JXB Advance Access originally published online on March 17, 2006
Journal of Experimental Botany 2006 57(6):1423-1430; doi:10.1093/jxb/erj122
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RESEARCH PAPER |
Hypoxia interferes with ABA metabolism and increases ABA sensitivity in embryos of dormant barley grains
1IFEVA-Cátedra de Cerealicultura Facultad de Agronomia, Universidad de Buenos Aires/CONICET, Av. San Martin 4453, 1417 Buenos Aires, Argentina
2Université Pierre et Marie Curie-Paris 6, Physiologie Végétale Appliquée, EA 2388 Physiologie des semences, Site d'Ivry, Boîte 152, 4 Place Jussieu, F-75252 Paris cedex 05, France
* To whom correspondence should be addressed. E-mail: benech{at}ifeva.edu.ar
Received 30 September 2005; Accepted 13 January 2006
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Abstract |
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Two mechanisms have been suggested as being responsible for dormancy in barley grain: (i) ABA in the embryo, and (ii) limitation of oxygen supply to the embryo by oxygen fixation as a result of the oxidation of phenolic compounds in the glumellae. The aim of the present work was to investigate whether hypoxia imposed by the glumellae interferes with ABA metabolism in the embryo, thus resulting in dormancy. In dormant and non-dormant grains incubated at 20 °C and in non-dormant grains incubated at 30 °C (i.e. when dormancy is not expressed), ABA content in the embryo decreased dramatically during the first 5 h of incubation before germination was detected. By contrast, germination of dormant grains was less than 2% within 48 h at 30 °C and embryo ABA content increased during the first hours of incubation and then remained 2–4 times higher than in embryos from grains in which dormancy was not expressed. Removal of the glumellae allowed germination of dormant grains at 30 °C and the embryos did not display the initial increase in ABA content. Incubation of de-hulled grains under 5% oxygen to mimic the effect of glumellae, restored the initial increase ABA in content and completely inhibited germination. Incubation of embryos isolated from dormant grains, in the presence of a wide range of ABA concentrations and under various oxygen tensions, revealed that hypoxia increased embryo sensitivity to ABA by 2-fold. This effect was more pronounced at 30 °C than at 20 °C. Furthermore, when embryos from dormant grains were incubated at 30 °C in the presence of 10 µM ABA, their endogenous ABA content remained constant after 48 h of incubation under air, while it increased dramatically in embryos incubated under hypoxia, indicating that the apparent increase in embryo ABA responsiveness induced by hypoxia was, in part, mediated by an inability of the embryo to inactivate ABA. Taken together these results suggest that hypoxia, either imposed artificially or by the glumellae, increases embryo sensitivity to ABA and interferes with ABA metabolism.
Key words: Abscisic acid, barley, glumellae, hypoxia, seed dormancy
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Introduction |
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Barley is one of the most important grain crops in the world. The barley grain is mostly used for malting, an industrial process which requires grain germination. For that reason, the presence of dormancy in the barley grain is a major constraint for its industrialization. Barley cultivars with low dormancy at harvest are usually welcomed by the industry because the grain can be malted soon after harvest, thus avoiding costs and deterioration derived from long-term storage until dormancy is terminated (Corbineau and Côme, 1996
; Benech-Arnold, 2004). On the other hand, barley cultivars that lose dormancy prior to harvest are prone to sprout. Therefore, the possibility of adjusting the timing of exit from dormancy to a narrow and precise time window (i.e. neither too early so as to subject the crop to the risk of sprouting, nor too late so as to have to store the grain until the germination capacity has been acquired) requires a thorough knowledge of the physiological mechanisms involved in dormancy imposition and maintenance.
Dormancy of the barley grain is typically imposed by the seed-covering structures (lemma and palea, pericarp plus seed coat). Indeed, embryos can germinate well from the very early stages of development if they are isolated from the rest of the grain and incubated in water (Benech-Arnold et al., 1999). Limitation of oxygen supply to the embryo by oxygen fixation as a result of oxidation of phenolic compounds in the lemma and palea (hereafter referred to as the glumellae or the hull) has been suggested to be responsible for the dormancy of dressed caryopses of cereals such as barley (Lenoir et al., 1986) and oat (Corbineau et al., 1986). In dormant grains of barley, for example, whole intact caryopses germinated with difficulty, even in the air, while de-hulled caryopses were all able to germinate under oxygen tensions of at least 10%, suggesting that oxygen concentration under the covering structures might be less than 10% (Lenoir et al., 1986). Calculations taking into account the diffusion coefficient of oxygen through the seed coats of various seeds, the thickness of the latter and the respiratory activity of the embryo have shown that the embryos enclosed in their seed coats are subjected to hypoxia (Edwards, 1973). Dormancy of barley and other cereals from temperate climates can be regarded as a relative phenomenon, the expression of which depends on the incubation temperature. It is usually not expressed at relatively low temperatures (10–20 °C) while its expression increases as the temperature rises (Lenoir et al., 1983; Corbineau and Côme, 1996; Benech-Arnold, 2004).
