Genes involved in ethylene and gibberellins metabolism are required for endosperm-limited germination of Sisymbrium officinale L. seeds: germination in Sisymbrium officinale L. seeds

Diverse functional interactions between ABA and ethylene in plant development and responses to stress

Abscisic acid (ABA) and ethylene are two essential hormones that play crucial roles throughout the entire plant life cycle and in their tolerance to abiotic or biotic stress. In recent decades, increasing research has revealed that, in addition to their individual roles, these two hormones are more likely to function through their interactions, forming a complex regulatory network. More importantly, their functions change and their interactions vary from synergistic to antagonistic depending on the specific plant organ and development stage, which is less focused, compared and systematically summarized. In this review, we first introduce the general synthesis and action signaling pathways of these two plant hormones individually and their interactions in relation to seed dormancy and germination, primary root growth, shoot development, fruit ripening, leaf senescence and abscission, and stomatal movement regulation under both normal and stress conditions. A better understanding of the complex interactions between ABA and ethylene will enhance our knowledge of how plant hormones regulate development and respond to stress and may facilitate the development of crops with higher yields and greater tolerance to stressful environments through tissue-specific genetic modifications in the future.

Ethylene, a Signaling Compound Involved in Seed Germination and Dormancy

The present review is focused on current findings on the involvement of ethylene in seed biology. The responsiveness of seeds to ethylene depends on the species and the dormancy status, improving concentrations ranging from 0.1 to 200 μL L-1. The signaling pathway of ethylene starts with its binding to five membrane-anchored receptors, which results in the deactivation of Constitutive Triple Response 1 (CTR1, a protein kinase) that does not exert its inhibitory effect on Ethylene Insensitive 2 (EIN2) by phosphorylating its cytosolic C-terminal domain. An analysis of germination in the presence of inhibitors of ethylene synthesis or action, and using seeds from mutant lines altered in terms of the genes involved in ethylene synthesis (acs) and the signaling pathway (etr1, ein2, ein4, ctr1 and erf1), demonstrates the involvement of ethylene in the regulation of seed dormancy. The promoting effect of ethylene is also regulated through crosstalk with abscisic acid (ABA) and gibberellins (GAs), essential hormones involved in seed germination and dormancy, and Reactive Oxygen Species (ROS). Using a mutant of the proteolytic N-degron pathway, Proteolysis (PRT6), the Ethylene Response Factors (ERFs) from group VII (HRE1, HRE2, RAP 2.2, RAP2.3 and RAP 2.12) have also been identified as being involved in seed insensitivity to ethylene. This review highlights the key roles of EIN2 and EIN3 in the ethylene signaling pathway and in interactions with different hormones and discusses the responsiveness of seeds to ethylene, depending on the species and the dormancy status.

Exploring fine tuning between phytohormones and ROS signaling cascade in regulation of seed dormancy, germination and seedling development

In higher plants, seed is a propagule which ensures dissemination and survival of species. Developmental phases of a seed comprise embryogenesis, maturation and germination paving a way to its final fate i.e. seedling establishment. The final stage of seed maturation is marked by dehydration, acquisition of dessication tolerance and induction of dormancy. A precise Abscisic acid (ABA) to Gibberellins (GA) ratio, accumulation of miRNA 156, low level of reactive oxygen species (ROS) and enzyme inactivity govern seed dormancy. This also prevent pre harvest sprouting of the seeds. Overtime, stored seed mRNAs and proteins are degraded through oxidation of specific nucleotides in response to ROS accumulation. This degradation alleviates seed dormancy and transforms a dormant seed into a germinating seed. At this stage, ABA catabolism and degradation accompanied by GA synthesis contribute to low ABA to GA ratio. GA as well as ROS acts downstream, to mobilize reserve food materials, rupture testa, enhance imbibition and protrude radicle. All these events mark seed germination. Further, seedling is established under the governance of auxin and light. ABA and GA are master regulators while auxin, cytokinins, ethylene, jasmonic acid, brassinosteroids act through interdependent pathways to tightly regulate seed dormancy, germination and seedling establishment. In this review, the role of phytohormones and ROS in accordance with environmental factors in governing seed dormancy, promoting seed germination and thus, establishing a seedling is discussed in detail.

The ethylene receptor mutation etr2b reveals crosstalk between ethylene and ABA in the control of Cucurbita pepo germination

The enhanced salt tolerance of squash ethylene-insensitive mutants during germination and early stages of seedling development suggested that abscisic acid (ABA) could mediate this tolerance. To gain insight into the crosstalk between ethylene and ABA in seed germination, the germination rate and early seedling growth of wild type (WT) and ethylene-insensitive etr2b mutant were compared in seeds germinated under water and exogenous ABA treatment. The etr2b seeds germinated earlier than WT under both water and ABA, and the effect of ABA on radicle length and seedling growth of etr2b was lower than in WT, indicating that etr2b is also insensitive to ABA. The comparison of ABA and ethylene contents and ABA and ethylene gene expression profiles in WT and etr2b dry and imbibed seeds in either water, NaCl or ABA demonstrated a clear crosstalk between ethylene and ABA in germination. The expression profiles of ethylene genes in WT and etr2b indicated that the role of ethylene in seed germination does not appear to follow the canonical ethylene signaling pathway. Instead, etr2b reduces ABA content during formation of the seeds (dry seeds) and in response to seed imbibition and germination, which means diminished dormancy in the ethylene mutant. The etr2b mutation downregulated the expression of ABA biosynthesis and signaling genes during germination, demonstrating the positive role of ethylene receptor gene CpETR2B on seed germination and early seedling growth in squash is mediated by ABA. The reduced effect of exogenous ABA on ethylene production and ethylene gene expression in etr2b seeds suggests that this regulation is also dependent on ethylene.

