莲种子萌发和幼苗生长时期营养物质的代谢变化

莲子叶细胞中储存了丰富的营养物质, 主要为蛋白质、淀粉和淀粉质体DNA。这些贮藏物质为种子萌发和幼苗的生长提供必需的能量和养料。通过组织化学和显微镜观察, 研究莲从种子萌发到植株生长至具有4个节时, 子叶中贮藏物质消耗的全过程。在此过程中, 子叶中的贮藏物质不断降解,营养物质发生转运。蛋白体首先发生降解, 其大量降解主要发生在幼苗三叶期。淀粉质体降解时会聚 集成团, 之后体积逐渐减小, 最后完全降解。种子萌发后65天是子叶贮藏物质消耗末期, 淀粉质体DNA的含量比萌发后20天的三叶期明显减少。细胞壁的形态结构发生多种形式的变化, 细胞壁发生的这些变化与子叶细胞间物质的运输有关。含多糖的球形颗粒通过维管束在子叶中运输。     

莲种子萌发和幼苗生长时期营养物质的代谢变化

莲子叶细胞中储存了丰富的营养物质,主要为蛋白质、淀粉和淀粉质体DNA.这些贮藏物质为种子萌发和幼苗的生长提供必需的能量和养料.通过组织化学和显微镜观察,研究莲从种子萌发到植株生长至具有4个节时,子叶中贮藏物质消耗的全过程.在此过程中,子叶中的贮藏物质不断降解,营养物质发生转运.蛋白体首先发生降解,其大量降解主要发生在幼苗三叶期.淀粉质体降解时会聚集成团,之后体积逐渐减小,最后完全降解.种子萌发后65天是子叶贮藏物质消耗末期,淀粉质体DNA的含量比萌发后20天的三叶期明显减少.细胞壁的形态结构发生多种形式的变化,细胞壁发生的这些变化与子叶细胞间物质的运输有关.含多糖的球形颗粒通过维管束在子叶中运输.     

Interconversion and translocation of free sugars during galactomannan utilization in germinating guar (Cyamopsis tetragonoloba) seed

With the onset of the degradation of galactomannan, the galactose and mannose levels increased in the endosperm. The hydrolysis of galactomannan was more or less complete within the first 3 days of germination. In the cotyledons, sucrose was the predominant free sugar during the period of rapid galactomannan hydrolysis and reducing sugars (glucose + fructose) were present in only 10–20% proportion. The level of soluble acid invertase activity was in the order of embryonic axis > endosperm > cotyledons. On the basis of (a) absence of galactose and mannose, (b) high proportion of sucrose, (c) very fast conversion of [¹⁴C]glucose and [¹⁴C]mannose to [¹⁴C]sucrose and (d) very low levels of both soluble and bound invertases in cotyledons, we conclude that there is an active synthesis of sucrose in this tissue where disaccharide seems to be least hydrolysed during the period of galactomannan mobilization. A rapid hydrolysis of galactomannan in endosperm during early germination resulted in the synthesis of some starch, as a temporary reserve, in cotyledons. When the cotyledons entered the phase of first leaf formation, cotyledonary sucrose was hydrolysed giving rise to invert sugars. In the embryonic axis, the increase in the ratio of reducing sugars to sucrose coupled with a higher level of invertase, compared with sucrose-UDP glucosyl transferase, indicated that free sugars from the cotyledons are translocated to the embryonic axis as sucrose.     

Sugar and starch changes in pea cotyledons during germination

Changes in starch and sugar contents in the cotyledons during germination have been compared in a smooth (cv. Alaska) and a wrinkled (cv. Progress) cultivar of the garden pea ( Pisum sativum L.). In both cultivars there was an initial accumulation of sucrose due to the hydrolysis of sucrosyl oligosaccharides, but galactose did not accumulate in the cotyledons. Starch mobilization in the Progress pea was linear with time and started before the rise in α-amylase (EC 3.2.1.1) activity in the cotyledons; sucrose was synthesized in the cotyledons, and their excision from the axis resulted in an additional accumulation of this sugar. In the Alaska pea, the onset of starch hydrolysis coincided with the rise in α-amylase activity; no accumulation of sucrose was found in excised cotyledons, whilst the sucrose content decreased continuously in attached cotyledons.
The same sugars were found in the cotyledons of both cultivars, suggesting a common pathway for starch breakdown. Maltose, maltotriose and linear malto-dextrins were not present and only trace amounts of glucose were detected, suggesting a degradation of starch by phosphorylase after an initial attack by α-amylase. α-Amylase activity in the cotyledons was higher in the presence of the axis, but was influenced by the water content of the cotyledons. Transient changes in α-amylase activity correlated well with changes in the rate of starch hydrolysis, but after 2–3 days starch mobilization was reduced in excised cotyledons probably due to the resynthesis of starch.     

