A critical role for disproportionating enzyme in starch breakdown is revealed by a knock-out mutation in Arabidopsis

Disproportionating enzyme (D-enzyme) is a plastidial alpha-1,4-glucanotransferase but its role in starch metabolism is unclear. Using a reverse genetics approach we have isolated a mutant of Arabidopsis thaliana in which the gene encoding this enzyme (DPE1) is disrupted by a T-DNA insertion. While D-enzyme activity is eliminated in the homozygous dpe1-1 mutant, changes in activities of other enzymes of starch metabolism are relatively small. During the diurnal cycle, the amount of leaf starch is higher in dpe1-1 than in wild type and the amylose to amylopectin ratio is increased, but amylopectin structure is unaltered. The amounts of starch synthesised and degraded are lower in dpe1-1 than in wild type. However, the lower amount of starch synthesised and the higher proportion of amylose are both eliminated when plants are completely de-starched by a period of prolonged darkness prior to the light period. During starch degradation, a large accumulation of malto-oligosaccharides occurs in dpe1-1 but not in wild type. These data show that D-enzyme is required for malto-oligosaccharide metabolism during starch degradation. The slower rate of starch degradation in dpe1-1 suggests that malto-oligosaccharides affect an enzyme that attacks the starch granule, or that D-enzyme itself can act directly on starch. The effects on starch synthesis and composition in dpe1-1 under normal diurnal conditions are probably a consequence of metabolism at the start of the light period, of the high levels of malto-oligosaccharides generated during the dark period. We conclude that the primary function of D-enzyme is in starch degradation.     

玉米淀粉生物合成及其遗传操纵

淀粉是许多植物重要的储藏物质。淀粉突变体以及转基因植物中淀粉变异的特点使我们对淀粉生物合成的过程有了较深入的了解,许多研究的结果揭示了玉米淀粉的生物合成涉及4类酶--ADPG焦磷酸化酶、淀粉合成酶、淀粉分支酶和去分支酶。随着编码这些酶的基因的克隆,利用转基因技术对淀粉合成过程进行遗传操纵业已成为可能,并且在提高淀粉产量以及不同特性淀粉品质的种质资源创新等方面展示出巨大的潜力。 Abstract:Starch is the most important source of calories and a vital storage component in plants.The characterization and production of starch variants from mutation and with transgenic technology has improved our understanding of the synthesis of starch granule.In starch biosynthesis in plants,four enzymes,including ADP-glucose pyrophosphorylase,starch synthase,starch branching enzyme and starch debranching enzyme,are widely accepted from an enormous amount of research aimed primarily at enzyme characterization.As many genes encoding the enzymes and their multiple isoforms in starch biosynthesis pathway have been isolated,genetic manipulation of the starch biosynthesis pathway shows to be a practical way by which starch quantity is increased and starch with novel properties can be created.     

Starch-related cytosolic heteroglycans in roots from Arabidopsis thaliana

Both photoautotrophic and heterotrophic plant cells are capable of accumulating starch inside the plastid. However, depending on the metabolic state of the respective cell the starch-related carbon fluxes are different. The vast majority of the transitory starch biosynthesis relies on the hexose phosphate pools derived from the reductive pentose phosphate cycle and, therefore, is restricted to ongoing photosynthesis. Transitory starch is usually degraded in the subsequent dark period and mainly results in the formation of neutral sugars, such as glucose and maltose, that both are exported into the cytosol. The cytosolic metabolism of the two carbohydrates includes reversible glucosyl transfer reactions to a heteroglycan that are mediated by two glucosyl transferases, DPE2 and PHS2 (or, in all other species, Pho2).In heterotrophic cells, accumulation of starch mostly depends on the long distance transport of reduced carbon compounds from source to sink organs and, therefore, includes as an essential step the import of carbohydrates from the cytosol into the starch forming plastids.In this communication, we focus on starch metabolism in heterotrophic tissues from Arabidopsis thaliana wild type plants (and in various starch-related mutants as well). By using hydroponically grown A. thaliana plants, we were able to analyse starch-related biochemical processes in leaves and roots from the same plants. Within the roots we determined starch levels and the morphology of native starch granules. Cytosolic and apoplastic heteroglycans were analysed in roots and compared with those from leaves of the same plants. A. thaliana mutants lacking functional enzymes either inside the plastid (such as phosphoglucomutase) or in the cytosol (disproportionating isoenzyme 2 or the phosphorylase isozyme, PHS2) were included in this study. In roots and leaves from the three mutants (and from the respective wild type organ as well), starch and heteroglycans as well as enzyme patterns were analysed.     

