Enzymes of carbohydrate metabolism in the developing endosperm of maize

Physicochemical changes and in vitro digestibility of three banana starches at different maturity stages

This study aimed to compare the changes in physicochemical properties of the starch isolated from three banana cultivars (Musa AAA group, Cavendish subgroup; Musa ABB group, Pisang Awak subgroup; Musa AA group, Huangdijiao subgroup) at five different maturity stages. The results revealed both similarities and significant differences in micromorphology and physicochemical characteristics of the three banana varieties during different growth stages. Apparent amylose content and particle size of the three starches increased with the ripeness of banana. Light microscopy and scanning electron microscopy revealed that starch particles of the three starches had different microscopic characteristics, and that banana starch morphology was basically unchanged at various growth stages. Moreover, the pasting and thermal properties of the banana starches were significantly different at various growth stages. The resistant starch content of the three banana cultivars was about 80% at all growth stages. Musa AAA group, Cavendish subgroup had the highest resistant starch content at stage Ⅴ. This study provides insights into the starch changes of three banana cultivars during ripening.

Soluble and insoluble α-glucan synthesis in yeast by enzyme suites derived exclusively from maize endosperm

Molecular mechanisms that distinguish the synthesis of semi-crystalline α-glucan polymers found in plant starch granules from the synthesis of water-soluble polymers by nonplant species are not well understood. To address this, starch biosynthetic enzymes from maize (Zea mays L.) endosperm were isolated in a reconstituted environment using yeast (Saccharomyces cerevisiae) as a test bed. Ninety strains were constructed containing unique combinations of 11 synthetic transcription units specifying maize starch synthase (SS), starch phosphorylase (PHO), starch branching enzyme (SBE), or isoamylase-type starch debranching enzyme (ISA). Soluble and insoluble branched α-glucans accumulated in varying proportions depending on the enzyme suite, with ISA function stimulating distribution into the insoluble form. Among the SS isoforms, SSIIa, SSIII, and SSIV individually supported the accumulation of glucan polymer. Neither SSI nor SSV alone produced polymers; however, synergistic effects demonstrated that both isoforms can stimulate α-glucan accumulation. PHO did not support α-glucan production by itself, but it had either positive or negative effects on polymer content depending on which SS or a combination thereof was present. The complete suite of maize enzymes generated insoluble particles resembling native starch granules in size, shape, and crystallinity. Ultrastructural analysis revealed a hierarchical assembly starting with subparticles of approximately 50 nm diameter that coalesce into discrete structures of approximately 200 nm diameter. These are assembled into semi-crystalline α-glucan superstructures up to 4 μm in length filling most of the yeast cytosol. ISA was not essential for the formation of such particles, but their abundance was increased dramatically by ISA presence.

Causes and consequences of endogenous hypoxia on growth and metabolism of developing maize kernels

Maize (Zea mays) kernels are the largest cereal grains, and their endosperm is severely oxygen deficient during grain fill. The causes, dynamics, and mechanisms of acclimation to hypoxia are minimally understood. Here, we demonstrate that hypoxia develops in the small, growing endosperm, but not the nucellus, and becomes the standard state, regardless of diverse structural and genetic perturbations in modern maize (B73, popcorn, sweet corn), mutants (sweet4c, glossy6, waxy), and non-domesticated wild relatives (teosintes and Tripsacum species). We also uncovered an interconnected void space at the chalazal pericarp, providing superior oxygen supply to the placental tissues and basal endosperm transfer layer. Modeling indicated a very high diffusion resistance inside the endosperm, which, together with internal oxygen consumption, could generate steep oxygen gradients at the endosperm surface. Manipulation of oxygen supply induced reciprocal shifts in gene expression implicated in controlling mitochondrial functions (23.6 kDa Heat-Shock Protein, Voltage-Dependent Anion Channel 2) and multiple signaling pathways (core hypoxia genes, cyclic nucleotide metabolism, ethylene synthesis). Metabolite profiling revealed oxygen-dependent shifts in mitochondrial pathways, ascorbate metabolism, starch synthesis, and auxin degradation. Long-term elevated oxygen supply enhanced the rate of kernel development. Altogether, evidence here supports a mechanistic framework for the establishment of and acclimation to hypoxia in the maize endosperm.

Genetic Perturbation of the Starch Biosynthesis in Maize Endosperm Reveals Sugar-Responsive Gene Networks

In maize, starch mutants have facilitated characterization of key genes involved in endosperm starch biosynthesis such as large subunit of AGPase Shrunken2 (Sh2) and isoamylase type DBE Sugary1 (Su1). While many starch biosynthesis enzymes have been characterized, the mechanisms of certain genes (including Sugary enhancer1) are yet undefined, and very little is understood about the regulation of starch biosynthesis. As a model, we utilize commercially important sweet corn mutations, sh2 and su1, to genetically perturb starch production in the endosperm. To characterize the transcriptomic response to starch mutations and identify potential regulators of this pathway, differential expression and coexpression network analysis was performed on near-isogenic lines (NILs) (wildtype, sh2, and su1) in six genetic backgrounds. Lines were grown in field conditions and kernels were sampled in consecutive developmental stages (blister stage at 14 days after pollination (DAP), milk stage at 21 DAP, and dent stage at 28 DAP). Kernels were dissected to separate embryo and pericarp from the endosperm tissue and 3' RNA-seq libraries were prepared. Mutation of the Su1 gene led to minimal changes in the endosperm transcriptome. Responses to loss of sh2 function include increased expression of sugar (SWEET) transporters and of genes for ABA signaling. Key regulators of starch biosynthesis and grain filling were identified. Notably, this includes Class II trehalose 6-phosphate synthases, Hexokinase1, and Apetala2 transcription factor-like (AP2/ERF) transcription factors. Additionally, our results provide insight into the mechanism of Sugary enhancer1, suggesting a potential role in regulating GA signaling via GRAS transcription factor Scarecrow-like1.

