Starch Granule Initiation in Arabidopsis Requires the Presence of Either Class IV or Class III Starch Synthases

The mechanisms underlying starch granule initiation remain unknown. We have recently reported that mutation of soluble starch synthase IV (SSIV) in Arabidopsis thaliana results in restriction of the number of starch granules to a single, large, particle per plastid, thereby defining an important component of the starch priming machinery. In this work, we provide further evidence for the function of SSIV in the priming process of starch granule formation and show that SSIV is necessary and sufficient to establish the correct number of starch granules observed in wild-type chloroplasts. The role of SSIV in granule seeding can be replaced, in part, by the phylogenetically related SSIII. Indeed, the simultaneous elimination of both proteins prevents Arabidopsis from synthesizing starch, thus demonstrating that other starch synthases cannot support starch synthesis despite remaining enzymatically active. Herein, we describe the substrate specificity and kinetic properties of SSIV and its subchloroplastic localization in specific regions associated with the edges of starch granules. The data presented in this work point to a complex mechanism for starch granule formation and to the different abilities of SSIV and SSIII to support this process in Arabidopsis leaves.     

Degradation of Glucan Primers in the Absence of Starch Synthase 4 Disrupts Starch Granule Initiation in Arabidopsis

Arabidopsis leaf chloroplasts typically contain five to seven semicrystalline starch granules. It is not understood how the synthesis of each granule is initiated or how starch granule number is determined within each chloroplast. An Arabidopsis mutant lacking the glucosyl-transferase, STARCH SYNTHASE 4 (SS4) is impaired in its ability to initiate starch granules; its chloroplasts rarely contain more than one large granule, and the plants have a pale appearance and reduced growth. Here we report that the chloroplastic α-amylase AMY3, a starch-degrading enzyme, interferes with granule initiation in the ss4 mutant background. The amy3 single mutant is similar in phenotype to the wild type under normal growth conditions, with comparable numbers of starch granules per chloroplast. Interestingly, the ss4 mutant displays a pleiotropic reduction in the activity of AMY3. Remarkably, complete abolition of AMY3 (in the amy3 ss4 double mutant) increases the number of starch granules produced in each chloroplast, suppresses the pale phenotype of ss4, and nearly restores normal growth. The amy3 mutation also restores starch synthesis in the ss3 ss4 double mutant, which lacks STARCH SYNTHASE 3 (SS3) in addition to SS4. The ss3 ss4 line is unable to initiate any starch granules and is thus starchless. We suggest that SS4 plays a key role in granule initiation, allowing it to proceed in a way that avoids premature degradation of primers by starch hydrolases, such as AMY3.     

Poloxomer 188 Has a Deleterious Effect on Dystrophic Skeletal Muscle Function

Duchenne muscular dystrophy (DMD) is an X-linked, fatal muscle wasting disease for which there is currently no cure and limited palliative treatments. Poloxomer 188 (P188) is a tri-block copolymer that has been proposed as a potential treatment for cardiomyopathy in DMD patients. Despite the reported beneficial effects of P188 on dystrophic cardiac muscle function, the effects of P188 on dystrophic skeletal muscle function are relatively unknown. Mdx mice were injected intraperitoneally with 460 mg/kg or 30 mg/kg P188 dissolved in saline, or saline alone (control). The effect of single-dose and 2-week daily treatment was assessed using a muscle function test on the Tibialis Anterior (TA) muscle in situ in anaesthetised mice. The test comprises a warm up, measurement of the force-frequency relationship and a series of eccentric contractions with a 10% stretch that have previously been shown to cause a drop in maximum force in mdx mice. After 2 weeks of P188 treatment at either 30 or 460 mg/kg/day the drop in maximum force produced following eccentric contractions was significantly greater than that seen in saline treated control mice (P = 0.0001). Two week P188 treatment at either dose did not significantly change the force-frequency relationship or maximum isometric specific force produced by the TA muscle. In conclusion P188 treatment increases susceptibility to contraction-induced injury following eccentric contractions in dystrophic skeletal muscle and hence its suitability as a potential therapeutic for DMD should be reconsidered.     