Effects of drought on primary photosynthetic processes of cotton leaves

The localization of enzymes involved in the flow of carbon into and out of starch was determined in guard cells of Commelina communis. The guard cell chloroplasts were separated from the rest of the cellular components by a modification of published microfuge methods. The enzymes of interest were then assayed in the supernatant and chloroplast fractions. The chloroplast yield averaged 75% with 10% cytoplasmic contamination. The enzymes involved in starch biosynthesis, ADPglucose pyrophosphorylase, starch synthase, and branching enzyme, are located exclusively in the chloroplast fraction. The enzymes involved in starch degradation show a more complex distribution. Phosphorylase is located in both the supernatant and chloroplast fraction, 50% in each fraction. Most of the amylase and debranching enzyme activity is present in the supernatant (70%) fraction. The majority of the rest of the enzymes involved in the degradation of starch to malate and synthesis of starch from a hexose precursor were also investigated. All of the enzymes were present in the chloroplast except for hexokinase and phosphofructokinase. The inability to assay these enzymes could possibly have been due to the lack of or low activity of the enzymes or to nonoptimal assay conditions.     

水稻淀粉合成的分子生物学研究进展

本文综述了水稻淀粉合成的分子生物学研究的最新进展,主要内容是参与淀粉合成的酶鉴定及其基因表达调控,也介绍了对这些酶的遗传操作改良稻米淀粉品质等内容     

植物支链淀粉合成的关键酶——淀粉分支酶

支链淀粉是植物淀粉的主要成分,而淀粉分支酶是其合成的关键酶。淀粉分支酶可分为两同形体家族,本文从酶学特性、染色体定位、基因及基因表达方面阐明了它们之间的联系和区别,并证实不同同形体在植物支链淀粉合成和结构决定上所起作用不同。开展对该酶的深入研究不论是在基础理论研究领域还是在现实应用方面都具重要意义。     

Engineering direct fructose production in processed potato tubers by expressing a bifunctional alpha-amylase/glucose isomerase gene complex

Manipulation of starch biosynthesis/degradation and formation of novel molecules in storage organs of plants through genetic engineering is an attractive but technically challenging goal. We report here, for the first time, that starch was degraded and glucose and fructose were produced directly when crushed potato tubers expressing a starch degrading bifunctional gene were heated for 45 minutes at 65 degrees C. To achieve this, we have constructed a fusion gene encoding the thermostable enzymes: alpha-amylase (Bacillus stearothermophilus) and glucose isomerase (Thermus thermophilus). The chimeric gene was placed under the control of the granule-bound-starch synthase promoter. This enzymatic complex produced in transgenic tubers was only active at high temperature (65 degrees C). More than 100 independent transgenic potato plants were regenerated. Molecular analyses confirmed the stable integration of the chimeric gene into the potato genome. The biochemical analyses performed on young and old tubers after high-temperature treatment (65 degrees C) revealed an increase in the formation rate of fructose and glucose by a factor of 16.4 and 5. 7, respectively, in the transgenic tubers as compared to untransformed control tubers. No adverse discernible effect on plant development and metabolism including tuber formation and starch accumulation was observed in the transgenic plants before heat treatment. Our results demonstrate that it is possible to replace starch degradation using microbial enzymes via a system where the enzymes are produced directly in the plants, but active only at high temperature, thus offering novel and viable strategies for starch-processing industries.     

水稻淀粉合成的分子生物学研究进展

本文综述了水稻淀粉合成的分子生物学研究的最新进展,主要内容是参与淀粉合成的酶鉴定及其基因表达调控,也介绍了对这些酶的遗传操作改良稻米淀粉品质等内容。     

Biosynthesis of alpha-amylase and protease by Streptomyces olivaceus 142. I. Regulation of alpha-amylase activity.

Streptomyces olivaceus 142 produces amylase in the logarithmic phase of growth of the culture. The synthesis of the enzyme is induced by maltose and starch. In the case of maltose the synthesis is induced by a contaminating compound, probably being a higher than maltose polymer of glucose. The synthesis of amylase is negatively controlled by catabolic repression. The level of the activity of the enzyme depends not only on the biosynthesis but also on it proteolytic degradation.     

Distribution of enzyme activities within the developing maize (Zea mays) kernel in relation to starch, oil and protein accumulation

The association of enzyme activities in developing kernels with specific storage product accumulation at maturity was analyzed in different parts of Zea mays inbred OH43 kernels. Maize kernels were harvested at 20 and 55 days post-pollination and dissected into basal region, pericarp, embryo, lower endosperm, middle endosperm and upper endosperm. Mature (55 days pos(-pollination) kernel parts were analyzed for starch, total protein, zein and oil content. Immature (20 days post-pollination) kernel parts were assayed for activities of 15 enzymes of sugar and amino acid metabolism. Statistical analyses of the data suggested that glucokinase (EC 2.7.1.2) fructokinase (EC 2.7.1.4) and phosphofructokinase (EC 2.7.1.1 11) activities were primarily associated with oil accumulation, whereas ADP'-glueose pyrophosphorylasc (EC 2.7.7.27) and sucrose synthase (EC 2.4.1.13) activities were associated with starch accumulation. The results suggest that oil biosynthesis utilizes inveitase-mediated sucrose degradation in a pathway not requiring pyrophosphatc. whereas starch biosynthesis utilizes a sucrose synthase-mediated pathway of sucrose degradation in a pathway requiring pyrophosphatc. Additional groups of enzyme activities were associated with each oilier but not with any specific storage product and appeared to be associated with general metabolic activity.     

