Granule-bound Starch Synthase Activity Research Articles

Effects of low temperature on grain filling, amylose content, and activity of starch biosynthesis enzymes in endosperm of basmati rice

The effects of low temperature on amylose contents and activities of key enzymes related to starch biosynthesis in basmati rice (Oryza sativa L.) endosperm were investigated. Low temperature treatment prolonged the grain-filling period from 32 days to 53 days, but had no significant effect on the final grain weight. Results showed that low temperature during grain filling had no significant effect on total starch content but increased the amylose content in the mature endosperm of hulled rice by 21%. The measurable activities of sucrose synthase (SuSy), ADP-glucose pyrophosphorylase (AGPase), starch phosphorylase (SPase), starch branching enzyme (SBE), and soluble starch synthase (SS) in endosperms developed at 12°C were lower than those at 22°C when compared at a similar ripening stage on an endosperm basis, but the activity of granule-bound starch synthase (GBSS) was significantly higher than the corresponding activity in endosperms developing at 22°C. These findings suggest that GBSS might play a crucial role in increasing amylose during low-temperature growth conditions.

Effects of High Temperature on Key Enzymes Involved in Starch and Protein Formation in Grains of Two Wheat Cultivars

AbstractHigh temperature is a major determinant of grain growth and yield formation in wheat. The present study was undertaken to investigate the effects of high temperature regimes on the activities of key regulatory enzymes involved in starch and protein accumulation in grains of two winter wheat (Triticum aestivum L.) cultivars Yangmai 9 and Xuzhou 26 with different protein contents. Four day/night temperature regimes of 34 °C/22 °C, 32 °C/24 °C, 26 °C/14 °C and 24 °C/16 °C were established after anthesis, resulting in two daily temperature levels of 28 °C and 20 °C and two diurnal temperature differences of 12 °C and 8 °C. The activities of glutamine synthase (GS) in flag leaves and glutamate pyruvic aminotransferase (GPT), sucrose synthase (SS), soluble starch synthase (SSS) and granule‐bound starch synthase (GBSS) in grains were measured during the periods of grain filling. High temperature reduced both content and yield of starch in grains, while enhanced protein content and reduced protein yield in grains. High temperature significantly enhanced the activities of SS and GBSS on 14 days after anthesis (DAA). High temperature affected SSS slightly in Yangmai 9, but reduced SSS activity markedly in Xuzhou 26 on 14 DAA. However, at the middle and late stages of grain filling, high temperature reduced the activities of SS, GBSS and SSS significantly in the two wheat cultivars. High temperature reduced GPT activity in grains in the two wheat cultivars, but reduced GS activity in flag leaves of Yangmai 9 and enhanced GS activity of Xuzhou 26 on 14 DAA. In addition, under the same high temperature level, SS activity was higher at 34 °C/22 °C, whereas the activities of SSS and GBSS were higher at 32 °C/24 °C. Also, diurnal temperature differences affected GPT and GS activities differently between the two cultivars. Under optimum temperature level, the activities of key enzymes for starch and protein synthesis were higher at 26 °C/14 °C. The activities of SS, SSS and GBSS significantly correlated with starch accumulation in grains, except for GBSS activity to starch content on 14 DAA. GPT activity was positively correlated with protein yield, and GS activity was negatively correlated with protein yield on 14 DAA, while the activities of both GPT and GS were negatively related to protein content in grains.

Molecular regulation of starch accumulation in rice seedling leaves in response to salt stress

In this report we show that the starch content decreased in NaCl-stressed rice (Oryza sativa L.) seedling leaves during the daytime. Because photosynthetic efficiency and starch degradation enzyme activity were not significantly affected by the high salt, it is likely that this effect results from repression of starch biosynthesis. To determine the regulatory mechanism, the activities of enzymes such as ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), soluble starch synthase (SSS) and starch branching enzyme (SBE) involved in starch synthetic pathway were examined. Data suggest that NaCl-induced repression of GBSS activity was the most significant factor reducing starch accumulation. Based on real-time RT-PCR analysis, the effect of salinity on GBSS expression was primarily controlled on the transcriptional level. Furthermore, the salt-induced decrease of both GBSSI and GBSSII gene expressions could be mostly contributed by ion-specific effect and not by osmotic stress. Although the mRNA accumulation of GBSSI and GBSSII can be down-regulated by exogenous ABA, the negative influence of salt stress on GBSSI and II gene expression could be chiefly mediated via an ABA-independent pathway.

