Seed dormancy and germination: the role of abscisic acid and gibberellins and the importance of hormone mutants

Over the past decades many studies have aimed at elucidating the regulation of seed dormancy and germination. Many hypotheses have been proposed and rejected but the regulatory principle behind changes in dormancy and induction of germination is still a black box. The majority of proposed mechanisms have a role for certain plant hormones in common. Abscisic acid and the gibberellins are the hormones most frequently suggested to control these processes. The development of hormone-deficient mutants made it possible to provide direct evidence for the involvement of hormones in germination and dormancy related processes.In the present paper an attempt is made to assess the role of abscisic acid and gibberellins in the transitions between dormant and non-dormant states and germination. First a conceptual framework is presented in which the different states of dormancy and germination are defined in order to contribute to a solution of the semantic confusion about these terms that has existed since the beginning of seed physiology.It is concluded that abscisic acid plays a pivotal role during the development of primary dormancy and gibberellins are involved in the induction of germination. Changes in sensitivity to these hormones occur during changes in dormancy. Both synthesis of and responsiveness to the hormones are controlled by natural environmental factors such as light, temperature and nitrate.     

拟南芥突变体种子休眠与萌发的研究进展

种子的休眠和萌发是一个复杂的过程, 至今尚未能清楚阐明其调控机制。目前已从拟南芥突变体中鉴定了一些与种子萌发和休眠相关的基因, 有助于阐明种子休眠和萌发的分子机制。本文综述了拟南芥突变体种子休眠与萌发方面的研究进展。赤霉素是促进种子萌发的主要因素之一, RGL、SPY、GCR、SLY和GAR等基因的表达参与赤霉素对种子萌发的调控。脱落酸与种子休眠有关, ABI1、ABI2、ABI3、ABI4、ABI5、FUS3、LEC、MARD和CIPK等基因参与了脱落酸的调控过程。对3类乙烯反应的突变体 (ein、etr和ctr) 以及油菜素内酯突变体 (det和bri) 的研究表明乙烯和油菜素内酯是通过拮抗脱落酸而促进种子萌发的。光对种子萌发的调节, 是通过具有Ser/Thr蛋白激酶活性的光敏色素PhyA、PhyB、 PhyC、PhyD和PhyE, 以磷酸化/去磷酸化方式调节其它与萌发相关基因的表达。含氮化合物对种子萌发的促进, 可能是以一种依赖一氧化氮的方式解除种子休眠。     

A potential role for endogenous microflora in dormancy release,cytokinin metabolism and the response to fluridone in Lolium rigidum seeds

Background and Aims Dormancy in Lolium rigidum (annual ryegrass) seeds can be alleviated by warm stratification in the dark or by application of fluridone, an inhibitor of plant abscisic acid (ABA) biosynthesis via phytoene desaturase. However, germination and absolute ABA concentration are not particularly strongly correlated. The aim of this study was to determine if cytokinins of both plant and bacterial origin are involved in mediating dormancy status and in the response to fluridone.Methods Seeds with normal or greatly decreased (by dry heat pre-treatment) bacterial populations were stratified in the light or dark and in the presence or absence of fluridone in order to modify their dormancy status. Germination was assessed and seed cytokinin concentration and composition were measured in embryo-containing or embryo-free seed portions.Key Results Seeds lacking bacteria were no longer able to lose dormancy in the dark unless supplied with exogenous gibberellin or fluridone. Although these seeds showed a dramatic switch from active cytokinin free bases to O-glucosylated storage forms, the concentrations of individual cytokinin species were only weakly correlated to dormancy status. However, cytokinins of apparently bacterial origin were affected by fluridone and light treatment of the seeds.Conclusions It is probable that resident microflora contribute to dormancy status in L. rigidum seeds via a complex interaction between hormones of both plant and bacterial origin. This interaction needs to be taken into account in studies on endogenous seed hormones or the response of seeds to plant growth regulators.     