不同浓度IAA对浮萍生长及淀粉积累的影响

以新型能源植物浮萍为试材,设置0（对照）、0.001、0.01、0.1、1 和10 μmol/L的IAA处理,通过光照培养室营养液水培试验,研究不同浓度IAA对浮萍生长及淀粉积累的影响。结果表明：（1）低浓度（0.001、0.01和0.1 μmol/L）的IAA可以有效促进浮萍叶绿素含量、净光合速率和干物质的积累,分别比对照显著增加14.7%、10.6%和28.1%,并以0.001 μmol/L IAA处理效果最佳。（2）高浓度（1 和10 μmol/L）IAA可以调节浮萍淀粉的积累,增强淀粉合成关键酶ADPG焦磷酸化酶（AGPase）的活性,淀粉含量和AGPase活性在10 μmol/L的IAA处理下分别比对照显著增加35.0%和14.1%。研究表明,低浓度IAA处理能有效促进浮萍叶绿素的合成和光合作用的进行,从而有利于生物量的积累;高浓度IAA可以通过增强浮萍AGPase的活性来促进淀粉含量和淀粉总量的积累。     

Characterization of an Arabidopsis thaliana Mutant that Has a Defect in ABA Accumulation: ABA-Dependent and ABA-Independent Accumulation of Free Amino Acids during Dehydration

In an attempt to elucidate the physiological role of ABA inseed dormancy and the adaptive response to dehydration, we isolatedan ABA-deficient mutant of Arabidopsis thaliana (L.) Heynh.which germinated in the presence of a gibberellin biosyntheticinhibitor. Genetic analysis showed this mutation is a new alleleof a recently reported locus aba2, and therefore has been designatedaba2-2. The levels of endogenous ABA in fresh and dehydratedtissues of the aba2-2 mutant were highly reduced compared tothose of wild-type plants. As a consequence, aba2-2 plants wiltand produce seeds with reduced dormancy. Dark germinated seedlingsof the aba2-2 mutant showed true leaves, which were not observedin those of the wild type, indicating that abal-2 em bryos grewprecociously during seed maturation. In the dehydrated tissuesof the wild-type plants, the levels of free proline, isoleucineand leucine were elevated to a content approximately 100-foldhigher than those in fresh tissues. In contrast to the wild-typeplants, dehydration-induced accumulation of proline was highlysuppressed in the aba2-2 mutant plants while that of leucineand isoleucine accumulated. Furthermore, exogenous applicationof ABA to wild-type plants promoted accumulation of free proline,but not leucine nor isoleucine. These results suggest that dehydration-inducedaccumulation of free leucine and isoleucine is achieved independentof ABA. (Received March 5, 1998; Accepted June 2, 1998)     

Effect of Time of Exposure to Salinity on Growth, Water Status, and Salt Accumulation in Bean Plants

Bean plants were exposed to salinity either during the lightor the dark period of the day. Under conditions of relativelymild evapotranspirative demand of the atmosphere, transpirationduring salt absorption did not markedly affect salt uptake orplant growth. In all cases duration of exposure to salinitywas the main factor. Under the experimental conditions, salinityreduced the transpiration rate. In these experiments full adjustmentto salinity was not achieved.     

Long-Term Nitrogen Addition Leads to Loss of Species Richness Due to Litter Accumulation and Soil Acidification in a Temperate Steppe

Background

Although community structure and species richness are known to respond to nitrogen fertilization dramatically, little is known about the mechanisms underlying specific species replacement and richness loss. In an experiment in semiarid temperate steppe of China, manipulative N addition with five treatments was conducted to evaluate the effect of N addition on the community structure and species richness.

Methodology/Principal Findings

Species richness and biomass of community in each plot were investigated in a randomly selected quadrat. Root element, available and total phosphorus (AP, TP) in rhizospheric soil, and soil moisture, pH, AP, TP and inorganic N in the soil were measured. The relationship between species richness and the measured factors was analyzed using bivariate correlations and stepwise multiple linear regressions. The two dominant species, a shrub Artemisia frigida and a grass Stipa krylovii, responded differently to N addition such that the former was gradually replaced by the latter. S. krylovii and A. frigida had highly-branched fibrous and un-branched tap root systems, respectively. S. krylovii had higher height than A. frigida in both control and N added plots. These differences may contribute to the observed species replacement. In addition, the analysis on root element and AP contents in rhizospheric soil suggests that different calcium acquisition strategies, and phosphorus and sodium responses of the two species may account for the replacement. Species richness was significantly reduced along the five N addition levels. Our results revealed a significant relationship between species richness and soil pH, litter amount, soil moisture, AP concentration and inorganic N concentration.