Leaf starch degradation comes out of the shadows

During the day, plants accumulate starch in their leaves as an energy source for the coming night. Based on recent findings, the prevailing view of how the transitory starch is remobilized needs considerable revision. Analyses of transgenic and mutant plants demonstrate that plastidic glucan phosphorylase is not required for normal starch breakdown and cast doubt on the presumed essential role of alpha-amylase but do show that beta-amylase is important. Repression of the activity of a plastidic beta-amylase, the export of its product (maltose) or further metabolism of maltose by a newly identified transglucosidase impairs starch degradation. Breakdown of particulate starch also depends on the activity of glucan-water dikinase, which phosphorylates glucosyl residues within the polymer.     

Starch synthesis in the cereal endosperm

The pathway of starch synthesis in the cereal endosperm is unique, and requires enzyme isoforms that are not present in other cereal tissues or non-cereal plants. Recent information on the functions of individual enzyme isoforms has provided insight into how the linear chains and branch linkages in cereal starch are synthesized and distributed. Genetic analyses have led to the formulation of models for the roles of de-branching enzymes in cereal starch production, and reveal pleiotropic effects that suggest that certain enzymes may be physically associated. For the first time, tools for global analyses of starch biosynthesis are available for cereal crops, and are heralded by the draft sequence of the rice genome.     

Starch metabolism in developing strawberry (Fragaria × ananassa) fruits

Fruit starch reserves can be an important contributor to the sugar content of some ripe fruit, and despite the relatively high financial premiums (compared to other fruit) commanded by ripe strawberries, neither their starch or sugar biochemistry has been examined in detail. This study assessed the rate of starch biosynthesis and breakdown in developing strawberry and sought to determine the temporal changes in the activities of selected enzymes known to be involved in sucrose-starch interconversions. Scanning electron microscopy revealed that starch levels appeared greatest in immature strawberry ( Fragaria × ananassa , cv. Elsanta) at 7 days postanthesis, as evidenced by a decrease in the number of cells containing starch granules as ripening progressed. Levels of key enzymes of starch and sugar metabolism estimated using Western blotting and enzyme activity analysis showed that activities did not correlate with antigen levels. In particular, enzyme activity recovery experiments indicated that losses were due to non-proteinaceous inhibitors, and in particular protein binding: highlighting the potential for misinterpretation of enzyme activity data gathered from ripening (strawberry) fruit tissue extracts. Consequently, in vitro experiments using [U-¹⁴C] glucose revealed that incorporation to starch is low (11%) at the earliest developmental stages when starch content is greatest. Starch synthesis rate then declines to non-detectable levels as fruit expand and ripen. These results show that starch accumulates extremely early in the fruit formation process and that starch degradation predominates during fruit growth and development. We estimate that breakdown of transient starch can contribute up to 3% of the sugar accumulated in ripe fruit.     

New look at starch degradation in Arabidopsis thaliana L. chloroplasts

Transitory starch is accumulated during the day and is the main source of energy for the cell metabolism during the night. The observed periodical starch degradation has become a model often used by scientist in their experiments. Starch granule degradation could be divided into 2 periods: initiation of degradation and digestion of amylopectin and amylose into maltooligosaccharide and their derivative. Key meaning is attributed in this process to beta-amylaze, product of its activity beta-maltose is transported to the cytosole and there it subjects farthest conversions. It has been demonstrated that a number of enzymes take part in the starch degradation process. However, the way of regulating their activity is still not fully explained. There is most important elements effecting rate of starch decomposition: day cycle, starch phosphorylation and regulation of enzyme activity. It proceeds through redox potential, pH changes and phosphorylation of protein involved in starch degradation due specific phosphatases. The purpose of the current work is to systematize the knowledge of the Arabidopsis thaliana L. leaf starch degradation. The results of the recent research cast a new light on the starch degradation process as well as on its control.     