A View into Seed Autophagy: From Development to Environmental Responses

Autophagy is a conserved cellular mechanism involved in the degradation and subsequent recycling of cytoplasmic components. It is also described as a catabolic process implicated in the specific degradation of proteins in response to several stimuli. In eukaryotes, the endoplasmic reticulum accumulates an excess of proteins in response to environmental changes, and is the major cellular organelle at the crossroads of stress responses. Return to proteostasis involves the activation of the Unfolded Protein Response (UPR) and eventually autophagy as a feedback mechanism to relieve protein overaccumulation. Recent publications have focused on the relevance of autophagy in two central processes of seed biology: (i) seed storage protein accumulation upon seed maturation and (ii) reserve mobilization during seed imbibition. Although ER-protein accumulation and the subsequent activation of autophagy resemble the Seed Storage Protein (SSP) deposition during seed maturation, the molecular connection between seed development, autophagy, and seed response to abiotic stresses is still an underexplored field. This mini-review presents current advances in autophagy in seeds, highlighting its participation in the normal course of seed development from embryogenesis to germination. Finally, the function of autophagy in response to the seed environment is also considered, as is its involvement in controlling seed dormancy and germination.

Overexpression of acdS gene encoding 1-aminocyclopropane-1-carboxylic acid deaminase enzyme in petunia negatively affects seed germination

Overexpression of acdS in petunia negatively affects seed germination by suppression of ethylene biosynthesis and signaling genes and induction of abscisic acid biosynthesis genes in the seeds. The acdS gene, which encodes 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, has been overexpressed in horticultural crops to improve their tolerance to abiotic stress. However, the role of acdS in the germination of crop seeds has not been investigated, despite its suppression of ethylene production. In this study, acdS overexpression significantly reduced seed weight and germination rate in transgenic petunia cv. Merage Rose (T5, T7, and T12) relative to wild type via the suppression of ethylene biosynthesis and signaling genes and induction of abscisic acid (ABA) biosynthesis genes. The germination rate of T7 was significantly lower than those of T5 and T12, which was linked to higher expression of acdS in the former than the latter. The addition of exogenous ACC and gibberellic acid (GA₃) to the germination medium improved the germination rate of T5 seeds and GA₃ promoted the germination rate of T12 seeds. However, neither ACC nor GA₃ promoted the germination rate of T7 seeds. The improved germination rates in T5 and T12 were associated with the transcriptional regulation of ethylene biosynthesis genes, particularly that of the ACO1 gene, signaling genes, and ABA biosynthesis genes. In this study, we discovered a negative role of acdS in seed germination in petunia. Thus, we highlight the need to consider the negative effect of acdS on seed germination when overexpressing the gene in horticultural crops to improve tolerance to abiotic stress.

Ethylene regulates post-germination seedling growth in wheat through spatial and temporal modulation of ABA/GA balance

This study aimed to gain insights into the molecular mechanisms underlying the role of ethylene in regulating germination and seedling growth in wheat by combining pharmacological, molecular, and metabolomics approaches. Our study showed that ethylene does not affect radicle protrusion but controls post-germination endospermic starch degradation through transcriptional regulation of specific α-amylase and α-glucosidase genes, and this effect is mediated by alteration of endospermic bioactive gibberellin (GA) levels, and GA sensitivity via expression of the GA signaling gene, TaGAMYB. Our data implicated ethylene as a positive regulator of embryo axis and coleoptile growth through transcriptional regulation of specific TaEXPA genes. These effects were associated with modulation of GA levels and sensitivity, through expression of GA metabolism (TaGA20ox1, TaGA3ox2, and TaGA2ox6) and signaling (TaGAMYB) genes, respectively, and/or the abscisic acid (ABA) level and sensitivity, via expression of specific ABA metabolism (TaNCED2 or TaCYP707A1) and signaling (TaABI3) genes, respectively. Ethylene appeared to regulate the expression of TaEXPA3 and thereby root growth through its control of coleoptile ABA metabolism, and root ABA signaling via expression of TaABI3 and TaABI5. These results show that spatiotemporal modulation of ABA/GA balance mediates the role of ethylene in regulating post-germination storage starch degradation and seedling growth in wheat.