Amylose in starch: towards an understanding of biosynthesis, structure and function

Starch granules are composed of two distinct glucose polymers - amylose and amylopectin. Amylose constitutes 5-35% of most natural starches and has a major influence over starch properties in foods. Its synthesis and storage occurs within the semicrystalline amylopectin matrix of starch granules, this poses a great challenge for biochemical and structural analyses. However, the last two decades have seen vast progress in understanding amylose synthesis, including new insights into the action of GRANULE BOUND STARCH SYNTHASE (GBSS), the major glucosyltransferase that synthesises amylose, and the discovery of PROTEIN TARGETING TO STARCH1 (PTST1) that targets GBSS to starch granules. Advances in analytical techniques have resolved the fine structure of amylose, raising new questions on how structure is determined during biosynthesis. Furthermore, the discovery of wild plants that do not produce amylose revives a long-standing question of why starch granules contain amylose, rather than amylopectin alone. Overall, these findings contribute towards a full understanding of amylose biosynthesis, structure and function that will be essential for future approaches to improve starch quality in crops.

A Rosaceae Family-Level Approach To Identify Loci Influencing Soluble Solids Content in Blackberry for DNA-Informed Breeding

A Rosaceae family-level candidate gene approach was used to identify genes associated with sugar content in blackberry (Rubus subgenus Rubus). Three regions conserved among apple (Malus × domestica), peach (Prunus persica), and alpine strawberry (Fragaria vesca) were identified that contained previously detected sweetness-related quantitative trait loci (QTL) in at least two of the crops. Sugar related genes from these conserved regions and 789 sugar-associated apple genes were used to identify 279 Rubus candidate transcripts. A Hyb-Seq approach was used in conjunction with PacBio sequencing to generate haplotype level sequence information of sugar-related genes for 40 cultivars with high and low soluble solids content from the University of Arkansas and USDA blackberry breeding programs. Polymorphisms were identified relative to the ‘Hillquist’ blackberry (R. argutus) and ORUS 4115-3 black raspberry (R. occidentalis) genomes and tested for their association with soluble solids content (SSC). A total of 173 alleles were identified that were significantly (α = 0.05) associated with SSC. KASP genotyping was conducted for 92 of these alleles on a validation set of blackberries from each breeding program and 48 markers were identified that were significantly associated with SSC. One QTL, qSSC-Ruh-ch1.1, identified in both breeding programs accounted for an increase of 1.5 °Brix and the polymorphisms were detected in the intron space of a sucrose synthase gene. This discovery represents the first environmentally stable sweetness QTL identified in blackberry. The approach demonstrated in this study can be used to develop breeding tools for other crops that have not yet benefited directly from the genomics revolution.

A Review of Starch Biosynthesis in Relation to the Building Block-Backbone Model

Starch is a water-insoluble polymer of glucose synthesized as discrete granules inside the stroma of plastids in plant cells. Starch reserves provide a source of carbohydrate for immediate growth and development, and act as long term carbon stores in endosperms and seed tissues for growth of the next generation, making starch of huge agricultural importance. The starch granule has a highly complex hierarchical structure arising from the combined actions of a large array of enzymes as well as physicochemical self-assembly mechanisms. Understanding the precise nature of granule architecture, and how both biological and abiotic factors determine this structure is of both fundamental and practical importance. This review outlines current knowledge of granule architecture and the starch biosynthesis pathway in relation to the building block-backbone model of starch structure. We highlight the gaps in our knowledge in relation to our understanding of the structure and synthesis of starch, and argue that the building block-backbone model takes accurate account of both structural and biochemical data.

Optimization of nucleotide sugar supply for polysaccharide formation via thermodynamic buffering

Plant polysaccharides (cellulose, hemicellulose, pectin, starch) are either direct (i.e. leaf starch) or indirect products of photosynthesis, and they belong to the most abundant organic compounds in nature. Although each of these polymers is made by a specific enzymatic machinery, frequently in different cell locations, details of their synthesis share certain common features. Thus, the production of these polysaccharides is preceded by the formation of nucleotide sugars catalyzed by fully reversible reactions of various enzymes, mostly pyrophosphorylases. These 'buffering' enzymes are, generally, quite active and operate close to equilibrium. The nucleotide sugars are then used as substrates for irreversible reactions of various polysaccharide-synthesizing glycosyltransferases ('engine' enzymes), e.g. plastidial starch synthases, or plasma membrane-bound cellulose synthase and callose synthase, or ER/Golgi-located variety of glycosyltransferases forming hemicellulose and pectin backbones. Alternatively, the irreversible step might also be provided by a carrier transporting a given immediate precursor across a membrane. Here, we argue that local equilibria, established within metabolic pathways and cycles resulting in polysaccharide production, bring stability to the system via the arrangement of a flexible supply of nucleotide sugars. This metabolic system is itself under control of adenylate kinase and nucleoside-diphosphate kinase, which determine the availability of nucleotides (adenylates, uridylates, guanylates and cytidylates) and Mg2+, the latter serving as a feedback signal from the nucleotide metabolome. Under these conditions, the supply of nucleotide sugars to engine enzymes is stable and constant, and the metabolic process becomes optimized in its load and consumption, making the system steady and self-regulated.

SH1-dependent maize seed development and starch synthesis via modulating carbohydrate flow and osmotic potential balance

BACKGROUND

As the main form of photoassimilates transported from vegetative tissues to the reproductive organs, sucrose and its degradation products are crucial for cell fate determination and development of maize kernels. Despite the relevance of sucrose synthase SH1 (shrunken 1)-mediated release of hexoses for kernel development, the underlying physiological and molecular mechanisms are not yet well understood in maize (Zea mays).

RESULTS

Here, we identified a new allelic mutant of SH1 generated by EMS mutagenesis, designated as sh1*. The mutation of SH1 caused more than 90% loss of sucrose synthase activity in sh1* endosperm, which resulted in a significant reduction in starch contents while a dramatic increase in soluble sugars. As a result, an extremely high osmolality in endosperm cells of sh1* was generated, which caused kernel swelling and affected the seed development. Quantitative measurement of phosphorylated sugars showed that Glc-1-P in endosperm of sh1* (17 μg g^- 1 FW) was only 5.2% of that of wild-type (326 μg g^- 1 FW). As a direct source of starch synthesis, the decrease of Glc-1-P may cause a significant reduction in carbohydrates that flow to starch synthesis, ultimately contributing to the defects in starch granule development and reduction of starch content.

