Starch Synthase Activity Research Articles

Biochemical and physiological responses of bulblets of Lycoris aurea to exogenously applied N-(2-chloro-4-pyridyl)-N1-phenylurea

SummaryOne-year-old bulblets of Lycoris aurea were obtained by propagation of cuttings. Different concentrations of N-(2-chloro-4-pyridyl)-N1-phenylurea (CPPU; 0, 1.0, 2.0, 5.0, 7.5, or 10.0 mg l 1) were sprayed on all the leaves on each plant at the green-leaf stage. Morphological indices including bulblet fresh weights (FW) and diameters, carbohydrate concentrations, the activities of enzymes related to starch metabolism, and the concentrations of endogenous hormones were determined in bulblets during dormancy. Bivariate correlation analysis was used to analyse the relationships of all parameters. The results showed that spray treatment with an appropriate concentration of CPPU was beneficial for the enlargement of L. aurea bulblets. The optimum concentration was 1.0 mg l 1. Bivariate correlation analysis showed that bulblet FW and diameter had a significant (P ≤ 0.05) positive correlation with starch concentration. The concentration of zeatin riboside (ZR) showed a significant (P ≤ 0.01) positive correlation with soluble starch synthase (SSS) and granule-bound starch synthase (GBSS) activities. Our study indicated that 1.0 mg l 1 CPPU promoted the activity of starch synthase and accelerated the accumulation of starch in bulblets, and that the observed increase in endogenous ZR may also have an important role.

Purification of Starch Granules from Arabidopsis Leaves and Determination of Granule-Bound Starch Synthase Activity

Purification of Starch Granules from Arabidopsis Leaves and Determination of Granule-Bound Starch Synthase Activity

Effects of Elevated Ozone Concentration on Starch and Starch Synthesis Enzymes of Yangmai 16 Under Fully Open-Air Field Conditions

O3 is not only greenhouse gas but also a primary gaseous contaminant in the atmosphere. It has long-lasting effects on crop growth, yield and quality, and brings a series of ecological and environmental problems. A free-air controlled enrichment (FACE) system was applied to study the effect of elevated ozone concentration on activities of key enzymes of starch synthesis of Yangmai 16 in 2009-2010. The main-plot treatment had two levels of O3: ambient level (A-O3) and 50% higher than ambient level (E-O3). The main results were that accumulation rate of amylose, amylopectin and starch were represented in a single peak curve, and their content and accumulation amount rose gradually. The O3 elevation decreased the accumulation rate of amylose, amylopectin and starch amylase, reduced the accumulation amount of amylopectin and starch, and decreased the content of amylopectin and starch, but increased the content of amylose. With the increase of O3 concentration, the enzyme activity of grain granule-bound starch synthase (GBSS), soluble starch synthase (SSS) and starch branching enzyme (SBE) decreased after anthesis. The activities of GBSS and SSS had highly significant correlations with amylose, amylopectin and starch accumulation rate, and the activity of SBE had significant correlations with these items. So the O3 elevation decreased the activity of key enzymes of starch synthesis, which led to the variation of starch synthesis.

Grain development in Brachypodium and other grasses: possible interactions between cell expansion, starch deposition, and cell-wall synthesis

To explain the low levels of starch, high levels of (1,3;1,4)-β-glucan, and thick cell walls in grains of Brachypodium distachyon L. relative to those in other Pooideae, aspects of grain development were compared between B. distachyon and barley (Hordeum vulgare L.). Cell proliferation, cell expansion, and endoreduplication were reduced in B. distachyon relative to barley and, consistent with these changes, transcriptional downregulation of the cell-cycle genes CDKB1 and cyclin A3 was observed. Similarly, reduced transcription of starch synthase I and starch-branching enzyme I was observed as well as reduced activity of starch synthase and ADP-glucose pyrophosphorylase, which are consistent with the lowered starch content in B. distachyon grains. No change was detected in transcription of the major gene involved in (1,3;1,4)-β-glucan synthesis, cellulose synthase-like F6. These results suggest that, while low starch content results from a reduced capacity for starch synthesis, the unusually thick cell walls in B. distachyon endosperm probably result from continuing (1,3;1,4)-β-glucan deposition in endosperm cells that fail to expand. This raises the possibility that endosperm expansion is linked to starch deposition.

