Effects of Photosystem II Herbicides on the Photosynthetic Membranes of the Cyanobacterium Aphanocapsa 6308

The effects of the photosystem II herbicides diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) on the photosynthetic membranes of a cyanobacterium, Aphanocapsa 6308, were compared to the effects on a higher plant, Spinacia oleracea. The inhibition of photosystem II electron transport by these herbicides was investigated by measuring the photoreduction of the dye 2,6-dichlorophenol-indophenol spectrophotometrically using isolated membranes. The concentration of herbicide that caused 50% inhibition of electron transport (I₅₀ value) in Aphanocapsa membranes for diuron was 6.8 × 10⁻⁹ molar and the I₅₀ value for atrazine was 8.8 × 10⁻⁸ molar. ¹⁴C-labeled diuron and atrazine were used to investigate herbicide binding with calculated binding constants (K) being 8.2 × 10⁻⁸ molar for atrazine and 1.7 × 10⁻⁷ molar for diuron. Competitive binding studies carried out on Aphanocapsa membranes using radiolabeled [¹⁴C]atrazine and unlabeled diuron revealed that diuron competed with atrazine for the herbicide-binding site. Experiments involving the photoaffinity label [¹⁴C]azidoatrazine (2-azido-4-ethylamino-6-isopropylamino-2-triazine) and autoradiography of polyacrylamide gels indicated that the herbicide atrazine binds to a 32-kilodalton protein in Aphanocapsa 6308 cell extracts.     

Changes in [C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

One hypothesis of triazine-type herbicide action in photosynthetic material is that the herbicide molecule competes with a secondary quinone acceptor, B, for a binding site at the reaction center of photosystem II. The binding affinity of B has been suggested to change with its level of reduction, being most strongly bound in its semiquinone form. To test this hypothesis, [¹⁴C]atrazine binding studies have been carried out under different photochemically induced levels of B reduction in Pisum sativum. It is found that herbicide binding is reduced in continuously illuminated samples compared to dark-adapted samples. Decreased binding of atrazine corresponds to an increase in the semiquinone form of B. With flash excitation, the herbicide binding oscillates with a cycle of two, being low on odd-numbered flashes when the amount of semiquinone form of B is greatest. Treatment with NH₂OH was found to significantly decrease the strength of herbicide binding in the dark as well as stop the ability of p-benzoquinone to oxidize the semiquinone form of B. It is suggested that the mode of action of NH₂OH is disruption of quinones or their environment on both the oxidizing and reducing sides of photosystem II. Herbicide binding was found to be unaltered under conditions when p-benzosemiquinone oxidation of the reduced primary acceptor, Q⁻, is herbicide insensitive; weak herbicide binding cannot explain this herbicide insensitivity. It is concluded that the quinone-herbicide competition theory of herbicide action is correct. Also, since quinones are lipophilic the importance of the lipid composition of the thylakoid membrane in herbicide interactions is stressed.     

Mechanism of Sulfonylurea Herbicide Resistance in the Broadleaf Weed, Kochia scoparia

Selection of kochia (Kochia scoparia) biotypes resistant to the sulfonylurea herbicide chlorsulfuron has occurred through the continued use of this herbicide in monoculture cereal-growing areas in the United States. The apparent sulfonylurea resistance observed in kochia was confirmed in greenhouse tests. Fresh and dry weight accumulation in the resistant kochia was 2- to >350-fold higher in the presence of four sulfonylurea herbicides as compared to the susceptible biotype. Acetolactate synthase (ALS) activity isolated from sulfonylurea-resistant kochia was less sensitive to inhibition by three classes of ALS-inhibiting herbicides, sulfonylureas, imidazolinones, and sulfonanilides. The decrease in ALS sensitivity to inhibition (as measured by the ratio of resistant I₅₀ to susceptible I₅₀) was 5- to 28-fold, 2- to 6-fold, and 20-fold for sulfonylurea herbicides, imidazolinone herbicides, and a sulfonanilide herbicide, respectively. No differences were observed in the ALS-specific activities or the rates of [¹⁴C]chlorsulfuron uptake, translocation, and metabolism between susceptible and resistant kochia biotypes. The K_m values for pyruvate using ALS from susceptible and resistant kochia were 2.13 and 1.74 mm, respectively. Based on these results, the mechanism of sulfonylurea resistance in this kochia biotype is due solely to a less sulfonylurea-sensitive ALS enzyme.     

