Isolation of a cDNA encoding a granule-bound 152-kilodalton starch-branching enzyme in wheat

Genome-specific granule-bound starch synthase I (GBSSI) influences starch biochemical and functional characteristics in near-isogenic wheat ( Triticum aestivum L.) lines

Near-isogenic wheat ( Triticum aestivum L.) lines differing at the Waxy locus were studied for the influence of genome-specific granule-bound starch synthase I (GBSSI/Waxy; Wx-A, Wx-B, Wx-D) on starch composition, structure, and in vitro starch enzymatic hydrolysis. Grain composition, amylose concentration, amylopectin unit-chain length distribution, and starch granule size distribution varied with the loss of functional GBSSI. Amylose concentration was more severely affected in genotypes with GBSSI missing from two genomes (double nulls) than from one genome (single nulls). Unit glucan chains (DP 6-8) of amylopectin were reduced with the complete loss of GBSSI as compared to wheat starch with a full complement of GBSSI. Wx-A and Wx-B had an additive effect toward short-chain phenotype of waxy amylopectin. Loss of Wx-D isoprotein alone significantly (p < 0.05) reduced the C-type starch granules. However, the absence of Wx-D in combination with Wx-A or Wx-B increased the B-type and C-type starch granules but decreased the volume of A-type starch granules. The rate of in vitro starch enzymatic hydrolysis was highest in completely waxy grain meal and purified starch. However, the presence of Wx-D reduced wheat starch hydrolysis as it increased the large A-type starch granule content (volume %) and reduced short chains (DP 6-8) in amylopectin. Factors such as small C-type starch granules, amylose concentration, and long chains of amylopectin (DP 23-45) also influenced wheat starch hydrolysis.

Transcriptomic analysis of starch biosynthesis in the developing grain of hexaploid wheat

The expression of genes involved in starch synthesis in wheat was analyzed together with the accumulation profiles of soluble sugars, starch, protein, and starch granule distribution in developing caryopses obtained from the same biological materials used for profiling of gene expression using DNA microarrays. Multiple expression patterns were detected for the different starch biosynthetic gene isoforms, suggesting their relative importance through caryopsis development. Members of the ADP-glucose pyrophosphorylase, starch synthase, starch branching enzyme, and sucrose synthase gene families showed different expression profiles; expression of some members of these gene families coincided with a period of high accumulation of starch while others did not. A biphasic pattern was observed in the rates of starch and protein accumulation which paralleled changes in global gene expression. Metabolic and regulatory genes that show a pattern of expression similar to starch accumulation and granule size distribution were identified, suggesting their coinvolvement in these biological processes.

Genomic isolation of genes encoding starch branching enzyme II (SBEII) in apple: toward characterization of evolutionary disparity in SbeII genes between monocots and eudicots

Two genes encoding starch branching enzyme II (SBEII) have been identified in apple. These genes share 94 and 92% identity in coding DNA sequences and amino acid sequences, respectively; moreover, they have similar expression patterns. Both genes are expressed in vegetative and reproductive tissues, including leaves, buds, flowers, and fruits. Based on genomic Southern blots, there are two copies of SbeII genes in the apple genome. Comparisons of genomic sequences between monocots and eudicots have revealed that the genomic structure of SbeII genes is conserved. However, the 5'-terminal region of coding DNA sequences of SbeII genes shows greater divergence than the 3'-terminal region between monocots and eudicots. Phylogenetic analysis of DNA sequences has demonstrated that the duplication patterns of SbeII genes are different between monocots and eudicots. In monocots, the duplication of SbeII genes must have occurred prior to the radiation of grasses (Poaceae); while, in eudicots, the expansion of SbeII genes must have followed the process of speciation.

Understanding storage starch biosynthesis in plants: a means to quality improvement

The many varied uses of starch in food and industrial applications often requires an understanding of its physicochemical properties and the detailed variations in granule structure that underpin these properties. The ability to manipulate storage starch structures depends on understanding the biosynthetic pathway, and in particular, how the many components of the pathway are coordinated and regulated. This article presents a current overview of starch structure and the known enzymes involved in the synthesis of the granule, with an emphasis on how current knowledge on the regulation of the pathway in cereals and other crops may be applied to the production of different functional starches.