CONCLUSIONS

Our results demonstrated that SH1-mediated sucrose degradation is critical for maize kernel development and starch synthesis by regulating the flow of carbohydrates and maintaining the balance of osmotic potential.

Starch formation inside plastids of higher plants

Starch is a water-insoluble polyglucan synthesized inside the plastid stroma within plant cells, serving a crucial role in the carbon budget of the whole plant by acting as a short-term and long-term store of energy. The highly complex, hierarchical structure of the starch granule arises from the actions of a large suite of enzyme activities, in addition to physicochemical self-assembly mechanisms. This review outlines current knowledge of the starch biosynthetic pathway operating in plant cells in relation to the micro- and macro-structures of the starch granule. We highlight the gaps in our knowledge, in particular, the relationship between enzyme function and operation at the molecular level and the formation of the final, macroscopic architecture of the granule.

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

The starch-rich endosperms of the Poaceae, which includes wild grasses and their domesticated descendents the cereals, have provided humankind and their livestock with the bulk of their daily calories since the dawn of civilization up to the present day. There are currently unprecedented pressures on global food supplies, largely resulting from population growth, loss of agricultural land that is linked to increased urbanization, and climate change. Since cereal yields essentially underpin world food and feed supply, it is critical that we understand the biological factors contributing to crop yields. In particular, it is important to understand the biochemical pathway that is involved in starch biosynthesis, since this pathway is the major yield determinant in the seeds of six out of the top seven crops grown worldwide. This review outlines the critical stages of growth and development of the endosperm tissue in the Poaceae, including discussion of carbon provision to the growing sink tissue. The main body of the review presents a current view of our understanding of storage starch biosynthesis, which occurs inside the amyloplasts of developing endosperms.

Starch Polysaccharides in the Human Diet: Effect of the Different Source and Processing on its Absorption

Starch is the main source of carbohydrates in human diet. It is widely used in food processing and non-food industrial applications. The effects on starch digestion and absorption in humans are reviewed in relation to the starch composition, sources, plant genetic variation, food processing and cooking. The impact of food industrial processing and starch modification on the digestibility of starch containing foods and on gut microbiota are discussed. Considering that the resistant starch (RS) fraction escaped from the small intestine is fermented in large intestine, all the variables that influence starch digestibility and absorption must be taken into account when discussing about healthy properties of fibers. Future trends in food industries are aimed to increase the RS fraction in processed foods in order to improve nutritional quality as well as to clarify the influence of RS3 and RS4 on gut microbiota.

Seed Coating Increases Seed Moisture Uptake and Restricts Embryonic Oxygen Availability in Germinating Cereal Seeds

Seed coating is a technology to improve germination and homogenize stand establishment. Although coating often results in lower germination rates, seeds that do germinate grow more vigorously and show strongly reduced respiratory losses during reserve mobilization. We hypothesize that the higher mobilization efficiency is due to a shift in the enzymatic cleavage of sucrose from invertase to sucrose synthase in the embryonic tissue caused by a reduced oxygen availability induced by oversaturation with water caused by the coating during early germination. We investigated the effect of coating on barley, rye, and wheat seed imbibition during the first 30 h after seeds were placed in moisture. We profiled oxygen in the embryos and measured sucrose and acid invertase levels as imbibition progressed. We found that seeds within coatings absorbed significantly more moisture than uncoated seeds. Coating resulted in near anoxic oxygen concentrations in the developing embryonic tissues in all three species. In barley, sucrose was not cleaved via the invertase pathway, despite the fact that invertase activity in coated seeds was increased. In rye and wheat, invertase activities were significantly lower in embryos from coated seeds without significantly changing the sugar composition.

Progress in High-Amylose Cereal Crops through Inactivation of Starch Branching Enzymes

High-amylose cereal starches provide many health benefits for humans. The inhibition or mutation of starch branching enzyme (SBE) genes is an effective method to develop high-amylose cereal crops. This review summarizes the development of high-amylose cereal crops through the inactivation of one or more SBE isoforms or combination with other genes. This review also reveals the causes of increase in amylose content in high-amylose crops. A series of changes, including amylopectin structure, crystalline structure, thermal properties, and hydrolysis properties, occurs as amylose content increases. The different morphological starch granules nominated as heterogeneous starch granules or differently stained starch granules are detected in high-amylose cereal crops. Detailed studies on four heterogeneous starch granules in high-amylose rice, which is developed by antisense RNA inhibition of SBEI/IIb, indicate that granules with different morphologies possess various molecular structures and physicochemical and functional properties. This variation diversifies their applications in food and non-food industries. However, current knowledge regarding how these heterogeneous starch granules form and why they exhibit regional distribution in endosperm remain largely unknown.

Transcriptome analysis of near-isogenic lines provides molecular insights into starch biosynthesis in maize kernel

Starch is the major component in maize kernels, providing a stable carbohydrate source for humans and livestock as well as raw material for the biofuel industry. Increasing maize kernel starch content will help meet industry demands and has the potential to increase overall yields. We developed a pair of maize near-isogenic lines (NILs) with different alleles for a starch quantitative trait locus on chromosome 3 (qHS3), resulting in different kernel starch content. To investigate the candidate genes for qHS3 and elucidate their effects on starch metabolism, RNA-Seq was performed for the developing kernels of the NILs at 14 and 21 d after pollination (DAP). Analysis of genomic and transcriptomic data identified 76 genes with nonsynonymous single nucleotide polymorphisms and 384 differentially expressed genes (DEGs) in the introgressed fragment, including a hexokinase gene, ZmHXK3a, which catalyzes the conversion of glucose to glucose-6-phosphate and may play a key role in starch metabolism. The expression pattern of all DEGs in starch metabolism shows that altered expression of the candidate genes for qHS3 promoted starch synthesis, with positive consequences for kernel starch content. These results expand the current understanding of starch biosynthesis and accumulation in maize kernels and provide potential candidate genes to increase starch content.

Physiological and comparative proteome analyses reveal low-phosphate tolerance and enhanced photosynthesis in a maize mutant owing to reinforced inorganic phosphate recycling

BACKGROUND

The low-phosphate-tolerant maize mutant Qi319-96 was obtained from Qi319 through cellular engineering. To elucidate the molecular mechanisms underlying the low-phosphate tolerance of this mutant, we performed comparative proteome analyses of the leaves of Qi319-96 and Qi319 under inorganic phosphate (Pi)-sufficient and Pi-deficient conditions.