Source-sink regulation of cotyledonary reserve mobilization during cashew (Anacardium occidentale) seedling establishment under NaCl salinity

Seedling establishment is a critical process to crop productivity, especially under saline conditions. This work was carried out to investigate the hypothesis that reserve mobilization is coordinated with salt-induced inhibition of seedling growth due to changes in source-sink relations. To test this hypothesis, cashew nuts (Anacardium occidentale) were sown in vermiculite irrigated daily with distilled water (control) or 50mM NaCl and they were evaluated at discrete developmental stages from the seed germination until the whole seedling establishment. The salt treatment coordinately delayed the seedling growth and the cotyledonary reserve mobilization. However, these effects were more pronounced at late seedling establishment than in earlier stages. The storage protein mobilization was affected by salt stress before the lipid and starch breakdown. The globulin fraction represented the most important storage proteins of cashew cotyledons, and its mobilization was markedly delayed by NaCl along the seedling establishment. Free amino acids were mostly retained in the cotyledons of salt-treated seedlings when the mobilization of storage proteins, lipids and starch was strongly delayed. Proline was not considerably accumulated in the cotyledons of cashew seedlings as a response to NaCl salinity. According to these results it is noteworthy that the salt-induced inhibition of seedling growth is narrowly coordinated with the delay of reserve mobilization and the accumulation of hydrolysis products in cotyledons. Also, it was evidenced that free amino acids, especially those related to nitrogen transport, are potential signals involved in the regulation of storage protein hydrolysis during cashew seedling establishment under NaCl salinity.     

Influence of Endogenous Cytokinins on Reverse Mobilization in Cotyledons of Cicer arietinum L: Reproduction of Endogenous Levels of Total Cytokinins, Zeatin, Zeatin Riboside, and Their Corresponding Glucosides

The embryonic axis plays an essential role in the mobilization of the main reserves of the cotyledons of seeds of Cicer arietinum L. cv Castellana. This control by the axis of the metabolism of the storage products of the cotyledons largely takes place through the cytokinins, which are transported from the embryonic axis to the cotyledons where the mobilization of reserves begins. The principal regulatory role of the endogenous cytokinins concerns the metabolism of carbohydrates and proteins; there is less influence on lipid metabolism. However, each cytokinin seems to have a different role in the mobilization processes. The glucosides, glucosyl zeatin riboside, and glucosyl zeatin act only as storage forms of the hormones. Zeatin riboside affects mainly the mobilization of carbohydrates and has less effect on protein mobilization. Zeatin regulates both the mobilization of carbohydrates and that of proteins and is more marked in the latter case.     

The control of storage xyloglucan mobilization in cotyledons of Hymenaea courbaril

Hymenaea courbaril is a leguminous tree species from the neotropical rain forests. Its cotyledons are largely enriched with a storage cell wall polysaccharide (xyloglucan). Studies of cell wall storage polymers have been focused mostly on the mechanisms of their disassembly, whereas the control of their mobilization and the relationship between their metabolism and seedling development is not well understood. Here, we show that xyloglucan mobilization is strictly controlled by the development of first leaves of the seedling, with the start of its degradation occurring after the beginning of eophyll (first leaves) expansion. During the period of storage mobilization, an increase in the levels of xyloglucan hydrolases, starch, and free sugars were observed in the cotyledons. Xyloglucan mobilization was inhibited by shoot excision, darkness, and by treatment with the auxin-transport inhibitor N-1-naphthylphthalamic acid. Analyses of endogenous indole-3-acetic acid in the cotyledons revealed that its increase in concentration is followed by the rise in xyloglucan hydrolase activities, indicating that auxin is directly related to xyloglucan mobilization. Cotyledons detached during xyloglucan mobilization and treated with 2,4-dichlorophenoxyacetic acid showed a similar mobilization rate as in attached cotyledons. This hormonal control is probably essential for the ecophysiological performance of this species in their natural environment since it is the main factor responsible for promoting synchronism between shoot growth and reserve degradation. This is likely to increase the efficiency of carbon reserves utilization by the growing seedling in the understorey light conditions of the rain forest.     

