study, <italic>Phyllostachys violascens</italic> <italic>PvSVP1</italic> is isolated by analysis of the <italic>P</italic>. <italic>edulis</italic> transcriptome database. Phylogenetic analysis shows that <italic>PvSVP1</italic> is closely related to <italic>OsMADS55</italic> (rice <italic>SVP</italic> homolog). <italic>PvSVP1</italic> is ubiquitously expressed in various tissues, predominantly in vegetative tissues. To investigate the function of <italic>PvSVP1</italic>, <italic>PvSVP1</italic> is overexpressed in <italic>Arabidopsis</italic> and rice under the influence of the 35S promoter. Overexpression of <italic>PvSVP1</italic> in <italic>Arabidopsis</italic> causes early flowering and produces abnormal petals and sepals. Quantitative real-time PCR reveals that overexpression in <italic>Arabidopsis</italic> produces an early flowering phenotype by downregulating <italic>FLC</italic> and upregulating <italic>FT</italic> and produces abnormal floral organs by upregulating <italic>AP1</italic>, <italic>AP3</italic> and <italic>PI</italic> expressions. Simultaneously, overexpression of <italic>PvSVP1</italic> in rice alters the expressions of flowering-related genes such as <italic>Hd3a</italic>, <italic>RFT1</italic>, <italic>OsMADS56</italic> and <italic>Ghd7</italic> and promotes flowering under field conditions. In addition, PvSVP1 may be a nuclear protein which interacts with PvVRN1 and PvMADS56 on the yeast two-hybrid and BiFC systems. Our study suggests that <italic>PvSVP1</italic> may play a vital role in flowering time and development by interacting with PvVRN1 and PvMADS56 in the nucleus. Furthermore, this study paves the way toward understanding the complex flowering process of bamboo. </p>.

PpyABF3 recruits the COMPASS-like complex to regulate bud dormancy maintenance via integrating ABA signaling and GA catabolism

Bud dormancy is essential for perennial trees that survive the cold winters and to flower on time in the following spring. Histone modifications have been reported to be involved in the control of the dormancy cycle and DAM/SVPs are considered targets. However, how the histone modification marks are added to the specific gene loci during bud dormancy cycle is still unknown. Using yeast-two hybrid library screening and co-immunoprecipitation assays, we found that PpyABF3, a key protein regulating bud dormancy, recruits Complex of Proteins Associated with Set1-like complex via interacting with PpyWDR5a, which increases the H3K4me3 deposition at DAM4 locus. Chromatin immunoprecipitation-quantitative polymerase chain reaction showed that PpyGA2OX1 was downstream gene of PpyABF3 and it was also activated by H3K4me3 deposition. Silencing of GA2OX1 in pear calli and pear buds resulted in a similar phenotype with silencing of ABF3. Furthermore, overexpression of PpyWDR5a increased H3K4me3 levels at DAM4 and GA2OX1 loci and inhibited the growth of pear calli, whereas silencing of PpyWDR5a in pear buds resulted in a higher bud-break percentage. Our findings provide new insights into how H3K4me3 marks are added to dormancy-related genes in perennial woody plants and reveal a novel mechanism by which ABF3 integrates abscisic acid signaling and gibberellic acid catabolism during bud dormancy maintenance.

Evidence of chromatin and transcriptional dynamics for cold development in peach flower bud

In temperate zones, fruit trees regulate their annual growth cycle to seasonal environmental changes. During the cold season, growth is limited by both environmental and genetic factors. After the exposure to low temperature and fulfillment of chilling requirements, mild temperatures promote the growth and flowering. However, an insufficient chilling exposure may lead to nonuniform blooming, with a negative impact on fruit set. To gain insights into flower development in the fruit tree buds, peach is an interesting model, the flower and vegetative bud being distinct organs. To understand how flower bud development is regulated, we integrated cytological observations and epigenetic and chromatin genome-wide data with transcriptional changes to identify the main regulatory factors involved in flower development during chilling accumulation. We demonstrated that growth cessation does not occur in peach flower buds during chilling accumulation, but that there are changes in transcript abundance of key genes of hormone metabolism and flower bud development, distribution of histone modifications (H3K4me3 and H3K27me3) and DNA methylation. Altogether, our findings indicate that during the cold season the flower bud is in a nondormant state and that the chilling experience allows flower differentiation to be completed.