RESULTS

Low-phosphorus levels limit plant growth and metabolism. Although the overall phosphorus contents of shoots were not significantly different between Qi319 and Qi319-96, the Pi level of Qi319-96 was 52.94 % higher than that of Qi319. Under low phosphorus conditions, Qi319-96 had increased chlorophyll levels and enhanced photosynthesis. The changes in starch and sucrose contents under these conditions also differed between genotypes. The proteomic changes included 29 (Pi-sufficient) and 71 (Pi-deficient) differentially expressed proteins involved in numerous metabolic processes. Proteome and physiological analyses revealed that Qi319-96 could better remodel the lipid composition of membranes and had higher V-ATPase activity levels than Qi319 under low-phosphate starvation, which enhanced the recycling of intracellular Pi, as reflected by its increased Pi levels. Chlorophyll biosynthesis was improved and the levels, and activities, of several Calvin cycle and "CO2 pump" enzymes were greater in Qi319-96 than in Qi319, which led to a higher rate of photosynthesis under low-phosphate stress in this line compared with in Qi319.

CONCLUSIONS

Our results suggest that the increased tolerance of the maize mutant Qi319-96 to low-phosphate levels is owing to its ability to increase Pi availability. Additionally, inbred lines of maize with low-P-tolerant traits could be obtained effectively through cellular engineering.

Transcriptomic Profiling Reveals Complex Molecular Regulation in Cotton Genic Male Sterile Mutant Yu98-8A

Although cotton genic male sterility (GMS) plays an important role in the utilization of hybrid vigor, its precise molecular mechanism remains unclear. To characterize the molecular events of pollen abortion, transcriptome analysis, combined with histological observations, was conducted in the cotton GMS line, Yu98-8A. A total of 2,412 genes were identified as significant differentially expressed genes (DEGs) before and during the critical pollen abortion stages. Bioinformatics and biochemical analysis showed that the DEGs mainly associated with sugars and starch metabolism, oxidative phosphorylation, and plant endogenous hormones play a critical and complicated role in pollen abortion. These findings extend a better understanding of the molecular events involved in the regulation of pollen abortion in genic male sterile cotton, which may provide a foundation for further research studies on cotton heterosis breeding.

Multimeric states of starch phosphorylase determine protein-protein interactions with starch biosynthetic enzymes in amyloplasts

Protein-protein interactions between starch phosphorylase (SP) and other starch biosynthetic enzymes were investigated using isolated maize endosperm amyloplasts and a recombinant maize enzyme. Plastidial SP is a stromal enzyme existing as a multimeric protein in amyloplasts. Biochemical analysis of the recombinant maize SP indicated that the tetrameric form was catalytically active in both glucan-synthetic and phosphorolytic directions. Protein-protein interaction experiments employing the recombinant SP as an affinity ligand with amyloplast extracts showed that the multimeric state of SP determined interactions with other enzymes of the starch biosynthetic pathway. The monomeric form of SP interacts with starch branching enzyme I (SBEI) and SBEIIb, whereas only SBEI interacts with the tetrameric form of SP. In all cases, protein-protein interactions were broken when amyloplast lysates were dephosphorylated in vitro, and enhanced following pre-treatment with ATP, suggesting a mechanism of protein complex formation regulated by protein phosphorylation. In vitro protein phosphorylation experiments with [γ-(32)P]-ATP show that SP is phosphorylated by a plastidial protein kinase. Evidence is presented which suggests SBEIIb modulates the catalytic activity of SP through the formation of a heteromeric protein complex.

Proteomic analysis of the enzymes involved in the starch biosynthesis of maize with different endosperm type and characterization of the starch

BACKGROUND

The characterization of starch maize with different endosperm type and the proteomic analysis of its biosynthetic enzymes at 20 and 50 days after pollination (DAP) was carried out.

RESULTS

There were differences between both endosperm types at 20 DAP, mainly in starch accumulation, amylose content, granule size and crystallinity percentage, whereas at 50 DAP the differences found were not relevant in the case of starch content, granule size, chain length distribution and thermal properties. SSSI, SBEIIb and GBSSI enzymes were identified; however, SBEIIb was only identified in two samples: floury endosperm at 20 DAP and vitreous at 50 DAP.

CONCLUSION

Starch did not show differences in its morphological or structural characteristics in either endosperm on reaching maturity. Starch biosynthetic enzymes identified by matrix-assisted laser desorption/ionization-time of flight did not show a relationship to starch structure.

Enhancing sucrose synthase activity results in increased levels of starch and ADP-glucose in maize (Zea mays L.) seed endosperms

Sucrose synthase (SuSy) is a highly regulated cytosolic enzyme that catalyzes the conversion of sucrose and a nucleoside diphosphate into the corresponding nucleoside diphosphate glucose and fructose. In cereal endosperms, it is widely assumed that the stepwise reactions of SuSy, UDPglucose pyrophosphorylase and ADPglucose (ADPG) pyrophosphorylase (AGP) take place in the cytosol to convert sucrose into ADPG necessary for starch biosynthesis, although it has also been suggested that SuSy may participate in the direct conversion of sucrose into ADPG. In this study, the levels of the major primary carbon metabolites, and the activities of starch metabolism-related enzymes were assessed in endosperms of transgenic maize plants ectopically expressing StSUS4, which encodes a potato SuSy isoform. A total of 29 fertile lines transformed with StSUS4 were obtained, five of them containing a single copy of the transgene that was still functional after five generations. The number of seeds per ear of the five transgenic lines containing a single StSUS4 copy was comparable with that of wild-type (WT) control seeds. However, transgenic seeds accumulated 10-15% more starch at the mature stage, and contained a higher amylose/amylopectin balance than WT seeds. Endosperms of developing StSUS4-expressing seeds exhibited a significant increase in SuSy activity, and in starch and ADPG contents when compared with WT endosperms. No significant changes could be detected in the transgenic seeds in the content of soluble sugars, and in activities of starch metabolism-related enzymes when compared with WT seeds. A suggested metabolic model is presented wherein both AGP and SuSy are involved in the production of ADPG linked to starch biosynthesis in maize endosperm cells.