Involvement of ethylene biosynthesis and perception during germination of dormant Avena fatua L. caryopses induced by KAR1 or GA3

Germination of primary dormant wild oat caused by KAR₁ or GA₃ is associated with ACC accumulation and increased ethylene production shortly before radicle protrusion as a result of the non-transcriptional and transcriptional activation of ACS and ACO enzymes, respectively. Response to both compounds involves the modulation of ethylene sensitivity through ethylene receptor genes. Harvested Avena fatua caryopses are primary dormant and, therefore, germinated poorly at 20 °C. Karrikin 1 (KAR₁), which action probably requires endogenous gibberellins (GAs), and gibberellin A₃ (GA₃) was found to induce dormant caryopses to germinate. The stimulatory effects were accompanied by the activation of the ethylene biosynthesis pathway and depended on undisturbed ethylene perception. KAR₁ and GA₃ promoted 1-aminocyclopropane-1-carboxylic acid (ACC) accumulation during coleorhizae emergence and ethylene production shortly prior to the radicle protrusion, which resulted from the enhanced activity of two ethylene biosynthesis enzymes, ACC synthase (ACS) and ACC oxidase (ACO). The inhibitor of ACS adversely affected beneficial impacts of both KAR₁ and GA₃ on A. fatua caryopses germination, while the inhibitor of ACO more efficiently impeded the GA₃ effect. The inhibitors of ethylene action markedly lowered germination in response to KAR₁ and GA₃. Gene expression studies preceded by the identification of several genes related to ethylene biosynthesis (AfACS6, AfACO1, and AfACO5) and perception (AfERS1b, AfERS1c, AfERS2, AfETR2, AfETR3, and AfETR4) provided further evidence for the engagement of ethylene in KAR₁ and GA₃ induced germination of A. fatua caryopses. Both AfACO1 and AfACO5 were upregulated, whereas AfACS6 remained unaffected by the treatment. This suggests the existence of different regulatory mechanisms of enzymatic activity, transcriptional for ACO and non-transcriptional for ACS. During imbibition in water, AfERS1b was stronger expressed than other receptor genes. In the presence of KAR₁ or GA₃, the expression of AfETR3 was substantially induced. Differential expression of ethylene receptor genes implies the modulation of caryopses sensitivity adjusted to ethylene availability and suggests the functional diversification of individual receptors.

Revisiting the Role of Ethylene and N-End Rule Pathway on Chilling-Induced Dormancy Release in Arabidopsis Seeds

Dormant Arabidopsis (Arabidopsis thaliana) seeds do not germinate easily at temperatures higher than 10–15 °C. Using mutants affected in ethylene signaling (etr1, ein2 and ein4) and in the N-end-rule pathway of the proteolysis (prt6 and ate1-ate2) we have investigated the effects of cold and ethylene on dormancy alleviation. Ethylene (10–100 ppm) and 2–4 days chilling (4 °C) strongly stimulate the germination of wild type (Col-0) seeds at 25 °C. Two to four days of chilling promote the germination at 25 °C of all the mutants suggesting that release of dormancy by cold did not require ethylene and did not require the N-end-rule pathway. One mutant (etr1) that did not respond to ethylene did not respond to GA₃ either. Mutants affected in the N-end rule (prt6 and ate1-ate2) did not respond to ethylene indicating that also this pathway is required for dormancy alleviation by ethylene; they germinated after chilling and in the presence of GA₃. Cold can activate the ethylene signaling pathway since it induced an accumulation of ETR1, EINI4, and EIN2 transcripts, the expression of which was not affected by ethylene and GA₃. Both cold followed by 10 h at 25 °C and ethylene downregulated the expression of PRT6, ATE1, ATE2, and of ABI5 involved in ABA signaling as compared to dormant seeds incubated at 25 °C. In opposite, the expression of RGA, GAI, and RGL2 encoding three DELLAs was induced at 4 °C but downregulated in the presence of ethylene.

Seed dormancy and control of germination in Sisymbrella dentata (L.) O.E. Schulz (Brassicaceae)

Seed germination responsiveness to environmental cues is crucial for plant species living in changeable habitats and can vary among populations within the same species as a result of adaptation or modulation to local climates. Here, we investigate the germination response to environmental cues of Sisymbrella dentata (L.) O.E. Schulz, an annual endemic to Sicily living in Mediterranean Temporary Ponds (MTP), a vulnerable ecosystem. Germination of the only two known populations, Gurrida and Pantano, was assessed over a broad range of conditions to understand the role of temperatures, nitrate, hormones (abscisic acid - ABA and gibberellins - GA) and after-ripening in dormancy release in this species. Seed germination responsiveness varied between the two populations, with seeds from Gurrida germinating under a narrower range of conditions. Overall, this process in S. dentata consisted of testa and endosperm rupture as two sequential events, influenced by ABA and GA biosynthesis. Nitrate addition caused an earlier testa rupture, after-ripening broadened the thermal conditions that allow germination, and alternating temperatures significantly promoted germination of non-after-ripened seeds. Primary dormancy in S. dentata seeds likely allows this plant to form a persistent seed bank that is responsive to specific environmental cues characteristic of MTP habitats.