Expression of granule‐bound starch synthase in developing rice grain

AbstractThe expression of granule‐bound starch synthase (GBSS) in rice (Oryza sativa) genotype Tainung 67 (200 g kg−1 grain amylose content) and its two NaN3‐induced mutants SA419 and SA418 were examined. G/T polymorphism analysis indicated that SA418 (300 g kg−1 grain amylose content) carried Wxa allele. The insertion of 23 base pairs sequence was found only in SA419 (80 g kg−1 grain amylose content), suggesting that it was a waxy mutant. Microsatellite polymorphisms (CT)n were also detected on the Wx gene encoding GBSS in the tested genotypes. The activities of several key enzymes involving starch biosynthesis in developing grains of field‐grown rice plants were also compared during grain filling period. Significant genotypic differences were only found in the expression of GBSS. The content of amylose in SA418 grain was higher than Tainung 67 and SA419 grains throughout the entire grain filling period, possibly due to its superiority to synthesize amylose through GBSS. The lowest amylose content of SA 419 grain was attributable to its extremely low activity of GBSS in comparison with the two other genotypes. The mutation effects on the expression of GBSS were confirmed by two‐dimensional electrophoresis and mass spectrometry. Copyright © 2007 Society of Chemical Industry

Starch Accumulation and Enzyme Activities Associated with Starch Synthesis in Maize Kernels

Abstract The filling rate and the starch accumulation in developing maize kernel were analyzed. The changes of enzyme activities associated with sucrose metabolism and starch biosynthesis were investigated. The purpose is to discuss the enzymatic mechanisms responsible for starch synthesis. Two types of maize cultivars ( Zea mays), high starch maize (Feiyu 3) and normal maize (Yuyu 22), were grown in a corn field. The factors involved in starch synthesis were performed during the growth period. The kernel filling rate, the sucrose content, the starch accumulating rates and the activities of SS (sucrose synthase), GBSS (granule-bound starch synthase), SBE (starch branching enzyme) of Feiyu 3, which has high starch content, were significantly higher than those of Yuyu 22, which has low starch content, after 10 DAP (days after pollination). Correlation analysis indicated that ADPGPPase (ADP-glucose pyrophosphorylase) and DBE (starch debranching enzyme) were not correlated with the starch accumulating rates and the kernel filling rate, but the SS activity at the middle and late period were highly significantly correlated with the starch accumulating rates and the kernel filling rate. The GBSS activity was highly significantly correlated with the amylose accumulating rate, but not correlated with the kernel filling rate. The SBE activity was highly significantly correlated with the amylopectin accumulating rate and the kernel filling rate. It was not ADPGPPase and DBE, but SS was the rate-limiting factor of starch biosynthesis in developing maize kernels. GBSS had an important effect on amylose accumulation, and SBE had a significant effect on amylopectin accumulation.

Temperature induced changes in the starch components and biosynthetic enzymes of two rice varieties

The effects of temperature on starch and amylose accumulation, fine structure of amylopectin and activities of some enzymes related to starch synthesis in developing rice endosperms was examined. Two early indica rice varieties were used, differing in amylose concentration (AC, %), namely Jia 935 (low AC) and Jia 353 (high AC). The results showed that the effects of high temperature on AC and amylopectin fine structure were variety-dependent. High temperature caused a reduction in amylose concentration and an increase in the short chain (CL<22) proportion of amylopectin for Jia 935; while opposite was true for Jia 353. High temperature also reduced and increased the activity of granule-bound starch synthase (GBSS) in Jia 935 and in Jia 353, respectively. This suggests that a change in the ratio of amylose/starch due to temperature was attributable to a change in GBSS activity. Moreover, obvious differences between the two rice varieties were detected in the activities of sucrose synthase (SuSy), ADP-glucose pyrophosphorylase (ADPG-Ppase), soluble starch synthase (SSS), starch branching enzyme (SBE), starch de-branching enzyme (SDBE) and starch phosphorylase (SPase) to high temperature. Accumulation rate of amylose was significantly and positively correlated with GBSS for Jia 935, but not for Jia 353. Amylose accumulation was also significantly and positively correlated with the activities of SDBE, SBE, ADPG-Ppase and SuSy for both varieties. The results suggest that the ratio of amylose to starch in rice endosperm is not only related to GBSS, but also affected by the activities of SDBE, SBE, ADPG-Ppase and SuSy.