拟南芥突变体种子休眠与萌发的研究进展

种子的休眠和萌发是一个复杂的过程,至今尚未能清楚阐明其调控机制。目前已从拟南芥突变体中鉴定了一些与种子萌发和休眠相关的基因,有助于阐明种子休眠和萌发的分子机制。本文综述了拟南芥突变体种子休眠与萌发方面的研究进展。赤霉素是促进种子萌发的主要因素之一,RGL、SPY、GCR、SLY和GAR等基因的表达参与赤霉素对种子萌发的调控。脱落酸与种子休眠有关,ABI1、ABI2、ABI3、ABI4、ABI5、FUS3、LEC、MARD和CIPK等基因参与了脱落酸的调控过程。对3类乙烯反应的突变体（ein、etr和ctr）以及油菜素内酯突变体（det和bri）的研究表明乙烯和油菜素内酯是通过拮抗脱落酸而促进种子萌发的。光对种子萌发的调节,是通过具有Ser／Thr蛋白激酶活性的光敏色素PhyA、PhyB、PhyC、PhyD和PhyE,以磷酸化／去磷酸化方式调节其它与萌发相关基因的表达。含氮化合物对种子萌发的促进,可能是以一种依赖一氧化氮的方式解除种子休眠。     

Molecular mechanisms of seed dormancy

Seed dormancy is an important component of plant fitness that causes a delay of germination until the arrival of a favourable growth season. Dormancy is a complex trait that is determined by genetic factors with a substantial environmental influence. Several of the tissues comprising a seed contribute to its final dormancy level. The roles of the plant hormones abscisic acid and gibberellin in the regulation of dormancy and germination have long been recognized. The last decade saw the identification of several additional factors that influence dormancy including dormancy-specific genes, chromatin factors and non-enzymatic processes. This review gives an overview of our present understanding of the mechanisms that control seed dormancy at the molecular level, with an emphasis on new insights. The various regulators that are involved in the induction and release of dormancy, the influence of environmental factors and the conservation of seed dormancy mechanisms between plant species are discussed. Finally, expected future directions in seed dormancy research are considered.     

The role of light in regulating seed dormancy and germination

Seed dormancy is an adaptive trait in plants. Breaking seed dormancy determines the timing of germination and is, thereby essential for ensuring plant survival and agricultural production. Seed dormancy and the subsequent germination are controlled by both internal cues (mainly hormones) and environmental signals. In the past few years, the roles of plant hormones in regulating seed dormancy and germination have been uncovered. However, we are only beginning to understand how light signaling pathways modulate seed dormancy and interaction with endogenous hormones. In this review, we summarize current views of the molecular mechanisms by which light controls the induction, maintenance and release of seed dormancy, as well as seed germination, by regulating hormone metabolism and signaling pathways.     

Role of Abscisic Acid in Seed Dormancy

Seed dormancy is an adaptive trait that improves survival of the next generation by optimizing the distribution of germination over time. The agricultural and forest industries rely on seeds that exhibit high rates of germination and vigorous, synchronous growth after germination; hence dormancy is sometimes considered an undesirable trait. The forest industry encounters problems with the pronounced dormancy of some conifer seeds, a feature that can lead to non-uniform germination and poor seedling vigor. In cereal crops, an optimum balance is most sought after; some dormancy at harvest is favored because it prevents germination of the physiologically mature grain in the head prior to harvest (that is, preharvest sprouting), a phenomenon that leads to considerable damage to grain quality and is especially prominent in cool moist environments. The sesquiterpene abscisic acid (ABA) regulates key events during seed formation, such as the deposition of storage reserves, prevention of precocious germination, acquisition of desiccation tolerance, and induction of primary dormancy. Its regulatory role is achieved in part by cross-talk with other hormones and their associated signaling networks, via mechanisms that are largely unknown. Quantitative genetics and functional genomics approaches will contribute to the elucidation of genes and proteins that control seed dormancy and germination, including components of the ABA signal transduction pathway. Dynamic changes in ABA biosynthesis and catabolism elicit hormone-signaling changes that affect downstream gene expression and thereby regulate critical checkpoints at the transitions from dormancy to germination and from germination to growth. Some of the recent developments in these areas are discussed.     