Nitric oxide regulates lateral root formation through modulation of ACC oxidase activity in sunflower seedlings under salt stress

Nitric oxide (NO) is established as a modulator of various developmental processes in plants through its interaction with multiple enzymatic and non-enzymatic biomolecules. Lateral root (LR) induction and extension in sunflower (Helianthus annuus L.) has been observed to be governed by a probable crosstalk between NO and ethylene biosynthesizing enzyme-ACC oxidase. NaCl (120 mM) stress not only lowers LR induction but also reduces their extension growth. Quenching of endogenous NO by raising seedlings in presence of 40 µM hemoglobin in the growth medium does not affect LR induction but lowers their extension growth. NaCl stress and NO depletion have additive effects on the enhancement of ACC oxidase activity, leading to enhanced ethylene biosynthesis. Role of NO has been further confirmed by raising sunflower seedlings in the presence of 20-60 µM of two NO donors, sodium nitroprusside (SNP) and diethylenetriamine NONOate (DETA). LR extension growth was higher with DETA than SNP as NO donor at 40 µM. Iron-deficiency also promoted LR proliferation. It also significantly lowered ACC oxidase activity in the seedling roots in response to salt stress. Based on the present findings it is proposed that salt stress-mediated LR proliferation is regulated by NO through its binding with ACC oxidase (an iron-containing enzyme). This results in the formation of a stable ternary complex (ACC-ACC oxidase-NO) which leads to the reduction in ethylene biosynthesis. Lesser availability of ethylene consequently brings about enhanced LR formation.

Mannans and endo-β-mannanase transcripts are located in different seed compartments during Brassicaceae germination

Mannans but not endo-β-mannanases are mainly found in the mucilage layer of two Brassicaceae seeds. Nonetheless, mannanase mobilization from inner to outer seed layers cannot be ruled out. The contribution of endo-β-mannanase (MAN) genes to the germination of the wild-type Sisymbrium officinale and cultivated Brassica rapa (Brassicaceae) species has been explored. In both species, mannans have been localized to the imbibed external seed coat layer (mucilage) by fluorescence immunolocalization and MAN enzymatic activity increases in seeds as imbibition progresses, reaching a peak before 100% germination is achieved. The MAN gene families have been annotated and the expression of their members analyzed in vegetative and reproductive organs. In S. officinale and B. rapa, MAN2, MAN5, MAN6, and MAN7 transcripts accumulate upon seed imbibition. SoMAN7 is the most expressed MAN gene in S. officinale germinating seeds, as occurs with its ortholog in Arabidopsis thaliana, but in B. rapa, the most abundant transcripts are BrMAN2 and BrMAN5. These genes (MAN2, MAN5, MAN6, and MAN7) are localized, by mRNA in situ hybridization, to the micropylar at the endosperm layer and to the radicle in S. officinale, but in B. rapa, these mRNAs are faintly found to the micropylar living seed coat layer and are mainly present at the radicle tip and the vascular bundles. If the domestication process undergone by B. rapa is responsible for these different MAN expression patterns, upon germination remains to be elucidated. Since mannans and MAN genes are not spatially distributed in the same seed tissues, a movement of MAN enzymes that are synthesized with typical signal peptides from the embryo tissues to the mucilage layer (via apoplastic space) is necessary for the mannans to be hydrolyzed.

Involvement of Polyamine Oxidase-Produced Hydrogen Peroxide during Coleorhiza-Limited Germination of Rice Seeds

Seed germination is a complicated biological process that requires regulated enzymatic and non-enzymatic reactions. The action of polyamine oxidase (PAO) produces hydrogen peroxide (H2O2), which promotes dicot seed germination. However, whether and, if so, how PAOs regulate monocot seed germination via H2O2 production is unclear. Herein, we report that the coleorhiza is the main physical barrier to radicle protrusion during germination of rice seed (a monocot seed) and that it does so in a manner similar to that of dicot seed micropylar endosperm. We found that H2O2 specifically and steadily accumulated in the coleorhizae and radicles of germinating rice seeds and was accompanied by increased PAO activity as the germination percentage increased. These physiological indexes were strongly decreased in number by guazatine, a PAO inhibitor. We also identified 11 PAO homologs (OsPAO1-11) in the rice genome, which could be classified into four subfamilies (I, IIa, IIb, and III). The OsPAO genes in subfamilies I, IIa, and IIb (OsPAO1-7) encode PAOs, whereas those in subfamily III (OsPAO8-11) encode histone lysine-specific demethylases. In silico-characterized expression profiles of OsPAO1-7 and those determined by qPCR revealed that OsPAO5 is markedly upregulated in imbibed seeds compared with dry seeds and that its transcript accumulated to a higher level in embryos than in the endosperm. Moreover, its transcriptional abundance increased gradually during seed germination in water and was inhibited by 5 mM guazatine. Taken together, these results suggest that PAO-generated H2O2 is involved in coleorhiza-limited rice seed germination and that OsPAO5 expression accounts for most PAO expression and activity during rice seed germination. These findings should facilitate further study of PAOs and provide valuable information for functional validation of these proteins during seed germination of monocot cereals.