Temporal and Spatial Variations of Carbohydrate Content in Rice Leaf Sheath and Their Varietal Differences

In rice plant, carbohydrates accumulated in leaf sheaths before heading are translocated to grain and affect yield formation greatly. To clarify the intrinsic mechanism of carbohydrate metabolism in the leaf sheath, we investigated the temporal and spatial variations of carbohydrate metabolism in the third leaf sheath counted from the top and their varietal differences. The results revealed that the amount of carbohydrate decreased from the base to the tip of the leaf sheath, irrespective of variety and developmental stage. However, the proportion of starch content in the basal one-fifth of the leaf sheath to that in the whole sheath varied from 35% to 60% with the variety. Comparing the activities of enzymes related to starch metabolism at the base, middle and tip of the leaf sheath in IR65598-112-2 (New plant type) with those in Nipponbare, the activities of ADP-glucose pyrophosphorylase, branching enzyme and granule-bound starch synthase (GBSS) showed varietal differences. Particularly, the activity of GBSS may play an important role in the varietal difference in spatial variation of starch content in the leaf sheath. In IR65598-112-2, the sucrose content in the leaf sheath was extremely high, suggesting that sucrose may be one of the carbohydrate reserves in this line.

Isolation and characterisation of novel starch mutants of oats

Abstract Mutagenised grains of the diploid oat, Avena strigosa Schreb., were screened for altered starch granules in the endosperm. Three mutant lines were identified: mutants lam-1 and lam-2 had granules that stained red with iodine solution and sga-1 had endosperm containing soluble material that stained red with iodine solution, as well as blue-staining starch granules. The lam-1 and lam-2 lines lack or have severely reduced amounts of granule-bound starch synthase activity, the granule-bound starch synthase I protein and the amylose component of starch in the endosperm. They therefore represent mutations of the waxy type. The red-staining, soluble material in the sga-1 mutant is very similar in appearance and chain-length profile to the phytoglycogen that accumulates in the endosperms of the sugary-1 mutants of cereals. All sugary-1 mutants lack or have reduced amounts of isoamylase activity. However, native gel assays reveal a normal isoamylase activity in the developing endosperms of sga-1 suggesting that it is not of the sugary-1 type. The nature of the mutation responsible for the sga-1 phenotype has yet to be discovered.

Oscillation of mRNA level and activity of granule-bound starch synthase I in Arabidopsis leaves during the day/night cycle.

Granule-bound starch synthase (GBSSI) is one of the most extensively studied enzymes of the starch synthesis pathway and its role in the synthesis of amylose has been well established. However, few studies have been carried out to characterize the regulation of GBSSI gene. Regulation of starch synthesis genes is especially interesting in photosynthetic tissues, where starch is subjected to a periodical alternation of synthesis and degradation during the day/night cycle. In this report we show a circadian oscillation of GBSSI mRNA levels in leaves of Arabidopsis during the day/night cycle, and provide evidence that GBSSI expression is controlled by the transcription factors CCA1 and LHY. Over-expression of both CCA1 and LHY genes causes the elimination of GBSSI mRNA oscillation. Binding shift assays indicate that this control may be exerted through a direct interaction of those regulatory proteins with the GBSSI promoter. Oscillation is not observed on the GBSSI protein levels, which remains constant along the cycle. However, GBSSI activity shows a clear oscillation with a period of 24 h that is altered in transgenic plants over-expressing CCA1. Possible mechanisms controlling GBSSI activity during the day/night cycle are discussed.

Research note: Activity of granule-bound starch synthase is an important determinant of amylose content in rice endosperm.

Effects of temperature on amylose synthesis in rice endosperm were investigated using 13 cultivars known to differ in amylose contents in endosperm. Both amylose content and granule-bound starch synthase (GBSS) activity was increased when endosperms were developed under lower temperature in low- and medium-amylose cultivars, while this was not the case for high-amylose cultivars. Amylose content in high-amylose cultivars was stable under different temperature, with varying GBSS activity. A nearly linear correlation between GBSS activity and amylose content was observed as far as activity levels about 250 nmol min-1 g-1 DW. These results suggest that GBSS activity is an important determinant of amylose content in endosperm of low- and medium-amylose cultivars, while factors other than the enzyme activity limit amylose synthesis in the high-amylose cultivars.