Growth and Dormancy in Lunularia cruciata (L.) Dum.: III. THE WAVELENGTHS OF LIGHT EFFECTIVE IN PHOTOPERIODIC CONTROL

Growth and dormancy in Lunularia are controlled by daylength,short-day promoting active growth, long-day or light-break treatmentinducing dormancy. Light-breaks of red light are highly effectivein inducing dormancy, while irradiation with other wavebandsis much less inhibitory to growth. Far-red light given afterred irradiation causes substantial reversal of the red-lighteffect, suggesting strongly that phytochrome is involved inthe photoperiodic response mechanism of Lunularia. However,even short(15 sec.) exposures to far-red light alone cause significantgrowth inhibition, and it is considered possible that far-redirradiation also leads to the formation of some of the P 730form of phytochrome.     

光信号与激素调控种子休眠和萌发研究进展

休眠是种子植物在长期进化过程中产生的适应性性状, 通过抑制种子在不适宜的环境中萌发进而保证植物能够在逆境中生存。此外, 休眠有助于种子的长距离运输和扩散, 因此休眠对种子延续和物种保存具有重要意义。种子由休眠向萌发的发育转变不仅关系到物种的繁衍, 而且对保证农业生产中作物的产量和品质也具有重要作用。种子的休眠和萌发受到内源激素和外源光信号的共同调控。其中, 外源光信号主要通过调控内源ABA和GA的生物合成及信号转导进而调控种子休眠和萌发。该文系统综述了外源光信号和内源激素调控种子休眠和萌发的作用通路以及两类信号通路之间的交互作用, 旨在为农业生产中利用光和激素调控种子休眠与萌发提供参考。     

光信号与激素调控种子休眠和萌发研究进展

休眠是种子植物在长期进化过程中产生的适应性性状, 通过抑制种子在不适宜的环境中萌发进而保证植物能够在逆境中生存。此外, 休眠有助于种子的长距离运输和扩散, 因此休眠对种子延续和物种保存具有重要意义。种子由休眠向萌发的发育转变不仅关系到物种的繁衍, 而且对保证农业生产中作物的产量和品质也具有重要作用。种子的休眠和萌发受到内源激素和外源光信号的共同调控。其中, 外源光信号主要通过调控内源ABA和GA的生物合成及信号转导进而调控种子休眠和萌发。该文系统综述了外源光信号和内源激素调控种子休眠和萌发的作用通路以及两类信号通路之间的交互作用, 旨在为农业生产中利用光和激素调控种子休眠与萌发提供参考。     

种子休眠与破眠机理研究进展

种子休眠机理主要围绕透性、抑制剂作用和光敏素转化等方面的研究而建立。种皮的阻碍作用可能是由于种皮的物理或化学特性引起．可导致对水、光、气体或溶质的透性改变。抑制剂作用机理是抑制物质可抵消促进细胞分裂和生长发育的激素的作用。光敏素转化机理来源于与休眠有关的生物活性化学物质的合成、活化或破坏受光诱导的观点,由于发现了光敏素蓝色蛋白的活化型（Pfr）和钝化型（Pr）而得到强有力的支持,种子光休眠取决于光敏素蓝色蛋白的活化型（Pfr）含量和Pfr／（Pr＋Pfr）比值。目前,打破休眠的方法一般有机械破皮法、激素处理法、分子生物学技术法、物理处理法（如激光、烟、热等处理技术）、CO2处理法等。激素的平衡由抑制剂占优势向促进物占优势的变化是打破休眠的决定因素。研究破眠机理的分子生物学技术有多种,包括ABA突变体的利用、分子标记、转基因技术、用反义RAN阻止基因的表达、cDNA克隆技术等。用激光照射种子,把适宜的光射入细胞,可增加细胞生物能,促进种子发育,从而可能打破休眠。热处理的机理是由于加热可以增加种皮的透气性。CO2之所以能提高某些物种的萌发率,在于其影响了种子内部乙烯的敏感性。     

