Conjugates of abscisic acid, brassinosteroids, ethylene, gibberellins, and jasmonates

Phytohormones, including auxins, abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellins, and jasmonates, are involved in all aspects of plant growth, and developmental processes as well as environmental responses. However, our understanding of hormonal homeostasis is far from complete. Phytohormone conjugation is considered as a part of the mechanism to control cellular levels of these compounds. Active phytohormones are changed into multiple forms by acylation, esterification or glycosylation, for example. It seems that conjugated compounds could serve as pool of inactive phytohormones that can be converted to active forms by de-conjugation reactions. Some conjugates are thought to be temporary storage forms, from which free active hormones can be released after hydrolysis. It is also believed that conjugation serves functions, such as irreversible inactivation, transport, compartmentalization, and protection against degradation. The nature of abscisic acid, brassinosteroid, ethylene, gibberellin, and jasmonate conjugates is discussed.     

Antagonism between abscisic acid and gibberellins is partially mediated by ascorbic acid during seed germination in rice

The antagonism between abscisic acid (ABA) and gibberellin (GA) plays a key role in controlling seed germination,^1,2 but the mechanism of antagonism during this process is not known. In the associated study,³ we investigated the relationship among ABA, reactive oxygen species (ROS), ascorbic acid (ASC) and GA during rice seed germination. ROS production is reduced by ABA, which hence results in decreasing ASC accumulation during imbibition. GA accumulation was also suppressed by a reduced ROS and ASC level, whereas application of exogenous ASC can partially rescue seed germination from ABA treatment. Further results show that production of ASC, which acts as a substrate in GA biosynthesis, was significantly inhibited by lycorine which thus suppressed the accumulation of GA. Consequently, expression of GA biosynthesis genes was suppressed by the low levels of ROS and ASC in ABA-treated seeds. These studies reveal a new role for ASC in mediating the antagonism between ABA and GA during seed germination in rice.     

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.     

Brassinosteroids and gibberellins promote tobacco seed germination by distinct pathways

Seed germination of Nicotiana tabacum L. cv. Havana 425 is determined by the balance of forces between the growth potential of the embryo and the mechanical restraint of the micropylar endosperm. In contrast to the gibberellin GA4, the brassinosteroid (BR) brassinolide (BL) did not release photodormancy of dark-imbibed photodormant seeds. Brassinolide promoted seedling elongation and germination of non-photodormant seeds, but did not appreciably affect the induction of class I beta-1,3-glucanase (betaGLU I) in the micropylar endosperm. Brassinolide, but not GA4, accelerated endosperm rupture of tobacco seeds imbibed in the light. Brassinolide and GA4 promoted endosperm rupture of dark-imbibed non-photodormant seeds, but only GA4 enhanced betaGLU I induction. Promotion of endosperm rupture by BL was dose-dependent and 0.01 microM BL was most effective. Brassinolide and GA4 promoted abscisic acid (ABA)-inhibited dark-germination of non-photodormant seeds, but only GA4 replaced light in inducing betaGLU I. These results indicate that BRs and GAs promote tobacco seed germination by distinct signal transduction pathways and distinct mechanisms. Gibberellins and light seem to act in a common pathway to release photodormancy, whereas BRs do not release photodormancy. Induction of betaGLU I in the micropylar endosperm and promotion of release of 'coat-enhanced' dormancy seem to be associated with the GA-dependent pathway, but not with BR signalling. It is proposed that BRs promote seed germination by directly enhancing the growth potential of the emerging embryo in a GA- and betaGLU I-independent manner.     

Nitrate,abscisic acid and gibberellin interactions on the thermoinhibition of lettuce seed germination