Conclusions/Significance

Our results indicate that litter accumulation and soil acidification accounted for 52.3% and 43.3% of the variation in species richness, respectively. These findings would advance our knowledge on the changes in species richness in semiarid temperate steppe of northern China under N deposition scenario.     

Effect of Gibberellin on Growth, Protein Secretion, and Starch Accumulation in Maize Endosperm Suspension Cells

The physiologic effect of gibberellins (GA) in seed development is poorly understood. We examined the effect of gibberellic acid (GA₃) on growth, protein secretion, and starch accumulation in cultured maize (Zea mays L.) endosperm suspension cells. GA₃ (5 and 30 μm) increased the fresh weight, dry weight, and protein content of the cultured cells, but the effect of GA₃ at 50 μm was not significantly different. However, the protein content in the culture medium was increased by these three concentrations of GA₃. The effect of GA₃ on the amount of cellular structural polysaccharides was not significant, but GA₃ had a dramatic effect on the starch content. At 5 μm, GA₃ caused an increase in the starch content, but at 50 μm the starch accumulation was reduced. Chlorocholine chloride (CCC), an inhibitor of GA biosynthesis, significantly increased the starch content and decreased the structural polysaccharide content of the cultured cells. The effects of CCC at 500 μm on the starch and polysaccharide content were partially reversed by 5 μm GA₃ applied exogenously. Based on these results we suggest that GA does not favor starch accumulation in the cell cultures and that the addition of lower concentrations of GA₃ in the medium may provide an improved balance among the endogenous GA in the cultured cells. Received October 31, 1995; accepted March 25, 1997     

Phosphorus Nutrition Influence on Starch and Sucrose Accumulation, and Activities of ADP-Glucose Pyrophosphorylase and Sucrose-Phosphate Synthase during the Grain Filling Period in Soybean

Several lines of evidence indicate that the partitioning of photosynthate between starch and sucrose is influenced by the relative concentrations of inorganic phosphate (Pi) in the cytosol and chloroplast. Two greenhouse experiments were conducted to determine the influence of long-term differences in soil P levels, ranging from deficient to supraoptimum, on leaf starch and sucrose concentrations, and activities of adenosine diphosphate glucose (ADPG) pyrophosphorylase and sucrose-phosphate synthase (SPS) during the grain filling period in soybean (Glycine max [L.] Merr.). It was hypothesized that, compared with optimum P nutrition, leaf starch and sucrose concentrations would be increased and decreased, respectively, for P deficiency and visa versa for supraoptimum P nutrition. Relative to the optimum soil P level, leaf Pi concentration was not altered by P deficiency but was increased two- to fourfold for the supraoptimum soil P treatment. The concentrations of leaf starch and sucrose were not markedly affected by any of the P fertility treatments and were not closely related to the activities of ADPG pyrophosphorylase and SPS. P deficiency resulted in increased activity of both enzymes in one of the experiments. The results indicated that long-term soil P treatments, that caused either large decreases in plant growth (P deficiency) or large increases in leaf Pi concentration (supraoptimum P), did not markedly alter starch and sucrose metabolism. Furthermore, it can be inferred that the method of plant culture and/or imposition of the P treatments is a critical factor in interpreting results of P nutrition studies.     

Starch Granule Biosynthesis in Arabidopsis Is Abolished by Removal of All Debranching Enzymes but Restored by the Subsequent Removal of an Endoamylase

Several studies have suggested that debranching enzymes (DBEs) are involved in the biosynthesis of amylopectin, the major constituent of starch granules. Our systematic analysis of all DBE mutants of Arabidopsis thaliana demonstrates that when any DBE activity remains, starch granules are still synthesized, albeit with altered amylopectin structure. Quadruple mutants lacking all four DBE proteins (Isoamylase1 [ISA1], ISA2, and ISA3, and Limit-Dextrinase) are devoid of starch granules and instead accumulate highly branched glucans, distinct from amylopectin and from previously described phytoglycogen. A fraction of these glucans are present as discrete, insoluble, nanometer-scale particles, but the structure and properties of this material are radically altered compared with wild-type amylopectin. Superficially, these data support the hypothesis that debranching is required for amylopectin synthesis. However, our analyses show that soluble glucans in the quadruple DBE mutant are degraded by α- and β-amylases during periods of net accumulation, giving rise to maltose and branched malto-oligosaccharides. The additional loss of the chloroplastic α-amylase AMY3 partially reverts the phenotype of the quadruple DBE mutant, restoring starch granule biosynthesis. We propose that DBEs function in normal amylopectin synthesis by promoting amylopectin crystallization but conclude that they are not mandatory for starch granule synthesis.     