ABA-stimulated SoDOG1 expression is after-ripening inhibited during early imbibition of germinating Sisymbrium officinale seeds

DELAY OF GERMINATION 1 (AtDOG1) was the first gene identified as dormancy-associated, but its physiological role in germination is far from being understood. Here, an orthologue of AtDOG1 in Sisymbrium officinale (SoDOG1; KM009050) is being reported. Phylogenetically, the SoDOG1 gene is included into the dicotyledonous group together with DOG1 from Arabidopsis thaliana (EF028470), Brassica rapa (AC189537), Lepidium papillosum (JX512183, JX512185) and Lepidium sativum (GQ411192). The SoDOG1 expression peaked at the onset of the silique maturation stage and there was presence of SoDOG1-mRNA in the freshly collected viable dry seed (i.e. AR0). The SoDOG1 transcripts were also found in other organs, such as open and closed flowers and to a lesser degree in roots and stems. We have previously reported in S. officinale seeds in which sensu stricto germination is positively affected by nitrate and both testa and micropylar endosperm ruptures are temporally separated. In dry viable seeds, the SoDOG1-mRNA level in three different after-ripening (AR) status was AR0 ≈ AR7 (optimal AR) < AR27 (optimal AR was almost lost). The presence of nitrate in the AR0 seed imbibition medium markedly decreased the SoDOG1 expression during sensu stricto germination. However, the nitrate stimulated the SoDOG1 expression during imbibition of AR7 compared to AR0. At the early AR0 seed imbibition (3-6 h), exogenous ABA provoked a very strong stimulation of the SoDOG1 expression. AR inhibits ABA-induced SoDOG1 expression during early germination and gibberellins (GA) can partially mimic this AR effect. A view on the integration of all found results in the sensu stricto germination of S. officinale was conducted.

Mannans and endo-β-mannanases (MAN) in Brachypodium distachyon: expression profiling and possible role of the BdMAN genes during coleorhiza-limited seed germination

Immunolocalization of mannans in the seeds of Brachypodium distachyon reveals the presence of these polysaccharides in the root embryo and in the coleorhiza in the early stages of germination (12h), decreasing thereafter to the point of being hardly detected at 27h. Concurrently, the activity of endo-β-mannanases (MANs; EC 3.2.1.78) that catalyse the hydrolysis of β-1,4 bonds in mannan polymers, increases as germination progresses. The MAN gene family is represented by six members in the Brachypodium genome, and their expression has been explored in different organs and especially in germinating seeds. Transcripts of BdMAN2, BdMAN4 and BdMAN6 accumulate in embryos, with a maximum at 24-30h, and are detected in the coleorhiza and in the root by in situ hybridization analyses, before root protrusion (germination sensu stricto). BdMAN4 is not only present in the embryo root and coleorhiza, but is abundant in the de-embryonated (endosperm) imbibed seeds, while BdMAN2 and BdMAN6 are faintly expressed in endosperm during post-germination (36-42h). BdMAN4 and BdMAN6 transcripts are detected in the aleurone layer. These data indicate that BdMAN2, BdMAN4 and BdMAN6 are important for germination sensu stricto and that BdMAN4 and BdMAN6 may also influence reserve mobilization. Whether the coleorhiza in monocots and the micropylar endosperm in eudicots have similar functions, is discussed.

Cytological and Proteomic Analyses of Osmunda cinnamomea Germinating Spores Reveal Characteristics of Fern Spore Germination and Rhizoid Tip Growth

Fern spore is a good single-cell model for studying the sophisticated molecular networks in asymmetric cell division, differentiation, and polar growth. Osmunda cinnamomea L. var. asiatica is one of the oldest fern species with typical separate-growing trophophyll and sporophyll. The chlorophyllous spores generated from sporophyll can germinate without dormancy. In this study, the spore ultrastructure, antioxidant enzyme activities, as well as protein and gene expression patterns were analyzed in the course of spore germination at five typical stages (i.e. mature spores, rehydrated spores, double-celled spores, germinated spores, and spores with protonemal cells). Proteomic analysis revealed 113 differentially expressed proteins, which were mainly involved in photosynthesis, reserve mobilization, energy supplying, protein synthesis and turnover, reactive oxygen species scavenging, signaling, and cell structure modulation. The presence of multiple proteoforms of 25 differentially expressed proteins implies that post-translational modification may play important roles in spore germination. The dynamic patterns of proteins and their encoding genes exhibited specific characteristics in the processes of cell division and rhizoid tip growth, which include heterotrophic and autotrophic metabolisms, de novo protein synthesis and active protein turnover, reactive oxygen species and hormone (brassinosteroid and ethylene) signaling, and vesicle trafficking and cytoskeleton dynamic. In addition, the function skew of proteins in fern spores highlights the unique and common mechanisms when compared with evolutionarily divergent spermatophyte pollen. These findings provide an improved understanding of the typical single-celled asymmetric division and polar growth during fern spore germination.

Ethylene, a key factor in the regulation of seed dormancy

Ethylene is an important component of the gaseous environment, and regulates numerous plant developmental processes including seed germination and seedling establishment. Dormancy, the inability to germinate in apparently favorable conditions, has been demonstrated to be regulated by the hormonal balance between abscisic acid (ABA) and gibberellins (GAs). Ethylene plays a key role in dormancy release in numerous species, the effective concentrations allowing the germination of dormant seeds ranging between 0.1 and 200 μL L(-1). Studies using inhibitors of ethylene biosynthesis or of ethylene action and analysis of mutant lines altered in genes involved in the ethylene signaling pathway (etr1, ein2, ain1, etr1, and erf1) demonstrate the involvement of ethylene in the regulation of germination and dormancy. Ethylene counteracts ABA effects through a regulation of ABA metabolism and signaling pathways. Moreover, ethylene insensitive mutants in Arabidopsis are more sensitive to ABA and the seeds are more dormant. Numerous data also show an interaction between ABA, GAs and ethylene metabolism and signaling pathways. It has been increasingly demonstrated that reactive oxygen species (ROS) may play a significant role in the regulation of seed germination interacting with hormonal signaling pathways. In the present review the responsiveness of seeds to ethylene will be described, and the key role of ethylene in the regulation of seed dormancy via a crosstalk between hormones and other signals will be discussed.