Starch synthase 4 is essential for coordination of starch granule formation with chloroplast division during Arabidopsis leaf expansion

Arabidopsis thaliana mutants lacking the SS4 isoform of starch synthase have strongly reduced numbers of starch granules per chloroplast, suggesting that SS4 is necessary for the normal generation of starch granules. To establish whether it plays a direct role in this process, we investigated the circumstances in which granules are formed in ss4 mutants.Starch granule numbers and distribution and the accumulation of starch synthase substrates and products were investigated during ss4 leaf development, and in ss4 mutants carrying mutations or transgenes that affect starch turnover or chloroplast volume.We found that immature ss4 leaves have no starch granules, but accumulate high concentrations of the starch synthase substrate ADPglucose. Granule numbers are partially restored by elevating the capacity for glucan synthesis (via expression of bacterial glycogen synthase) or by increasing the volumes of individual chloroplasts (via introduction of arc mutations). However, these granules are abnormal in distribution, size and shape.SS4 is an essential component of a mechanism that coordinates granule formation with chloroplast division during leaf expansion and determines the abundance and the flattened, discoid shape of leaf starch granules.

Analysis of gene expression and enzyme activities related to starch metabolism in Lycoris sprengeri bulbs of different sizes

Differentially expressed genes related to starch metabolism in mature and juvenile bulbs of Lycoris sprengeri were screened via tag-based digital gene expression profiling. The relative expression levels of these genes were further verified by quantitative real-time polymerase chain reaction. Related enzyme activities and carbohydrate concentrations were determined to study the molecular mechanism of starch accumulation during bulb development. Four starch metabolic genes (LsBAM1, LsAMY1, LsAMY3 and LsSS1) were differentially expressed between the mature and juvenile bulbs. LsSS1 is utilised during starch synthesis and the three other genes are utilised for starch degradation. The expressions of the four genes were significantly higher in juvenile bulbs than in mature bulbs. Along with bulb development (seven sizes, from bulblets to mature bulbs), β-amylase (BAM) activity significantly increased whereas α-amylase (AMY) activity slightly fluctuated. The expression levels of LsBAM1, LsAMY1 and LsAMY3 were either negatively correlated or not correlated with BAM and AMY activities. Soluble starch synthase (SSS) activity increased with decreased LsSS1 expression whereas granule-bound starch synthase (GBSS) activity decreased, indicating a positive correlation with the decreased expression level of LsSS1. During bulb development, the carbohydrate concentrations increased. In the largest mature bulbs, the highest concentration of total soluble sugar coincided with the highest β-amylase activity. However, no evidence was found to confirm the relationship between this phenomenon and related gene expression. The results revealed preliminary characteristics related to starch metabolism during the bulb development of L. sprengeri.

Multigene engineering of starch biosynthesis in maize endosperm increases the total starch content and the proportion of amylose

Maize (Zea mays spp. mays) is a staple crop for more than 900 million people. The seeds or kernels provide a rich source of calories because ~70% of the weight is carbohydrate, mostly in the form of starch. The content and composition of starch are complex traits controlled by many genes, offering multiple potential targets for intervention. We used a multigene engineering approach combining the overexpression of Bt2, Sh2, Sh1 and GbssIIa (to enhance the activity of sucrose synthase, AGPase and granule-bound starch synthase) with the suppression of SbeI and SbeIIb by RNA interference (to reduce the activity of starch branching enzyme). Maize plants expressing all six genes plus the selectable marker showed a 2.8-7.7% increase in the endosperm starch content and a 37.8-43.7% increase in the proportion of amylose, which was significant compared to untransformed control plants. We also observed improvements in other agronomic traits, such as a 20.1-34.7% increase in 100-grain weight, a 13.9-19.0% increase in ear weight, and larger kernels with a better appearance, presumably reflecting the modified starch structure within the kernels. Our results confirm that multigene engineering applied to the starch biosynthesis pathway can not only modulate the quality and quantity of starch but can also improve starch-dependent agronomic traits.

Structure of starch synthase I from barley: insight into regulatory mechanisms of starch synthase activity

Starch, a polymer of glucose, is the major source of calories in the human diet. It has numerous industrial uses, including as a raw material for the production of first-generation bioethanol. Several classes of enzymes take part in starch biosynthesis, of which starch synthases (SSs) carry out chain elongation of both amylose and amylopectin. Plants have five classes of SS, each with different roles. The products of the reaction of SS are well known, but details of the reaction mechanism remain obscure and even less is known of how different SSs select different substrates for elongation, how they compete with each other and how their activities are regulated. Here, the first crystal structure of a soluble starch synthase is presented: that of starch synthase I (SSI) from barley refined to 2.7 Å resolution. The structure captures an open conformation of the enzyme with a surface-bound maltooligosaccharide and a disulfide bridge that precludes formation of the active site. The maltooligosaccharide-binding site is involved in substrate recognition, while the disulfide bridge is reflective of redox regulation of SSI. Activity measurements on several SSI mutants supporting these roles are also presented.

