Starch granules: structure and biosynthesis.

The triazine herbicide atrazine (2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine) is one of the most used pesticides in North America. Atrazine is principally used for control of certain annual broadleaf and grass weeds, primarily in corn but also in sorghum, sugarcane, and, to a lesser extent, other crops and landscaping. Atrazine is found in many surface and ground waters in North America, and aquatic ecological effects are a possible concern for the regulatory and regulated communities. To address these concerns an expert panel (the Panel) was convened to conduct a comprehensive aquatic ecological risk assessment. This assessment was based on several newly suggested procedures and included exposure and hazard subcomponents as well as the overall risk assessment. The Panel determined that use of probabilistic risk assessment techniques was appropriate. Here, the results of this assessment are presented as a case study for these techniques. The environmental exposure assessment concentrated on monitoring data from Midwestern watersheds, the area of greatest atrazine use in North America. This analysis revealed that atrazine concentrations rarely exceed 20 μg/L in rivers and streams that were the main focus of the aquatic ecological risk assessment. Following storm runoff, biota in lower-order streams may be exposed to pulses of atrazine greater than 20 μg/L, but these exposures are short-lived. The assessment also considered exposures in lakes and reservoirs. The principal data set was developed by the U.S. Geological Survey, which monitored residues in 76 Midwestern reservoirs in 11 states in 1992-1993. Residue concentrations in some reservoirs were similar to those in streams but persisted longer. Atrazine residues were widespread in reservoirs (92% occurrence), and the 90th percentile of this exposure distribution for early June to July was about 5 μg/L. Mathematical simulation models of chemical fate were used to generalize the exposure analysis to other sites and to assess the potential effects of reduction in the application rates. Models were evaluated, modified, and calibrated against available monitoring data to validate that these models could predict atrazine runoff. PRZM-2 overpredicted atrazine concentrations by about an order of magnitude, whereas GLEAMS underpredicted by a factor of 2 to 5. Thus, exposure models were not used to extrapolate to other regions of atrazine use in this assessment. The effects assessment considered both freshwater and saltwater toxicity test results. Phytoplankton were the most sensitive organisms, followed, in decreasing order of sensitivity, by macrophytes, benthic invertebrates, zooplankton, and fish. Atrazine inhibits photophosphorylation but typically does not result in lethality or permanent cell damage in the short term. This characteristic of atrazine required a different model than typically used for understanding the potential impact in aquatic systems, where lethality or nonreversible effects are usually assumed. In addition, recovery of phytoplankton from exposure to 5 to 20 μg/L atrazine was demonstrated. In some mesocosm field experiments, phytoplankton and macrophytes were reduced after atrazine exposures greater than 20 μg/L. However, populations were quickly reestablished, even while atrazine residues persisted in the water. Effects in field studies were judged to be ecologically important only at exposures of 50 μg/L or greater. Mesocosm experiments did not reveal disruption of either ecosystem structure or function at atrazine concentrations typically encountered in the environment (generally 5 μg/L or less). Based on an integration of laboratory bioassay data, field effects studies, and environmental monitoring data from watersheds in high-use areas in the Midwestern United States, the Panel concluded that atrazine does not pose a significant risk to the aquatic environment. Although some inhibitory effects on algae, phytoplankton, or macrophyte production may occur in small streams vulnerable to agricultural runoff, these effects are likely to be transient, and quick recovery of the ecological system is expected. A subset of surface waters, principally small reservoirs in areas with intensive use of atrazine, may be at greater risk of exposure to atrazine. Therefore, it is recommended that site-specific risk assessments be conducted at these sites to assess possible ecological effects in the context of the uses to which these ecosystems are put and the effectiveness and cost-benefit aspect of any risk mitigation measures that may be applied.

https://setac.onlinelibrary.wiley.com/doi/pdf/10.1002/etc.5620150105

Ecological risk assessment of atrazine in North American surface waters.

The instantaneous rate of photosynthetic CO2 assimilation in C3 plants has generally been studied in model systems such as isolated chloroplasts and algae. From these studies and from theoretical analyses of gas exchange behavior it is now possible to study the biochemistry of photosynthesis in intact leaves using a combination of methods, most of which are nondestructive. The limitations to the rate of photosynthesis can be divided among three general classes: (1) the supply or utilization of CO2, (2) the supply or utilization of light, and (3) the supply or utilization of phosphate. The first limitation is most readily studied by determining how the CO2 assimilation rate varies with the partial pressure of CO2 inside the leaf. The second limitation can be studied by determining the quantum requirement of photosynthesis. The third limitation is most easily detected as a loss of O2 sensitivity of photosynthesis. Measurement of fluorescence from intact leaves can give additional information about the various limitations. These methods are all non-destructive and so can be observed repeatedly as the environment of a leaf is changed. In addition, leaves can be quick-frozen and metabolite concentrations then measured to give more information about the limitations to intact leaf photosynthesis rates. In this review the physics and biochemistry of photosynthesis in intact C3 leaves, and the interface between physiology and photosynthesis—triose phosphate utilization—are discussed.