Germination of lettuce seeds has obvious thermoinhibition, but the mechanism for thermoinhibition of seed germination is poorly understood. Here, we investigated the interactions of nitrate, abscisic acid (ABA) and gibberellin on seed germination at high temperatures to understand further the mechanism for thermoinhibition of seed germination. Our results showed that lettuce (Lactuca sativa L. ‘Jianye Xianfeng No. 1’) seeds exhibited notable thermoinhibiton of germination at ≥17°C in darkness, and at ≥23°C in light, but the thermoinhibited seeds did not exhibit secondary dormancy. Thermoinhibition of seed germination at 23 or 25°C in light was notably decreased by 5 and 10 mM nitrate, and the stimulatory effects were markedly prevented by nitric oxide (NO) scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. The sensitivity of seed germination to exogenous ABA increased with increasing temperature. Thermoinhibition of seed germination was markedly decreased by fluridone (an inhibitor of ABA biosynthesis) and GA₃, and was increased by diniconazole (an inhibitor of the ABA-catabolizing enzyme ABA 8′-hydroxylase) and paclobutrazol (an inhibitor of GA biosynthetic pathway). The effect of fluridone in decreasing thermoinhibition of seed germination was obviously antagonized by paclobutrazol, and that of GA₃ was notably added to by fluridone, and that of nitrate was antagonized by paclobutrazol, diniconazole and ABA and was added to by GA₃ and fluridone. Our data show that thermoinhibition of lettuce seed germination is decreased by nitrate in a NO-dependent manner, which is antagonized by ABA, diniconazole and paclobutrazol and added by fluridone.     

Control of macaw palm seed germination by the gibberellin/abscisic acid balance

The hormonal mechanisms involved in palm seed germination are not fully understood. To better understand how germination is regulated in Arecaceae, we used macaw palm (Acrocomia aculeata (Jacq.) Lodd. Ex Mart.) seed as a model. Endogenous hormone concentrations, tocopherol and tocotrienol and lipid peroxidation during germination were studied separately in the embryo and endosperm. Evaluations were performed in dry (D), imbibed (I), germinated (G) and non‐germinated (NG) seeds treated (+GA₃) or not treated (control) with gibberellins (GA). With GA₃ treatment, seeds germinated faster and to a higher percentage than control seeds. The +GA₃ treatment increased total bioactive GA in the embryo during germination relative to the control. Abscisic acid (ABA) concentrations decreased gradually from D to G in both tissues. Embryos of G seeds had a lower ABA content than NG seeds in both treatments. The GA/ABA ratio in the embryo was significantly higher in G than NG seeds. The +GA₃ treatment did not significantly affect the GA/ABA ratio in either treatment. Cytokinin content increased from dry to germinated seeds. Jasmonic acid (JA) increased and 1‐aminocyclopropane‐1‐carboylic acid (ACC) decreased after imbibition. In addition, α‐tocopherol and α‐tocotrienol decreased, while lipid peroxidation increased in the embryo during germination. We conclude that germination in macaw palm seed involves reductions in ABA content and, consequently, increased GA/ABA in the embryo. Furthermore, the imbibition process generates oxidative stress (as observed by changes in vitamin E and MDA).     

The effect of drought on levels of abscisic acid,cytokinins, gibberellins and ethylene in aeroponically-grown sunflower plants

Abscisic acid (ABA), cytokinins and gibberellin-like substances (GAs) were extracted from the roots and shoots of 17-day-old sunflower seedlings which had been droughted or were unstressed. Plants were grown in an aeroponic chamber which allowed for good control over degree of water stress and easy access to roots. Following methanolic extraction of lyophilized material, cytokinins were separated from the acidic growth-regulators on a cellulose PO₄ cationic exchange column. The cytokinins were analysed by paper chromatography and HPLC and the soybean hypocotyl section assay. Semipurified acidic regulators were chromatographed on SiO₂ columns and HPLC and aliquots assayed with the dwarf rice cv. Tan-ginbozu bioassay for GAs. Fractions known to contain ABA were purified by sequential reverse-phase HPLC of the acid and then of the methyl ester forms followed by quantitation as Me-ABA on GLC-EC. ABA losses were measured by using an internal standard [³H]-ABA). Ethylene production was also monitored in stressed and unstressed seedlings.The effect of drought on GAs and ethylene was minimal. The ABA levels were markedly higher in droughted plants. Stressed roots had 32 times more ABA than controls. The levels of cytokinins in the shoots of droughted plants were about half those in unstressed shoots, and qualitative differences occurred in the roots. Under stress a large peak of activity was present similar to zeatin glucoside which was not present in the unstressed condition. The results are discussed in relation to drought-effects on metabolism.     