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.

Physiological and metabolic enzymes activity changes in transgenic rice plants with increased phosphoenolpyruvate carboxylase activity during the flowering stage

To compare the differences in physiology and metabolism between phosphoenolpyruvate carboxylase (PEPC) transgenic rice and its control, untransformed wild rice, dry matter accumulation, soluble sugar, starch and protein contents and enzyme activities were determined in different plant parts during flowering. Results revealed that PEPC transgenic rice had higher dry weights for leaf, stem and sheath as well as panicle than the untransformed wild rice did, with the largest increase in the panicle. Soluble sugar and protein content in the grains of PEPC transgenic rice were significantly enhanced while starch content changed less. PEPC transgenic rice exhibited high levels of PEPC activity, manifesting in high net photosynthetic rates during flowering. Moreover, transgenic rice with high PEPC expression levels also had elevated levels of the enzymes such as sucrose-p-synthase and sucrose synthase, which may confer a higher capacity to assimilate CO2 into sucrose. Little increase in grain starch content was observed in transgenic plants due to the stable activities of starch synthase and Q enzyme. However, the PEPC transgenic rice plant induced the activities of nitrate reductase, glutamate oxaloacetate transaminase, glutamate pyruvate transaminase, glutamine synthetase, and asparagine synthase to high levels, as compared with the untransformed rice plant. PEPC activity was correlated with protein content in grains and the enzymes of nitrogen metabolism, suggesting that high PEPC activity in transgenic rice might be able to redirect carbon and nitrogen flow by regulating some enzymes related to carbon or nitrogen metabolisms. These results may help to understand how the C3 plants possessing a C4-like photosynthesis pathway worked by expression of PEPC.

RNA interference-mediated silencing of the starch branching enzyme gene improves amylose content in rice

Amylose and amylopectin are the 2 major components of plant storage starch. The rice starch branching enzyme (RBE) plays an important role in the starch components of rice. In the present study, we selected a specific 195-bp segment from the RBE3 gene to construct hairpin DNA, which was driven by an endosperm-specific high molecular weight glutenin promoter to regulate the biosynthesis of starch. An RNA interference plasmid for the RBE3 gene was constructed to form double-stranded RNA. Following Agrobacterium-mediated rice transformation (in the cultivar Zhonghua 11), 41 transgenic plants were identified using PCR and Southern blot analysis. Semi-quantitative real-time PCR revealed that RBE3 gene expression was significantly reduced in immature transgenic seeds. Transgenic rice amylose content had an average increase of 140%. The highest rice amylose content was 47.61% and the growth rate increased 238% compared to the non-transgenic controls. Branching enzyme II activity was notably reduced, and ADP-glucose pyrophosphorylase, soluble starch synthase, isoamylase, and pullulanase enzyme activity was markedly reduced in T3 seeds. Relative enzyme activity change explained the reduction in thousand-grain weight in transgenic plants. The present study indicated that amylose content was negatively correlated with branching enzyme II activity, spike size, and thousand-grain weight.

Characterization of Carbohydrates and Their Metabolizing Enzymes Related to the Eating Quality of Postharvest Fresh Waxy Corn