Transcriptome mining of hormonal and floral integrators in the leafless flowers of three cymbidium orchids

Flowering is the most studied ornamental trait in orchids where long vegetative phase may span up to three years. Cymbidium orchids produce beautiful flowers with astonishing shapes and pleasant scent. However, an unusually long vegetative phase is a major drawback to their ornamental value. We observed that under certain culture conditions, three cymbidium species (Cymbidium ensifolium, C. goeringii and C. sinense) skipped vegetative growth phase and directly flowered within six months, that could be a breakthrough for future orchids with limited vegetative growth. Hormonal and floral regulators could be the key factors arresting vegetative phase. Therefore, transcriptomic analyses were performed for leafless flowers and normal vegetative leaves to ascertain differentially expressed genes (DEGs) related to hormones (auxin, cytokinin, gibberellin, abscisic acid and ethylene), floral integrators and MADS-box genes. A significant difference of cytokinin and floral regulators was observed among three species as compared to other hormones. The MADS-box genes were significantly expressed in the leafless flowers of C. sinense as compared to other species. Among the key floral regulators, CONSTANS and AGAMOUS-like genes showed the most differential expression in the leafless flowers as compared to leaves where the expression was negligible. However, CONSTANS also showed downregulation. Auxin efflux carriers were mainly downregulated in the leafless flowers of C. ensifolium and C. sinense, while they were upregulated in C. goeringii. Moreover, gibberellin and cytokinin genes were also downregulated in C. ensifolium and C. sinense flowers, while they were upregulated in C. goeringii, suggesting that species may vary in their responses. The data mining thus, outsources the valuable information to direct future research on orchids at industrial levels.

Winter warming post floral initiation delays flowering via bud dormancy activation and affects yield in a winter annual crop

In temperate climates many plant species use long-term detection of winter chilling as a seasonal cue. Previously the timing of flowering in winter annual plants has been shown to be controlled by the promotion of the floral transition by chilling, known as vernalization. In contrast, many temperate perennial species produce flower buds prior to winter and require winter chilling to break bud dormancy to enable bud break and flowering in the following spring. Here we show that flowering time in winter annuals can be controlled by bud dormancy and that in winter oilseed rape–reduced chilling during flower bud dormancy is associated with yield declines.

Winter annual life history is conferred by the requirement for vernalization to promote the floral transition and control the timing of flowering. Here we show using winter oilseed rape that flowering time is controlled by inflorescence bud dormancy in addition to vernalization. Winter warming treatments given to plants in the laboratory and field increase flower bud abscisic acid levels and delay flowering in spring. We show that the promotive effect of chilling reproductive tissues on flowering time is associated with the activity of two FLC genes specifically silenced in response to winter temperatures in developing inflorescences, coupled with activation of a BRANCHED1-dependent bud dormancy transcriptional module. We show that adequate winter chilling is required for normal inflorescence development and high yields in addition to the control of flowering time. Because warming during winter flower development is associated with yield losses at the landscape scale, our work suggests that bud dormancy activation may be important for effects of climate change on winter arable crop yields.

Later Growth Cessation and Increased Freezing Tolerance Potentially Result in Later Dormancy in Evergreen Iris Compared with Deciduous Iris

Winter dormancy is a protective survival strategy for plants to resist harsh natural environments. In the context of global warming, the progression of dormancy has been significantly affected in perennials, which requires further research. Here, a systematic study was performed to compare the induction of dormancy in two closely related iris species with an ecodormancy-only process, the evergreen Iris japonica Thunb. and the deciduous Iris tectorum Maxim. under artificial conditions. Firstly, morphological and physiological observations were evaluated to ensure the developmental status of the two iris species. Furthermore, the expression patterns of the genes involved in key pathways related to plant winter dormancy were determined, and correlation analyses with dormancy marker genes were conducted. We found that deciduous iris entered dormancy earlier than evergreen iris under artificial dormancy induction conditions. Phytohormones and carbohydrates play roles in coordinating growth and stress responses during dormancy induction in both iris species. Moreover, dormancy-related MADS-box genes and SnRKs (Snf1-related protein kinase) might represent a bridge between carbohydrate and phytohormone interaction during iris dormancy. These findings provide a hypothetical model explaining the later dormancy in evergreen iris compared with deciduous iris under artificial dormancy induction conditions and reveal some candidate genes. The findings of this study could provide new insights into the research of dormancy in perennial plants with an ecodormancy-only process and contribute to effectively managing iris production, postharvest storage, and shipping.