Influence of an abscisic acid (ABA) seed coating on seed germination rate and timing of Bluebunch Wheatgrass

Semi‐arid rangeland degradation is a reoccurring issue throughout the world. In the Great Basin of North America, seeds sown in the fall to restore degraded sagebrush (Artemisia spp.) steppe plant communities may experience high mortality in winter due to exposure of seedlings to freezing temperatures and other stressors. Delaying germination until early spring when conditions are more suitable for growth may increase survival. We evaluated the use of BioNik? (Valent BioSciences LLC) abscisic acid (ABA) to delay germination of bluebunch wheatgrass (Pseudoroegneria spicata). Seed was either left untreated or coated at five separate rates of ABA ranging from 0.25 to 6.0 g 100 g^?1 of seed. Seeds were incubated at five separate constant temperatures from 5 to 25°C. From the resultant germination data, we developed quadratic thermal accumulation models for each treatment and applied them to 4 years of historic soil moisture and temperature data across six sagebrush steppe sites to predict germination timing. Total germination percentage remained similar across all temperatures except at 25°C, where high ABA rates had slightly lower values. All ABA doses delayed germination, with the greatest delays at 5–10°C. For example, the time required for 50% of the seeds to germinate at 5°C was increased by 16–46 d, depending on the amount of ABA applied. Seed germination models predicted that the majority of untreated seed would germinate 5–11 weeks after a 15 October simulated planting date. In contrast, seeds treated with ABA were predicted to delay germination to late winter or early spring. These results indicate that ABA coatings may delay germination of fall planted seed until conditions are more suitable for plant survival and growth.     

Stimulation of lettuce seed germination by ethylene

Ethylene increased the germination of freshly imbibed lettuce (Lactuca sativa L. var. Grand Rapids) seeds. Seeds receiving either red or far-red light or darkness all showed a positive response to the gas. However, ethylene was apparently without effect on dormant seeds, those which failed to germinate after an initial red or far-red treatment. Carbon dioxide, which often acts as a competitive inhibitor of ethylene, failed to clearly reverse ethylene-enhanced seed germination. While light doubled ethylene production from the lettuce seeds, its effect was not mediated by the phytochrome system since both red and far-red light had a similar effect.     

Gibberellin requirement for Arabidopsis seed germination is determined both by testa characteristics and embryonic abscisic acid

The mechanisms imposing a gibberellin (GA) requirement to promote the germination of dormant and non-dormant Arabidopsis seeds were analyzed using the GA-deficient mutant ga1, several seed coat pigmentation and structure mutants, and the abscisic acid (ABA)-deficient mutant aba1. Testa mutants, which exhibit reduced seed dormancy, were not resistant to GA biosynthesis inhibitors such as tetcyclacis and paclobutrazol, contrarily to what was found before for other non-dormant mutants in Arabidopsis. However, testa mutants were more sensitive to exogenous GAs than the wild-types in the presence of the inhibitors or when transferred to a GA-deficient background. The germination capacity of the ga1-1 mutant could be integrally restored, without the help of exogenous GAs, by removing the envelopes or by transferring the mutation to a tt background (tt4 and ttg1). The double mutants still required light and chilling for dormancy breaking, which may indicate that both agents can have an effect independently of GA biosynthesis. The ABA biosynthesis inhibitor norflurazon was partially efficient in releasing the dormancy of wild-type and mutant seeds. These results suggest that GAs are required to overcome the germination constraints imposed both by the seed coat and ABA-related embryo dormancy.     

Quantitative models characterizing seed germination responses to abscisic Acid and osmoticum

Mathematical models were developed to characterize the physiological bases of the responses of tomato (Lycopersicon esculentum Mill. cv T5) seed germination to water potential (ψ) and abscisic acid (ABA). Using probit analysis, three parameters were derived that can describe the germination time courses of a seed population at different ψ or ABA levels. For the response of seed germination to reduced ψ, these parameters are the mean base water potential (¯ψ_b, MPa), the standard deviation of the base water potential among seeds in the population (σ_ψb, MPa), and the “hydrotime constant” (θ_H, MPa·h). For the response to ABA, they are the log of the mean base ABA concentration ([unk]ABA_b, m), the standard deviation of the base ABA concentration among seeds in the population (σ_ABAb, log[m]), and the “ABA-time constant” (θ_ABA, log[m]·h). The values of ¯ψ_b and [unk]ABA_b provide quantitative estimates of the mean sensitivity of germination rate to ψ or ABA, whereas σ^ψ_b and σ_ABAb account for the variation in sensitivity among seeds in the population. The time constants, θ_H and θ_ABA, indicate the extent to which germination rate will be affected by a given change in ψ or ABA. Using only these parameters, germination time courses can be predicted with reasonable accuracy at any medium ψ according to the equation probit(g) = [ψ - (θ_H/t_g) - ¯ψ_b]/σ_ψb, or at any ABA concentration according to the equation probit(g) = [log[ABA] - (θ_ABA/t_g) - log[[unk]ABA_b]]/σ_ABAb, where t_g is the time to radicle emergence of percentage g, and ABA is the ABA concentration (m) in the incubation solution. In the presence of both ABA and reduced ψ, the same parameters can be used to predict seed germination time courses based upon strictly additive effects of ψ and ABA in delaying the time of radicle emergence. Further analysis indicates that ABA and ψ can act both independently and interactively to influence physiological processes preparatory for radicle growth, such as the accumulation of osmotic solutes in the embryo. The models provide quantitative values for the sensitivity of germination to ABA or ψ, allow evaluation of independent and interactive effects of the two factors, and have implications for understanding how ABA and ψ may regulate growth and development.     

