Which hormone promotes dormancy

In PD, lateral bud growth is suppressed by the terminal bud, a phenomenon known as apical dominance. Endodormant buds track chilling units and will not resume growth until the fulfillment of the chilling requirement. ECD marks the last stage of dormancy where buds resume the ability to grow but are inhibited by unfavorable weather conditions. While this classification allows convenient references to the different stages of the dormancy—growth cycle of deciduous perennials, the nomenclature of PD and ECD has raised some concerns and confusions as they lack many genetic and biological hallmarks that are characteristic of a true dormancy.

In view of this, several authors have advocated the prudent use of dormancy, especially in the discussion of dormancy mechanisms at the molecular or cellular levels Rohde and Bhalerao, ; Cooke et al.

Dormancy is a highly regulated and complex process and is subjected to the influences of many internal and external factors. Plant hormones have been shown to be the most significant internal mediators in the control of dormancy cycle in deciduous trees.

Plant hormones, or phytohormones, are naturally occurring small signaling molecules that affect plant physiological metabolism at low concentrations Davies, Other major plant-produced substances known to exert hormone-like functions in plants include jasmonates JAs , brassinosteroids BRs , strigolactones SL , and salicylic acid SA.

Many of these plant hormones have been found to participate in the highly complex orchestration of bud dormancy. Thus far, our knowledge in deciphering the mechanisms by which dormancy is regulated by plant hormones still remains limited.

However, several breakthroughs associated with mechanistic aspects of hormone signaling at the molecular and subcellular levels have elucidated hormone perception, signal transduction, and signal interplay in several major hormones Chow and Mccourt, ; Santner and Estelle, ; Santner et al.

Genetic mutagenesis in model plant species have also allowed us to examine the function of a different element of hormone biosynthesis and signaling pathways under various conditions, including dormancy. This, along with the recent advances in omics and bioinformatics that provide global views of all relevant genetic events at a specific time, has considerably advanced our knowledge of hormone regulation as it applies to bud dormancy.

Such quick gains in research, knowledge, and technologies necessitate a summarization of recent findings and suggestions for future research. A comprehensive overview of the underlying molecular and biochemical mechanisms involved in bud dormancy will also help researchers design practical approaches to address critical issues in agriculture, horticulture, and forestry, such as global warming and spring frost.

In this review, we focus on topics related to recent findings associated with bud dormancy, with an emphasis on the functions and interactions of plant hormones during bud dormancy, particularly in deciduous fruit trees. Plant hormone ABA regulates a great number of aspects in plant growth and development and is also an important messenger of stress responses Finkelstein, The primary role of ABA in plants is to repress growth and to promote organ senescence and abscission Finkelstein, ; Zheng et al.

In light of this, ABA is of particular importance in regulating dormancy, since dormancy in essence is the suspension of meristematic growth and successful dormancy establishment entails cessation of the overall plant growth Cooke et al. The central role of ABA in regulating bud dormancy has been extensively documented in many physiological, genetic, and molecular studies. It has been widely observed that endogenous ABA levels increase at dormancy establishment and decrease towards dormancy release transition from ED to ECD.

For example, ABA content in grapevine Vitis vinifera buds increases up to threefold at the onset of dormancy and then decreases gradually towards the release of dormancy, indicated by the increasing bud break rate from node cuttings in forcing conditions Zheng et al.

Similar results were also observed in some other woody species including peach Prunus persica Wang et al. It has been proposed that the increase of ABA content is triggered by SD photoperiod prior to the establishment of dormancy Ruttink et al. The importance of ABA in dormancy regulation is also evidenced by the precocious release of dormancy when ABA content in dormant buds is artificially reduced. Early study in rose Rosa hybrida indicated that application of ABA synthesis inhibitor fluridone to dormant buds initiated the growth of new leaf primordia Le Bris et al.

Furthermore, ABA catabolism in grapevine was effectively activated, and its content was reduced after treatment of buds with hydrogen cyanamide HC , an efficient bud-breaking chemical Ophir et al.