The phytohormone abscisic acid (ABA) has largely been implicated in both the imposition and the maintenance of dormancy (Fong et al., 1983; Karssen et al., 1983; Walker-Simmons, 1987; Bewley, 1997; Steinbach et al., 1997). The role of ABA in dormancy imposition is clear from the low dormancy displayed by freshly harvested seeds of ABA-deficient or -insensitive mutants of Arabidopsis and other species whose seeds normally present primary dormancy. Moreover, application of the ABA-synthesis inhibitor, fluridone, has been shown to anticipate the release from dormancy in developing seeds of a number of species (Fong et al., 1983; Le Page-Degivry and Garello, 1992; Steinbach et al., 1997). The role of ABA in dormancy maintenance is a bit more controversial, although some reports have shown that dormancy expression in seeds of some species is associated with ABA de novo synthesis and/or a lower rate of ABA inactivation upon imbibition, which does not occur in seeds that do not express dormancy (Le Page-Degivry and Garello, 1992; Bianco et al., 1997; Le Page-Degivry et al., 1997; Yoshioka et al., 1998; Grappin et al., 2000). Dormancy of the barley grain also appears to be under ABA control: termination of glumellae-imposed dormancy during grain development was shown to be correlated with a sharp decline both in ABA embryonic content and sensitivity (Benech-Arnold et al., 1999). A role for ABA in dormancy maintenance of the barley grain has also been suggested: ABA embryonic content declines during the first hours of incubation of non-dormant seeds, whereas it remains at high levels in embryos of dormant grains (Wang et al., 1998). However, most functional aspects of this role of ABA in dormancy maintenance in barley have not been explored so far. Although it is known that incubation temperature can affect embryo responsiveness to ABA (Walker-Simmons, 1988; Poljakoff-Mayber et al., 1990; Corbineau and Côme, 2000), it is not known whether the differential expression of dormancy with temperature is also mediated by a different rate of ABA inactivation (i.e. catabolism/conjugation). The way in which oxygen influences germination of dormant seeds is largely unknown, but it has been proposed that oxygen concentration might determine the rate with which germination inhibitors or ABA are catabolized (Neill and Horgan, 1987; Barthe et al., 2000). If indeed ABA is implicated in dormancy maintenance, it could be hypothesized that, under conditions that allow dormancy expression (i.e. incubation at high temperatures) the hypoxia condition imposed by the glumellae is instrumental for such expression by interfering with ABA inactivation and, perhaps, by enhancing embryo sensitivity to the hormone.
In this paper these possibilities are explored. A correlation was determined between dormancy expression (i.e. through incubation at different temperatures) and ABA content evolution pattern during grain incubation in hulled and de-hulled grains. In addition, through the incubation of de-hulled grains under hypoxia, the effect of the glumellae was mimicked both in the expression of dormancy and in the evolution of ABA content, thus suggesting that glumellae interfere with ABA inactivation through oxygen deprivation. It was also tested whether the hypoxic condition imposed on the embryo can modify its responsiveness to ABA.
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Materials and methods |
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Plant material
Barley (Hordeum vulgare L., cv. Pewter) grains harvested in 2002 and received from the ‘Coopérative Agricole de Toury’ (western France) were used throughout this study. Experiments were carried out with dormant grains, which were maintained at –20 °C from harvest until the experiments began in order to maintain their initial dormancy (Lenoir et al., 1983
), and non-dormant grains, which were stored dry in the open air for at least 3 months at 25 °C in order to break their dormancy (Corbineau and Côme, 1996).