The Ethylene Receptors ETHYLENE RESPONSE1 and ETHYLENE RESPONSE2 Have Contrasting Roles in Seed Germination of Arabidopsis during Salt Stress

In Arabidopsis (Arabidopsis thaliana), ethylene responses are mediated by a family of five receptors that have both overlapping and nonoverlapping roles. In this study, we used loss-of-function mutants for each receptor isoform to determine the role of individual isoforms in seed germination under salt stress. From this analysis, we found subfunctionalization of the receptors in the control of seed germination during salt stress. Specifically, loss of ETHYLENE RESPONSE1 (ETR1) or ETHYLENE INSENSITIVE4 (EIN4) leads to accelerated germination, loss of ETR2 delays germination, and loss of either ETHYLENE RESPONSE SENSOR1 (ERS1) or ERS2 has no measurable effect on germination. Epistasis analysis indicates that ETR1 and EIN4 function additively with ETR2 to control this trait. Interestingly, regulation of germination by ETR1 requires the full-length receptor. The differences in germination between etr1 and etr2 loss-of-function mutants under salt stress could not be explained by differences in the production of or sensitivity to ethylene, gibberellin, or cytokinin. Instead, etr1 loss-of-function mutants have reduced sensitivity to abscisic acid (ABA) and germinate earlier than the wild type, whereas etr2 loss-of-function mutants have increased sensitivity to ABA and germinate slower than the wild type. Additionally, the differences in seed germination on salt between the two mutants and the wild type are eliminated by the ABA biosynthetic inhibitor norflurazon. These data suggest that ETR1 and ETR2 have roles independent of ethylene signaling that affect ABA signaling and result in altered germination during salt stress.

Nitrate-induced early transcriptional changes during imbibition in non-after-ripened Sisymbrium officinale seeds

We have here demonstrated for the first time that nitrate not only accelerates testa rupture of non- AR seeds but also modifies expression pattern of the cell-wall remodeling proteins (mannanases; SoMAN6 and SoMAN7) and key genes belonging to metabolism and signaling of ABA (SoNCED6, SoNCED9, SoCYP707A2 and SoABI5) and GAs (SoGA3ox, SoGA20ox, SoGA2ox and SoRGL2). These results were obtained during Sisymbrium officinale seed imbibition in the absence of endosperm rupture. Exogenous ABA induced a notable inhibition of testa rupture in both absence and presence of nitrate being this effect sharply reversed by GA(4+7). However, nitrate was capable to provoke testa rupture in absence of ABA synthesis. The expression of SoMAN6 and SoMAN7 were positively altered by nitrate. Although ABA synthesis seems apparent at the start of non-AR seed imbibition, taken together the results of SoNCED6, SoNCED9 and SoCYP707A2 expression seem to suggest that nitrate leads to a strong net ABA decrease. Likewise, nitrate positively affected the SoABI5 expression when the SoNCED9 expression was also stimulated. By contrast, at the early and final of imbibition, nitrate clearly inhibited the SoABI5 expression. The expression of SoGA2ox6 and SoGA3ox2 are strongly inhibited by nitrate whereas of SoGA20ox6 was stimulated. On the other hand, SoRGL2 transcript level decreased in the presence of nitrate. Taken together, the results presented here suggest that the nitrate signaling is already operative during the non-AR S. officinale seeds imbibition. The nitrate, in cross-talk with the AR network likely increases the favorable molecular conditions that trigger germination.

Arabidopsis thaliana bZIP44: a transcription factor affecting seed germination and expression of the mannanase-encoding gene AtMAN7

Endo-β-mannanases (MAN; EC. 3.2.1.78) catalyze the cleavage of β1→4 bonds in mannan polymers and have been associated with the process of weakening the tissues surrounding the embryo during seed germination. In germinating Arabidopsis thaliana seeds, the most highly expressed MAN gene is AtMAN7 and its transcripts are restricted to the micropylar endosperm and to the radicle tip just before radicle emergence. Mutants with a T-DNA insertion in AtMAN7 have a slower germination than the wild type. To gain insight into the transcriptional regulation of the AtMAN7 gene, a bioinformatic search for conserved non-coding cis-elements (phylogenetic shadowing) within the Brassicaceae MAN7 gene promoters has been done, and these conserved motifs have been used as bait to look for their interacting transcription factors (TFs), using as a prey an arrayed yeast library from A. thaliana. The basic-leucine zipper TF AtbZIP44, but not the closely related AtbZIP11, has thus been identified and its transcriptional activation upon AtMAN7 has been validated at the molecular level. In the knock-out lines of AtbZIP44, not only is the expression of the AtMAN7 gene drastically reduced, but these mutants have a significantly slower germination than the wild type, being affected in the two phases of the germination process, both in the rupture of the seed coat and in the breakage of the micropylar endosperm cell walls. In the over-expression lines the opposite phenotype is observed.