Winter dormancy in trees

Plants growing in temperate and boreal regions of the world have to face strikingly different environmental conditions during summer and winter. Being sessile organisms, plants have had to develop various strategies to adapt to these changes in light, temperature, and water availability, thereby optimizing their 'economy of growth'. While annual plants can endure unfavorable winter conditions in the form of a seed, or under a protective cover of thick snow, perennial plants such as trees adapt by going into a stage of deep sleep called winter dormancy. To enter dormancy, vegetative growth is stopped in the late summer or early autumn and the shoots are converted into buds, where the shoot apical meristems are protected by tightly closed and hardened bud scales (Figures 1 and 2). At the same time, cold hardiness develops and the need for water and nutrient uptake is drastically reduced. Deciduous trees also go through leaf senescence whereby the leaves develop their autumn colors and are shed (Figure 1A). The trees then spend the beginning of the winter in a state of deep sleep in which they are completely unreceptive to any environmental signals telling them to wake up. However, as winter progresses, the trees are gradually released from this slumber and will eventually flush their buds in the spring. Vegetative growth then resumes with the formation of new leaves and shoots during summer until the trees again go into growth cessation and the cycle is closed (Figures 1 and 2). This cycle of growth and dormancy is central for the ability of trees to adapt to growth at different latitudes and elevations. The further north, or the higher the elevation at which the trees grow, the earlier in the season the trees enter growth cessation and the later they flush their buds in the spring. This is because meteorological winter arrives earlier in the season and lasts longer into the spring. The trees therefore have to stop growth earlier in the season to ensure that they have enough time to complete bud formation and to develop cold hardiness and dormancy. They also have to be sure that winter is really over before flushing their buds. Winter dormancy is therefore a clear case of a trade-off between the length of the growing season and the protection against winter damage - a nice example of 'economy in biology', the theme of this special issue. This primer will briefly summarize what we know about the environmental signals that influence the annual growth cycle in trees, as well as our current understanding of the genetic pathways and molecular mechanisms regulated by these signals.

Fall Applications of Ethephon Modulates Gene Networks Controlling Bud Development during Dormancy in Peach (Prunus Persica)

Ethephon (ET) is an ethylene-releasing plant growth regulator (PGR) that can delay the bloom time in Prunus, thus reducing the risk of spring frost, which is exacerbated by global climate change. However, the adoption of ET is hindered by its detrimental effects on tree health. Little knowledge is available regarding the mechanism of how ET shifts dormancy and flowering phenology in peach. This study aimed to further characterize the dormancy regulation network at the transcriptional level by profiling the gene expression of dormant peach buds from ET-treated and untreated trees using RNA-Seq data. The results revealed that ET triggered stress responses during endodormancy, delaying biological processes related to cell division and intercellular transportation, which are essential for the floral organ development. During ecodormancy, ET mainly impeded pathways related to antioxidants and cell wall formation, both of which are closely associated with dormancy release and budburst. In contrast, the expression of dormancy-associated MADS (DAM) genes remained relatively unaffected by ET, suggesting their conserved nature. The findings of this study signify the importance of floral organogenesis during dormancy and shed light on several key processes that are subject to the influence of ET, therefore opening up new avenues for the development of effective strategies to mitigate frost risks.