Germination assays
Germination assays were carried out using whole grains, de-hulled grains (seeds without glumellae), and isolated embryos which had been separated from the endosperm of dry seeds by a sharp scalpel blade, with as little endosperm as possible. Both types of grains, and embryos were germinated in 9 cm Petri dishes (50 individuals per dish, two replicates) on a layer of cotton wool imbibed with deionized water. To test for embryo sensitivity to ABA, isolated embryos were incubated in aqueous solutions of abscisic acid (mixed isomers, Sigma) at the concentrations indicated in the text, ranging from 0 mM (water) to 1 mM. Assays were performed in darkness at 20 °C, a temperature at which dormant and non-dormant grains germinated, and 30 °C, a temperature at which only non-dormant grains were able to germinate.
Germination in atmospheres with different controlled oxygen tensions was performed at both temperatures in darkness, using the procedure of Côme and Tissaoui (1968). Gas mixtures containing 5–21% oxygen were obtained through capillary tubes from compressed air and nitrogen. The gaseous atmospheres were passed continuously through germination chambers at a constant flow rate (4.0 l h–1). The oxygen tensions were measured daily using a Servomex analyser (Servomex, type 570A).
A grain or an isolated embryo was regarded as germinated when the radicle had pierced the coats in de-hulled or whole grains, or had reached 2–3 mm in length in isolated embryos. Germination counts were made every day during 7 d. The results presented are the means of the germination percentages obtained after various durations in two replicates ±arithmetical spread.
ABA quantitation
In parallel with all the germination assays carried out, batches of whole seeds, de-hulled grains and embryos were incubated and sampled at different times after incubation. Thirty embryos were dissected from the rest of the seed structure (except when isolated embryos were incubated) and immediately frozen in liquid N2. The embryos were then lyophilized, powdered, weighed, and stored at –30 °C until assayed for ABA content with a radioimmunoassay as described elsewhere (Steinbach et al., 1995). The results presented are the means of three measurements carried out in duplicate ±standard deviation.
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Results |
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Germination and embryo ABA content of dormant and non-dormant whole grains incubated at 20 °C and 30 °C
Figure 1a shows the germination, as a function of time, of dormant and non-dormant whole grains at 20 °C and 30 °C. Both dormant and non-dormant grains germinated easily at 20 °C, although dormant grains reached a somewhat lower germination percentage (76%) than non-dormant ones (100%) and also displayed a slower germination rate. Maximum germination percentage was reached in less than 24 h by non-dormant grains while it took 48 h in the case of dormant ones. By contrast, more than 80% of non-dormant grains were able to germinate when incubated at 30 °C, while almost no dormant grain germinated at this temperature. Embryo ABA content throughout the incubation period also depended on grain dormancy level and the incubation temperature (Fig. 1b). While embryo ABA content declined sharply a few hours after incubation in embryos from grains that germinated (i.e. non-dormant grains incubated at 20 °C and 30 °C, and dormant grains incubated at 20 °C), it increased during the first hours of incubation at 30 °C in dormant grains and decreased smoothly afterwards. Twenty-four hours after incubation, ABA content in embryos from dormant grains incubated at 30 °C was between 2 and 4 times higher than that measured in embryos from dormant and non-dormant grains incubated at 20 °C and non-dormant grains incubated at 30 °C. ABA depletion was evident 6 h after the initiation of incubation, well before any visible germination was detected. The close association found between germination behaviour and the changes in embryo ABA content during incubation, suggests a role for ABA in dormancy maintenance of the barley grain.
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Germination and embryo ABA content of de-hulled dormant grains incubated at 30 °C in the air
Since incubation at 30 °C allowed the expression of dormancy in dormant grains, and this expression appeared to be mediated by changes in ABA content that are substantially different from that determined in embryos from grains in which dormancy is not expressed, it was investigated whether the glumellae interfered with germination through ABA metabolism. Therefore, intact and de-hulled dormant grains were incubated at 30 °C and embryo ABA content was followed throughout incubation.
Removal of the glumellae strongly reduced the inhibition of germination imposed by incubation at 30 °C; de-hulled grains reached almost 100% germination after about 5–6 d of incubation (Fig. 2a), whereas intact grains almost did not germinate throughout the experimental period (Fig. 1a). Changes in ABA content during incubation were also different between de-hulled and intact grains: although ABA embryo content did not decrease as sharply as observed in dormant and non-dormant whole grains incubated at 20 °C and in non-dormant ones incubated at 30 °C (Fig. 1b), an initial increase in ABA content was not detected in de-hulled dormant grains (Fig. 2b) as observed in intact grains (Fig. 1b). After 24 h of incubation, ABA content in embryos from intact dormant grains (Fig. 1b) remained similar (around 350 pg mg–1 DW) to that measured in embryos from de-hulled grains (Fig. 2b). These results strongly suggest that the glumellae, which are involved in the expression of dormancy of dormant grains incubated at 30 °C, interfere with ABA metabolism.