Photocontrol of bud burst involves gibberellin biosynthesis in Rosa sp

Light is a critical determinant of plant shape by controlling branching patterns and bud burst in many species. To gain insight into how light induces bud burst, we investigated whether its inductive effect in rose was related to gibberellin (GA) biosynthesis. In axillary buds of beheaded plants subject to light, the expression of two GA biosynthesis genes (RoGA20ox and RoGA3ox) was promptly and strongly induced, while that of a GA-catabolism genes (RoGA2ox) was reduced. By contrast, lower expression levels of these two GA biosynthesis genes were found in darkness, and correlated with a total inhibition of bud burst. This effect was dependent on both light intensity and quality. In in vitro cultured buds, the inductive effect of light on the growth of preformed leaves and SAM organogenic activity was inhibited by ancymidol and paclobutrazol, two effectors of GA biosynthesis. This effect was concentration-dependent, and negated by GA(3). However, GA(3) alone could not rescue bud burst in the dark. GA biosynthesis was also required for the expression and activity of a vacuolar invertase, and therefore for light-induced sugar metabolism within buds. These findings are evidence that GA biosynthesis contributes to the light effect on bud burst and lay the foundations of a better understanding of its exact role in plant branching.

Embryo growth, testa permeability, and endosperm weakening are major targets for the environmentally regulated inhibition of Lepidium sativum seed germination by myrigalone A

Myrigalone A (MyA) is a rare flavonoid in fruit leachates of Myrica gale, a deciduous shrub adapted to flood-prone habitats. As a putative allelochemical it inhibits seed germination and seedling growth. Using Lepidium sativum as a model target species, experiments were conducted to investigate how environmental cues modulate MyA's interference with key processes of seed germination. Time course analyses of L. sativum testa and endosperm rupture under different light conditions and water potentials were combined with quantifying testa permeability, endosperm weakening, tissue-specific gibberellin (GA) and abscisic acid (ABA) contents, as well as embryo growth and apoplastic superoxide production important for cell expansion growth. Lepidium sativum testa permeability and early water uptake by imbibition is enhanced by MyA. During late germination, MyA inhibits endosperm weakening and embryo growth, both processes required for endosperm rupture. Inhibition of embryo cell expansion by MyA depends on environmental cues, which is evident from the light-modulated severity of the MyA-mediated inhibition of apoplastic superoxide accumulation. Several important key weakening and growth processes during early and late germination are targets for MyA. These effects are modulated by light conditions and ambient water potential. It is speculated that MyA is a soil seed bank-destroying allelochemical that secures the persistence of M. gale in its flood-prone environment.

Softening-up mannan-rich cell walls

The softening and degradation of the cell wall (CW), often mannan enriched, is involved in several processes during development of higher plants, such as meristematic growth, fruit ripening, programmed cell death, and endosperm rupture upon germination. Mannans are also the predominant hemicellulosic CW polymers in many genera of green algae. The endosperm CWs of dry seeds often contain mannan polymers, sometimes in the form of galactomannans (Gal-mannans). The endo-β-mannanases (MANs) that catalyse the random hydrolysis of the β-linkage in the mannan backbone are one of the main hydrolytic enzymes involved in the loosening and remodelling of CWs. In germinating seeds, the softening of the endosperm seed CWs facilitates the emergence of the elongating radicle. Hydrolysis and mobilization of endosperm Gal-mannans by MANs also provides a source of nutrients for early seedling growth, since Gal-mannan, besides its structural role, serves as a storage polysaccharide. Therefore, the role of mannans and of their hydrolytic enzymes is decisive in the life cycle of seeds. This review updates and discusses the significance of mannans and MANs in seeds and explores the increasing biotechnological potential of MAN enzymes.

Beyond gibberellins and abscisic acid: how ethylene and jasmonates control seed germination

Appropriate responses of seeds and fruits to environmental factors are key traits that control the establishment of a species in a particular ecosystem. Adaptation of germination to abiotic stresses and changing environmental conditions is decisive for fitness and survival of a species. Two opposing forces provide the basic physiological mechanism for the control of seed germination: the increasing growth potential of the embryo and the restraint weakening of the various covering layers (seed envelopes), including the endosperm which is present to a various extent in the mature seeds of most angiosperms. Gibberellins (GA), abscisic acid (ABA) and ethylene signaling and metabolism mediate environmental cues and in turn influence developmental processes like seed germination. Cross-species work has demonstrated that GA, ABA and ethylene interact during the regulation of endosperm weakening, which is at least partly based on evolutionarily conserved mechanisms. We summarize the recent progress made in unraveling how ethylene promotes germination and acts as an antagonist of ABA. Far less is known about jasmonates in seeds for which we summarize the current knowledge about their role in seeds. While it seems very clear that jasmonates inhibit germination, the results obtained so far are partly contradictory and depend on future research to reach final conclusions on the mode of jasmonate action during seed germination. Understanding the mechanisms underlying the control of seed germination and its hormonal regulation is not only of academic interest, but is also the ultimate basis for further improving crop establishment and yield, and is therefore of common importance.