These findings are also supported by a transgenic study in pear P. These studies support the notion that ABA is an effective suppressor of primordia growth during dormancy and even though ABA is on the decline after dormancy has been established, basal levels of ABA and de novo ABA production may still be required to keep the buds at dormancy, and the continuous reduction of ABAs contributes to the release of dormancy.

In several woody species, exogenous ABA application was found to promote dormancy initiation and to delay bud break Dutcher and Powell, ; Mielke and Dennis, ; Lionakis and Schwabe, ; Li et al. However, closer examination shows that the inhibitory effect of exogenous ABA on bud cuttings diminishes as dormancy intensifies and disappears after the dormancy is released. ABA inhibitory effect on bud break is also affected by chilling accumulation, as injection of ABA only inhibited bud break of Japanese pear P yrus fauriei shoots that were exposed to — chilling hours, but not those exposed to —1, chilling hours Tamura et al.

Together, this proposes that a non-ABA-mediated regulatory mechanism, which controls the dormancy transition at certain phases or when given requirements have been met, may exist. This can be supported by the observation that the variation of ABA levels is not always in exact synchronization with dormancy progression.

For instance, while the decrease in ABA levels was found to commence somewhat prior to the release of dormancy Or et al. In contrast to applications of exogenous ABA to cuttings, effects of ABA applied on whole plants produced inconsistent results, with application timing appearing to be critical.

Foliar application of ABA to dormant peach trees prior to budburst slightly accelerated bloom progression Parker et al. Similarly, spring application of ABA on field-grown grapevine produced little or no inhibitory effect on budburst Hellman et al. On the other hand, fall application of ABA on nursery apple trees Malus domestica promoted the occurrence of the physiological events preceding dormancy commencement such as N mobilization from leaves to stems, cold acclimation in stems, and shoot growth cessation Guak and Fuchigami, ABA application on grapevines was more effective in inducing deeper dormancy during early autumn between the veraison and post veraison stages compared to mid-autumn applications Li and Dami, Furthermore, leaf age was also found to influence the effectiveness of exogenous ABA, as grapevines with older leaves were more responsive to exogenous ABA in inducing dormancy compared to those with younger leaves Zhang et al.

This may explain why cuttings are more consistently responsive to ABA applications compared to whole-plant applications, as cuttings are usually incubated with ABA solution for an extended period of time to facilitate cuticular penetration of ABA. In contrast, uptake of ABA by whole-plant application is more dependent on successful cuticular penetration, frequency of applications, and several environmental variables, which may all contribute to the inconsistency of ABA effect on whole-plant application.

In higher plants, ABA metabolism is finely coordinated to ensure proper growth and effective stress responses. The ABA biosynthesis pathway has been reviewed extensively Marin et al.

Next, antheraxanthin is converted to neoxanthin or violaxanthin, which are both cleaved to form xanthoxin by 9- cis -epoxycarotenoid dioxygenase NCED Liotenberg et al.

In the following two steps, xanthoxin is first dehydrated by alcohol dehydrogenase and then oxidized to ABA by aldehyde oxidase Bittner et al. In contrast, the NCED-mediated violaxanthin cleavage is a rate-limiting and committed step, constituting a regulation pivot in controlling ABA biosynthesis, and has received substantial attention in the ABA-related studies Nambara and Marion-Poll, Accumulating evidence has indicated that ABA biosynthesis is involved in controlling dormancy.

Upregulation of ABA synthetic enzyme NCED at the onset of dormancy and its downregulation during dormancy release were observed in many species such as peach Wang et al. However, it was noted that various NCED homologs follow distinct expression patterns in these plant species during dormancy. These findings indicate the existence of a complex regulatory network of ABA biosynthesis, in which NCED genes are probably regulated by relatively independent mechanisms and are expressed in an organ-specific manner.