Increased Na+ and Cl− accumulation induced by NaCl salinity inhibits cotyledonary reserve mobilization and alters the source-sink relationship in establishing dwarf cashew seedlings

We studied the NaCl-induced changes in cotyledons and the embryonic axis of establishing dwarf cashew (Anacardium occidentale) seedlings. The salt stress reduced the growth of dwarf cashew seedlings, and this response was related to the inhibition of cotyledonary reserve depletion. Lipid mobilization was inhibited by NaCl due to reduced lipase activity in the emerging and establishing seedlings. Additionally, there was reduced transient starch accumulation in the cotyledons of the salt-stressed seedlings that was associated with lower starch synthase activity at the early developmental stages and inhibited amylolytic and starch phosphorylase activities at the established seedling stage. The NaCl-induced changes in lipid and starch metabolism influenced the soluble sugar content in the cotyledons. Protein mobilization was inhibited by NaCl, and we observed the accumulation of amino acids and the inhibition of proteolytic activity in the cotyledons of the salt-stressed established seedlings. Salinity significantly reduced the free amino acid and reducing sugar contents in the embryonic axes of both emerged and established seedlings, whereas the non-reducing sugar content was affected by this stress only in the established seedlings. The Na⁺ and Cl^? contents progressively increased in the cotyledons and embryonic axis of the seedlings as the salinity increased. We conclude that salt stress inhibits dwarf cashew seedling establishment by inhibiting the mobilization of reserves, an inhibition that was related to increased Na⁺ and Cl^? accumulation in the cotyledons. Additionally, these toxic ions reduced the sink strength of the embryonic axis with regard to the products of cotyledonary reserve mobilization.     

Response of sugar interconversion, starch and protein accumulation to supplied metabolites during pod filling in chickpea

Detached chickpea inflorescences bearing pods at 20 days after flowering (DAF) were cultured for 5 days in complete liquid medium supplemented separately with asparate, myo-inositol, alpha-ketoglutarate and phytic acid. Effect of these metabolites on sugar interconvestion and starch and protein accumulation in developing pods was studied. Substituting asparate (62.5 mM) for glutamine in culture medium decreased relative proportion of sucrose in all pod tissues but increased the level of sugars, starch and protein in pod wall and cotyledons. In cotyledons, whereas myo-inositol (75 mM) reduced the accumulation of starch without affecting protein level, alpha-ketoglutarate (44 mM) increased both starch and protein accumulation. Both myo-inositol and alpha-ketoglutarate increased relative proportion of sucrose in cotyledons. Phytic acid (1 mM) decreased in cotyledons 14C incorporation from glucose into EtOH extract (principally constituted by sugars), amino acids and proteins but increased the same into starch. In cotyledons, phytic acid also increased 14C incorporation from glutamate into amino acids but this increase was negatively correlated with protein synthesis. Phytic acid decreased the relative distribution of 14C from glucose and glutamate into sucrose from pod wall but enhanced the same into EtOH extract from embryo. Based on the results, it is suggested that mode of metabolic response to exogenously supplied metabolites widely differs in pod tissues of chickpea.     

Ultrastructural and biochemical investigations of protein mobilization of Mucuna pruriens (L.) DC. cotyledons and embryo axis

The mobilization of storage reserves, with particular emphasis on storage proteins of Mucuna pruriens (L.) DC., cotyledons, and embryo was investigated from the ultrastructural and biochemical points of view. Proteins and starch were the two main storage substances in cotyledons, and proteins and lipids were the main ones in the embryo. Embryo protein bodies were smaller and fewer in number than those of cotyledons. Structural and ultrastructural data determined between 24 and 48 h after imbibition and between 48 and 72 h after imbibition, the end of significant embryo and cotyledon protein mobilization, respectively, indicating more precocious storage protein mobilization in the axis than cotyledons. Moreover, storage protein mobilization in embryo and cotyledons occurred before the end of germination. Water soluble proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, producing 29 bands with molecular weights from 14 to 90 KDa. Embryo extract contained more proteins than cotyledon extract, contained seven characteristic bands, and showed a higher variability of the optical density trend than cotyledon.     