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Germination and embryo ABA content of de-hulled dormant grains incubated at 30 °C under hypoxia
To determine whether the interference of the glumellae with grain germination and ABA inactivation was through the hypoxia condition imposed on the embryo, de-hulled dormant grains were incubated under 5% oxygen and both germination and ABA content were followed throughout incubation.
Incubation of de-hulled dormant grains under 5% oxygen suppressed germination at 30 °C (Fig. 2a) resembling the behaviour of intact dormant grains incubated in air at this temperature (Fig. 1a). Whereas the ABA content in embryos from de-hulled grains decreased steadily throughout incubation in the air (Fig. 2b), changes in ABA content in embryos from de-hulled grains incubated under 5% oxygen was similar to that of embryos from intact dormant grains incubated at 30 °C in air as described in Fig. 1b: ABA content increased immediately after incubation, stayed high until 16 h and then decreased, reaching values similar to those observed in de-hulled grains incubated under 21% oxygen (Fig. 2b).
These results show that incubation under hypoxia of de-hulled dormant grains can mimic the presence of the glumellae in terms of both germination of the grains and changes in ABA content. Thus, it seems likely that interference with grain germination and ABA metabolism exerted by the presence of the glumellae is through the hypoxia condition imposed on the embryo by this structure.
Effects of oxygen supply on embryo responsiveness to ABA
The possibility was explored that hypoxia, in addition to interfering with ABA inactivation, can modify embryo responsiveness to ABA. With this aim, embryos isolated from dormant and non-dormant grains were incubated under a range of ABA concentrations, at 20 °C and 30 °C and under different oxygen tensions.
Embryos from dormant grains were able to germinate under a wide range of ABA concentrations if they were incubated at 20 °C and under 21% oxygen; only when ABA concentration was as high as 1 mM did the hormone produce some degree of inhibition (Fig. 3a). Reducing the oxygen concentration to 10% increased embryo sensitivity 2-fold; an ABA concentration of 0.1 mM was sufficient to suppress germination almost completely (Fig. 3a).
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Incubation at 30 °C markedly increased embryo sensitivity to ABA and the effect of hypoxia was even more evident than at 20 °C. When incubation was performed under 21% oxygen, embryo germination was suppressed with 1 mM ABA and highly inhibited with 0.1 mM ABA (Fig. 3b). However, if incubation was carried out under 15% and 10% oxygen, embryo responsiveness to ABA increased more than 1- and 2-fold, respectively (Fig. 3b). For example, with embryos incubated under 10% oxygen, an ABA concentration as low as 0.01 mM (10 µM) was nearly sufficient to suppress germination and it was severely inhibited with 0.001 mM ABA (Fig. 3b). As in de-hulled dormant grains (Fig. 2a) the germination of isolated embryos was strongly inhibited under 5% oxygen (Fig. 3b) regardless of ABA concentration.
Embryos from non-dormant grains were tested at 30 °C only. Overall, embryo responsiveness to ABA was lower than in embryos coming from dormant grains and the effect of hypoxia was also to a lesser extent than observed in those embryos. Embryo germination was maximum with ABA concentrations up to 0.1 mM and germination was only partially reduced at 1 mM ABA if incubation was performed under 21% oxygen (Fig. 4). When embryos were incubated under 10% and 5% oxygen, germination was partially inhibited (more than 50% embryos germinated) at ABA concentrations of 0.1 and 0.01 mM, respectively (Fig. 4).
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These results demonstrate that hypoxic conditions markedly increased embryo responsiveness to ABA and that this effect is partially overcome by afterripening.