Influence of maize starch granule-associated protein on the rheological properties of starch pastes. Part I. Large deformation measurements of paste properties

Starch granule-associated proteins, mainly consisting of granule bound starch synthase (GBSS), were shown to influence the rheological properties of starch pastes measured under large deformation steady shear conditions. A normal, a waxy null mutant, and two GBSS-containing waxy mutant maize starches (waxy protein was synthesized with no GBSS activity) were isolated (isolation procedure I, IP I) and further purified (isolation procedure II, IP II) using the toluene method. Further purification removed additional residual proteins in starches including the majority of GBSS in the two GBSS-containing waxy starches, and increased the paste viscosity in all four starches, especially at high shear rates. IP I starch paste of the waxy null mutant exhibited higher viscosity at high shear rate than starch pastes of the GBSS-containing waxy mutants that was attributed to the absence of GBSS in the former. Shear sweep tests (up to a shear rate of 120 l/s) showed that the IP I GBSS-containing waxy mutant starches had less shear-induced breakdown than the waxy null mutant starch; removal of additional granule-associated protein in the former, including GBSS, caused an increase in shear-induced breakdown. This study showed that granule-associated protein reduced paste viscosity, and GBSS specifically reduced shear-induced breakdown of starch pastes.

Influence of maize starch granule-associated protein on the rheological properties of starch pastes. Part II. Dynamic measurements of viscoelastic properties of starch pastes

The influence of granule-bound starch synthase (GBSS) on viscoelastic properties of gelatinized starch pastes was studied using a normal, a waxy null, and two GBSS-containing waxy mutant maize starches (waxy protein was synthesized with no GBSS activity). The four starches were isolated using the toluene method (isolation procedure I) (IP I) and were then further purified using an extended period of washing (isolation procedure II) (IP II). Dynamic rheological measurements of IP I gelatinized starches showed that storage moduli ( G′) were higher in the two GBSS-containing waxy starches than in the waxy null mutant starch not containing GBSS. The total non-GBSS granule-associated proteins of the three waxy starches were similar. Further removal of granule-associated proteins by purification decreased the G′, with decreases higher in the two GBSS-containing waxy mutants than in the waxy null mutant starch. This was interpreted to be due to the removal of GBSS during purification of the former. High shear broke gelatinized IP I waxy and normal maize starches starch granule or remnant structure, which caused a decrease in G′. Cox–Merz plots showed that the elastic properties of the starch pastes were reduced by removal of starch granule-associated proteins.

Multiple allelism as a control mechanism in metabolic pathways: GBSSI allelic composition affects the activity of granule-bound starch synthase I and starch composition in potato.

Multiple allelism in heterozygous autopolyploid species like potato not only occurs for genes that affect morphological characteristics but also for genes involved in metabolic pathways. Based on a combination of Southern and PCR analyses, at least eight alleles encoding granule-bound starch synthase I (GBSSI), which is responsible for amylose biosynthesis, have been identified in potato. These alleles were grouped into four classes, distinguishable by Southern analysis, and subdivided based on PCR. Despite the heterozygous and polyploid character of potato it was possible to assign variation in GBSSI activity to the allelic composition at the GBSSI loci within a large population of Solanum tuberosum cultivars and Solanum breeding lines. Moreover, the availability of an amf allele made it possible to reduce heterogeneity and enabled us to demonstrate an effect of GBSSI allelic composition on amylose content. The major difference between the alleles identified was the absence or presence of a 140-bp fragment at a site 0.5 kb upstream of the ATG start codon of the gene for GBSSI. The absence of this 140-bp fragment had a major effect on GBSSI activity and amylose content, while the presence of small deletions and simple sequence repeats had no obvious effect.