Different functions of vicilin and legumin are reflected in the histopattern of globulin mobilization during germination of vetch (Vicia sativa L.)

The temporal and spatial patterns of storage-globulin mobilization were immunohistochemically pursued in the embryonic axis and cotyledons of vetch seed (Vicia sativa L.) during germination and early seedling growth. Embryonic axes as well as cotyledons of mature seeds contain protein bodies with stored globulins. Prevascular strands of axes and cotyledons, the radicle and epidermal layers of axis organs were nearly exclusively stained by vicilin antibodies whereas the cotyledonous storage mesophyll gave similar staining for vicilin and legumin. Globulin breakdown started locally where growth and differentiation commenced in the axis. There, vicilin mobilization preceded legumin mobilization. Thus vicilin represents the initial source of amino acids for early growth and differentiation processes in vetch. Legumin presumably only serves as a bulk amino acid source for subsequent seedling growth during postgerminative globulin degradation. During the first 2–3 d after the start of imbibition the axis was depleted of globulins whereas no decrease in immunostainability was detected in the cotyledons except in their vascular strands where immunostainability was almost completely lost at this time. Continuous vascular strands were established at the third day when globulin breakdown was finished in the axis but had just started in the cotyledon mesophyll. Protein mobilization proceeded in a small zone from the epidermis towards the vascular strands in the center of the cotyledons. In this zone the storage cells, which initially appeared densely packed with starch grains and protein bodies, concomitantly transformed into cells with a large central vacuole and only a thin cytoplasmic layer attached to the cell wall. These results agree well with the hypothesis that during the first 2 d after imbibition the axis is autonomous in amino acid provision. After the endogenous reserves of the axis are depleted and the conductive tissue has differentiated, globulins are mobilized in the cotyledons, suggesting that then the amino acid supply is taken over by the cotyledons. For comparison with other degradation patterns we used garden bean (Phaseolus vulgaris L) and rape (Brassica napus L.) as reference plants. Received: 3 August 1999 / Accepted: 11 December 1999     

AtDSEL, an Arabidopsis cytosolic DAD1-like acylhydrolase, is involved in negative regulation of storage oil mobilization during seedling establishment

Mobilization of seed storage reserves is essential for seed germination and seedling establishment. Here, we report that AtDSEL, an Arabidopsis thalianaDAD1-like Seedling Establishment-related Lipase, is involved in the mobilization of storage oils for early seedling establishment. AtDSEL is a cytosolic member of the DAD1-like acylhydrolase family encoded by At4g18550. Bacterially expressed AtDSEL preferentially hydrolyzed 1,3-diacylglycerol and 1-monoacylglycerol, suggesting that AtDSEL is an sn-1-specific lipase. AtDSEL-overexpressing transgenic Arabidopsis plants (35S:AtDSEL) were defective in post-germinative seedling growth in medium without an exogenous carbon source. This phenotype was rescued by the addition of sucrose to the growth medium. In contrast, loss-of-function mutant plants (atdsel-1 and atdsel-2) had a mildly fast-growing phenotype regardless of the presence of an exogenous carbon source. Electron microscopy revealed that 5-day-old 35S:AtDSEL cotyledons retained numerous peroxisomes and oil bodies, which were exhausted in wild-type and mutant cotyledons. The impaired seedling establishment of 35S:AtDSEL was not rescued by the addition of an exogenous fatty acid source, and 35S:AtDSEL seedling growth was insensitive to 2,4-dichlorophenoxybutyric acid, indicating that β-oxidation was blocked in AtDSEL-overexpressers. These results suggest that AtDSEL is involved in the negative regulation of seedling establishment by inhibiting the breakdown of storage oils.     