Li et al. Homeostasis of ABA in plants is essential for normal growth and development, in which buds are both the target site for ABA to act upon and the principal location of ABA metabolism and catabolism. Transcription studies indicate that the alteration of one process is often accompanied by an opposite change in the other process, suggesting that these two processes are closely co-regulated. Furthermore, Li et al. This observation suggests that ABA may stimulate its own degradation via a negative feedback pathway.

This mechanism seems to function only under certain genetic backgrounds such as aba mutants and stress conditions such as drought Xiong et al. As a plant growth regulator, ABA can be used in agricultural practices to manipulate dormancy release and bloom date, thus saving early-bloom varieties and species from potential spring freezes. However, the systems by which endogenous ABA and its signal pathway are affected by exogenous ABA during dormancy need further elucidation.

ABA deactivation can also be achieved through conjugation. ABA-GE lacks a direct biological function and is generally believed to serve as the storage form of ABA, which can be relocated and disrupted to release ABA in response to stresses, such as dehydration Sauter et al. A recent study has indicated that the glucosyl ester may also act as an ABA antagonist by regulating ABA supply during bud dormancy. Similarly, a homolog of glucosyltransferase was found to be highly expressed in Prunus mume during later stages of dormancy Zhang et al.

While these reports confirm the role of ABA glucosylation in ABA deactivation during dormancy release, it still remains unclear to what extent this mechanism supplements the oxidative catabolism of ABA and how it is spatially and temporally regulated. Genetic studies have indicated that transcript levels of these central components of ABA signaling pathway are modulated by environmental signals and levels of ABA as well. In a transcriptomic study with pear, Li et al.

A similar result was also obtained in Japanese pear, in which PP2C genes were upregulated and SnRK2s downregulated after buds exited from dormancy Bai et al.

Ruttink et al. Taken together, these results suggest that the ABA signaling pathway is subject to the influence of both seasonal variation and ABA contents during dormancy. Early studies in Arabidopsis have demonstrated that transcriptional factors from the ABA-insensitive ABI group can individually or collaboratively mediate the expression of ABA-inducible genes Nakamura et al. Among them, ABI3 is believed to play an important role in seed embryo maturation and dormancy by positively regulating the ABA signaling pathway Nambara et al.

In poplar Populus trichocarpa , ABI3 was found to be expressed in the embryonic leaves inside the bud during bud set, and the overexpression or silencing of ABI3 caused alterations in bud development, indicating its crucial role in bud formation Rohde et al. Several recent studies have indicated that ABA may regulate dormancy through modulating the intercellular communication.

In plants, the cell-to-cell transport in the symplastic continuum relies on the connectivity of specialized channels between adjacent cells called plasmodesmata. Symplastic closure caused by callose deposition is a major mechanism in plants to defend pathogen invasion. In fact, it is also a critical step in the establishment of dormancy and is triggered by SD photoperiod events Rinne and Schoot, In a recent study, ABA-mediated plasmodesmata constriction in hybrid aspen was shown to prevent dormancy release by limiting the passage of growth factors such as flowering locus T FT into the dormant buds Tylewicz et al.

In this study, an ABA-insensitive mutant abi failed to produce plasmodesmatal callose and exhibited compromised dormancy in SD treatment, whereas the plasmodesmata closure and dormancy were restored through downregulation of a chromodomain remodeling factor PICKLE PKL or ectopic expression of plasmodesmata located protein 1 PDLP1 ; PKL is a chromatin remodeler that facilitates epigenetic marks e. Taken together, these results suggest that plasmodesmata blockage is an integral mechanism in the establishment of dormancy and ABA is central to the regulation to this process Figure 1.

Figure 1 Schematic diagram integrating major components of ABA biosynthesis, signaling, and catabolism during the establishment of bud endodormancy. Solid and dashed lines indicate direct and indirect regulation; respectively. Arrowed blue and barred red lines indicate activation and inhibition, respectively.