Interactions between abscisic acid and ethylene signaling cascades

We screened for mutations that either enhanced or suppressed the abscisic acid (ABA)-resistant seed germination phenotype of the Arabidopsis abi1-1 mutant. Alleles of the constitutive ethylene response mutant ctr1 and ethylene-insensitive mutant ein2 were recovered as enhancer and suppressor mutations, respectively. Using these and other ethylene response mutants, we showed that the ethylene signaling cascade defined by the ETR1, CTR1, and EIN2 genes inhibits ABA signaling in seeds. Furthermore, epistasis analysis between ethylene- and ABA-insensitive mutations indicated that endogenous ethylene promotes seed germination by decreasing sensitivity to endogenous ABA. In marked contrast to the situation in seeds, ein2 and etr1-1 roots were resistant to both ABA and ethylene. Our data indicate that ABA inhibition of root growth requires a functional ethylene signaling cascade, although this inhibition is apparently not mediated by an increase in ethylene biosynthesis. These results are discussed in the context of the other hormonal regulations controlling seed germination and root growth.     

Effect of low-temperature stress on abscisic acid,jasmonates, and polyamines in apples

The effect of low temperatures on polyamines, jasmonates, abscisic acid (ABA), and antioxidant activities was investigated in apple fruitlets. Although endogenous ABA concentrations were not significantly different between untreated control fruit kept at −2°C and those kept at 20°C, endogenous jasmonic acid (JA), putrescine, and spermidin concentrations at −2°C were generally higher than those at 20°C. Endogenous ABA concentrations increased in n-propyl dihydrojasmonate (PDJ)—or spermine-treated fruit in comparison to the untreated control at 20 and −2°C. The applications of PDJ or spermine decreased low-temperature injuries such as splitting and spotting in fruit. Although the IC₅₀ of 1,1-diphenil-2-pycrylhydrazyl (DPPH)-radical scavenging activities was not significantly different among the treatments, the IC₅₀ of O₂ ⁻-scavenging activities in PDJ-treated or Spm-treated fruit at 5 days after the low-temperature treatment was lower than in the untreated control at 20 and −2°C. The expression of MdCHS increased in Spm-treated fruit. The concentrations of ascorbic acid, catechin, chlorogenic acid, epi-catechin, and phloridzin in Spm-treated fruit were higher than in the untreated control at −2 or 20°C. These facts suggest that ABA, jasmonates and polyamines may be associated with low-temperature stress tolerance in apple fruitlets.     

The regulator of G-protein signaling proteins involved in sugar and abscisic acid signaling in Arabidopsis seed germination

The regulator of G-protein signaling (RGS) proteins, recently identified in Arabidopsis (Arabidopsis thaliana; named as AtRGS1), has a predicted seven-transmembrane structure as well as an RGS box with GTPase-accelerating activity and thus desensitizes the G-protein-mediated signaling. The roles of AtRGS1 proteins in Arabidopsis seed germination and their possible interactions with sugars and abscisic acid (ABA) were investigated in this study. Using seeds that carry a null mutation in the genes encoding RGS protein (AtRGS1) and the alpha-subunit (AtGPA1) of the G protein in Arabidopsis (named rgs1-2 and gpa1-3, respectively), our genetic evidence proved the involvement of the AtRGS1 protein in the modulation of seed germination. In contrast to wild-type Columbia-0 and gpa1-3, stratification was found not to be required and the after-ripening process had no effect on the rgs1-2 seed germination. In addition, rgs1-2 seed germination was insensitive to glucose (Glc) and sucrose. The insensitivities of rgs1-2 to Glc and sucrose were not due to a possible osmotic stress because the germination of rgs1-2 mutant seeds showed the same response as those of gpa1-3 mutants and wild type when treated with the same concentrations of mannitol and sorbitol. The gpa1-3 seed germination was hypersensitive while rgs1-2 was less sensitive to exogenous ABA. The different responses to ABA largely diminished and the inhibitory effects on seed germination by exogenous ABA and Glc were markedly alleviated when endogenous ABA biosynthesis was inhibited. Hypersensitive responses of seed germination to both Glc and ABA were also observed in the overexpressor of AtRGS1. Analysis of the active endogenous ABA levels and the expression of NCED3 and ABA2 genes showed that Glc significantly stimulated the ABA biosynthesis and increased the expression of NCED3 and ABA2 genes in germinating Columbia seeds, but not in rgs1-2 mutant seeds. These data suggest that AtRGS1 proteins are involved in the regulation of seed germination. The hyposensitivity of rgs1-2 mutant seed germination to Glc might be the result of the impairment of ABA biosynthesis during seed germination.     