Carbohydrates and their metabolizing enzymes related to sweetness and glutinosity were measured in the fresh waxy corn of “Zhongnuo 1” and “Jingkenuo 2000” cultivars during storage for 4 days at 20C. The contents of sugars decreased to a lesser degree in “Jingkenuo 2000” compared to those of “Zhongnuo 1,” and both cultivars exhibited similar trends in starch content. The amylopectin and total starch contents both increased significantly and then gradually decreased, whereas the amylose content significantly increased. Furthermore, the activities of both uridine diphosphate glucose pyrophosphorylase (UGPase) and starch debranched enzyme exhibited a similar pattern with a single peak curve. The activities of adenosine diphosphate glucose pyrophosphorylase (AGPase) and soluble starch synthase decreased, whereas the activity of granule-bound starch synthase (GBSS) increased significantly. The activity of the starch-branching enzyme was mostly constant. Correlation analysis between carbohydrates and metabolizing enzymes indicates that UGPase, AGPase and GBSS may be important enzymes. Practical Applications Fresh waxy corn is widely eaten because of its sweetness and glutinosity, which are the eternal behaviors of related carbohydrate character. Contrary to other fruits and vegetables with high soluble sugar and water content, fresh waxy corn shows high starch content and low water content. Thus, the physiological basis for changes in eating quality may differ between fresh waxy corn and other fruits and vegetables. That means the postharvest changes in carbohydrates of common fruits and vegetables are not applicable to instruct the storage and retail of fresh waxy corn. In this study, we report the changes of related carbohydrates and associated enzyme activities and the correlations between them. The research realizes the reason for eating quality changing and controlling factors. These results can promote the quality of fresh waxy corn in the processing, storage and retail.

Glucan affinity of starch synthase IIa determines binding of starch synthase I and starch-branching enzyme IIb to starch granules

The sugary-2 mutation in maize (Zea mays L.) is a result of the loss of catalytic activity of the endosperm-specific SS (starch synthase) IIa isoform causing major alterations to amylopectin architecture. The present study reports a biochemical and molecular analysis of an allelic variant of the sugary-2 mutation expressing a catalytically inactive form of SSIIa and sheds new light on its central role in protein-protein interactions and determination of the starch granule proteome. The mutant SSIIa revealed two amino acid substitutions, one being a highly conserved residue (Gly522→Arg) responsible for the loss of catalytic activity and the inability of the mutant SSIIa to bind to starch. Analysis of protein-protein interactions in sugary-2 amyloplasts revealed the same trimeric assembly of soluble SSI, SSIIa and SBE (starch-branching enzyme) IIb found in wild-type amyloplasts, but with greatly reduced activities of SSI and SBEIIb. Chemical cross-linking studies demonstrated that SSIIa is at the core of the complex, interacting with SSI and SBEIIb, which do not interact directly with each other. The sugary-2 mutant starch granules were devoid of amylopectin-synthesizing enzymes, despite the fact that the respective affinities of SSI and SBEIIb from sugary-2 for amylopectin were the same as observed in wild-type. The data support a model whereby granule-bound proteins involved in amylopectin synthesis are partitioned into the starch granule as a result of their association within protein complexes, and that SSIIa plays a crucial role in trafficking SSI and SBEIIb into the granule matrix.

Biomolecular Analyses of Starch and Starch Granule Proteins in the High-Amylose Rice Mutant Goami 2

Elevated proportions of amylose in cereals are commonly associated with either the loss of starch branching or starch synthase activity. Goami 2 is a high-amylose mutant of the temperate japonica rice variety Ilpumbyeo. Genotyping revealed that Goami 2 and Ilpumbyeo carry the same alleles for starch synthase IIa and granule-bound starch synthase I genes. Analyses of granule-bound proteins revealed that SSI and SSIIa accumulate inside the mature starch granules of Goami 2, which is similar to the amylose extender mutant IR36ae. However, unlike the amylose extender mutants, SBEIIb was still detectable inside the starch granules of Goami 2. Detection of SBEIIb after protein fractionation revealed that most of the SBEIIb in Goami 2 accumulates inside the starch granules, whereas most of it accumulates at the granule surface in Ilpumbyeo. Exhaustive mass spectrometric characterisations of granule-bound proteins failed to detect any peptide sequence mutation or major post-translational modifications in Goami 2. Moreover, the signal peptide was found to be cleaved normally from the precursor protein, and there is no apparent N-linked glycosylation. Finally, no difference was found in the SBEIIb structural gene sequence of Goami 2 compared with Ilpumbyeo. In contrast, a G-to-A mutation was detected in the SBEIIb gene of IR36ae located at the splice site between exon and intron 11, which could potentially introduce a premature stop codon and produce a truncated form of SBEIIb. It is suggested that the mutation responsible for producing high amylose in Goami 2 is not due to a defect in SBEIIb gene as was observed in IR36ae, even though it produces a phenotype analogous to the amylose extender mutation. Understanding the molecular genetic basis of this mutation will be important in identifying novel targets for increasing amylose and resistant starch contents in rice and other cereals.