Some reflections on the relationship between endogenous hormones and light-mediated seed dormancy

The relationships between phytochrome and endogenous hormones in the light-mediated control of seed dormancy are discussed. It is concluded that gibberellins are primarily involved in post-dormancy metabolic processes leading to embryo growth and radicle emergence, such as food reserve mobilisation and endosperm softening. Evidence is considered that germination inhibitors, particularly abscisic acid, are involved in the establishment and maintenance of primary dormancy. The role of cytokinins not fully elucidated but there is considerable evidence to suggest that phytochrome control may involve cytokinin effects on transmembrane ion fluxes. In terms of hormonal control, phytochrome mediated dormancy is a complex phenomenon. There is a need for molecular studies of processes controlled by phytochrome, GAs, CKs and ABA during dormancy and germination to unravel the complexities of the dormancy mechanisms. Such studies would be facilitated by the availability of CK-deficient mutants of classical light-sensitive species.     

The physiological basis of seed dormancy in Avena fatua III. Action of nitrogenous compounds

Sodium nitrate and nitrite (50–100 m M ) induced germination in three out of four genetically pure dormant lines of Avena fatua L. The sensitivity to these treatments was low immediately ater harvest and increased markedly after six months of dry after-ripening. The observation that a fourth dormant line failed to respond suggests at least two metabolic blocks may be involved in expression of dormancy. An inhibitor of gibberellin biosynthesis, 2-chloroethyl trimethylammonium chloride, completely inhibited the dormancy-breaking effect by nitrate and nitrite, indicating a requirement for gibberellin biosynthesis. Among reduced nitrogenous compounds, ammonium chloride and urea failed to break dormancy in all partly after-ripened lines, suggesting that nitrate and nitrite may induce germination through their ability to act as electron acceptors. The sensitivity to all nitrogenous compounds tested increased with the length of after-ripening indicating that the depth of the second dormancy block amy decrease with the time of after-ripening. Other reduced nitrogenous compounds, thiourea and hydroxylamine hydrochloride, promoted some germination in the least dormant, partially after-ripened lines. The function of these compounds as electron acceptors and their similarity in activity to the cytochrome oxidase inhibitor, sodium azide, is discussed with reference to dormancy and the possible involvement of the alternative pathway of respiration.     

DELLA-mediated cotyledon expansion breaks coat-imposed seed dormancy

Seed dormancy is a key adaptive trait in plants responsible for the soil seed bank. The long established hormone-balance theory describes the antagonistic roles of the dormancy promoting plant hormone abscisic acid (ABA), and the germination promoting hormone gibberellin (GA) in dormancy control. Light, temperature, and other dormancy-breaking signals function to modulate the synthesis and perception of these hormones in the seed. However, the way in which these hormones control dormancy in the imbibed seed remains unknown. Here, we show that the DELLA protein regulators of the GA response are required for dormancy and describe a model through which hormone signal integration and dormancy regulation is achieved. We demonstrate that cotyledon expansion precedes radicle emergence during Arabidopsis seed germination and that a striking correlation exists between final seedling cotyledon size and seed dormancy in the DELLA mutants. Furthermore, twelve previously characterized seed-dormancy mutants are also defective in the control of cotyledon size in a manner consistent with their effect on germination potential. We propose that DELLA-mediated, light-, temperature-, and hormone-responsive cotyledon expansion prior to radicle emergence overcomes dormancy imposed by the seed coat and underlies seed-dormancy control in Arabidopsis.     