Regulation of starch and protein accumulation in alfalfa (Medicago sativa L.) somatic embryos

Compared to seeds, somatic embryos accumulated relatively low levels and different types of storage carbohydrates. The regulation of starch accumulation was studied to determine its effects on desiccation tolerance and vigor of dry somatic embryos. Somatic embryos of Medicago sativa are routinely matured through three phases: 7 days of development; 10 days of phase I maturation, a rapid growth phase; and 10 days of phase II maturation, a phase leading to the acquisition of desiccation tolerance. The control of starch deposition was investigated in alfalfa somatic embryos by manipulating the composition of the phase I maturation medium with different levels of sucrose, abscisic acid, glutamine and different types of carbohydrates and amino acids. After phase II maturation, mature somatic embryos were collected for desiccation and subsequent conversion, or for biochemical analyses. Starch deposition occurred primarily during phase I maturation, and variations in the composition of this medium influenced embryo quality, storage protein and starch accumulation. A factorial experiment with two levels of glutamine × three levels of sucrose showed that increasing the sucrose concentration from 30 to 80 g/l increased embryo size and starch content, but had minimal effect on accumulation of storage proteins; glutamine also increased embryo size, but decreased starch content and increased accumulation of the high salt soluble S-2 (medicagin) storage proteins. ABA did not influence any of the parameters tested when included in phase I maturation at concentration up to 10 μM. Replicating sucrose with maltose, glucose, or glucose and fructose did not alter embryo size or starch accumulation (mg/g fresh weight), but replacement with fructose alone reduced embryo size, and replacement with glucose alone reduced germination. Suplementation with the amino acids, asparagine, aspartic acid and glutamine increased seedling vigor, but decreased the starch content of embryos. The data indicate that starch accumulation in somatic embryos is regulated by the relative availability of carbon versus nitrogen nutrients in the maturation medium. The quality of mature somatic embryos, determined by the rate of seedling development (conversion and vigor), correlated with embryo size, storage protein and free amino acid but not with starch. Therefore, further improvements in the quality of somatic embryo may be achieved through manipulation of the maturation medium in order to increase storage protein, but not starch deposition.     

Degradation of the endosperm cell walls of Lactuca sativa L., cv. Grand Rapids

The timing of mobilisation of lipid, sucrose, raffinose and phytate in lettuce seeds (achenes) (cv. Grand Rapids) has been examined. These reserves (33%, 1.5%, 0.7%, 1.4% of achene dry weight, respectively) are stored mostly in the cotyledons. Except for a slight degradation of raffinose and increase in sucrose, there is no detectable reserve mobilisation during germination. The endosperm (8% of seed dry weight), which has thick, mannan-containing cell walls (carbohydrate, 3,4% of seed dry weight), is completely degraded within about 15h following germination. Mannanase activity increases about 100-fold during the same period and arises in all regions of the endosperm. Also during this period sucrose and raffinose are degraded and fructose and glucose accumulate in the embryo. The endosperm hydrolysis products are taken up by the embryo, and are probably used as an additional reserve to support early seedling growth. However, endosperm cell-wall carbohydrates, such as mannose, are not found as free sugars. Lipid and phytate are degraded in a later, second phase of mobilisation. Low levels of sucrose are present in the embryo, mostly in the cotyledons, and large amounts of fractose and glucose (14% of seedling dry weight at 3 days after sowing) accumulate in the hypocotyl and radicle. It is suggested that sucrose, produced in the cotyledons by gluco-neogenesis, is translocated to the axis and converted there to fructose and glucose.     

Photosynthesis, reserve mobilization and enzymes of sucrose metabolism in soybean (Glycine max) cotyledons