Lack of starch synthase IIIa and high expression of granule-bound starch synthase I synergistically increase the apparent amylose content in rice endosperm

Rice endosperm starch is composed of 0-30% linear amylose, which is entirely synthesized by granule-bound starch synthase I (GBSSI: encoded by Waxy, Wx). The remainder consists of branched amylopectin and is elongated by multiple starch synthases (SS) including SSI, IIa and IIIa. Typical japonica rice lacks active SSIIa and contains a low expressing Wx(b) causing a low amylose content (ca. 20%). WAB2-3 (SS3a/Wx(a)) lines generated by the introduction of a dominant indica Wx(a) into a japonica waxy mutant (SS3a/wx) exhibit elevated GBSSI and amylose content (ca. 25%). The japonica ss3a mutant (ss3a/Wx(b)) shows a high amylose content (ca. 30%), decreased long chains of amylopectin and increased GBSSI levels. To investigate the functional relationship between the ss3a and Wx(a) genes, the ss3a/Wx(a) line was generated by crossing ss3a/Wx(b) with SS3a/Wx(a), and the starch properties of this line were examined. The results show that the apparent amylose content of the ss3a/Wx(a) line was increased (41.3%) compared to the parental lines. However, the GBSSI quantity did not increase compared to the SS3a/Wx(a) line. The amylopectin branch structures were similar to the ss3a/Wx(b) mutant. Therefore, Wx(a) and ss3a synergistically increase the apparent amylose content in rice endosperm, and the possible reasons for this increase are discussed.

Sugar-hormone cross-talk in seed development: two redundant pathways of IAA biosynthesis are regulated differentially in the invertase-deficient miniature1 (mn1) seed mutant in maize

The miniature1 (mn1) seed phenotype is a loss-of-function mutation at the Mn1 locus that encodes a cell wall invertase; its deficiency leads to pleiotropic changes including altered sugar levels and decreased levels of IAA throughout seed development. To understand the molecular details of such a sugar-hormone relationship, we have initiated studies on IAA biosynthesis genes in developing seeds of maize. Two tryptophan-dependent pathways of IAA biosynthesis, tryptamine (TAM) and indole-3-pyruvic acid (IPA), are of particular interest. We report on molecular isolation and characterization of an endosperm-specific ZmTARelated1 (ZmTar1) gene of the IPA branch; we have also reported recently on ZmYuc1 gene in the TAM branch. Comparative gene expression analyses here have shown that (1) the ZmTar1 transcripts were approximately 10-fold higher levels than the ZmYuc1; (2) although both genes showed the highest level of expression at 8-12 d after pollination (DAP) coincident with an early peak in IAA levels, the two showed highly divergent (antagonistic) response at 12 and 16 DAP but similar patterns at 20 and 28 DAP in the Mn1 and mn1 endosperm. The Western blot analyses for the ZmTAR1 protein, however, displayed disconcordant protein/transcript expression patterns. Overall, these data report novel observations on redundant trp-dependent pathways of auxin biosynthesis in developing seeds of maize, and suggest that homeostatic control of IAA in this important sink is highly complex and may be regulated by both sucrose metabolism and developmental signals.

A study of enzyme activities in a dosage series of the long arm of chromosome one in maize

The enzyme activity levels of alcohol, malate, isocitrate, glucose-6-phosphate and 6-phosphogluconate dehydrogenases were determined in mature maize scutella in a series of one to four doses of the long arm of chromosome 1, produced by the B-A translocation 1La. Although the Adh structural locus was varied, ADH levels did not exhibit a gene-dosage effect. The levels of G6PDH, 6PGDH and IDH were negatively correlated with the dosage of 1L. MDH was unresponsive. The esterase-8 enzyme, whose structural locus was demonstrated to be elsewhere in the genome, was also negatively correlated with 1L dosage. The portion of the B chromosome involved in the translocation was shown to have no effect on the enzyme levels. Measurements of cell size and hydrolysable DNA per mg dry weight revealed no change in the number of cells through the one, two and three dose series. The topic of enzyme alterations in aneuploids is reviewed.

The amylose extender mutant of maize conditions novel protein-protein interactions between starch biosynthetic enzymes in amyloplasts

The amylose extender (ae(-)) mutant of maize lacks starch branching enzyme IIb (SBEIIb) activity, resulting in amylopectin with reduced branch point frequency, and longer glucan chains. Recent studies indicate isozymes of soluble starch synthases form high molecular weight complexes with SBEII isoforms. This study investigated the effect of the loss of SBEIIb activity on interactions between starch biosynthetic enzymes in maize endosperm amyloplasts. Results show distinct patterns of protein-protein interactions in amyloplasts of ae(-) mutants compared with the wild type, suggesting functional complementation for loss of SBEIIb by SBEI, SBEIIa, and SP. Coimmunoprecipitation experiments and affinity chromatography using recombinant proteins showed that, in amyloplasts from normal endosperm, protein-protein interactions involving starch synthase I (SSI), SSIIa, and SBEIIb could be detected. By contrast, in ae(-) amyloplasts, SSI and SSIIa interacted with SBEI, SBEIIa, and SP. All interactions in the wild-type were strongly enhanced by ATP, and broken by alkaline phosphatase, indicating a role for protein phosphorylation in their assembly. Whilst ATP and alkaline phosphatase had no effect on the stability of the protein complexes from ae(-) endosperm, radiolabelling experiments showed SP and SBEI were both phosphorylated within the mutant protein complex. It is proposed that, during amylopectin biosynthesis, SSI and SSIIa form the core of a phosphorylation-dependent glucan-synthesizing protein complex which, in normal endosperm, recruits SBEIIb, but when SBEIIb is absent (ae(-)), recruits SBEI, SBEIIa, and SP. Differences in stromal protein complexes are mirrored in the complement of the starch synthesizing enzymes detected in the starch granules of each genotype, reinforcing the hypothesis that the complexes play a functional role in starch biosynthesis.

