Biosynthesis of starch in chloroplasts

PEP-ASSOCIATED PROTEIN 3 regulates rice tiller formation and grain yield by controlling chloroplast biogenesis

Plastid-encoded RNA polymerase (PEP) plays a pivotal role in chloroplast development by governing the transcription of chloroplast genes, and PEP-associated proteins (PAPs) modulate PEP transcriptional activity. Therefore, PAPs provide an intriguing target for those efforts to improve yield, by enhancing chloroplast development. In this study, we identified the rice (Oryza sativa) OsPAP3 gene and characterized its function in chloroplast development. OsPAP3 expression was light-dependent and leaf-specific, similar to the PEP-dependent chloroplast gene RUBISCO LARGE SUBUNIT (OsRbcL), and OsPAP3 protein localized to chloroplast nucleoids where PEP functions. Analysis of loss-of-function and gain-of-function mutants showed that the expression of OsPAP3 is tightly linked to chloroplast gene expression and chloroplast biogenesis in rice. Homozygous knockout mutants of OsPAP3 had fewer chloroplasts than wild type, whereas plants overexpressing OsPAP3 had more chloroplasts. Also, OsPAP3 knockout suppressed the PEP-dependent expression of chloroplast genes, but OsPAP3 overexpression increased their expression. These findings indicate that OsPAP3 regulates chloroplast biogenesis in rice by controlling the PEP-dependent expression of chloroplast genes. More importantly, data from 3 seasons of field cultivation revealed that the overexpression of OsPAP3 improves rice grain yield by approximately 25%, largely due to increased tiller formation. Collectively, these observations suggest that OsPAP3 regulates rice growth and productivity by promoting chloroplast development.

Identification of Chemicals That Abrogate Folate-Dependent Inhibition of Starch Accumulation in Non-Photosynthetic Plastids of Arabidopsis

Folate, also known as vitamin B9, is an essential cofactor for a variety of enzymes and plays a crucial role in many biological processes. We previously reported that plastidial folate prevents starch biosynthesis triggered by the influx of sugar into non-starch-accumulating plastids, such as etioplasts, and chloroplasts under darkness; hence the loss of plastidial folate induces the accumulation of starch in plastids. To understand the molecular mechanism underlying this phenomenon, we screened our in-house chemical library and searched their derivatives to identify chemicals capable of inducing starch accumulation in etioplasts. The results revealed four chemicals, compounds #120 and #375 and their derivatives, compounds #120d and #375d, respectively. The derivative compounds induced starch accumulation in etioplasts and suppressed hypocotyl elongation in dark-grown Arabidopsis seedlings. They also inhibited the post-germinative growth of seedlings under illumination. All four chemicals contained the sulfonamide group as a consensus structure. The sulfonamide group is also found in sulfa drugs, which exhibit antifolate activity, and in sulfonylurea herbicides. Further analyses revealed that compound #375d induces starch accumulation by inhibiting folate biosynthesis. By contrast, compound #120d neither inhibited folate biosynthesis nor exhibited the herbicide activity. Protein and metabolite analyses suggest that compound #120d abrogates folate-dependent inhibition of starch accumulation in etioplasts by enhancing starch biosynthesis.

A chloroplastic UDP-glucose pyrophosphorylase from Arabidopsis is the committed enzyme for the first step of sulfolipid biosynthesis

Plants synthesize a sulfur-containing lipid, sulfoquinovosyldiacylglycerol, which is one of three nonphosphorus glycerolipids that provide the bulk of the structural lipids in photosynthetic membranes. Here, the identification of a novel gene, UDP-glucose pyrophosphorylase3 (UGP3), required for sulfolipid biosynthesis is described. Transcriptome coexpression analysis demonstrated highly correlated expression of UGP3 with known genes for sulfolipid biosynthesis in Arabidopsis thaliana. Liquid chromatography-mass spectrometry analysis of leaf lipids in two Arabidopsis ugp3 mutants revealed that no sulfolipid was accumulated in these mutants, indicating the participation of UGP3 in sulfolipid biosynthesis. From the deduced amino acid sequence, UGP3 was presumed to be a UDP-glucose pyrophosphorylase (UGPase) involved in the generation of UDP-glucose, serving as the precursor of the polar head of sulfolipid. Recombinant UGP3 was able to catalyze the formation of UDP-glucose from glucose-1-phosphate and UTP. A transient assay using fluorescence fusion proteins and UGPase activity in isolated chloroplasts indicated chloroplastic localization of UGP3. The transcription level of UGP3 was increased by phosphate starvation. A comparative genomics study on UGP3 homologs across different plant species suggested the structural and functional conservation of the proteins and, thus, a committing role for UGP3 in sulfolipid synthesis.