Multiple isoforms of starch branching enzyme-I in wheat: lack of the major SBE-I isoform does not alter starch phenotype

The role of starch branching enzyme-I (SBE-I) in determining starch structure in the endosperm has been investigated. Null mutations of SBE-I at the A, B and D genomes of wheat were identified in Australian wheat varieties by immunoblotting. By combining individual null mutations at the B and D genomes through hybridisation, a double-null mutant wheat, which lacks the B and D isoforms of SBE-I, was developed. Wheat mutants lacking all the three isoforms of SBE-I were generated from a doubled haploid progeny of a cross between the BD double-null mutant line and a Chinese Spring (CS) deletion line lacking the A genome isoform. Comparison of starch from this mutant wheat to that from wild type revealed no substantial alteration in any of the structural or functional properties analysed. Further analysis of this triple-null mutant line revealed the presence of another residual peak of SBE-I activity, referred to as SBE-I^r, in wheat endosperm representing < 3% of the activity of SBE-I in wild type endosperm.

Protein phosphorylation in amyloplasts regulates starch branching enzyme activity and protein-protein interactions

Protein phosphorylation in amyloplasts and chloroplasts of Triticum aestivum (wheat) was investigated after the incubation of intact plastids with gamma-(32)P-ATP. Among the soluble phosphoproteins detected in plastids, three forms of starch branching enzyme (SBE) were phosphorylated in amyloplasts (SBEI, SBEIIa, and SBEIIb), and both forms of SBE in chloroplasts (SBEI and SBEIIa) were shown to be phosphorylated after sequencing of the immunoprecipitated (32)P-labeled phosphoproteins using quadrupole-orthogonal acceleration time of flight mass spectrometry. Phosphoamino acid analysis of the phosphorylated SBE forms indicated that the proteins are all phosphorylated on Ser residues. Analysis of starch granule-associated phosphoproteins after incubation of intact amyloplasts with gamma-(32)P-ATP indicated that the granule-associated forms of SBEII and two granule-associated forms of starch synthase (SS) are phosphorylated, including SSIIa. Measurement of SBE activity in amyloplasts and chloroplasts showed that phosphorylation activated SBEIIa (and SBEIIb in amyloplasts), whereas dephosphorylation using alkaline phosphatase reduced the catalytic activity of both enzymes. Phosphorylation and dephosphorylation had no effect on the measurable activity of SBEI in amyloplasts and chloroplasts, and the activities of both granule-bound forms of SBEII in amyloplasts were unaffected by dephosphorylation. Immunoprecipitation experiments using peptide-specific anti-SBE antibodies showed that SBEIIb and starch phosphorylase each coimmunoprecipitated with SBEI in a phosphorylation-dependent manner, suggesting that these enzymes may form protein complexes within the amyloplast in vivo. Conversely, dephosphorylation of immunoprecipitated protein complex led to its disassembly. This article reports direct evidence that enzymes of starch metabolism (amylopectin synthesis) are regulated by protein phosphorylation and indicate a wider role for protein phosphorylation and protein-protein interactions in the control of starch anabolism and catabolism.

Three nicotianamine synthase genes isolated from maize are differentially regulated by iron nutritional status

Nicotianamine synthase (NAS) is an enzyme that is critical for the biosynthesis of the mugineic acid family of phytosiderophores in graminaceous plants, and for the homeostasis of metal ions in nongraminaceous plants. We isolated one genomic NAS clone, ZmNAS3, and two cDNA NAS clones, ZmNAS1 and ZmNAS2, from maize (Zea mays cv Alice). In agreement with the increased secretion of phytosiderophores with Fe deficiency, ZmNAS1 and ZmNAS2 were positively expressed only in Fe-deficient roots. In contrast, ZmNAS3 was expressed under Fe-sufficient conditions, and was negatively regulated by Fe deficiency. This is the first report describing down-regulation of NAS gene expression in response to Fe deficiency in plants, shedding light on the role of nicotianamine in graminaceous plants, other than as a precursor in phytosiderophore production. ZmNAS1-green fluorescent protein (sGFP) and ZmNAS2-sGFP were localized at spots in the cytoplasm of onion (Allium cepa) epidermal cells, whereas ZmNAS3-sGFP was distributed throughout the cytoplasm of these cells. ZmNAS1 and ZmNAS3 showed NAS activity in vitro, whereas ZmNAS2 showed none. Due to its duplicated structure, ZmNAS2 was much larger (65.8 kD) than ZmNAS1, ZmNAS3, and previously characterized NAS proteins (30-38 kD) from other plant species. We reveal that maize has two types of NAS proteins based on their expression pattern and subcellular localization.