Photosynthesis in intact leaves of C3 plants: Physics, physiology and rate limitations

Publisher Summary This chapter focuses on the polymeric reserve materials and thus excludes the consideration of lipids other than poly-β-hydroxybutyrate, and the disaccharide trehalose, which plays an important role in the economy of the yeast cell, but displays certain features that are at variance with the normally accepted behavior of storage compounds. The chapter considers the evidence for the energy-storage roles of glycogen, polyphosphates and poly-p-hydroxybutyrate, and the current state of knowledge concerning the regulation of their biosynthesis and degradation in the microbial cell. The intracellular location of the enzymes concerned with polymer synthesis and degradation is an intriguing problem. The most detailed work has been carried out with PHB granules, which have PHB synthetase and a factor involved in depolymerase activity associated with the membrane that bounds the granule. The depolymerase system is a complex one that also involves a soluble fraction, underlining the importance of reactions at liquid-solid interfaces in the process and emphasizing that relatively little work has been done to isolate polyphosphate or glycogen granules in their native state in order to examine the possibility of regulation by a membranous system.

The Role and Regulation of Energy Reserve Polymers in Micro-organisms

Interaction of chlamydiae and host cells in vitro.

Interaction of spinach leaf adenosine diphosphate glucose α-1, 4-glucan α-4-glucosyl transferase and α-1, 4-glucan α-1, 4-glucan-6-glycosyl transferase in synthesis of branched α-glucan

Biosynthesis of Bacterial Glycogen. IV. Activation and Inhibition of the Adenosine Diphosphate Glucose Pyrophosphorylase of Escherichia coli B

The subcellular localization of the starch biosynthetic and degradative enzymes of spinach leaves was carried out by measuring the distribution of the enzymes in a crude chloroplast pellet and soluble protein fraction, and by the separation on sucrose density gradients of intact organelles, chloroplasts, peroxisomes, and mitochondria of a protoplast lysate. ADP-Glucose pyrophosphorylase, starch synthase, and starch-branching enzymes are quantitatively associated with the chloroplasts. The starch degradative enzymes amylase, R-enzyme (debranching activity), phosphorylase, and D-enzyme (transglycosylase) are observed both in the chloroplast and soluble protein fractions, the bulk of the degradative enzyme activities reside in the latter fraction. Chromatography of a chloroplast extract on diethylaminoethyl-cellulose resolves the R- and D-enzymes from amylase and phosphorylase activities although the two latter enzyme activities coeluted. The digestion pattern of amylase with amylopectin as a substrate indicates an endolytic activity but displays properties unlike the typical α-amylase as isolated from endosperm tissue.

Subcellular Localization of the Starch Degradative and Biosynthetic Enzymes of Spinach Leaves

The ADPglucose pyrophosphorylases of 7 plant-leaf tissues were partially purified and characterized. In all cases the enzymes showed stability to heat treatment at 65° for 5 minutes in the presence of 0.02 m phosphate buffer, pH 7.0. The leaf ADPglucose pyrophosphorylases were activated 5 to 15-fold by 3-phosphoglycerate. Fructose-6-phosphate and fructose 1, 6-diphosphate stimulated ADPglucose pyrophosphorylase to lesser extents. The A 0.5 (conc of activator required to give 50% of the observed maximal activation) of 3-phosphoglycerate for the barley enzyme was 7 × 10 −6 m while for the sorghum enzyme it was 3.7 × 10 −4 m. Inorganic phosphate proved to be an effective inhibitor of ADPglucose synthesis. The I 0.5 (conc of inhibitor that gave 50% inhibition of activity for the various leaf enzymes varied from 2 × 10 −5 m (barley) to 1.9 × 10 −4 m (sorghum). This inhibition was reversed or antagonized by the activator 3-phosphoglycerate. These results form the basis for an hypothesis of the regulation of leaf starch biosynthesis.

Regulation of Starch Biosynthesis in Plant Leaves: Activation and Inhibition of ADPglucose Pyrophosphorylase

Soluble ADPglucose-α-glucan 4-α-glucosyltransferase (starch synthetase), ADPglucose pyrophosphorylase, UDPglucose pyrophosphorylase and phosphorylase were assayed in extracts from developing kernels of maize (Zea mays). Normal, waxy and amylose-extender maize at stages of development ranging from 8 days to 28 days after pollination were studied. Shrunken-4 maize at the 22-day stage was also studied. There is adequate activity of both ADPglucose pyrophosphorylase and starch synthetase at all stages of development to account for the synthesis of starch. Thus all starch could be synthesized via the ADPglucose pathway. High levels of UDPglucose pyrophosphorylase and of phosphorylase activities were also found at all stages of development. The possible role of phosphorylase in starch synthesis could not be discounted. The levels of phosphorylase, ADPglucose pyrophosphorylase, starch synthetase, and UDPglucose pyrophosphorylase activities in shrunken-4 kernels were about 20 to 40% of that found in normal maize kernels. It appears that the mutation in shrunken-4 affects the activities of more than one enzyme. The defective starch synthesis seen in this mutant could be due to the low activities of ADPglucose pyrophosphorylase and starch synthetase rather than the low activity of phosphorylase.

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Elaine Greenberg

Papers

Interaction of spinach leaf adenosine diphosphate glucose α-1, 4-glucan α-4-glucosyl transferase and α-1, 4-glucan α-1, 4-glucan-6-glycosyl transferase in synthesis of branched α-glucan

Biosynthesis of Bacterial Glycogen. IV. Activation and Inhibition of the Adenosine Diphosphate Glucose Pyrophosphorylase of Escherichia coli B

Subcellular Localization of the Starch Degradative and Biosynthetic Enzymes of Spinach Leaves

Regulation of Starch Biosynthesis in Plant Leaves: Activation and Inhibition of ADPglucose Pyrophosphorylase

Starch Synthetase, Phosphorylase, ADPglucose Pyrophosphorylase, and UDPglucose Pyrophosphorylase in Developing Maize Kernels.