Binding of ABI4 to a CACCG motif mediates the ABA-induced expression of the ZmSSI gene in maize (Zea mays L.) endosperm

Starch synthase I (SSI) contributes the majority of the starch synthase activity in developing maize endosperm. In this work, the effects of various plant hormones and sugars on the expression of the starch synthase I gene (ZmSSI) in developing maize endosperms were examined. The accumulation of ZmSSI mRNA was induced using abscisic acid (ABA) but not with glucose, sucrose, or gibberellin treatment. To investigate the molecular mechanism underlying this effect, the ZmSSI promoter region (-1537 to +51) was isolated and analysed. A transient expression assay in maize endosperm tissue showed that the full-length ZmSSI promoter is activated by ABA. The results of deletion and mutation assays demonstrated that a CACCG motif in the ZmSSI promoter is responsible for the ABA inducibility. The results of binding shift assays indicated that this CACCG motif interacts with the maize ABI4 protein in vitro. The overexpression of ABI4 in endosperm tissue enhanced the activity of a promoter containing the CACCG motif in the absence of ABA treatment. Expression pattern analysis indicated that the transcription pattern of ABI4 in the developing maize endosperm was induced by ABA treatment but was only slightly affected by glucose or sucrose treatment. Taken together, these data indicate that ABI4 binds to the CACCG motif in the ZmSSI promoter and mediates its ABA inducibility.

An efficient and thermostable soluble starch synthase in developing maize (Zea mays) grains

High temperature during grain-filling stage is an important yield-limiting factor in wheat. It was reported earlier that a decrease in grain growth under high temperature is associated with a decrease in soluble starch synthase (SSS) activity, as this enzyme is extremely sensitive to high temperature. In attempt to identify thermostable SSS, a maize variety, in which grain development takes place under relatively warmer conditions, was compared with durum and aestivum wheat varieties in the present study. Maize was grown in rabi season so that grain development takes place in an increasing temperature as in wheat. The mean of maximum temperature during grain development period was highest for maize. In spite of such a high temperature exposure during grain development, the maize grains had a remarkably higher (3-4 times high) SSS activity as compared to wheat. Granule bound starch synthase (GBSS) activity was considerably lower than SSS activity, indicating SSS is the major enzyme for starch synthesis. High temperature exposure of excised developing grains showed no significant decrease in SSS activity in maize, whereas, T. aestivum var. Lok Bold showed a significant decrease. The present study reveals an efficient and relatively thermostable SSS in maize var. HQPM7 as compared to wheat, which could possibly be utilized in improving thermotolerance for grain growth in wheat.

Potent inhibition of starch-synthase by Tris-type buffers is responsible for the perpetuation of the primer myth for starch biosynthesis

Mukerjea and Robyt [Carbohydr. Res.2012, 352, 137–142] showed that a primer-free potato starch-synthase synthesized starch chains de novo, without the addition of a primer. A dichotomy arises as to why 61 studies from 1964 to the present have had to add a carbohydrate primer to obtain starch-synthase activity. All of these studies used 25–100mM Tris, Bicine, or Tricine buffers. We have found that the Tris-type buffers completely inhibit starch-synthase at these concentrations. The addition of 10mg/mL of the putative primers, glycogen and maltotetraose, gave a partial reversal of the inhibition, with glycogen being better than maltotetraose. It has been found that the Tris-type buffers form a complex with the ADPGlc substrate, removing it from the digest, causing the inhibition. The addition of the putative primers releases some of the ADPGlc from the complex, permitting it to act as a substrate for starch-synthase. The study definitively shows that the need for primers for starch-synthase by many investigators from 1964 to the present has been caused by Tris-type buffer inhibition that was partially reversed by putative primers. This has led to the perpetuation of the primer myth for the biosynthesis of starch chains by starch-synthase.

Two carbon fluxes to reserve starch in potato (Solanum tuberosum L.) tuber cells are closely interconnected but differently modulated by temperature