Sugar levels modulate sorbitol dehydrogenase expression in maize

The first step in sucrose use by maize kernels produces fructose, regardless of whether the initial reaction is catalyzed by an invertase or the reversible sucrose synthase. This fructose can enter subsequent metabolism via hexokinase, or in maize kernels, by a sorbitol dehydrogenase that reversibly converts fructose + NADH to sorbitol + NAD. High levels of SDH activity suggest that kernels synthesize considerable amounts of sorbitol, but the molecular mechanism and functional role for this process have remained equivocal. To gain insights on the role of sorbitol synthesis in maize endosperm we cloned and characterized the transcriptional control of the maize sorbitol dehydrogenase (Sdh1) gene. Data indicated that Sdh1 was essentially kernel- and endosperm-specific, with maximal expression at both the mRNA and enzyme activity levels during early kernel development. Expression was elevated in high-sugar mutants (sugary1, shrunken2), also by sugar injections, and was more pronounced when transfected tissues were incubated at low oxygen concentrations. Control of Sdh1 expression in our transient assays was largely dependent on the first intron of Sdh1. We speculate that SDH activity may represent an adaptation to the high-sugar/low-oxygen environment of the endosperm. Under these conditions, the NADH-dependent reduction of fructose to sorbitol would regenerate NAD[+], thus contributing to the maintenance of the redox and energy status of the cell.

The complexities of starch biosynthesis in cereal endosperms

Starch serves not only as an energy source for plants, animals, and humans but also as an environmentally friendly alternative for fossil fuels. Here, we describe recent findings concerning the synthesis of this important molecule in the cereal endosperm. Results from six separate transgenic reports point to the importance of adenosine diphosphate glucose pyrophosphorylase in controlling the amount of starch synthesized. The unexpected cause underlying the contrast in sequence divergence of its two subunits is also described. A major unresolved question concerning the synthesis of starch is the origin of nonrandom or clustered alpha-1,6 branch-points within the major component of starch, amylopectin. Developing evidence that several of the starch biosynthetic enzymes involved in amylopectin synthesis occur in complexes is reviewed. These complexes may provide the specificity for the formation of nonrandom branch-points.

Expression of cell wall invertase and several other genes of sugar metabolism in relation to seed development in sorghum (Sorghum bicolor)

We report expression profiles of several genes of carbohydrate metabolism, cell wall invertase (CWI) in particular, to better understand sugar transport and its utilization in developing caryopses of grain sorghum [Sorghum bicolor (L.) Moench]. Gene expression analyses for CWI using RNA gel blot and real-time quantitative PCR approaches on developing caryopses, including the glumes (maternal tissue appended to the seeds), showed expression of SbIncw (ZmIncw2 ortholog) primarily in the basal sugar unloading zone of endosperm. The expression of ZmIncw1 ortholog was significantly less abundant and restricted to the glumes. The protein and enzyme activity data corroborated the temporal transcript expression profile that showed maximal CWI protein (INCW) expression preceding the starch-filling phase of endosperm development, i.e. 6-12d-after-pollination (DAP). Protein gel blot analysis using polyclonal maize INCW1 antibodies showed a single polypeptide of 72kDa. The highest level of enzyme activity was unique to the basal part of the endosperm, in particular the basal endosperm transfer cell (BETC) layer and the maternal pedicel region that were highly enriched for the INCW protein, as seen by immunolocalization. High hexose-to-sucrose ratio in 6-12 DAP seeds, and negligible starch deposition in glumes corroborated the CWI activity data. Additionally, we report transcription profiles of several other genes related to sugar-to-starch metabolism in developing sorghum endosperm. As in maize, the INCW-mediated apoplastic cleavage of sucrose in the BETC and pedicel during the early developmental stages of caryopses is essential for the normal development of filial tissues. The unique cell-specificity of the INCW protein to both proximal and distal ends of placental sac shown here for the first time is likely to greatly increase uptakes of both hexose sugars and water through turgor sensing into developing seed. This trait is unique to sorghum among cereals and may facilitate its survival in drought environment.

Analysis of the sucrose synthase gene family in Arabidopsis

The properties and expression patterns of the six isoforms of sucrose synthase in Arabidopsis are described, and their functions are explored through analysis of T-DNA insertion mutants. The isoforms have generally similar kinetic properties. Although there is variation in sensitivity to substrate inhibition by fructose this is unlikely to be of major physiological significance. No two isoforms have the same spatial and temporal expression patterns. Some are highly expressed in specific locations, whereas others are more generally expressed. More than one isoform is expressed in all organs examined. Mutant plants lacking individual isoforms have no obvious growth phenotypes, and are not significantly different from wild-type plants in starch, sugar and cellulose content, seed weight or seed composition under the growth conditions employed. Double mutants lacking the pairs of similar isoforms sus2 and sus3, and sus5 and sus6, are also not significantly different in these respects from wild-type plants. These results are surprising in the light of the marked phenotypes observed when individual isoforms are eliminated in crop plants including pea, maize, potato and cotton. A sus1/sus4 double mutant grows normally in well-aerated conditions, but shows marked growth retardation and accumulation of sugars when roots are subjected to hypoxia. The sucrose synthase activity in roots of this mutant is 3% or less of wild-type activity. Thus under well-aerated conditions sucrose mobilization in the root can proceed almost entirely via invertases without obvious detriment to the plant, but under hypoxia there is a specific requirement for sucrose synthase activity.

Genetic control of cell wall invertases in developing endosperm of maize

We show here that the total invertase activity in developing seeds of maize is due to two cell wall invertase (CWI) genes, Incw1 and Incw2 (Mn1). Our previous results have shown that loss-of-function mutations at the Mn1 locus lead to the miniature-1 (mn1) seed phenotype, marked by a loss of >70% of seed weight at maturity. The mn1 seed mutant is, however, non-lethal presumably because it retains a residual low level, approximately 1%, of the total CWI activity relative to the Mn1 endosperm throughout seed development. Evidence here shows that the residual activity in the mn1 mutant is encoded by the Incw1 gene. RNA level analyses, especially quantitative real-time PCR studies, showed significant spatial and temporal heterogeneity in the expression of the two CWI genes in the developing endosperm. The Mn1-encoded Incw2 transcripts were seen at the highest levels in the basal region (the sugar unloading zone) during the early phase of cell division and elongation in the endosperm. In contrast, the highest levels of Incw1 transcripts were seen in the storage phase in both the upper (storage cells) and the lower parts of the endosperm. Protein and enzyme level analyses, however, appeared to show a lack of concordance with the RNA level of expression in both the Mn1 and mn1 endosperms, indicating a possibility of post-transcriptional control in the expression of these two genes. Collectively, the data suggest an important role for apoplastic cleavage of sucrose throughout the duration of seed development; and, of the two isoforms, the INCW2 appears to control metabolic flux of sugar utilization in the developing endosperm.