The effect of water stress on the activities of key regulatory enzymes of the sucrose to starch pathway in wheat

Developmental changes in the starch and sucrose content of grains andthe activities of enzymes of starch synthesis in wheat were studied under waterstress conditions. Water stress caused a marked reduction in the sucrose andstarch content of the grains. Sucrose synthase (SS) and UDP-glucosepyrophosphorylase (UDP-Gppase), showed higher catalytic activity and moreresistance to water stress compared with amyloplastic enzymes. ADP-glucosepyrophosphorylase (ADP-Gppase) activity was reduced to a low level under bothin situ and osmotic stress conditions in which grainsfailed to accumulate dry matter in vivo. Granule-boundstarch synthase (GBSS) also responded rapidly to in situwater stress treatments as did ADP-Gppase. Reduction in GBSS activity at thetime of growth cessation in situ was less than that ofADP-Gppase and the enzyme did not respond to severe osmotic stress. Solublestarch synthase (SSS) was the enzyme most sensitive to water stress in that itresponded earlier, and to a greater extent, than the other enzymes. However,under severe dehydration conditions, leading to cessation of growth, thedeclinein SSS activity was less than that for ADP-Gppase. SSS showed the lowestin vitro activity followed by GBSS. These results suggestthat SSS is the site of response to water stress by which the rate of graingrowth can be affected, whereas growth cessation is due mainly to theinactivation of ADP-Gppase.

Comparison Between Amylose‐free and Amylose Containing Potato Starches

AbstractThis paper describes the comparison of potato amylose‐free starches obtained by two different approaches: mutation induction of the gene encoding the key‐enzyme in amylose biosynthesis Granule Bound Starch Synthase (GBSS) and antisense inhibition of the expression of GBSS, with that of normal amylose containing potato starch. One of the most prominent differences between the amylose‐free and amylose‐containing starches was their phenotype after staining with iodine. The amylose‐free starch obtained via mutation induction stained completely red whereas the amylose‐free starch obtained by the antisense approach always had a very small dark‐blue staining core in a further entirely red staining granule. Amylose‐containing starch stained blue with iodine. Differences were also observed with regard to the absence of GBSS protein and activity in the amylose‐free potatoes. Furthermore the lambda max and the blue value of the amylose‐free starches was markedly different as would be expected since they almost completely lacked (&lt; 0.1%) amylose in both types of amylose‐free starches. Determination of a number of different chemical parameters including particle size distribution, ion content, as well as phosphate levels of the starch indicated that these were not different in the amylose‐free containing starches as compared to the normal amylose containing starches. The absence of amylose resulted in improved gel stability and paste clarity of gels even after prolonged durations at 5°C and at room temperature.

Field evaluation of transgenic potato plants expressing an antisense granule-bound starch synthase gene: increase of the antisense effect during tuber growth

Transgenic plants of a tetraploid potato cultivar were obtained in which the amylose content of tuber starch was reduced via antisense RNA-mediated inhibition of the expression of the gene encoding granule-bound starch synthase (GBSS). GBSS is one of the key enzymes in the biosynthesis of starch and catalyses the formation of amylose. The antisense GBSS genes, based on the full-length GBSS cDNA driven by the 35S CaMV promoter or the potato GBSS promoter, were introduced into the potato genome by Agrobacterium tumefaciens-mediated transformation. Expression of each of these genes resulted in the complete inhibition of GBSS gene expression, and thus in the production of amylose-free tuber starch, in mature field-grown plants originating from rooted in vitro plantlets of 4 out of 66 transgenic clones. Clones in which the GBSS gene expression was incompletely inhibited showed an increase of the extent of inhibition during tuber growth. This is likely to be due to the increase of starch granule size during tuber growth and the specific distribution pattern of starch components in granules of clones with reduced GBSS activity. Expression of the antisense GBSS gene from the GBSS promoter resulted in a higher stability of inhibition in tubers of field-grown plants as compared to expression from the 35S CaMV promoter. Field analysis of the transgenic clones indicated that inhibition of GBSS gene expression could be achieved without significantly affecting the starch and sugar content of transgenic tubers, the expression level of other genes involved in starch and tuber metabolism and agronomic characteristics such as yield and dry matter content.

Effect of grain location on the panicle on activities involved in starch synthesis in rice endosperm

Starch metabolism in rice endosperm of both the superior and inferior caryopses in the panicle was characterized by comparing changes in activities of five major enzymes associated with starch synthesis during endosperm development; sucrose synthase, ADPglucose pyrophosphorylase, Q-enzyme (starch branching enzyme), and soluble and starch granule-bound forms of starch synthase. Activities of ADPglucose pyrophosphorylase of both the superior and inferior caryopses increased markedly between 11 and 14 days after pollination. The developmental patterns of Q-enzyme activities were quite different between the superior and inferior caryopses. In addition, it was found that eight-14 days after pollination the activity of the starch granule-bound starch synthase in the inferior caryopsis was markedly lower than that in the superior caryopsis. These results suggest that the low activity of the granule-bound starch synthase is related to the lower content of amylose in the inferior caryopsis.