Effect of Chrysanthemum Plant Extract on Flower Initiation in Short-Day Plants

An extract was prepared from Chrysanthemum plants in flower. It was separated into eight different fractions putting emphasis on the sterol components of the extracts. Fractions 5 and 6 were found to contain active substance (s), which to some extent, brought about floral bud initiation in both Chrysanthemum and Xanthium. The active substance (s) present in the above two fractions might be a sterol (s), similar to those of sitosterol and stigmasterol or other related compounds. Gibberellic acid brought about stem elongation as usual, but was found ineffective in causing any tissue differentiation in both Chrysanthemum and Xanthium. Investigations are underway to separate and identify the different compounds present in fractions 5 and 6. The effects of these compounds on test plants, both individually and in combinations, might provide some valuable information as to the nature of the flowering hormone.     

Plastidic Phosphoglucose Isomerase Is an Important Determinant of Starch Accumulation in Mesophyll Cells,Growth, Photosynthetic Capacity,and Biosynthesis of Plastidic Cytokinins in Arabidopsis

Phosphoglucose isomerase (PGI) catalyzes the reversible isomerization of glucose-6-phosphate and fructose-6-phosphate. It is involved in glycolysis and in the regeneration of glucose-6-P molecules in the oxidative pentose phosphate pathway (OPPP). In chloroplasts of illuminated mesophyll cells PGI also connects the Calvin-Benson cycle with the starch biosynthetic pathway. In this work we isolated pgi1-3, a mutant totally lacking pPGI activity as a consequence of aberrant intron splicing of the pPGI encoding gene, PGI1. Starch content in pgi1-3 source leaves was ca. 10-15% of that of wild type (WT) leaves, which was similar to that of leaves of pgi1-2, a T-DNA insertion pPGI null mutant. Starch deficiency of pgi1 leaves could be reverted by the introduction of a sex1 null mutation impeding β-amylolytic starch breakdown. Although previous studies showed that starch granules of pgi1-2 leaves are restricted to both bundle sheath cells adjacent to the mesophyll and stomata guard cells, microscopy analyses carried out in this work revealed the presence of starch granules in the chloroplasts of pgi1-2 and pgi1-3 mesophyll cells. RT-PCR analyses showed high expression levels of plastidic and extra-plastidic β-amylase encoding genes in pgi1 leaves, which was accompanied by increased β-amylase activity. Both pgi1-2 and pgi1-3 mutants displayed slow growth and reduced photosynthetic capacity phenotypes even under continuous light conditions. Metabolic analyses revealed that the adenylate energy charge and the NAD(P)H/NAD(P) ratios in pgi1 leaves were lower than those of WT leaves. These analyses also revealed that the content of plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP)-pathway derived cytokinins (CKs) in pgi1 leaves were exceedingly lower than in WT leaves. Noteworthy, exogenous application of CKs largely reverted the low starch content phenotype of pgi1 leaves. The overall data show that pPGI is an important determinant of photosynthesis, energy status, growth and starch accumulation in mesophyll cells likely as a consequence of its involvement in the production of OPPP/glycolysis intermediates necessary for the synthesis of plastidic MEP-pathway derived hormones such as CKs.     

Focus on Metabolism: Molecular Genetic Analysis of Glucan Branching Enzymes from Plants and Bacteria in Arabidopsis Reveals Marked Differences in Their Functions and Capacity to Mediate Starch Granule Formation