Upward and downward arrows indicate upregulation and downregulation, respectively. Blue and yellow arrows indicate cold- and SD-mediated activation, respectively, whereas the black arrow indicates an undetermined signal source.

Question marks indicate the unconfirmed mechanisms. Towards the end of the growing season, SD photoperiod and short-term low temperature activate both ABA biosynthesis and signaling, possibly via kinase cascades. The ABA signaling pathway can also upregulate CALS1, which in turn produces callose at the plasmodesmata to block the intercellular communication, contributing to the establishment of endodormancy.

Activated ABA responses, repressed FT, and callose deposition all lead to the establishment of endodormancy. Cell cycle is closely related to bud dormancy. In cell cycle, G1 is the interphase at which cells accomplish most of the growth and preparation for DNA synthesis, and it is also the phase where the cells may exit from cell cycle and enter stasis of G0.

Previous studies indicate that cell cycle arrests at the G1 stage in the dormant buds Devitt and Stafstrom, ; Gutierrez et al. Swiatek et al. Further, SD photoperiod has been shown to keep more cells in the G1 stage than the G2 stage and is associated with an increase in ABA levels Rohde et al.

Recently, Vergara et al. VvICK5 in grapevine V. Together, these findings indicate that ABA-modulated cell cycle arrest may be central to the overall development of ED in woody buds. GAs are a large group of tetracyclic diterpenoid compounds that exert significant effects on a broad spectrum of biological processes in plants.

Of all the numerically coded GAs, only a few have been found to have bioactivity. GAs promote both vegetative and reproductive growth in plants by modulating leaf morphology, stem elongation, sex expression, seed germination, floral development, and dormancy Takatsuka and Umeda, ; Hedden and Sponsel, GAs are a leading phytohormone that modulates bud dormancy as significant changes of bioactive GA levels before and after dormancy have been widely noted Cooke et al.

In general, GA levels are downregulated at the induction of dormancy and upregulated during dormancy release or bud burst, and such dynamics of GA contents have been reported in many woody species such as sweet cherry P. The role of GAs especially GA3 and GA4 in promoting bud dormancy release is also evident and has been demonstrated in several woody species Zhuang et al. In Populus , exogenous GA4 was found to substitute for the effect of chilling by upregulating several chilling-responsive genes e.

FT and induce bud burst Rinne et al. First, GAs control dormancy cycle through modulating the intercellular communication. In plants, dormancy cycle progression is highly dependent on the mobility of molecules such as FT, auxin, sugars, and possibly ABA Cooke et al.

When active GA content is extremely low during the deep dormancy period, the rate of substance exchange in buds with adjacent organs declines remarkably Hao et al. In a recent RNAi study, Singh et al.

Secondly, GA was found to enhance the production of reactive oxygen species ROS , which are of particular importance in dormancy breaking Zhuang et al. In grapevine V. Production of ROS at dormancy release has been documented in several plant systems and considered to play a central role in bud break Sudawan et al. Finally, GA may activate the metabolic pathways leading to dormancy release. For instance, in Japanese apricot P. Soluble sugars are considered to be an important energy source to sustain bud growth during dormancy release.

Additionally, sucrose is a potential signaling element that can indirectly enhance the expression of the genes that are related to cell division and cell cycle Ruan et al. The pathways of GA biosynthesis and catabolism have been extensively investigated by a combination of biochemical and molecular techniques. In GA biosynthesis, three classes of enzymes have been identified that correspond to the three stages of conversion. Next, ent -kaurene is oxidized to GA12 through stepwise oxidation via two cytochrome P monooxygenases: ent -kaurene oxidase KO and ent -kaurenoic acid oxidase KAO.

The GA biosynthesis occurs primarily within the vicinity of its action Kaneko et al. Two novel mechanisms that were found to reduce the bioactivity of GAs are the epoxidation by a P monooxygenase Zhu et al. The metabolic enzymes of GA are crucial players in the maintenance of GA homeostasis and the regulation bud dormancy. Specifically, the synthetic genes GA20ox and GA3ox and the catabolism gene GA2ox are of particular importance in regulating GA levels, and they are all encoded by multigene families.