The VvWRKY37 Regulates Bud Break in Grape Vine Through ABA-Mediated Signaling Pathways

Dormancy is a common survival strategy in plants to temporarily suspend visible growth under unsuitable conditions. The elaborate mechanism underlying bud break in perennial woody plants is gradually illustrated. Here, we identified a grape vine WRKY transcription factor, VvWRKY37, which was highly expressed in dormant buds. It was particularly induced by the application of exogenous abscisic acid, and depressed on exposure to gibberellin and low temperature (4°C) stress at the transcript level. The yeast one-hybrid assay confirmed that VvWRKY37 had a transcriptional activity. Ectopic over-expression of VvWRKY37 significantly delayed bud break of transgenic poplar plants. As an ABA-inducible gene, VvWRKY37 also depressed the expression of ABA catabolic gene CYP707As and enhanced the accumulation of endogenous ABA in transgenic poplar plants. The molecular pieces of evidence showed that VvWRKY37 preferentially recognized and bound W-box 5'-G/CATTGACT/C/G-3' cis-element in vitro. Additionally, VvABI5 and VvABF2 acted as the upstream transcriptional activators of VvWRKY37 via protein-DNA interactions. Taken together, our findings provided valuable insights into a new regulatory mechanism of WRKY TF by which it modulates bud break through ABA-mediated signaling pathways.

Gentian FLOWERING LOCUS T orthologs regulate phase transitions: floral induction and endodormancy release

Perennial plants undergo a dormant period in addition to the growth and flowering phases that are commonly observed in annuals and perennials. Consequently, the regulation of these phase transitions in perennials is believed to be complicated. Previous studies have proposed that orthologs of FLOWERING LOCUS T (FT) regulate not only floral initiation but also dormancy. We, therefore, investigated the involvement of FT orthologs (GtFT1 and GtFT2) during the phase transitions of the herbaceous perennial gentian (Gentiana triflora). Analysis of seasonal fluctuations in the expression of these genes revealed that GtFT1 expression increased prior to budbreak and flowering, whereas GtFT2 expression was induced by chilling temperatures with the highest expression occurring when endodormancy was released. The expression of FT-related transcription factors, reportedly involved in flowering, also fluctuated during each phase transition. These results suggested the involvement of GtFT1 in budbreak and floral induction and GtFT2 in dormancy regulation, implying that the two gentian FT orthologs activated a different set of transcription factors. Gentian ft2 mutants generated by CRISPR/Cas9-mediated genome editing had a lower frequency of budbreak and budbreak delay in overwintering buds caused by an incomplete endodormancy release. Our results highlighted that the gentian orthologs of FRUITFULL (GtFUL) and SHORT VEGETATIVE PHASE-like 1 (GtSVP-L1) act downstream of GtFT2, probably to prevent untimely budbreak during ecodormancy. These results suggest that each gentian FT ortholog regulates a different phase transition by having variable responses to endogenous or environmental cues, leading to their ability to induce the expression of distinct downstream genes.

Hybrid RNA Sequencing Strategy for the Dynamic Transcriptomes of Winter Dormancy in an Evergreen Herbaceous Perennial, Iris japonica

Japanese iris (Iris japonica) is a popular perennial ornamental that originated in China; it has a long display period and remains green outdoors throughout the year. winter dormancy characteristics contribute greatly to the evergreenness of herbaceous perennials. Thus, it is crucial to explore the mechanism of winter dormancy in this evergreen herbaceous perennial. Here, we used the hybrid RNA-seq strategy including single-molecule real-time (SMRT) and next-generation sequencing (NGS) technologies to generate large-scale Full-length transcripts to examine the shoot apical meristems of Japanese iris. A total of 10.57 Gb clean data for SMRT and over 142 Gb clean data for NGS were generated. Using hybrid error correction, 58,654 full-length transcripts were acquired and comprehensively analysed, and their expression levels were validated by real-time qPCR. This is the first full-length RNA-seq study in the Iris genus; our results provide a valuable resource and improve understanding of RNA processing in this genus, for which little genomic information is available as yet. In addition, our data will facilitate in-depth analyses of winter dormancy mechanisms in herbaceous perennials, especially evergreen monocotyledons.