The major component of starch is the branched glucan amylopectin, the branching pattern of which is one of the key factors determining its ability to form semicrystalline starch granules. Here, we investigated the functions of different branching enzyme (BE) types by expressing proteins from maize (Zea mays BE2a), potato (Solanum tuberosum BE1), and Escherichia coli (glycogen BE [EcGLGB]) in Arabidopsis (Arabidopsis thaliana) mutant plants that are deficient in their endogenous BEs and therefore, cannot make starch. The expression of each of these three BE types restored starch biosynthesis to differing degrees. Full complementation was achieved using the class II BE ZmBE2a, which is most similar to the two endogenous Arabidopsis isoforms. Expression of the class I BE from potato, StBE1, resulted in partial complementation and high amylose starch. Expression of the glycogen BE EcGLGB restored only minimal amounts of starch production, which had unusual chain length distribution, branch point distribution, and granule morphology. Nevertheless, each type of BE together with the starch synthases and debranching enyzmes were able to create crystallization-competent amylopectin polymers. These data add to the knowledge of how the properties of the BE influence the final composition of starch and fine structure of amylopectin.Starch is composed of two glucan polymers: amylopectin and amylose. Amylopectin constitutes around 80% of the mass of most starches and is a large, branched polymer with a tree-like architecture. The positioning and frequency of branch points together with the distribution of chain lengths are thought to be critical factors allowing amylopectin to adopt a semicrystalline state. Within amylopectin molecules, clusters of unbranched chain segments align, and adjacent chains form double helices. These pack into crystalline lamellae that alternate with amorphous regions containing the branch points. Longer chain segments span from one cluster to the next (Zeeman et al., 2010).Amylopectin is synthesized by three enzyme activities. First, starch synthases (SSs) transfer the glucosyl part of ADP-Glc to the nonreducing end of existing glucan chains, forming new α-1,4 glucosidic bonds. Second, branching enzymes (BEs) cleave part of an α-1,4-linked chain and through an inter- or intramolecular transfer reaction, reattach it, creating α-1,6-branch points. This reaction creates additional nonreducing ends on which SSs can act. Third, debranching enzymes (DBEs) hydrolyze some of these branches, tailoring the structure of the polymer to promote its crystallization.Several SS and BE isoforms are involved in starch synthesis in plants. There are five conserved classes of SSs (granule-bound starch synthase [GBSS] and SS1–SS4) and two conserved classes of BEs (classes I and II; also referred to as classes B and A, respectively; Nougué et al., 2014). In addition, plants contain two classes of DBEs: isoamylases (ISAs) and limit dextrinases (LDAs; also called pullulanases). One ISA, a multimeric enzyme composed of either a mixture of ISA1 and ISA2 subunits or just ISA1 subunits, is primarily involved in amylopectin synthesis (James et al., 1995; Mouille et al., 1996; Nakamura et al., 1996; Delatte et al., 2005). The other DBEs (i.e. ISA3 and LDA) are primarily involved in starch degradation (Wattebled et al., 2005; Delatte et al., 2006).Based on the in vitro analysis of purified or recombinant proteins and the phenotypes of mutant plants, the different SS isoforms are proposed to have distinct, albeit overlapping, functions. SS1 is thought to preferentially elongate short chains produced by the branching reactions to between 8 and 12 Glc units (Delvallé et al., 2005; Fujita et al., 2006). SS2 is proposed to elongate such chains farther to about 20 Glc units, optimal for cluster formation (Edwards et al., 1999; Umemoto et al., 2002; Zhang et al., 2008). The precise role of SS3 is less clear, although it has been proposed to generate long, cluster-spanning chains (Fujita et al., 2007). SS4 has a distinct role in initiating and/or coordinating granule formation (Roldán et al., 2007; Crumpton-Taylor et al., 2013).The two different BE classes are also proposed to have distinct functions in amylopectin synthesis. In vitro analyses of maize (Zea mays), rice (Oryza sativa), and potato (Solanum tuberosum) enzymes suggest that the class I enzymes preferentially act on amylose and transfer longer chains, whereas class II enzymes preferentially act on branched substrates, such as amylopectin, and transfer shorter chains (Guan and Preiss, 1993; Rydberg et al., 2001; Nakamura et al., 2010). This knowledge derives largely from experiments where linear or branched substrates were provided to recombinant or purified enzymes and the increased degree of branching was monitored. Similar conclusions were gained by recombinant protein expression in Escherichia coli and yeast (Saccharomyces cerevisiae) strains deficient in their endogenous glycogen BEs (Guan et al., 1995; Seo et al., 2002), where chain elongation by glycogen synthases occurred concurrently with branching.Models have been proposed in which both BE classes help create the final cluster structure of amylopectin: class I BEs initiate branching by transferring long or branched chains, which are subsequently acted on by class II BEs to create more numerous shorter chains. These shorter chains are then elaborated by the SSs to create the clusters (Nakamura et al., 2010). After the branching reactions, a degree of debranching occurs, which is thought to control branch number and positioning and thereby, facilitate amylopectin crystallization (Myers et al., 2000; Zeeman et al., 2010). Several