Transcription studies have shown the close correlation between the expression of GA20ox , GA3ox , and GA2ox and the GA level variation during bud dormancy in woody species such as rose Rosa sp. Choubane et al. In particular, the expression of GA2ox was found to be significantly upregulated by SD photoperiod, and its overexpression resulted in accelerated bud set and delayed bud flush Zawaski et al.

This indicates that the GA2ox -mediated catabolism is the key mechanism that reduces GA levels for the establishment and maintenance of bud dormancy. In a comprehensive transcription study in the tea plant Camellia sinensis , the expression of the GA synthetic enzyme genes KAO and KO was in line with the progression of bud dormancy Yue et al. When multiple genes from the GA20oxs , GA3oxs , and GA2oxs families were examined, some genes were expressed in correlation with the bud dormancy progression, and others showed differential expression Yue et al.

Similarly, these genes were differentially expressed when tea plants were treated with exogenous GA3. Such redundant roles and possibly specialized functions of GA metabolism gene families contribute to the complexity of the underlying mechanism of GA metabolism. In addition to GA metabolism, recent genetic studies also indicated the involvement of the GA signaling pathway in dormancy. For example, the expression of GID1 in Chinese cherry Prunus pseudocerasus is significantly downregulated at the early stage of dormancy development and rapidly increases when buds enter the ECD stage Zhu et al.

Similar increase of GID1 expression prior to bud break was also found in tea plant Yue et al. In poplar P. Thus far, it is still unclear if these GA signaling genes respond directly to the environmental signals or if their expression merely reflects the developmental events during dormancy. The antagonism between ABA and GA marks the main feature of their interactions in modulating biological processes, in which the metabolism and signaling of these two phytohormones respond oppositely to environmental cues.

For example, in Japanese apricot, the decrease of ABA level is accompanied by a gradual increase in GA level from dormancy through dormancy release Wen et al. Transgenic studies showed that the alternation of one hormone may affect the metabolism of the other. Further evidence was obtained in a recent transcriptome study with tea plant Yue et al. Recently, Yue et al. Figure 2 Proposed schematic model of hormone interactions during bud dormancy induction and release in woody perennials.

Solid arrows and lines indicate actions or interactions among hormones, pathways, and environmental cues that have been documented in the literature. Red color indicates the substance or process that induces dormancy, and green color indicates those that promote dormancy release. This pathway probably serves as a negative feedback regulation mechanism. ABA was also found to suppress the intercellular communication during dormancy by enhancing the expression of callose synthase, leading to callose deposition and blockage of plasmodesmata.

This alteration of plasmodesmata during dormancy is reversed by GA as buds transition to bud break. GAs induce the expression of glucanases which degrade callose, allowing for the passage of sugars and other growth-promoting factors. Under SD conditions, GA biosynthesis is inhibited through phytochrome and phytochrome-interacting factors. ET is induced by SD and has a negative impact on GA biosynthesis and signaling, which nominates it as a dormancy inducer. IAA facilitates dormancy release through promoting GA biosynthesis and callose degradation.

Although it is recognized as the ripening hormone, ET has wide-ranging effects on a number of other biological processes including, but are not limited to, seed germination, flowering, abscission, senescence, and stress responses Bleecker and Kende, The function of ET in dormancy is closely related to its biosynthesis and signaling transduction. The results of several studies have converged to indicate ET and its response pathway is involved in dormancy regulation. Early evidence showed that ET levels are elevated at both dormancy initiation and bud break stages and application of ET antagonist 2,5-norbornadiene NBD causes premature dormancy break in potato microtuber Suttle, Later on, Ruonala et al.