Starch branching enzymes in sorghum (Sorghum bicolor) and barley (Hordeum vulgare): comparative analyses of enzyme structure and gene expression

A genomic clone for starch branching enzyme (SBE) IIb was isolated from a sorghum bacterial artificial chromosome (BAC) library. The promoter and 5' flanking sequence, the first four exons and introns as well as the last exon and the 3' untranslated region were sequenced. The tentative transcription start site of sorghum sbeIIb was mapped based on alignment with the maize sbeIIb gene. The exon-intron structure of the 5' portion of sorghum sbeIIb was similar to that of maize sbeIIb but differed from that of barley sbeIIb. Specifically, the intronic BbI element involved in the endosperm specific expression of barley sbeIIb was lacking in the sorghum gene. A cDNA clone for sorghum sbeIIb was reverse PCR amplified and found to encode an 803 amino acids long protein. The amino acid sequence of sorghum SBEIIb was compared to that of sorghum SBEI and corresponding enzymes in barley. The overall identity in amino acid sequence was 54% in the central portion of the enzymes. A major difference between the SBEII and SBEI isoforms was a 67 amino acids-long C-terminal extension in the SBEIs. The spatial and temporal expression patterns of sorghum sbeIIb was determined and compared with those of the sorghum gene for SBEI and the barley genes for SBEIIB and SBEI. All four genes exhibited a seed specific expression. However, while barley sbeIIb and sbeI transcripts were detected exclusively in the endosperm, the sorghum genes were expressed also in the embryo. The activity of sorghum sbeIIb and sbeI exhibited a late onset, with a peak of transcription at around 22 days after pollination. This is similar to the pattern of barley sbeI but different from that of barley sbeIIb, which showed a peak of transcription at 12 days after pollination.

Differential characteristics and subcellular localization of two starch-branching enzyme isoforms encoded by a single gene in Phaseolus vulgaris L

Starch-branching enzymes (SBE) have a dominant role for amylopectin structure as they define chain length and frequency of branch points. We have previously shown that one of the SBE isoforms of kidney bean (Phaseolus vulgaris L.), designated PvSBE2, has a molecular mass (82 kDa) significantly smaller than those reported for isologous SBEs from pea (SBEI), maize (BEIIb), and rice (RBE3). Additionally, in contrast to the dual location of the pea SBEI in both the soluble and starch granule fractions, PvSBE2 was found only in the soluble fraction during seed development. Analysis of a pvsbe2 cDNA suggested that PvSBE2 is generated from a larger precursor with a putative plastid targeting sequence of 156 residues. Here we describe the occurrence of a larger 100-kDa form (LF-PvSBE2) of PvSBE2 found both in the soluble and starch granule fractions of the developing seeds. The determined N-terminal sequence, VKSSHDSD, of LF-PvSBE2 corresponded to a peptide sequence located 111 amino acids upstream from the N terminus of purified PvSBE2, suggesting that LF-PvSBE2 and PvSBE2 are products of the same gene. Analysis of the products by 5'-RACE (rapid amplification of cDNA ends) and reverse transcription PCR indicated that the two transcripts for pre-LF-PvSBE2 and pre-PvSBE2 are generated by alternative splicing. Recombinant LF-PvSBE2 (rLF-PvSBE2) was purified from Escherichia coli and the kinetic properties were compared with those of recombinant PvSBE2 (rPvSBE2). rLF-PvSBE2 had much higher affinity for amylopectin (K(m) = 4.4 mg/ml) than rPvSBE2 (18.4 mg/ml), whereas the V(max) of rLF-PvSBE2 (135 units/mg) for this substrate was much lower than that of rPvSBE2 (561 units/mg). These results suggest that the N-terminal extension of LF-PvSBE2 plays a critical role for localization in starch granules by altering its enzymatic properties.