MADS-box transcription factors determine the duration of temporary winter dormancy in closely related evergreen and deciduous Iris spp

Winter dormancy (WD) is a crucial strategy for plants coping with potentially deadly environments. In recent decades, this process has been extensively studied in economically important perennial eudicots due to changing climate. However, in evergreen monocots with no chilling requirements, dormancy processes are so far a mystery. In this study, we compared the WD process in closely related evergreen (Iris japonica) and deciduous (I. tectorum) iris species across crucial developmental time points. Both iris species exhibit a 'temporary' WD process with distinct durations, and could easily resume growth under warm conditions. To decipher transcriptional changes, full-length sequencing for evergreen iris and short read RNA sequencing for deciduous iris were applied to generate respective reference transcriptomes. Combining results from a multipronged approach, SHORT VEGETATIVE PHASE and FRUITFULL (FUL) from MADS-box was associated with a dormancy- and a growth-related module, respectively. They were co-expressed with genes involved in phytohormone signaling, carbohydrate metabolism, and environmental adaptation. Also, gene expression patterns and physiological changes in the above pathways highlighted potential abscisic acid and jasmonic acid antagonism in coordinating growth and stress responses, whereas differences in carbohydrate metabolism and reactive oxygen species scavenging might lead to species-specific WD durations. Moreover, a detailed analysis of MIKCCMADS-box in irises revealed common features described in eudicots as well as possible new roles for monocots during temporary WD, such as FLOWERING LOCUS C and FUL. In essence, our results not only provide a portrait of temporary WD in perennial monocots but also offer new insights into the regulatory mechanism underlying WD in plants.

A MADS-box gene with similarity to FLC is induced by cold and correlated with epigenetic changes to control budbreak in kiwifruit

Temperate perennials require exposure to chilling temperatures to resume growth in the following spring. Growth and dormancy cycles are controlled by complex genetic regulatory networks and are governed by epigenetic mechanisms, but the specific genes and mechanisms remain poorly understood. To understand how seasonal changes and chilling regulate dormancy and growth in the woody perennial vine kiwifruit (Ac, Actinidia chinensis), a transcriptome study of kiwifruit buds in the field and controlled conditions was performed. A MADS-box gene with homology to Arabidopsis FLOWERING LOCUS C (FLC) was identified and characterized. Elevated expression of AcFLC-like (AcFLCL) was detected during bud dormancy and chilling. A long noncoding (lnc) antisense transcript with an expression pattern opposite to AcFLCL and shorter sense noncoding RNAs were identified. Chilling induced an increase in trimethylation of lysine-4 of histone H3 (H3K4me3) in the 5' end of the gene, indicating multiple layers of epigenetic regulation in response to cold. Overexpression of AcFLCL in kiwifruit gave rise to plants with earlier budbreak, whilst gene editing using CRISPR-Cas9 resulted in transgenic lines with substantially delayed budbreak, suggesting a role in activation of growth. These results have implications for the future management and breeding of perennials for resilience to changing climate.

Populus SVL Acts in Leaves to Modulate the Timing of Growth Cessation and Bud Set

SHORT VEGETATIVE PHASE (SVP) is an important regulator of FLOWERING LOCUS T (FT) in the thermosensory pathway of Arabidopsis. It is a negative regulator of flowering and represses FT transcription. In poplar trees, FT2 is central for the photoperiodic control of growth cessation, which also requires the decrease of bioactive gibberellins (GAs). In angiosperm trees, genes similar to SVP, sometimes named DORMANCY-ASSOCIATED MADS-BOX genes, control temperature-mediated bud dormancy. Here we show that SVL, an SVP ortholog in aspen trees, besides its role in controlling dormancy through its expression in buds, is also contributing to the regulation of short day induced growth cessation and bud set through its expression in leaves. SVL is upregulated during short days in leaves and binds to the FT2 promoter to repress its transcription. It furthermore decreases the amount of active GAs, whose downregulation is essential for growth cessation, by repressing the transcription of GA20 oxidase. Finally, the SVL protein is more stable in colder temperatures, thus integrating the temperature signal into the response. We conclude that the molecular function of SVL in the photoperiodic pathway has been conserved between Arabidopsis and poplar trees, albeit the physiological process it controls has changed. SVL is thus both involved in regulating the photoperiod response in leaves, modulating the timing of growth cessation and bud set, and in the subsequent temperature regulation of dormancy in the buds.