Parenchyma cells from tubers of Solanum tuberosum L. convert several externally supplied sugars to starch but the rates vary largely. Conversion of glucose 1-phosphate to starch is exceptionally efficient. In this communication, tuber slices were incubated with either of four solutions containing equimolar [U-14C]glucose 1-phosphate, [U-14C]sucrose, [U-14C]glucose 1-phosphate plus unlabelled equimolar sucrose or [U-14C]sucrose plus unlabelled equimolar glucose 1-phosphate. 14C-incorporation into starch was monitored. In slices from freshly harvested tubers each unlabelled compound strongly enhanced 14C incorporation into starch indicating closely interacting paths of starch biosynthesis. However, enhancement disappeared when the tubers were stored. The two paths (and, consequently, the mutual enhancement effect) differ in temperature dependence. At lower temperatures, the glucose 1-phosphate-dependent path is functional, reaching maximal activity at approximately 20 °C but the flux of the sucrose-dependent route strongly increases above 20 °C. Results are confirmed by in vitro experiments using [U-14C]glucose 1-phosphate or adenosine-[U-14C]glucose and by quantitative zymograms of starch synthase or phosphorylase activity. In mutants almost completely lacking the plastidial phosphorylase isozyme(s), the glucose 1-phosphate-dependent path is largely impeded. Irrespective of the size of the granules, glucose 1-phosphate-dependent incorporation per granule surface area is essentially equal. Furthermore, within the granules no preference of distinct glucosyl acceptor sites was detectable. Thus, the path is integrated into the entire granule biosynthesis. In vitro 14C-incorporation into starch granules mediated by the recombinant plastidial phosphorylase isozyme clearly differed from the in situ results. Taken together, the data clearly demonstrate that two closely but flexibly interacting general paths of starch biosynthesis are functional in potato tuber cells.

Starch biosynthesis in rice endosperm requires the presence of either starch synthase I or IIIa

Starch synthase (SS) I and IIIa are the first and second largest components of total soluble SS activity, respectively, in developing japonica rice (Oryza sativa L.) endosperm. To elucidate the distinct and overlapping functions of these enzymes, double mutants were created by crossing the ss1 null mutant with the ss3a null mutant. In the F2 generation, two opaque seed types were found to have either the ss1ss1/SS3ass3a or the SS1ss1/ss3ass3a genotype. Phenotypic analyses revealed lower SS activity in the endosperm of these lines than in those of the parent mutant lines since these seeds had different copies of SSI and SSIIIa genes in a heterozygous state. The endosperm of the two types of opaque seeds contained the unique starch with modified fine structure, round-shaped starch granules, high amylose content, and specific physicochemical properties. The seed weight was ∼90% of that of the wild type. The amount of granule-bound starch synthase I (GBSSI) and the activity of ADP-glucose pyrophosphorylase (AGPase) were higher than in the wild type and parent mutant lines. The double-recessive homozygous mutant prepared from both ss1 and ss3a null mutants was considered sterile, while the mutant produced by the leaky ss1 mutant×ss3a null mutant cross was fertile. This present study strongly suggests that at least SSI or SSIIIa is required for starch biosynthesis in rice endosperm.

Comparison of Kernel Starch Accumulation and Related Enzyme Activities among Six Maize Cultivars of Different Yield Types

Six maize cultivars were used to compare the difference of the related enzyme activities in kernel starch accumulation and starch synthesis among different yield cultivars.Sucrose synthase(SS),UDPG pyrophosphorylase(UGPase)and starch branching enzyme(SBE)were the key enzymes in maize starch synthesis.High activities of SS,UGPase and SBE were beneficial to the accumulation of starch and the improvement of kernel weight.The time of amylose accumulation in low yield cultivars(LYV)(30 d)was shorter than that in middle yield cultivars(MYV)(40 d)and high yield cultivars(HYV)(50 d).Amylopectin accumulation was slower in LYV than in MYV and HYV during the earlier stage of kernel-filling,but the difference among them became smaller during the later stage.SS activity in LYV kernel was higher than that in MYV and HYV during 10-30 days after pollination,but decreased rapidly at 30 days later after pollination and had larger decreasing range than that in HYV.The peak of UGPase activity appeared on the 20th day after pollination in LYV,while on the 40th day after pollination in MYV and HYV,and the activity in LYV was obviously lower than that in MYV and HYV during the later stage of kernel-filling stage.The SBE activity during the later stage of kernel-filling(30-50 days after pollination)was HYVMYVLYV,and the decreasing range was 72.44%(LYV),44.54%(MYV),and 30.21%(HYV).The activity of soluble starch synthase(SSS)in MYV and HYV showed"N" type curve changes with the first peak on the 30th day after pollination,while it showed single peak curve in LYV.The activity of ADPG pyrophosphorylase(ADPGPPase)in LYV was higher than that in MYV and HYV.The activity of starch debranching enzyme(DBE)in the three yield types of cultivar all decreased rapidly during the whole process of kernel-filling,and the difference among different types was not obvious.