Similar result was also found in chrysanthemum, in which mutants with impaired ET receptor gene DG-ERS1 fail to enter dormancy at dormancy inducing temperature Sumitomo et al. The requirement of ET in the induction of dormancy was further confirmed by microarray and transcriptomic studies, in which ET biosynthesis gene set and signaling component genes e. Moreover, Achard et al. More evidence of the interaction between ET and GA was obtained when the interaction between ET action and phytochrome signaling was examined.

In tobacco, low red to far-red light ratios R:FR was found to trigger ET biosynthesis and ET insensitive transgenic lines exhibit no shade avoidance, which can be rescued by application of GA3 Pierik et al. This suggests that the early signal transduction of phytochrome-mediated light responses might trigger ET accumulation and GA reduction in response to SD, leading to growth cessation and dormancy inductions.

Interestingly, ET was also indicated to participate in the dormancy release. Further evidence was supported by the following observations in grape buds: 1 ET biosynthesis can be temporarily activated by dormancy break stimuli such as HC, heat shock and sodium azid; 2 exogenous ET application enhances bud break; and 3 dormancy release is severely delayed when of ET signaling is blocked by NBD Ophir et al.

These findings suggest that ET has complex actions during bud dormancy, in which ET interacts with ABA synergistically during dormancy initiation, but antagonistically during dormancy release. These studies also suggest that ET and its signaling pathway is essential for the development of dormancy, and both ABA and GA respond downstream of ET mediated dormancy regulation.

How these opposite processes are integrated in a stage-dependent way warrants further investigation. Increasing evidence has suggested that bud break is triggered by elevated levels of ROS, and ET is actively involved in this process.

Differential expression of genes induced by HC revealed connection between dormancy release and oxidative stress, hypoxia, mitochondrial activity, ET biosynthesis and signaling pathways Ophir et al. In response to HC, plants transiently elicit ROS, such as H 2 O 2 , and subsequently activating many pathways that are related to dormancy release, including antioxidant systems.

ET biosynthesis has been suggested to increase oxidative stress in plants due to production of hydrogen cyanide Ionescu et al. Indeed, the accumulation of ROS and their elimination has been proposed to be pivotal steps in releasing dormancy Sudawan et al.

The effects of CKs are highly dependent on cell and tissue types, developmental stage and environmental conditions, thus CKs are particularly important in modulating meristem activity and morphogenesis. At the cellular level, CKs can activate cell cycle regulator, CDK by dephosphorylating its tyrosine Tyr , and this CK effect is considered to be primary and required for the proper progression of cell cycle, which would otherwise arrest at the G2 phase Zhang et al.

Natural CKs differ greatly in the side chains, which are attached to the parental compound adenine, and this structural diversity provides high specificity of the interaction between CKs and the receptors Kieber and Schaller, Though CKs are highly mobile in plants and can be transported in the xylem sap over long distances, locally synthesized CKs were suggested to be critical in regulating dormancy Tanaka et al.

Similar to its effect in releasing latent buds from PD, CKs are also implicated in the regulation of dormancy release Faust, Early research showed that CK concentration in xylem sap increases rapidly in response to bud breaking chemicals and reaches a maximum level at the budburst in apple Cutting et al.

In support of these findings, Hartmann et al. Using loss-of-function approach, Tran et al. Dobisova et al. This result was supported by the finding in rose Rosa hybrida , in which CK was found to participate in the initial responses of the light signaling pathway that promotes bud outgrowth Roman et al. These results suggest the light-mediated increase of CKs during the dormancy release may contribute to reduce ABA levels.

Taken together, these results suggest that CK is an essential regulator in the dormancy release and CK acts upstream of GA and ABA response pathways in stimulating meristematic activity. Auxin has long been known to promote stem elongation and to suppress the growth of lateral buds, in a phenomenon of the apical dominance.

Recent findings indicate auxin is also involved in plant senescence, blooming and stress responses Di et al. Auxin biosynthesis occurs primarily in shoot apex and young leaves. Being a major growth promoter, IAA has been implicated in the dormancy release in many species. Early study showed that exogenous auxin can promote the degradation of dormancy callose in the phloem of magnolia Magnolia kobus and lead to the restoration of the symplastic paths Aloni and Peterson, , which is the preparatory step of bud break.