The Genetic and Hormonal Inducers of Continuous Flowering in Orchids: An Emerging View

Orchids are the flowers of magnetic beauty. Vivid and attractive flowers with magnificent shapes make them the king of the floriculture industry. However, the long-awaited flowering is a drawback to their market success, and therefore, flowering time regulation is the key to studies about orchid flower development. Although there are some rare orchids with a continuous flowering pattern, the molecular regulatory mechanisms are yet to be elucidated to find applicable solutions to other orchid species. Multiple regulatory pathways, such as photoperiod, vernalization, circadian clock, temperature and hormonal pathways are thought to signalize flower timing using a group of floral integrators. This mini review, thus, organizes the current knowledge of floral time regulators to suggest future perspectives on the continuous flowering mechanism that may help to plan functional studies to induce flowering revolution in precious orchid species.

Persian Walnut (Juglans regia L.) Bud Dormancy Dynamics in Northern Patagonia, Argentina

Temperate deciduous fruit trees survive winter temperatures by entering a dormant phase in their aerial meristematic organs. Release from bud dormancy occurs after chill requirements (CR) have been satisfied, whereas bud burst/flowering follows heat requirement (HR) fulfillment. The physiological basis behind these metrics remains elusive. In this study, we are presenting the first multidisciplinary dormancy progression analysis in northern Patagonia, linking (1) forcing/field phenology, (2) bud anatomical development, and (3) soluble sugar (sucrose, glucose, and fructose) dynamics in Juglans regia L. CR and HR were determined for 'Chandler' and 'Franquette,' two walnut cultivars with markedly different CR, in artificial chill/forced heat trials (three seasons) and in-field chill/forced heat tests (five seasons) using excised twigs either with or without apical buds (non-decapitated and decapitated). The soluble sugar dynamics of 'Chandler' (high-performance liquid chromatography) and the anatomical changes of the buds (light microscopy) of the two cultivars were analyzed during endo-ecodormancy progression in one and two seasons, respectively. The CR defined by artificial chill tests proved to be an overestimation compared to the field determinations. Moreover, HR was the main driver in the phenology dynamics, as expected for a high-chill region. 'Chandler' showed an average of 10.3 field chill portions (CP) and 2,163 Growing Degree Hours (GDH°C) less than 'Franquette' for dormancy release and bud burst, respectively. These results were consistent with the transition of the shoot apex from the vegetative to the reproductive phase and the soluble sugar profile. The decrease in sucrose between 15 and 30 days after CR fulfillment could be a reliable biological marker for endodormancy release in walnut, while the increase in fructose and glucose is likely an osmolyte and cellulosic carbon source in pre-sprouting. In addition, we discuss the effect of paradormancy thanks to our apical bud experiment (with or without). Our results improve the current understanding of endo-ecodormancy progression in walnut and provide insightful results for walnut production (i.e., cultivation practices such as pruning) as well as for further application in dormancy modeling, to infer the ideotypes that should be bred for future climate conditions.

Transcriptional Proposition for Uniquely Developed Protocorm Flowering in Three Orchid Species: Resources for Innovative Breeding

During orchid seed culture, seeds germinate as protocorms, and protocorms normally develop into plant with leaves and roots. Orchids require many years of vegetative development for flowering. However, under a certain combination of growth cultures, we observed that protocorms can directly flower without leaves and roots. Therefore, we performed comparative transcriptome analysis to identify the different transcriptional regulators of two types of protocorms of Cymbidium ensifolium, Cymbidium sinense, and Cymbidium goeringii. Zinc finger, MYB, AP2, and bHLH were the most abundant transcription factor (TF) families in the transcriptome. Weighted gene coexpression network analysis (WGCNA) was performed to identify hub genes related to leaf and flower development. The key hubs included SPL6, SVP, SEP2, KNOX1, AP2, OFP1, COL12, MYB13, MYB36, MYB59, bHLH086, and ARF7. The hub genes were further validated through statistical tools to propose the roles of key TFs. Therefore, this study initiates to answer that why there is no leaf initiation and root development and how can protocorm bypass the vegetative phase to flower? The outcomes can direct future research on short-span flowering in orchids through protocorms.