Translation of Zea mays endosperm sucrose-synthase mRNA in vitro

mRNA of 23-day-old maize endosperm was translated both in wheat germ extracts and rabbit reticulocyte lysates. A protein with an apparent molecular weight of 88,000 comigrates in dodecylsulfate/polyacrylamide electrophoresis with sucrose synthase. This protein is precipitated with an antiserum against sucrose synthase and shows the same protease digestion pattern as the enzyme. It is not synthesized with mRNA extracted from sh/sh mutant kernels lacking sucrose synthase. By these criteria, the protein is the translation product in vitro of sucrose synthase mRNA. The separation of mRNA in methylmercury-hydroxide--agarose gels and subsequent translation indicates a length of sucrose synthase mRNA of 2800 nucleotides which is compatible with the coding length necessary for a protein with a molecular weight of 88,000 plus untranslated sequences.

Expression of a bacterial xylose isomerase in potato tubers results in an altered hexose composition and a consequent induction of metabolism

Here we investigate the role of hexoses in the metabolism of the developing potato (Solanum tuberosum) tuber by the expression of a bacterial xylose isomerase which catalyzes the interconversion of glucose and fructose. Previously, we found that glycolysis was induced in transgenic tubers expressing a yeast invertase in the cytosol and postulated that this was due either to the decreased levels of sucrose or to effects downstream of the sucrose cleavage. In the present study xylose isomerase was expressed under the control of the tuber-specific patatin promoter. Selected transformants exhibited minor changes in the levels of tuber glucose and fructose but not in sucrose. Analysis of the enzyme activities of the glycolytic pathway revealed minor yet significant increases in the maximal catalytic activities of aldolase and glyceraldehyde 3-phosphate dehydrogenase but no increase in the activities of other enzymes of glycolysis. These lines were also characterized by an elevated tuber number, glycolytic and sucrose synthetic fluxes and in some metabolite levels downstream of glycolysis. When considered together these data suggest that the perturbation of hexose levels can result in increased glycolytic and sucrose (re)synthetic fluxes in the potato tuber even in the absence of changes in the level of sucrose. The consequences of altering hexose levels in the tuber are, however, not as severe as those observed following perturbation of the level of tuber sucrose.

A bypass of sucrose synthase leads to low internal oxygen and impaired metabolic performance in growing potato tubers

Plants possess two alternative biochemical pathways for sucrose (Suc) degradation. One involves hydrolysis by invertase followed by phosphorylation via hexokinase and fructokinase, and the other route-which is unique to plants-involves a UDP-dependent cleavage of Suc that is catalyzed by Suc synthase (SuSy). In the present work, we tested directly whether a bypass of the endogenous SuSy route by ectopic overexpression of invertase or Suc phosphorylase affects internal oxygen levels in growing tubers and whether this is responsible for their decreased starch content. (a) Oxygen tensions were lower within transgenic tubers than in wild-type tubers. Oxygen tensions decreased within the first 10 mm of tuber tissue, and this gradient was steeper in transgenic tubers. (b) Invertase-overexpressing tubers had higher activities of glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, and alcohol dehydrogenase, and (c) higher levels of lactate. (d) Expression of a low-oxygen-sensitive Adh1-beta-glucuronidase reporter gene construct was more strongly induced in the invertase-overexpressing background compared with wild-type background. (e) Intact transgenic tubers had lower ATP to ADP ratios than the wild type. ATP to ADP ratio was restored to wild type, when discs of transgenic tubers were incubated at 21% (v/v) oxygen. (f) Starch decreased from the periphery to the center of the tuber. This decrease was much steeper in the transgenic lines, leading to lower starch content especially near the center of the tuber. (g) Metabolic fluxes (based on redistribution of (14)C-glucose) and ATP to ADP ratios were analyzed in more detail, comparing discs incubated at various external oxygen tensions (0%, 1%, 4%, 8%, 12%, and 21% [v/v]) with intact tubers. Discs of Suc phosphorylase-expressing lines had similar ATP to ADP ratios and made starch as fast as wild type in high oxygen but had lower ATP to ADP ratios and lower rates of starch synthesis than wild type at low-oxygen tensions typical to those found inside an intact tuber. (h) In discs of wild-type tubers, subambient oxygen concentrations led to a selective increase in the mRNA levels of specific SuSy genes, whereas the mRNA levels of genes encoding vacuolar and apoplastic invertases decreased. (i) These results imply that repression of invertase and mobilization of Suc via the energetically less costly route provided by SuSy is important in growing tubers because it conserves oxygen and allows higher internal oxygen tensions to be maintained than would otherwise be possible.

Response of plant metabolism to too little oxygen

Oxygen can fall to low concentrations within plant tissues, either because of environmental factors that decrease the external oxygen concentration or because the movement of oxygen through the plant tissues cannot keep pace with the rate of oxygen consumption. Recent studies document that plants can decrease their oxygen consumption in response to low oxygen concentrations to avoid internal anoxia. This adaptive response involves a restriction of respiration and a concomitant decrease in ATP consumption that results from the inhibition of a wide range of biosynthetic processes. The inhibition of respiration is rapid and occurs at oxygen concentrations well above the K(m)(oxygen) of cytochrome oxidase, indicating that an oxygen-sensing system triggers a coordinated inhibition of ATP formation and consumption. In addition to this, low oxygen concentrations lead to the induction of a plant-specific and energy-conserving pathway of sucrose degradation, which decreases oxygen consumption and improves plant performance. Low oxygen concentrations also lead to long-term morphological adaptations, which allow respiration per volume tissue to be decreased and oxygen entry to be increased. Recently, advances have been made in elucidating possible oxygen-sensing systems and regulatory components that are involved in these responses.