studies have shown that isa1-deficient mutants produce starch with an altered amylopectin, accumulate a related soluble polymer (phytoglycogen), or both (James et al., 1995; Mouille et al., 1996; Nakamura et al., 1996; Delatte et al., 2005).Despite the wide conservation of the two BE classes, major alterations in starch properties are only observed when genes encoding class II enzymes are mutated or repressed. Loss of class I BE activity in maize endosperm, rice endosperm, or potato tuber did not alter starch content and caused only minor differences in amylopectin structure (e.g. the distribution of chain lengths and branch points) and/or starch properties (e.g. gelatinization or digestibility; Safford et al., 1998; Blauth et al., 2002; Satoh et al., 2003; Xia et al., 2011). In contrast, loss of class II BE results in significant changes, such as decreased starch content and a high apparent amylose content. This has been observed in several species, including maize (Stinard et al., 1993), potato (Jobling et al., 1999), pea (Pisum sativum; Bhattacharyya et al., 1990), rice (Mizuno et al., 1993), barley (Hordeum vulgare; Regina et al., 2010), and wheat (Triticum aestivum; Regina et al., 2006). The high apparent amylose content was caused at least in part by the accumulation of less-frequently branched amylopectin that stains with a higher wavelength of maximal absorption (λ_max) than that of the wild type (Boyer et al., 1976). In potato, this phenotype was enhanced by the simultaneous suppression of BE1 (Schwall et al., 2000), a result also shown recently in barley (Carciofi et al., 2012).Arabidopsis (Arabidopsis thaliana) has three genes annotated as BEs, At3g20440 (BE1), At5g03650 (BE2), and At2g36390 (BE3), but it seems that only BE2 and BE3 are active. Both BE2 and BE3 are class II BEs, making Arabidopsis somewhat unusual in not possessing a class I BE. The gene annotated as BE1 encodes a related protein that falls into a separate clade to either class I or II BEs (Dumez et al., 2006; Han et al., 2007; Wang et al., 2010). It was initially suggested that plants with mutations in this gene had a wild-type phenotype (Dumez et al., 2006), but subsequent work indicated that homozygous be1 mutation causes embryo lethality (hence, its alternative name EMBRYO DEFECTIVE2729; Wang et al., 2010). Thus, the function of the protein encoded at At3g20440 is currently unknown but unlikely to be a functional BE.The be2 and be3 single mutants have phenotypes that closely resemble the wild type, indicating that there is a high degree of redundancy between the enzymes. However, be2be3 double mutants lack starch (Dumez et al., 2006). Instead, the plants accumulate large amounts of maltose and other linear malto-oligosaccharides (MOSs). This is presumably because linear chains produced by the SSs are cleaved by starch-degrading enzymes (α- and β-amylases; Dumez et al., 2006). The altered metabolism of these double-mutant plants impedes growth, and they are smaller and paler than the wild type. The precise reason for this is unclear.In addition to mutagenesis, there have been several studies where BEs were overexpressed in transgenic plants. Overexpression of the E. coli glycogen BE (EcGLGB) in potato tubers or rice endosperm resulted in an increased degree of branching of amylopectin (Shewmaker et al., 1994; Kortstee et al., 1996; Kim et al., 2005). Overexpression of endogenous plant BE2 genes has also been performed in both rice and potato, increasing the proportion of shorter amylopectin chains (Tanaka et al., 2004; Brummell et al., 2015), and rice, leading to the accumulation of highly branched, water-soluble polysaccharides (Tanaka et al., 2004). Transgenic expression of genes from different photosynthetic organisms has also shown the degree of functional conservation within the plant BE classes. Sawada et al. (2009) showed that class II BE from Chlorella kessleri could rescue the BE2b-deficient phenotype in rice endosperm.The aim of this work was to investigate the capacity of different types of BEs to mediate starch granule formation by assessing their ability to function in the context of an otherwise intact starch biosynthesis pathway. To do this, we used the Arabidopsis be2be3 double mutants as a line in which to express three types of BEs. We chose BE2a from maize (required for leaf starch synthesis and similar to the endogenous Arabidopsis proteins; Yandeau-Nelson et al., 2011), BE1 from potato (represents the plant class I BEs that Arabidopsis lacks; Safford et al., 1998), and GLGB (the BE from E. coli involved in glycogen biosynthesis). This approach differs from previous investigations, because the activity of each BE type (working in planta with the same set of SSs and DBEs) can be assessed, and the results can be directly compared. In addition, we sought to address whether a glycogen BE was sufficient for starch production—in other words, whether the remaining starch biosynthetic enzymes are capable of generating a crystallization competent polymer, even when partnered with a BE with a different specificity. In previously described transgenic plants expressing E. coli GLGB, the endogenous plant BEs were still present (Shewmaker et al., 1994; Kortstee et al., 1996; Kim et al., 2005).In the transgenic lines generated here, we analyzed glucan synthesis, starch structure, and composition. Our results show that all three BE types can mediate starch granule production but to differing degrees. In each case, the structure of amylopectin and the amylose content depend on the type of BE present, as does starch granule morphology. We discuss the reasons for these differences in relation to previously reported BE properties.     