Soybean (Glycine max L. [Merr] cv. Ransom II) seedlings were grown under a light/ dark regime or in continuous darkness. Cotyledons were harvested daily for measurements of reserve mobilization, net carbon exchange rate, chlorophyll content and activities of certain enzymes involved in sucrose metabolism. Seedlings lost dry weight for the first 3 to 4 days after planting, then maintained a constant dry weight in the etiolated seedlings, and gained dry weight (via net fixation of CO₂) in the light-grown seedlings. In general, the patterns of reserve mobilization were as expected based on the collective work of other investigators. Soluble sugars were mobilized first, followed by protein and lipid. Galactinol, previously uncharacterized in soybean cotyledons, was present at low concentrations and was rapidly depleted within 2 days after planting. Mobilization of reserves was most important during the first 8 days after planting, whereas net cotyledonary photosynthesis began at 6 days after planting and was the primary source of assimilates after 8 days. Maximum rates of cotyledon photosynthesis were higher [up to 18 mg CO₂ (g dry weight)^?1 h^?1] than previously reported and accounted for about 75% of the assimilates transported from the cotyledons to the growing seedling during the functional life of the cotyledon. Enzyme activities in light-grown cotyledons peaked 7 to 10 days after planting and then declined. Sucrose phosphate synthase (EC 2.4.1.14) and sucrose synthase (EC 2.4.1.13) activities were similar in etiolated and light-grown seedlings, whereas uridine-5′-di-phosphatase (EC 3.6.1.6) activity was substantially higher in light-grown seedlings. During the period of reserve mobilization, the maximum sucrose phosphate synthase activity in cotyledonary extracts was in excess of the calculated rate of sucrose formation. However, when the cotyledons had highest net photosynthetic rates (14 days after planting), sucrose phosphate synthase activity was similar to the rate of carbon assimilation. It appears that soybean cotyledons are adapted for high rates of sucrose formation (from reserve mobilization and/or photosynthesis) for export to the rapidly growing tissues of the seedling.     

Reserve mobilization and starch formation in soybean (Glycine max) cotyledons in relation to seedling growth

Brown, C. S. and Huber, S. C. 1988. Reserve mobilization and starch formation in soybean ( Glycine max ) cotyledons in relation to seedling growth. - Physiol. Plant. 72: 518–524.
In germinating soybean ( Glycine max [L.] Merr.) seedlings, starch accumulated in the cotyledons during the first 5 days of seedling growth. Among 10 genotypes, the amount of starch accumulated after 5 days was relatively independent of light and appeared to be primarily related to the amount of sucrose depleted from the cotyledons during the same time period. Depletion of other reserves (e.g. protein and lipid) were not closely correlated with starch formation. In addition, the differences in starch formation were not related to differences in activities of certain enzymes that may be involved in the conversion of sucrose to starch, namely starch synthase (EC 2.4.1.21), ADP-glucose pyrophosphorylase (EC 2.7.7.27), sucrose synthase (EC 2.4.1.13), neutral invertase (EC 3.2.1.26), and PPi-linked phosphofructokinase (PFP) (EC 2.7.1.90). Starch synthesis did not compete with seedling growth, because among 10 genotypes, transient starch formation was correlated positively with seedling growth and cotyledonary photosynthetic rates. We postulate that starch is derived primarily from stored sucrose in the cotyledons and is not merely a result of 'overflow' carbon from other reserves. Starch formation also appeared to have a positive relationship with both early (0 to 5 days) and later (5 to 13 days) seedling growth, the latter perhaps due to enhanced cotyledon photosynthetic rates.     

Hydrolytic enzyme activities and degradation of storage components in cotyledons of germinatingPhaseolus mungo seeds

Phaseolus mungo seeds were allowed to germinate in the dark, and time-course changes in contents of protein fractions, starch, soluble α-amino nitrogen and reducing sugars and in hydrolytic enzyme activities in cotyledons were investigated. In cotyledons of germinated seeds, marked increases in proteolytic (caseolytic, globulytic and gelatin-hydrolyzing) activities and amylolytic activity occurred with concurrent mobilization of storage proteins and starch. Removal of axis organs from seeds at very early stages of germination caused the deteriorated breakdown of storage components and decreased development of proteolytic enzymes in the cotyledons, but this treatment did not significantly affect the appearance of amylolytic activity. The experimental results are discussed in comparison with the hydrolytic enzyme activities of germinating seeds of other leguminous species.     

Effects of Gibberellic Acid on the Transport of Nitrogen from the Cotyledons of Young Pea Seedlings

Gibberellic acid increased shoot length in tall and dwarf peaseedlings, but whilst in the former it had no effect on therate of breakdown of reserve proteins nor on the transport ofnitrogen to the axis and its final distribution between theshoot and the root, it promoted these processes in the dwarfpea. The results are discussed in relation to the overall controlof the mobilization of reserves in the cotyledons during germination.