Expression of a wild-type GBSS gene introduced into an amylose-free potato mutant by Agrobacterium tumefaciens and the inheritance of the inserts at the microsporic level

Granule-bound starch synthase (GBSS) catalyses the synthesis of amylose in starch granules. Transformation of a diploid amylose-free (amf) potato mutant with the gene encoding GBSS leads to the restoration of amylose synthesis. Transformants were obtained which had wild-type levels of both GBSS activity and amylose content. It proved to be difficult to increase the amylose content above that of the wild-type potato by the introduction of additional copies of the wild-type GBSS gene. Staining of starch with iodine was suitable for investigating the degree of expression of the inserted GBSS gene in transgenic amf plants. Of the 19 investigated transformants, four had only red-staining starch in tubers indicating that no complementation of the amf mutation had occured. Fifteen complemented transformants had only blue-staining starch in tubers or tubers of different staining categories (blue, mixed and red), caused either by full or partial expression of the inserted gene. Complementation was also found in the microspores. The segregation of blue- and red-staining microspores was used to analyse the inheritance of the introduced GBSS genes. A comparison of the results from microspore staining and Southern hybridisation indicated that, in three tetraploid transgenics, the gene was probably inserted before (duplex), and in all others after, chromosome doubling (simplex). The partial complementation was not due to methylation of the HPAII/MSPI site in the promoter region. Partially complemented plants had low levels of mRNA as was found when the GBSS expression levels were inhibited by anti-sense technology.

Formation and Deposition of Amylose in the Potato Tuber Starch Granule Are Affected by the Reduction of Granule-Bound Starch Synthase Gene Expression.

The synthesis of amylose in amyloplasts is catalyzed by granule-bound starch synthase (GBSS). GBSS gene expression was inhibited via antisense RNA in Agrobacterium rhizogenes-transformed potato plants. Analysis of starch production and starch granule composition in transgenic tubers revealed that reduction of GBSS activity always resulted in a reduction of the production of amylose. Field experiments, performed over a 2-year period, showed that stable inhibition of GBSS gene expression can be obtained. Microscopic evaluation of iodine-stained starch granules was shown to be a sensitive system for qualitative and quantitative examination of amylose formation in starch granules of transgenic potato tubers. In plants showing inhibition of GBSS gene expression, the reduced amylose content in tuber starch was not a consequence of a lower amylose content throughout the entire starch granule. Starch granules of transgenic tubers were found to contain amylose at a percentage similar to wild-type starch in a core of varying size at the hilum of each granule. This indicated that reduced GBSS gene expression results in amylose formation in a restricted zone of the granules. The size of this zone is suggested to be dependent on the GBSS protein level. During development of the granules, the available GBSS protein is thought to become limiting, resulting in the formation of starch that lacks amylose. RNA gel blot analysis of tuber tissue showed that inhibition of GBSS gene expression resulted in a reduced GBSS mRNA level but did not affect the expression level of other starch synthesizing enzymes. Antisense RNA could only be detected in leaf tissue of the transgenic plants.

Waxy Chlamydomonas reinhardtii: monocellular algal mutants defective in amylose biosynthesis and granule-bound starch synthase activity accumulate a structurally modified amylopectin.

Amylose-defective mutants were selected after UV mutagenesis of Chlamydomonas reinhardtii cells. Two recessive nuclear alleles of the ST-2 gene led to the disappearance not only of amylose but also of a fraction of the amylopectin. Granule-bound starch synthase activities were markedly reduced in strains carrying either st-2-1 or st-2-2, as is the case for amylose-deficient (waxy) endosperm mutants of higher plants. The main 76-kDa protein associated with the starch granule was either missing or greatly diminished in both mutants, while st-2-1-carrying strains displayed a novel 56-kDa major protein. Methylation and nuclear magnetic resonance analysis of wild-type algal storage polysaccharide revealed a structure identical to that of higher-plant starch, while amylose-defective mutants retained a modified amylopectin fraction. We thus propose that the waxy gene product conditions not only the synthesis of amylose from endosperm storage tissue in higher-plant amyloplasts but also that of amylose and a fraction of amylopectin in all starch-accumulating plastids. The nature of the ST-2 (waxy) gene product with respect to the granule-bound starch synthase activities is discussed.