Sucrose phosphate synthetase : Separation from sucrose synthetase and a study of its properties

A method for the complete separation of sucrose phosphate synthetase (EC 2.4.1.14) and sucrose synthetase (EC 2.4.1.13) from wheat (Triticum aestivum L.) germ is described. The separation is achieved by chromatography on DEAE-cellulose at pH 6.5. The sucrose phosphate synthetase obtained can be further purified by gel filtration. Disc electrophoresis of sucrose phosphate preparations reveals the presence of isoenzymes. Molecular weight estimates of sucrose phosphate synthetase by gel filtration and sedimentation velocity give a value of 380,000. The enzyme is inhibited by various anions, particularly citrate, maleate, and phosphate. Activity estimate should be carried out with Good's buffers in order to avoid inhibition. Nucleoside triphosphates are competitive inhibitors toward UDP-glucose. The enzyme is sensitive to sulfhydryl reagents, but activity can be restored with DTT or β-mercapto ethanol. The fact that the enzyme is inhibited by δ-gluconolactone suggests that the reaction occurs through the formation of an unstable glucose-enzyme complex. Mg(2+) can restore enzyme activity to control values when inhibited by nucleoside triphosphates, citrate, or phosphate.

Pyrophosphorylases in Solanum tuberosum: I. Changes in ADP-Glucose and UDP-Glucose Pyrophosphorylase Activities Associated with Starch Biosynthesis during Tuberization, Maturation, and Storage of Potatoes

Changes in ADP-glucose and UDP-glucose pyrophosphorylase activities were followed during tuber development of Solanum tuberosum and prolonged storage at 4 and 11 C. Potato tuberization was accompanied by a sharp increase in starch synthesis simultaneous with a marked rise in ADP-glucose pyrophosphorylase activity. When tubers reached an average diameter of 1 centimeter (0.5 gram average tuber weight) and had already established 58% starch on a dry weight basis, ADP-glucose pyrophosphorylase increased 16- to 24-fold over its activity seen in low starch containing stolon tissue. During this same period UDP-glucose pyrophosphorylase increased approximately 2- to 3-fold. Although participation of UDP-glucose in starch formation can not be neglected, it is suggested that the onset of rapid non-photosynthetic potato tuber starch biosynthesis may be closely related to the simultaneous increase in ADP-glucose pyrophosphorylase activity.Evidence that UDP-glucose and ADP-glucose pyrophosphorylases are separate protein entities was indicated by their (a) activity ratio variations during tuber development and storage, (b) extraction stabilities, (c) morphological localization, (d) separation with ammonium sulfate, (e) pH optima, and (f) differential activation with 3-P-glycerate.

Starch metabolism in the leaf sheaths and culm of rice

The levels of starch and dextrin, free sugars, soluble protein, and enzymes involved in starch metabolism-alpha-amylase, beta-amylase, phosphorylase, Q-enzyme, R-enzyme, and ADP-glucose starch synthetases-were assayed in the leaf sheaths and culm of the rice plant (Oryza sativa L., variety IR8) during growth.Starch accumulation in the leaf sheaths reached a maximum 10 to 11 weeks after transplanting, the time of development of the rice panicle. Maximal concentration of free sugars occurred earlier. Starch and sugars in the leaf sheaths and culm decreased rapidly during grain development.During starch accumulation, the starch granules of the leaf sheaths increased slightly in size and its gelatinization temperature decreased. The molecular size of amylose and amylopectin and amylose content of the starch were similar in both culm and leaf sheaths.Changes in the level of soluble protein paralleled changes in starch level in the leaf sheaths. Among the enzymes, only synthetase bound to the starch granule paralleled the level of starch in the leaf sheaths and in the culm. ADP-glucose, but not UDP-glucose, was utilized as a glucosyl donor by these starch synthetases. Zymograms of these extracts showed only one alpha-amylase band, one beta-amylase band, two phosphorylase bands, and one Q-enzyme band.

[The role of phosphorylase in starch metabolism in plastids]

Two phosphorylases could be detected on gel-electropherograms of leaf-extracts of Spinacia oleracea and of immature cotyledons of Vicia faba. These two phosphorylases could be separated by means of ammonium sulfate fractionation. Both the slower migrating phosphorylases from spinach and from beans, but not the fast one from beans, could be adsorbed on amyloplasts. This process takes place only when the amyloplasts are suspended in a salt medium. The slow phosphorylases can also be adsorbed on chloroplasts. The specific activity of the amyloplast-adsorbable phosphorylase in spinach leaves is about ten times higher in the cytoplasmatic fraction than in chloroplasts, a fact which suggests that this phosphorylase is localised in the cytoplasma. The addition of ADP or ATP to the reaction mixture had no influence on the synthesizing activity of the slow phosphorylases when they were tested with soluble amylopectin as a primer or while they were adsorbed on amyloplasts. The presence of ADPG and UDPG was inhibitory.The results reported above suggest that phosphorylase could catalyse the synthesis of starch in the plastids when photophosphorylation or oxidative phosphorylation occurs. This starch synthesis could be controlled by the concentration of ADPG. When, on the other hand, the ATP/Pi ratio is low, phosphorylase would be involved in starch breakdown. This reverse reaction is also regulated by the concentration of glucosylnucleotides.