Towards understanding the biological foundations of perenniality

Perennial life cycles enable plants to have remarkably long lifespans, as exemplified by trees that can live for thousands of years. For this, they require sophisticated regulatory networks that sense environmental changes and initiate adaptive responses in their growth patterns. Recent research has gradually elucidated fundamental mechanisms underlying the perennial life cycle. Intriguingly, several conserved components of the floral transition pathway in annuals such as Arabidopsis thaliana also participate in these regulatory mechanisms underpinning perenniality. Here, we provide an overview of perennials' physiological features and summarise their recently discovered molecular foundations. We also highlight the importance of deepening our understanding of perenniality in the development of perennial grain crops, which are promising elements of future sustainable agriculture.

Genetic insights into the regulatory pathways for continuous flowering in a unique orchid Arundina graminifolia

BACKGROUND

Manipulation of flowering time and frequency of blooming is key to enhancing the ornamental value of orchids. Arundina graminifolia is a unique orchid that flowers year round, although the molecular basis of this flowering pattern remains poorly understood.

RESULTS

We compared the A. graminifolia transcriptome across tissue types and floral developmental stages to elucidate important genetic regulators of flowering and hormones. Clustering analyses identified modules specific to floral transition and floral morphogenesis, providing a set of candidate regulators for the floral initiation and timing. Among candidate floral homeotic genes, the expression of two FT genes was positively correlated with flower development. Assessment of the endogenous hormone levels and qRT-PCR analysis of 32 pathway-responsive genes supported a role for the regulatory networks in floral bud control in A. graminifolia. Moreover, WGCNA showed that flowering control can be delineated by modules of coexpressed genes; especially, MEgreen presented group of genes specific to flowering.

CONCLUSIONS

Candidate gene selection coupled with hormonal regulators brings a robust source to understand the intricate molecular regulation of flowering in precious orchids.

Phytochrome B and PHYTOCHROME INTERACTING FACTOR8 modulate seasonal growth in trees

The seasonally synchronized annual growth cycle that is regulated mainly by photoperiod and temperature cues is a crucial adaptive strategy for perennial plants in boreal and temperate ecosystems. Phytochrome B (phyB), as a light and thermal sensor, has been extensively studied in Arabidopsis. However, the specific mechanisms for how the phytochrome photoreceptors control the phenology in tree species remain poorly understood. We characterized the functions of PHYB genes and their downstream PHYTOCHROME INTERACTING FACTOR (PIF) targets in the regulation of shade avoidance and seasonal growth in hybrid aspen trees. We show that while phyB1 and phyB2, as phyB in other plants, act as suppressors of shoot elongation during vegetative growth, they act as promoters of tree seasonal growth. Furthermore, while the Populus homologs of both PIF4 and PIF8 are involved in the shade avoidance syndrome (SAS), only PIF8 plays a major role as a suppressor of seasonal growth. Our data suggest that the PHYB-PIF8 regulon controls seasonal growth through the regulation of FT and CENL1 expression while a genome-wide transcriptome analysis suggests how, in Populus trees, phyB coordinately regulates SAS responses and seasonal growth cessation.

Plasmodesmata and their role in the regulation of phloem unloading during fruit development

Fruit consumption is fundamental to a balanced diet. The contemporary challenge of maintaining a steady food supply to meet the demands of a growing population is driving the development of strategies to improve the production and nutritional quality of fruit. Plasmodesmata, the structures that mediate symplasmic transport between plant cells, play an important role in phloem unloading and distribution of sugars and signalling molecules into developing organs. Targeted modifications to the structures and functioning of plasmodesmata have the potential to improve fruit development; however, knowledge on the mechanisms underpinning plasmodesmata regulation in this context is scarce. In this review, we have compiled current knowledge on plasmodesmata and their structural characterisation during the development of fruit organs. We discuss key questions on phloem unloading, including the pathway shift from symplasmic to apoplastic that takes place during the onset of ripening as potential targets for improving fruit quality.