A cytosolic glucosyltransferase is required for conversion of starch to sucrose in Arabidopsis leaves at night

Maltose is exported from the Arabidopsis chloroplast as the main product of starch degradation at night. To investigate its fate in the cytosol, we characterised plants with mutations in a gene encoding a putative glucanotransferase (disproportionating enzyme; DPE2), a protein similar to the maltase Q (MalQ) gene product involved in maltose metabolism in bacteria. Use of a DPE2 antiserum revealed that the DPE2 protein is cytosolic. Four independent mutant lines lacked this protein and displayed a decreased capacity for both starch synthesis and starch degradation in leaves. They contained exceptionally high levels of maltose, and elevated levels of glucose, fructose and other malto-oligosaccharides. Sucrose levels were lower than those in wild-type plants, especially at the start of the dark period. A glucosyltransferase activity, capable of transferring one of the glucosyl units of maltose to glycogen or amylopectin and releasing the other, was identified in leaves of wild-type plants. Its activity was sufficient to account for the rate of starch degradation. This activity was absent from dpe2 mutant plants. Based on these results, we suggest that DPE2 is an essential component of the pathway from starch to sucrose and cellular metabolism in leaves at night. Its role is probably to metabolise maltose exported from the chloroplast. We propose a pathway for the conversion of starch to sucrose in an Arabidopsis leaf.     

EFFECT OF NUTRIENT DEPRIVATION AND RESUPPLY ON METABOLITES AND ENZYMES RELATED TO CARBON ALLOCATION IN GRACILARIA TENUISTIPITATA (RHODOPHYTA)1

The starch content of red algae normally increases during nitrogen limitation. Based on this we hypothesized that nutrient deprivation would result in an increased activity of starch‐synthesizing enzymes and a decrease in the activity of starch‐degrading enzymes, with the opposite scenario when nutrients were sufficient. We therefore examined the effect of the nutrient status of Gracilaria tenuistipitata Chang et Xia on the content of starch and floridoside and on the activity of enzymes involved in the allocation of carbon into starch, floridoside, and agar; floridoside phosphate synthase and α‐galactosidase involved in synthesis and degradation of floridoside; starch synthase and starch phosphorylase involved in the metabolism of starch; uridine 5′‐diphosphate (UDP)‐glucose pyrophosphorylase; adenosine 5′‐diphosphate‐glucose pyrophosphorylase; UDP‐glucose 4‐epimerase; and phosphoglucomutase. During the period of nutrient limitation the starch and floridoside content increased, as did dry weight and C/N ratio, whereas growth rate and protein content decreased. A general decrease in the enzyme activities during nutrient limitation was also observed, indicating a decrease in overall cellular metabolism. The addition of nutrients caused an increase in enzyme activities and a decrease in the contents of starch and floridoside. Of the enzymes examined, only the activity of UDP‐glucose pyrophosphorylase increased during nutrient limitation and decreased abruptly after nutrient addition. This implies a regulatory role for this enzyme in the supply of UDP‐glucose for starch synthesis. It also supports our suggestion that UDP‐glucose is the substrate for starch synthesis in red algae. This assertion is further strengthened by the observation that of the potential starch synthases only the UDP‐glucose starch synthase could support the observed rate of starch synthesis.     

Characterization of Starch-Debranching Enzymes in Pea Embryos

Two distinct types of debranching enzymes have been identified in developing pea (Pisum sativum L.) embryos using native gel analysis and tests of substrate preference on purified or partially purified activities. An isoamylase-like activity capable of hydrolyzing amylopectin and glycogen but not pullulan is present throughout development and is largely or entirely confined to the plastid. Activities capable of hydrolyzing pullulan are present both inside and outside of the plastid, and extraplastidial activity increases relative to the plastidial activity during development. Both types of debranching enzyme are also present in germinating embryos. We argue that debranching enzymes are likely to have a role in starch metabolism in the plastid of the developing embryo and in starch degradation during germination.     

Analyses of isoamylase gene activity in wild-type barley indicate its involvement in starch synthesis

The notion of debranching enzyme activity as a participant in starch synthesis is gaining acceptance. Inconsistent reports from mutant analyses implicate either isoamylase or pullulanase as a determinant in amylopectin formation and whether wild-type plants utilize one or the other, or both, of these debranching enzymes in starch synthesis is unclear. Recent results on the su1 mutant in maize suggest that both forms of debranching enzymes might be involved in amylopectin formation. We wished to find out if isoamylase takes part in starch synthesis by comparing isoamylase gene activity under three conditions: (1) during starch accumulation in developing sink tissues; (2) during starch degradation in germinating seeds; (3) in ectopic expression after applying sucrose, a starch precursor. We isolated the gene for barley isoamylase, iso1, and analysed its expression and regulation in germinating seeds, developing endosperm and vegetative tissues, and compared the isoamylase gene expression in sink tissues from three different species. Our results indicate that isoamylase gene activity is involved in starch synthesis in wild-type plants and is modulated by sucrose.