In tea plant C. This study also revealed that free IAA content changes in opposite to its conjugated form, suggesting conjugation may serve as a major mechanism in maintaining the homeostasis of endogenous IAA during dormancy. Transcriptomic data revealed differential expression of the main components in the IAA signaling pathway during the transition from dormancy to active growth, and their potential roles remain yet to be further elucidated Qiu et al.

On the other hand, IAA also appears to be an integrator of environmental signals during the dormancy establishment. When exposed to low temperature or in combination with SD photoperiod, IAA levels in strawberry and the transcript levels of polar auxin transport PAT -related gene e.

Transcriptome data implied that the majority of auxin-associated genes are downregulated during dormancy in Japanese apricot and poplar Zhong et al. These results suggest that modulation of cellular auxin content, auxin responsiveness, auxin transport capacity, and conjugation could all be integrated in the regulation network of dormancy.

JAs are a class of lipid-based plant hormones that regulate diverse processes in plants development and defense Browse, Bioactive forms of JA include jasmonic acids, its biosynthetic precursor oxophytodienoic acid OPDA and the conjugate form JA-Ile, which all have been shown to be effective signaling compounds. Similar to ABA, JAs prevent plant growth by repressing meristem activity, and some stress related genes can be activated by both JAs and ABA, indicating their synergism in certain processes Swiatek et al.

Indeed, JA can induce leaf senescence and control the expression of senescence-related genes in many species Ueda and Kato, ; Shan et al. However, JAs seem to have opposite effects during bud dormancy. In beech trees Fagus sylvatica , JA levels were found to increase remarkably during bud burst Juvany et al. In agreement with this, contents of JA-Ile in potato tubers increase gradually as the buds transition from dormancy to active sprouting Suttle et al.

Transcriptomic analysis showed that JA pathway is repressed during dormancy but activated during ECD stage and bud break Hao et al. These results suggest that JAs may play a role other than inhibiting growth during dormancy release. Recent research indicates that JA signaling pathway is actively involved in cold acclimation process, which is closely related to dormancy. Third, In Arabidopsis , it was noticed that exogenous JA application enhanced plant freezing tolerance and JA biosynthesis was triggered upon cold exposure Hu et al.

However, whether such cross talks between JA and cold-responsive pathways would function during dormancy, especially in woody species, still needs further validation. The interactions between JAZ proteins and other hormonal signaling components e.

The first DAM genes were identified in a peach mutant Evergrowing EVG , in which deletion of EVG locus rendered complete loss of growth cessation and bud formation in dormancy inductive conditions Bielenberg et al.

Based on the phylogenetic data, no consensus has been reached weather SVL genes represent a distinct group apart from DAMs or they belong to one group. Nevertheless, the fact that DAM and SVL both exhibit growth inhibitory effect across different species demonstrated by transgenic studies Sasaki et al. Though DAM genes are highly conserved and closely arranged, their expression patterns are generally distinct in response to seasonal changes and developmental signals.

In peach P. It is thus suggested that DAM5 and DAM6 in peach operate downstream of the circadian perception of photoperiodic stimuli, which occurs prior to the onset of chilling. As the dormancy progresses, the expression of these genes decreases concomitant with chilling accumulation and reaches the minimum at bud break. In light of these findings, DAM5 and DAM6 are believed to be the primary internal regulators of dormancy induction and maintenance in peach and probably other stone fruit species.

Following the identification of peach DAM genes, identification and characterization of more DAM and SVP homologs have been reported in other perennial species including but are not limited to raspberry Rubus idaeus Mazzitelli et al.