To investigate the extent to which this increase in embryo responsiveness to ABA caused by hypoxia was in fact the result of hypoxia interference with ABA inactivation, embryos isolated from dormant grains were incubated in the presence of 0.01 mM ABA, at 20 °C and 30 °C and under 21% or 5% oxygen, and embryo ABA content was determined at different times after incubation. Embryos incubated at 20 °C displayed an initial increase in ABA content (i.e. until 6 h of incubation) which was evident both under 21% and 5% oxygen (Fig. 5a). After this increase, and irrespective of oxygen concentration of the atmosphere, ABA content stabilized around 1000 pg ABA mg–1 dry weight until the end of the experiment (i.e. 48 h of incubation under 5% oxygen and 24 h under 21% oxygen since embryos germinated at that time in this condition) (Fig. 5a). By contrast, changes in ABA content in embryos incubated at 30 °C depended on the oxygen concentration of the atmosphere: while in embryos incubated under 21% oxygen this pattern was similar to that observed in embryos incubated at 20 °C (i.e. an initial increase followed by stabilization around 1000 pg ABA mg–1 dry weight), embryos incubated under 5% oxygen appeared to have lost their ability to inactivate ABA beyond 24 h of incubation (Fig. 5b). Indeed, when measured after 48 h of incubation, ABA content had increased almost five times with respect to the value measured after 24 h of incubation.
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Taken together, these results suggest that part of the enhanced responsiveness to ABA observed in embryos from dormant grains, incubated at 30 °C and under hypoxia, might be due to an interference of hypoxia in the capacity of the embryo to inactivate ABA. By contrast, the increase in sensitivity to ABA displayed by embryos incubated under hypoxia but at 20 °C, would not include this interference on ABA inactivation.
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Discussion |
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Barley grains exhibited a dormancy, which was expressed at high temperature (30 °C), but hardly at all at 20 °C (Fig. 1a). As in other cereal seeds, this dormancy expression mainly resulted from an inhibitory action of the glumellae, since de-hulled grains germinated at 30 °C (Fig. 2a). It was suggested by Lenoir et al. (1983)
that the covering structures (glumellae adhering to the grain and pericarp+seed coat) prevent germination by placing the embryo under hypoxia, because they fixed oxygen through polyphenol oxidase-mediated oxidation of phenolic compounds present in high amounts (Lenoir et al., 1986). Even though incubation under high oxygen tensions (i.e. >21%) did not overcome the inhibition to germination imposed by the glumellae, this failure was attributed to a concomitant increase in polyphenol oxidase activity in the hull with oxygen concentrations higher than 21% (Lenoir et al., 1986). Only incubation in the presence of a strong oxidant like hydrogen peroxide can overcome hull-imposed dormancy (Wang et al., 1998). This barrier to oxygen supply to the embryo might play a role in the metabolic regulation of the embryo (Lecat et al., 1992; Corbineau and Côme, 1996, 2003) or hormonal regulation by preventing ABA leaching into the medium (Bianco et al., 1997; Wang et al., 1998) or by interfering with ABA inactivation (Neill and Horgan, 1987; Barthe et al., 2000). However, the inability of dormant seeds placed at 30 °C to germinate did not result from an inability of the embryo to synthesize ATP, since the energy charge is similar in dormant and non-dormant grains until germination occurred (Lecat et al., 1992; Corbineau and Côme, 2003).
Although the embryo ABA content was similar in dry dormant and non-dormant grains, these results clearly demonstrated that a relationship exists between the dormancy level of the barley grain and the evolution of embryo ABA content during incubation (Fig. 1b). The ABA level of the embryo largely decreased during the first hours of incubation and prior to any visible germination in both dormant and non-dormant grains at 20 °C, the temperature at which dormancy is not expressed, and in non-dormant grains incubated at 30 °C, suggesting that ABA catabolism and/or conjugation exceeds ABA biosynthesis. By contrast, at 30 °C dormancy expression was associated with a maintenance of ABA at high levels: after an initial increase, ABA content decreased very smoothly and was always between 2- and 4-fold higher than in embryos from grains in which dormancy was not expressed (Fig. 1b). It appears, then, that a high dormancy level, together with incubation at high temperatures (i.e. 30 °C), promotes ABA biosynthesis and prevents ABA inactivation through metabolism and/or conjugation. The possibility of de novo ABA synthesis upon incubation of dormant grains at 30 °C is supported by the observation that germination at this temperature was improved in the presence of fluridone, an ABA synthesis inhibitor (data not shown). On the other hand, the difference with respect to ABA content in embryos from grains that did not strongly express dormancy, might result from a reduction of ABA inactivation, either through catabolism or conjugation. In most plants, inactivation of ABA occurs via the oxidative pathway, the key step being the hydroxylation of the 8' methyl group of ABA through the action of 8'-hydroxylase (Saito et al., 2004). For example, in Arabidopsis, this enzyme is known to play a major role in the decline of ABA content during seed imbibition (Kushiro et al., 2004).