Two starch-branching-enzyme isoforms occur in different fractions of developing seeds of kidney bean

The nature and enzymic properties of starch-branching enzyme (SBE) are two of the dominant factors influencing the fine structure of starch. To understand the role of this enzyme's activity in the formation of starch in kidney bean (Phaseolus vulgaris L.), a study was undertaken to identify the major SBE sequences expressed during seed development and to characterize the enzymic properties of the coded recombinant enzymes. Two SBE cDNA species (designated pvsbe2 and pvsbe1) that displayed significant similarity (more than 70%) to other family A and B SBEs respectively were isolated. Northern blot analysis revealed that pvsbe1 and pvsbe2 were differentially expressed during seed development. pvsbe2 showed maximum steady-state transcript levels at the mid-stage of seed maturation, whereas pvsbe1 reached peak levels at a later stage. Western blot analysis with antisera raised against both recombinant proteins (rPvSBE1 and rPvSBE2) showed that these two SBEs were located in different amyloplast fractions of developing seeds of kidney bean. PvSBE2 was present in the soluble fraction, whereas PvSBE1 was associated with the starch granule fraction. The differences in location suggest that these two SBE isoenzymes have different roles in amylopectin synthesis in kidney bean seeds. rPvSBE1 and rPvSBE2 were purified from Escherichia coli and their kinetic properties were determined. The affinity of rPvSBE2 for amylose (K(m) 1.27 mg/ml) was lower than that of rPvSBE1 (0.46 mg/ml). The activity of rPvSBE2 was stimulated more than 3-fold in the presence of 0.3 M citrate, whereas rPvSBE1 activity was not affected. The implications of the enzymic properties and the distribution of SBEs and amylopectin structure are discussed.

Comparison of starch-branching enzyme genes reveals evolutionary relationships among isoforms. Characterization of a gene for starch-branching enzyme IIa from the wheat genome donor Aegilops tauschii

Genes and cDNAs for starch-branching enzyme II (SBEII) have been isolated from libraries constructed from Aegilops tauschii and wheat (Triticum aestivum) endosperm, respectively. One class of genes has been termed wSBEII-DA1 and encodes the N terminus reported for an SBEII from wheat endosperm. On the basis of phylogenetic comparisons with other branching enzyme sequences, wSBEII-DA1 is considered to be a member of the SBEIIa class. The wSBEII-DA1 gene consists of 22 exons with exons 4 to 21 being identical in length to the maize (Zea mays) SBEIIb gene, and the gene is located in the proximal region of the long arm of chromosome 2 at a locus designated sbe2a. RNA encoding SBEIIa can be detected in the endosperm from 6 d after flowering and is at its maximum level from 15 to 18 d after anthesis. Use of antibodies specific for SBEIIa demonstrated that this protein was present in both the soluble and granule bound fractions in developing wheat endosperm. We also report a cDNA sequence for SBEIIa that could arise by variant transcription/splicing. A second gene, termed wSBEII-DB1, was isolated and encodes an SBEII, which shows greater sequence identity with SBEIIb-type sequences than with SBEIIa-type sequences. Comparisons of SBEII gene structures among wheat, maize, and Arabidopsis indicate the lineage of the SBEII genes.

Starch-branching enzymes preferentially associated with A-type starch granules in wheat endosperm

Two starch granule-bound proteins (SGP), SGP-140 and SGP-145, were preferentially associated with A-type starch granules (>10 microm) in developing and mature wheat (Triticum aestivum) kernels. Immunoblotting and N-terminal sequencing suggested that the two proteins were different variants of SBEIc, a 152-kD isoform of wheat starch-branching enzyme. Both SGP-140 and SGP-145 were localized to the endosperm starch granules but were not found in the endosperm soluble fraction or pericarp starch granules younger than 15 d post anthesis (DPA). Small-size starch granules (<10 microm) initiated before 15 DPA incorporated SGP-140 and SGP-145 throughout endosperm development and grew into full-size A-type starch granules (>10 microm). In contrast, small-size starch granules harvested after 15 DPA contained only low amounts of SGP-140 and SGP-145 and developed mainly into B-type starch granules (<10 microm). Polypeptides of similar mass and immunologically related to SGP-140 and/or SGP-145 were also preferentially incorporated into A-type starch granules of barley (Hordeum vulgare), rye (Secale cereale), and triticale (x Triticosecale Wittmack) endosperm, which like wheat endosperm have a bimodal starch granule size distribution.