Myrigalone A inhibits Lepidium sativum seed germination by interference with gibberellin metabolism and apoplastic superoxide production required for embryo extension growth and endosperm rupture

Myrica gale L. (sweet gale) fruit leachate contains myrigalone A (MyA), a rare C-methylated dihydrochalcone and putative allelochemical, which is known to be a phytotoxin impeding seedling growth. We found that MyA inhibited Lepidium sativum L. seed germination in a dose-dependent manner. MyA did not affect testa rupture, but inhibited endosperm rupture and the transition to subsequent seedling growth. MyA inhibited micropylar endosperm cap (CAP) weakening and the increase in the growth potential of the radical/hypocotyl region (RAD) of the embryo, both being key processes required for endosperm rupture. We compared the contents of abscisic acid (ABA) and gibberellins in the tissues and found that the major bioactive forms of gibberellin in L. sativum seed tissues were GA(4) and GA(6), while GA(8) and GA(13) were abundant inactive metabolites. MyA did not appreciably affect the ABA contents, but severely interfered with gibberellin metabolism and signaling by inhibiting important steps catalyzed by GA3 oxidase, as well as by interfering with the GID1-type gibberellin signaling pathway. The hormonally and developmentally regulated formation of apoplastic superoxide radicals is important for embryo growth. Specific zones within the RAD were associated with accumulation of apoplastic superoxide radicals and endoreduplication indicative of embryo cell extension. MyA negatively affected both of these processes and acted as a scavenger of apoplastic reactive oxygen species. We propose that MyA is an allelochemical with a novel mode of action on seed germination.

Members of the gibberellin receptor gene family GID1 (GIBBERELLIN INSENSITIVE DWARF1) play distinct roles during Lepidium sativum and Arabidopsis thaliana seed germination

Germination of endospermic seeds is partly regulated by the micropylar endosperm, which acts as constraint to radicle protrusion. Gibberellin (GA) signalling pathways control coat-dormancy release, endosperm weakening, and organ expansion during seed germination. Three GIBBERELLIN INSENSITIVE DWARF1 (GID1) GA receptors are known in Arabidopsis thaliana: GID1a, GID1b, and GID1c. Molecular phylogenetic analysis of angiosperm GID1s reveals that they cluster into two eudicot (GID1ac, GID1b) groups and one monocot group. Eudicots have at least one gene from each of the two groups, indicating that the different GID1 receptors fulfil distinct roles during plant development. A comparative Brassicaceae approach was used, in which gid1 mutant and whole-seed transcript analyses in Arabidopsis were combined with seed-tissue-specific analyses of its close relative Lepidium sativum (garden cress), for which three GID1 orthologues were cloned. GA signalling via the GID1ac receptors is required for Arabidopsis seed germination, GID1b cannot compensate for the impaired germination of the gid1agid1c mutant. Transcript expression patterns differed temporarily, spatially, and hormonally, with GID1b being distinct from GID1ac in both species. Endosperm weakening is mediated, at least in part, through GA-induced genes encoding cell-wall-modifying proteins. A suppression subtraction hybridization (SSH) cDNA library enriched for sequences that are highly expressed during early germination in the micropylar endosperm contained expansins and xyloglucan endo-transglycosylases/hydrolases (XTHs). Their transcript expression patterns in both species strongly suggest that they are regulated by distinct GID1-mediated GA signalling pathways. The GID1ac and GID1b pathways seem to fulfil distinct regulatory roles during Brassicaceae seed germination and seem to control their downstream targets distinctly.

Three endo-β-mannanase genes expressed in the micropylar endosperm and in the radicle influence germination of Arabidopsis thaliana seeds

Mannans are hemicellulosic polysaccharides in the plant primary cell wall (CW). Mature seeds, specially their endosperm cells, have CWs rich in mannan-based polymers that confer a strong mechanical resistance for the radicle protrusion upon germination. The rupture of the seed coat and endosperm are two sequential events during the germination of Arabidopsis thaliana. Endo-β-mannanases (MAN; EC. 3.2.1.78) are hydrolytic enzymes that catalyze cleavage of β1 → 4 bonds in the mannan-polymer. In the genome of Arabidopsis, the endo-β-mannanase (MAN) family is represented by eight members. The expression of these eight MAN genes has been systematically explored in different organs of this plant and only four of them (AtMAN7, AtMAN6, AtMAN2 and AtMAN5) are expressed in the germinating seeds. Moreover, in situ hybridization analysis shows that their transcript accumulation is restricted to the micropylar endosperm and to the radicle and this expression disappears soon after radicle emergence. T-DNA insertion mutants in these genes (K.O. MAN7, K.O. MAN6, K.O. MAN5), except that corresponding to AtMAN2 (K.O. MAN2), germinate later than the wild type (Wt). K.O. MAN6 is the most affected in the germination time course with a t (50) almost double than that of the Wt. These data suggest that AtMAN7, AtMAN5 and specially AtMAN6 are important for the germination of A. thaliana seeds by facilitating the hydrolysis of the mannan-rich endosperm cell walls.