Convergent Evolution of Polysaccharide Debranching Defines a Common Mechanism for Starch Accumulation in Cyanobacteria and Plants

Starch, unlike hydrosoluble glycogen particles, aggregates into insoluble, semicrystalline granules. In photosynthetic eukaryotes, the transition to starch accumulation occurred after plastid endosymbiosis from a preexisting cytosolic host glycogen metabolism network. This involved the recruitment of a debranching enzyme of chlamydial pathogen origin. The latter is thought to be responsible for removing misplaced branches that would otherwise yield a water-soluble polysaccharide. We now report the implication of starch debranching enzyme in the aggregation of semicrystalline granules of single-cell cyanobacteria that accumulate both glycogen and starch-like polymers. We show that an enzyme of analogous nature to the plant debranching enzyme but of a different bacterial origin was recruited for the same purpose in these organisms. Remarkably, both the plant and cyanobacterial enzymes have evolved through convergent evolution, showing novel yet identical substrate specificities from a preexisting enzyme that originally displayed the much narrower substrate preferences required for glycogen catabolism.     

The Effect of Deleterious Concentrations of Mercury on the Photosynthesis and Growth of Chlorella pyrenoidosa

The effect of mercury ions on the photosynthesis and growth of Chlorella pyrenoidosa in a balanced medium has been studied and compared with the effect of copper. In many ways Hg treated algae behave like algae treated with Cu in concentrations of the same molarity, but important deviations occur. Hg acts at a lower and in a more narrow range of concentrations than does Cu due to a more specific binding. The depressing effect of Hg is not counteracted by other cations such as potassium and sodium, and iron has only a slight effect. Cell division is stopped after Hg addition and there is no accumulation of assimilation products. On the contrary the cells become pale yellow. Preliminary studies indicate a light-independent leakage in the cytoplasmatic membrane leading to an outflow of potassium ions. This is the primary action of both Hg and Cu poisoning, but the leakage does not seem to be correlated with the decrease of photosynthesis. After a lapse of time — dependent on the mercury/cell concentration ratio — a detoxication takes place probably due to the binding of Hg to “insensitive sites” in the cells. Probable mechanisms for the action of Hg on photosynthesis and growth are discussed.     

LED照明对植物体内功能性化学物质积累的影响

LED照明对植物生长发育作用的研究已进行了多年, 但之前学者们多从植物形态学角度进行研究。近年来, 该项研究已逐步转向LED照明对植物体内功能性化学物质积累的影响这一领域。该文就这方面研究的最新进展进行了评述, 并指出了目前存在的问题及今后研究的思路和可能方向, 以供生产和研究部门科研人员参考。     

The Effect of an Incandescent Supplement on the Growth of Tomato Plants in Low Light

Young tomato plants were grown at low light flux densities (21W m^-2 for 8 h days) in growth cabinets under three types offluorescent lamps or under a fluorescent/incandescent mixedsource. Whilst net assimilation rates under the fluorescentlamps were in agreement with those calculated from the lampcharacteristics and the photosynthetic action spectrum, therate under the mixed source was about 20 per cent higher thanexpected. Relative growth rates and relative leaf area growthrates were also higher and leaf area ratios lower under thefluorescent/incandescent lamp combination than under the purefluorescent sources. Small differences in stem elongation, leaftemperature and dry weight distribution which were associatedwith the addition of incandescent radiation were not consideredto be responsible for these increases. When the light flux densityfrom the mixed source was reduced by 20 per cent, the plantgrowth parameters were then similar to those in fluorescentlight alone.