Both the removal of the glumellae and the incubation of de-hulled grains under low oxygen concentrations modified grain germination behaviour and ABA content evolution throughout incubation at 30 °C (Fig. 2). Although the potential multiplicity of factors might contribute to the regulation of germination, these results indicate that the presence of the glumellae is instrumental for dormancy maintenance because it imposes oxygen deprivation to the embryo which, in turn, promotes ABA synthesis and/or inhibits ABA inactivation. Indeed, removal of the glumellae in dormant grains incubated at 30 °C under 21% oxygen (Fig. 2B) suppressed the initial increase in ABA content that was observed in dressed dormant grains incubated at the same temperature (Fig. 2b). By contrast, incubation under hypoxia (5% oxygen) of de-hulled grains restored it (Fig. 2b) and inhibited germination (Fig. 2a). The physiological significance of this initial peak is not known, but its existence seemed to be closely associated with dormancy maintenance. Although evidence exists showing that hypoxia can impair ABA catabolism, the authors are not aware of any previous report showing that hypoxia can promote ABA biosynthesis (Xiong and Zhu, 2003; Nambara and Marion-Poll, 2005). Hypoxia interference with ABA catabolism could be through the regulation of ABA 8'-hydroxylase activity. However, evidence exists showing that the enzyme ABA 8'-hydroxylase works in a wide range of temperatures and oxygen availabilities (Krochko et al., 1998). Therefore, it could be that temperature and oxygen concentration regulate the functioning of this enzyme at the level of expression of the gene that codifies it. The authors current efforts are devoted to clone this gene in barley and to study its expression under different temperatures and oxygen supplies as related to grain dormancy. The biochemical and molecular bases of afterripening are largely unknown, but it seems from these results that breaking of dormancy results in a loss of sensitivity of ABA metabolism to the temperature of incubation and to oxygen deprivation.
Involvement of embryo responsiveness to ABA in the regulation of seed dormancy has been reported for various cereals, such as wheat (Walker-Simmons, 1987; Corbineau et al., 2000), barley (Wang et al., 1995; Benech-Arnold et al., 1999; Corbineau and Côme, 2000), oat (Corbineau et al., 1991), and sorghum (Steinbach et al., 1995) and other species (Groot and Karssen, 1992; Le Page-Degivry et al., 1996; Corbineau et al., 2002). It is also known to be modulated by incubation temperature as reported by other authors (Poljakoff-Mayber et al., 1990; Corbineau et al., 1993) and our results (Fig. 3). In this paper it is clearly demonstrated that embryo sensitivity to ABA can also be affected by oxygen availability (Figs 3, 4). Incubation under low oxygen tensions enhanced the inhibitory effect of ABA on embryo germination by several degrees of magnitude. To the best of our knowledge, this is the first time that such an effect is reported. These results suggest that, in addition to interference with ABA metabolism, the presence of the glumellae increases embryo responsiveness to the phytohormone. It is also demonstrated that, in embryos coming from dormant grains and incubated at 30 °C, this apparent increase in embryo sensitivity to ABA is, in part, mediated by an inability of the embryo to inactivate ABA. Consistently, with results reported by Grappin et al. (2000) in Nicotiana plumbaginifolia and by Le Page-Degivry et al. (1996) in sunflower, the results seem to indicate that there exists a correlation between ABA responsiveness and capacity for ABA biosynthesis and metabolism.
Overall, the results reported in this paper shed new light on the role of ABA in dormancy maintenance in the barley grain, and on the way its synthesis and catabolism is modulated by the environment imposed to the embryo by the surrounding structures. Since germination was scored as radicle elongation, the possibility cannot be ruled out that hypoxia imposed by the glumellae, even if it did not reduce the embryo energy charge (Corbineau and Côme, 1996, 2003), might affect various biochemical processes or expression of genes involved in cell elongation independently of ABA metabolism. However, hypoxia imposed by the glumellae seems to be of paramount importance for controlling hormone-mediated dormancy maintenance through content and tissue responsiveness. The authors' present work is focused on elucidating the molecular features underlying these mechanisms.
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Acknowledgements |
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The authors want to thank Silvina Enciso for excellent technical assistance with ABA determinations. This work has been financed by Université Pierre et Marie Curie, University of Buenos Aires (G012) and Agencia Nacional de Promocion Cientifica y Tecnica (Argentina) (Project 08-14402).
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References |
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