Wu et al. In a recent review, Falavigna et al. The majority of these DAM genes fall in the group that are highly expressed during the intensified stage of dormancy, characteristic of peach DAM 5 and DAM 6 ; whereas other DAM genes are asynchronous with the progression of dormancy. More definitive role of DAM genes in bud dormancy regulation were demonstrated in a number of recent transgenic studies. In poplar, Sasaki et al. Overexpression of DAM6 in Japanese apricot resulted in inhibited shoot growth, early bud set, repressed bud break competency and delayed bud break Yamane et al.

The high similarity of the DAM genes characteristics across species indicate that bud dormancy may be regulated by a highly conserved mechanism that is shared by most perennial species.

Though the role of DAM genes in dormancy regulation has been extensively verified, the underlying molecular processes are still largely unknown. Recent studies suggested that flowering regulating gene FT and cold response transcription factors CBFs may act as major mediators in the DAM regulation pathway. FT encodes a small globular protein that has been implicated in flowering, prevention of growth cessation and proper induction of dormancy Bohlenius et al.

The CBF genes can be quickly activated by cold stress, and in turn trigger many downstream genes involving growth cessation, cold acclimation and dormancy Kendall et al. In a transient transformation and luciferase assay, Saito et al.

This notion was supported by Singh et al. Using chromatin immunoprecipitation and ChIP-seq technique, Wu et al. However, this notion was challenged by Tuan et al. The finding of the versatility of SVL in targeting multiple dormancy-related genes raised the hypnosis that SVL may function as a hub gene that dictates both dormancy establishment and release by connecting ABA and GA, and cold perception pathways Busov, In addition to the environmental and hormonal regulation, epigenetic modification has also been found to play an important role in regulating DAM expression during the dormancy phase transition.

In the chromatin modification, histone H3 trimethylation at K4 H3K4me3 and H3 acetylation H3ac are associated with activation of the nearby genes, whereas histone trimethylation at K27 H3K27me3 represses transcription Shilatifard, Concomitant with cold accumulation, several chromatin regions including the promoter of DAM6 genes in peach are marked by the enrichment of H3K27me3 and removal of H3K4me3 and H3ac Leida et al.

In Japanese pear, Saito et al. Similarly, Singh et al. It was reported that H3K27me3 was specific to peach cultivars with high chilling requirement Leida et al.

These findings indicate the epigenetic modification is a main mechanism that regulates DAM genes and highlight the involvement and importance of DAM genes during dormancy. Although many plant hormones are implicated in dormancy regulation, some aspects are still open to questions. ABA is the central regulator of dormancy, and its repression of cell cycle and intercellular communication via plasmodesmata appears to be important mechanisms leading to dormancy induction.

The role of GA is largely within its conventionally defined function: promotion of cell division and elongation, which is an essential step in dormancy release. However, whether GA directly unlock dormancy still needs further investigations. ET acts antagonistically with GA during dormancy induction, but the byproduct of its metabolism seems able to promote dormancy release.

The potential roles of DAM and SVL in the mediation of dormancy, though seemingly distinct, are expected to be integrated in a broader context of an overarching theme of growth inhibition, rather than dormancy per se. Thanks to the fast-growing research on bud dormancy in recent years, much have been learned about the hormone induction and repression kinetics during dormancy, the role of hormone biosynthesis and signaling-related genes, effects of hormones and their cross talks during the initiation, progression and release of bud dormancy.

However, the fundamental molecular and cellular mechanisms that initiate the transitions between meristematic growth, bud arrest, dormancy and resumption of metabolic activities still require further elucidation. JL and SS have contributed equally to the writing, editing and preparation of this review article. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Reprints and Permissions. Hilhorst, H. Seed dormancy and germination: the role of abscisic acid and gibberellins and the importance of hormone mutants. Plant Growth Regul 11, — Download citation. Received : 25 February Accepted : 20 March Issue Date : August Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search SpringerLink Search. Abstract Over the past decades many studies have aimed at elucidating the regulation of seed dormancy and germination. References 1. Physiol Plant 15—20 Google Scholar 2. Physiol Plant — Google Scholar 3. Planta — Google Scholar 4.