Transcriptomic Analysis Reveals the Positive Role of Abscisic Acid in Endodormancy Maintenance of Leaf Buds of Magnolia wufengensis

Magnolia wufengensis (Magnoliaceae) is a deciduous landscape species, known for its ornamental value with uniquely shaped and coloured tepals. The species has been introduced to many cities in south China, but low temperatures limit the expansion of this species in cold regions. Bud dormancy is critical for plants to survive in cold environments during the winter. In this study, we performed transcriptomic analysis of leaf buds using RNA sequencing and compared their gene expression during endodormancy, endodormancy release, and ecodormancy. A total of 187,406 unigenes were generated with an average length of 621.82 bp (N50 = 895 bp). In the transcriptomic analysis, differentially expressed genes involved in metabolism and signal transduction of hormones especially abscisic acid (ABA) were substantially annotated during dormancy transition. Our results showed that ABA at a concentration of 100 μM promoted dormancy maintenance in buds of M. wufengensis. Furthermore, the expression of genes related to ABA biosynthesis, catabolism, and signalling pathway was analysed by qPCR. We found that the expression of MwCYP707A-1-2 was consistent with ABA content and the dormancy transition phase, indicating that MwCYP707A-1-2 played a role in endodormancy release. In addition, the upregulation of MwCBF1 during dormancy release highlighted the enhancement of cold resistance. This study provides new insights into the cold tolerance of M. wufengensis in the winter from bud dormancy based on RNA-sequencing and offers fundamental data for further research on breeding improvement of M. wufengensis.

ABA and Bud Dormancy in Perennials: Current Knowledge and Future Perspective

Bud dormancy is an evolved trait that confers adaptation to harsh environments, and affects flower differentiation, crop yield and vegetative growth in perennials. ABA is a stress hormone and a major regulator of dormancy. Although the physiology of bud dormancy is complex, several advancements have been achieved in this field recently by using genetics, omics and bioinformatics methods. Here, we review the current knowledge on the role of ABA and environmental signals, as well as the interplay of other hormones and sucrose, in the regulation of this process. We also discuss emerging potential mechanisms in this physiological process, including epigenetic regulation.

Chilling and gibberellin acids hyperinduce β-1,3-glucanases to reopen transport corridor and break endodormancy in tree peony (Paeonia suffruticosa)

Bud endodormancy is accompanied by transport channel apertures blockage through callose deposition, and its resume to growth requires evoking β-1,3-glucanases (BGs) to unchoke the conduit. To understand out its working manner, the statuses of the transport channels were evaluated and candidate BGs were identified during chilling and gibberellin acids (GA) induced dormancy release in tree peony. Calcein reflects plasmodesmata permeability, and no calcein was observed in the bud together with density aniline blue fluorescent around the stem phloem at 0 d chilling. With the increase of chilling accumulation, the contents of glucan declined and the activities of gulcanase increased gradually in buds, and the calcein reached the top of flower primordia at 21 d chilled bud. Both GA₃ and GA₄ feedings promoted bud sprouting and growth along with rapidly unchoking the transport channels, and GA₃ was more effective. Several candidate β-1,3-glucanase genes were detected, combining transcriptional profiling and quantitative PCR analysis. PsBG1, PsBG3, PsBG6, PsBG8 and PsBG9 were inducible by chilling accumulation and presented laminarin hydrolyzing activities after prokaryotically expression, while PsBG1, PsBG3, PsBG8 and PsBG9 responded to GAs application. Subcellular localizations revealed that PsBG6 and PsBG9 were plasmodesmata residents. It was concluded that PsBG6 played a vital role in chilling accumulation response and PsBG9 was central in GAs-induced dormancy release, and they could be used as marker genes for dormancy release in tree peony. These results were of great value to understand the mechanism of dormancy regulation and as an important fundamental for forcing culture technology in tree peony.

Molecular responses to chilling in a warming climate and their impacts on plant reproductive development and yield

Responses to prolonged winter chilling are universal in temperate plants which use seasonal temperature cues in the seed, vegetative and reproductive phases to align development with the earth's orbit. Climate change is driving a decline in reliable winter chill and affecting the sub-tropical extent of cultivation for temperate over-wintering crops. Here we explore molecular aspects of plant responses to winter chill including seasonal bud break and flowering, and how variation in the intensity of winter chilling or de-vernalisation can lead to effects on post-chilling plant development, including that of structures necessary for crop yields.