Effect of jasmonic acid on endogenous gibberellins and abscisic acid in rice under NaCl stress

Content of endogenous abscisic acid (ABA) increased in rice plants under salt stress. Pre- or post-treatment by jasmonic acid (JA) mostly further increased ABA content. In the presence of salt stress also content of gibberellins (GAs) mostly increased more after treatment by JA. Endogenous content of bioactive GA₁ was higher in post-treatment by JA than in pre-treatment by JA.This study was supported by the Korea Science and Engineering Foundation (2000-2-20100-001-3)     

Ascorbic acid and reactive oxygen species are involved in the inhibition of seed germination by abscisic acid in rice seeds

The antagonism between abscisic acid (ABA) and gibberellin (GA) plays a key role in controlling seed germination, but the mechanism of antagonism during this process is not known. The possible links among ABA, reactive oxygen species (ROS), ascorbic acid (ASC), and GA during rice seed germination were investigated. Unlike in non-seed tissues where ROS production is increased by ABA, ABA reduced ROS production in imbibed rice seeds, especially in the embryo region. Such reduced ROS also led to an inhibition of ASC production. GA accumulation was also suppressed by a reduced ROS and ASC level, which was indicated by the inhibited expression of GA biosynthesis genes, amylase genes, and enzyme activity. Application of exogenous ASC can partially rescue seed germination from ABA treatment. Production of ASC, which acts as a substrate in GA biosynthesis, was significantly inhibited by lycorine which thus suppressed the accumulation of GA. Consequently, expression of GA biosynthesis genes was suppressed by the low levels of ROS and ASC in ABA-treated seeds. It can be concluded that ABA regulates seed germination in multiple dimensions. ROS and ASC are involved in its inhibition of GA biosynthesis.     

Metabolic control of seed germination

We have used proteomics to better characterize germination and early seedling vigor in sugarbeet. Our strategy includes (1) construction of proteome reference maps for dry and germinating seeds of a high-vigor reference seed lot; (2) investigation of the specific tissue accumulation of proteins (root, cotyledon, perisperm); (3) investigation of changes in protein expression profiles detected in the reference seed lot subjected to different vigor-modifying treatments, e.g. aging and/or priming. More than 1 000 sugarbeet seed proteins have been identified by LC/MS-MS mass spectrometry (albumins, globulins and glutelins have been analyzed separately). Due to the conservation of protein sequences and the quality of MS sequencing (more than 10 000 peptide sequences have been obtained), the success rate of protein identification was on the average of 80%. This is to our knowledge the best detailed proteome analysis ever carried out in seeds. The data allowed us to build a detailed metabolic chart of the sugarbeet seed, generating new insights into the molecular mechanisms determining the development of a new seedling. Also, the proteome of a seed-storage tissue as the perisperm is described for the first time.     

The reciprocal regulation of abscisic acid and ethylene biosyntheses

Ethylene and abscisic acid (ABA) have compact effects on plant development and stress responses. It is not well understood about the mechanism of ABA modulation in ethylene biosynthesis. In our recent research, HY5-AtERF11 regulon was evidenced to connect the ABA action and ethylene biosynthesis. In this paper, by analyzing the expression of ABA biosynthesis genes and the ABA concentration in ethylene over-production mutants, we demonstrated that ethylene production affected by HY5-AtERF11 regulon targeted gene increased the expression of ABA biosynthesis genes and its contents. In addition, we discussed that HY5 might function as a convergence point of multiple hormones in response to light.