Germination ecophysiology of Annona crassiflora seeds

BACKGROUND AND AIMS: Little is known about environmental factors that break morphophysiological dormancy in seeds of the Annonaceae and the mechanisms involved. The aim of this study was to characterize the morphological and physiological components of dormancy of Annona crassiflora, a tree species native to the Cerrado of Brazil, in an ecophysiological context. METHODS: Morphological and biochemical characteristics of both embryo and endosperm were monitored during dormancy break and germination at field conditions. Seeds were buried in the field and exhumed monthly for 2 years. Germination, embryo length and endosperm digestion, with endo-beta-mannanase activity as a marker, were measured in exhumed seeds, and scanning electron microscopy was used to detect cell division. The effect of constant low and high temperatures and exogenous gibberellins on dormancy break and germination was also tested under laboratory conditions. KEY RESULTS: After burial in April, A. crassiflora seeds lost their physiological dormancy in the winter months with lowest monthly average minimum temperatures (May-August) prior to the first rainfall of the wet season. The loss of physiological dormancy enabled initiation of embryo growth within the seed during the first 2 months of the rainy season (September-October), resulting in a germination peak in November. Embryo growth occurred mainly through cell expansion but some dividing cells were also observed. Endosperm digestion started at the micropylar side around the embryo and diffused to the rest of the endosperm. Exogenous gibberellins induced both embryo growth and endo-beta-mannanase activity in dormant seeds. CONCLUSIONS: The physiological dormancy component is broken by low temperature and/or temperature fluctuations preceding the rainy season. Subsequent embryo growth and digestion of the endosperm are both likely to be controlled by gibberellins synthesized during the breaking of physiological dormancy. Radicle protrusion thus occurred at the beginning of the rainy season, thereby maximizing the opportunity for seedlings to emerge and establish.     

桃芽休眠的自然诱导因子及钙在休眠诱导中的作用

以2年生低需冷量设施栽培适宜品种“春捷毛桃”为试材,研究桃植株进入休眠和发展抗冻性的自然诱导因子及Ca²⁺在此过程中的作用.结果表明：短日照和自然低温单个因子或其共同作用均能诱导桃植株停止生长,然后进入休眠状态和发展抗冻性,但短日照和自然低温的作用机制不同.短日照首先诱导桃植株进入休眠状态,然后诱导抗冻性发展;而自然低温则是首先诱导桃植株抗冻性发展,而后诱导其进入休眠状态;当短日照和自然低温共同作用即自然条件下时,短日照起主导作用,自然低温起辅助促进作用.短日照处理研究表明,短日照诱导桃植株停止生长、休眠诱导和抗冻性发展离不开Ca²⁺的作用,在此过程中Ca²⁺起着传递短日照信号的作用;在人工补光长日照条件下,随着温度的降低,Ca²⁺逐渐由细胞间隙、细胞壁和液泡流入细胞质和核内,同时植株生长速度变缓并最终完全停止,随后进入休眠状态并发展抗冻性.表明Ca²⁺作为传递自然低温信号的信使在自然低温诱导桃植株停止生长、抗冻性发展和休眠过程中起着重要作用.     

Spatial and temporal changes in multiple hormone groups during lateral bud release shortly following apex decapitation of chickpea (Cicer arietinum) seedlings

Although the co-ordination of promotive root-sourced cytokinin (CK) and inhibitory shoot apex-sourced auxin (IAA) is central to all current models on lateral bud dormancy release, control by those hormones alone has appeared inadequate in many studies. Thus it was hypothesized that the IAA : CK model is the central control but that it must be considered within the relevant timeframe leading to lateral bud release and against a backdrop of interactions with other hormone groups. Therefore, IAA and a wide survey of cytokinins (CKs), were examined along with abscisic acid (ABA) and polyamines (PAs) in released buds, tissue surrounding buds and xylem sap at 1 and 4 h after apex removal, when lateral buds of chickpea are known to break dormancy. Three potential lateral bud growth inhibitors, IAA, ABA and cis -zeatin 9-riboside (ZR), declined sharply in the released buds and xylem following decapitation. This is in contrast to potential dormancy breaking CKs like trans -ZR and trans -zeantin 9-riboside 5'phosphate (ZRMP), which represented the strongest correlative changes by increasing 3.5-fold in xylem sap and 22-fold in buds. PAs had not changed significantly in buds or other tissues after 4 h, so they were not directly involved in the breaking of bud dormancy. Results from the xylem and surrounding tissues indicated that bud CK increases resulted from a combination synthesis in the bud and selective loading of CK nucleotides into the xylem from the root.