Sucrose synthase catalyzes the de novo production of ADPglucose linked to starch biosynthesis in heterotrophic tissues of plants

By using barley seeds, developmental changes of ADPglucose (ADPG)-producing sucrose synthase (SS) and ADPG pyrophosphorylase (AGPase) have been compared with those of UDPglucose (UDPG), ADPG, sucrose (Suc) and starch contents. Both ADPG-synthesizing SS and AGPase activity patterns were found to correlate well with those of ADPG and starch contents. Remarkably, however, maximal activities of ADPG-synthesizing SS were found to be several fold higher than those of AGPase throughout seed development, the highest rate of starch accumulation being well accounted for by SS. Kinetic analyses of SS from barley endosperms and potato tubers in the Suc cleavage direction showed similar K(m) values for ADP and UDP, whereas apparent affinity for Suc was shown to be higher in the presence of UDP than with ADP. Moreover, measurements of transglucosylation activities in starch granules incubated with purified SS, ADP and [U-(14)C]Suc revealed a low inhibitory effect of UDP. The ADPG and UDPG contents in the transgenic S-112 SS and starch deficient potato mutant [Zrenner et al. (1995) Plant J. 7: 97] were found to be 35% and 30% of those measured in wild-type plants, whereas both glucose-1-phosphate and glucose-6-phosphate contents were found to be normal as compared with those of wild-type plants. The overall results thus strongly support a novel gluconeogenic mechanism reported previously [Pozueta-Romero et al. (1999) CRIT: Rev. Plant Sci. 18: 489] wherein SS catalyses directly the de novo production of ADPG linked to starch biosynthesis in heterotrophic tissues of plants.

Starch biosynthesis during pollen maturation is associated with altered patterns of gene expression in maize

Starch biosynthesis during pollen maturation is not well understood in terms of genes/proteins and intracellular controls that regulate it in developing pollen. We have studied two specific developmental stages: "early," characterized by the lack of starch, before or during pollen mitosis I; and "late," an actively starch-filling post-pollen mitosis I phase in S-type cytoplasmic male-sterile (S-CMS) and two related male-fertile genotypes. The male-fertile starch-positive, but not the CMS starch-deficient, genotypes showed changes in the expression patterns of a large number of genes during this metabolic transition. In addition to a battery of housekeeping genes of carbohydrate metabolism, we observed changes in hexose transporter, plasma membrane H(+)-ATPase, ZmMADS1, and 14-3-3 proteins. Reduction or deficiency in 14-3-3 protein levels in all three major cellular sites (amyloplasts [starch], mitochondria, and cytosol) in male-sterile relative to male-fertile genotypes are of potential interest because of interorganellar communication in this CMS system. Further, the levels of hexose sugars were significantly reduced in male-sterile as compared with male-fertile tissues, not only at "early" and "late" stages but also at an earlier point during meiosis. Collectively, these data suggest that combined effects of both reduced sugars and their reduced flux in starch biosynthesis along with a strong possibility for altered redox passage may lead to the observed temporal changes in gene expressions, and ultimately pollen sterility.

Soluble invertase expression is an early target of drought stress during the critical, abortion-sensitive phase of young ovary development in maize

To distinguish their roles in early kernel development and stress, expression of soluble (Ivr2) and insoluble (Incw2) acid invertases was analyzed in young ovaries of maize (Zea mays) from 6 d before (-6 d) to 7 d after pollination (+7 d) and in response to perturbation by drought stress treatments. The Ivr2 soluble invertase mRNA was more abundant than the Incw2 mRNA throughout pre- and early post-pollination development (peaking at +3 d). In contrast, Incw2 mRNAs increased only after pollination. Drought repression of the Ivr2 soluble invertase also preceded changes in Incw2, with soluble activity responding before pollination (-4 d). Distinct profiles of Ivr2 and Incw2 mRNAs correlated with respective enzyme activities and indicated separate roles for these invertases during ovary development and stress. In addition, the drought-induced decrease and developmental changes of ovary hexose to sucrose ratio correlated with activity of soluble but not insoluble invertase. Ovary abscisic acid levels were increased by severe drought only at -6 d and did not appear to directly affect Ivr2 expression. In situ analysis showed localized activity and Ivr2 mRNA for soluble invertase at sites of phloem-unloading and expanding maternal tissues (greatest in terminal vascular zones and nearby cells of pericarp, pedicel, and basal nucellus). This early pattern of maternal invertase localization is clearly distinct from the well-characterized association of insoluble invertase with the basal endosperm later in development. This localization, the shifts in endogenous hexose to sucrose environment, and the distinct timing of soluble and insoluble invertase expression during development and stress collectively indicate a key role and critical sensitivity of the Ivr2 soluble invertase gene during the early, abortion-susceptible phase of development.

Gene expression studies on developing kernels of maize sucrose synthase (SuSy) mutants show evidence for a third SuSy gene

Previous studies have identified two tissue- and cell-specific, yet functionally redundant, sucrose synthase (SuSy) genes, Sh1 and Sus1, which encode biochemically similar isozymes, SH1 and SUS1 (previously referred to as SS1 and SS2, respectively). Here we report evidence for a third SuSy gene in maize, Sus3, which is more similar to dicot than to monocot SuSys. RNA and/or protein blot analyses on developing kernels and other tissues show evidence of expression of Sus3, although at the lowest steady-state levels of the three SuSy gene products and without a unique pattern of tissue specificity. Immunoblots of sh1sus1-1 embryos that are either lacking or deficient for the embryo-specific SUS1 protein have shown a protein band which we attribute to the Sus3 gene, and may contribute to the residual enzyme activity seen in embryos of the double mutant. We also studied developing seeds of the double mutant sh1sus1-1, which is missing 99.5% of SuSy enzyme activity, for evidence of co-regulation of several genes of sugar metabolism. We found a significant reduction in the steady-state levels of Miniature-1 encoded cell wall invertase2, and Sucrose transporter (Sut) mRNAs in the double mutant, relative to the lineage-related sh1Sus1 and sh1Sus1 kernels. Down-regulation of the Mn1 gene was also reflected in significant reductions in cell wall invertase activity. Co-regulatory changes were not seen in the expression of Sucrose phosphate synthase, UDP-glucose pyrophosphorylase, and ADP-glucose pyrophosphorylase.