Initiation of Activation of a Preemergent Herbicide by a Novel Alkylsulfatase of Pseudomonas putida FLA

The activation of the preemergent herbicide 2-(2,4-dichlorophenoxy)ethyl sulfate (Crag herbicide) is initiated by soil microorganisms that are presumed to act by removing the ester sulfate group via some type of sulfatase enzyme. An enrichment technique with the herbicide as the sole source of sulfur led to the isolation of several pure cultures that could produce 2-(2,4-dichlorophenoxy)ethanol from the herbicide. One of these, a strain of Pseudomonas putida, was particularly active. Polyacrylamide gel zymograms of extracts of cells grown on nutrient broth showed the presence of three secondary and three primary alkylsulfatases. One of the latter enzymes was active toward Crag herbicide as well as sodium dodecyl sulfate. Maximum activity was obtained in the late-stationary phase of growth, and enzyme yields were not affected by either the presence or the absence of the herbicide in the growth medium. The enzyme was purified 2,670-fold to homogeneity by a combination of streptomycin sulfate treatment, heat treatment, and column chromatography on DEAE-cellulose, Sephacryl 200-S, and butyl agarose. The pure enzyme was tetrameric (molecular weight, 295,000) and most active at pH 6.0. Saturation kinetics with inhibition by excess substrate were observed for Crag herbicide and octyl sulfate. 2-Butox-yethyl sulfate was a relatively poor substrate, and dodecyltriethoxy sulfate was not hydrolyzed at all. Enzymatic hydrolysis of each substrate in the presence of H₂¹⁸O led to incorporation of ¹⁸O exclusively into SO₄²⁻ ions in all three cases. The Crag herbicide sulfatase therefore acts by cleaving the O-S bond of the C-O-S ester linkage, in contrast with other alkylsulfatases acting on long-chain alkyl sulfates.     

The interaction between bicarbonate and the herbicide ioxynil in the thylakoid membrane and the effects of amino acid modification on bicarbonate action

Bicarbonate depletion of chloroplast thylakoids reduces the affinity of the herbicide, ioxynil, to its binding site in Photosystem (PS) II. This herbicide is found to be a relatively more efficient inhibitor of the Hill reaction when HCO^?₃ is added to CO₂-depleted thylakoids in subsaturating rather than in saturating concentrations. The reason for this dependence of the inhibitor efficiency on the HCO^?₃ concentration is that the inactive HCO^?₃-deficient PS II reaction chains bind less ioxynil than the active PS II electron-transport chains that have bound HCO^?₃, and, thus, after addition of a certain amount of ioxynil the concentration of the free herbicide increases when the HCO^?₃ concentration decreases. Therefore, the inhibition of electron transport by ioxynil increases at decreasing HCO^?₃ levels. Measurements on the effects of modification of lysine and arginine residues on the rate of electron transport are also presented: the rate of modification is faster in the presence than in the absence of HCO^?₃. Therefore, we suggest that surface-exposed lysine or arginine residues are not involved in binding of HCO^?₃ (or CO₂ or CO^2?₃) to its binding protein, but that HCO^?₃ influences the conformation of its binding environment such that the affinity for certain herbicides and the accessibility for amino acid modifiers are changed.     

Transplasma-membrane ferricyanide reduction in sycamore cells. Characterization of the system and inhibition by some phenyl biscarbamates

Evidence is presented for the presence of a transplasma-membrane redox system in sycamore cells, which were found able to reduce external ferricyanide. This reduction induced a simultaneous acidification of the external medium, with a H⁺/e⁻ stoichiometry of 0.925. Ferricyanide reduction was accompanied by a slight reduction of potassium uptake (15% inhibition). The transmembrane electron transport was inhibited by oligomycin and quinacrine; the effect of the latter inhibitor was only temporary, owing to its progressive absorption by the cells.At 100 μM, several derivatives of the herbicide phenmedipham were able to inhibit the growth of sycamore cells. These compounds inhibited the external acidification induced by fusicoccin, without interfering with the integrity of the plasmalemma. They had no effect on the phosphohydrolase activity of a microsomal fraction enriched in plasmalemma ATPase. They were not uncouplers of oxidative phosphorylation, and appeared as poor inhibitors of mitochondrial electron transfer (I₅₀ > 100 μM). In intact cell suspension, they inhibited both the reduction of ferricyanide and the accompanying acidification of the external medium (I₅₀ = 32 μM). Moreover, they could induce a reversal of the functioning of the redox system, which consequently induced ferrocyanide oxidation.     

Caffeic acid derivatives as growth inhibitors of Setaria viridis: Structure-activity relationships and mechanisms

Weeds bring severe threats to agricultural production and most traditional herbicides cause serious pollution to the environment. Caffeic acid is a potential allelochemical of many crops. To develop novel herbicides, a series of trans-caffeic acid analogues were prepared to evaluate their growth inhibition on S. viridis and investigate structure-activity relationships. Presence of hydroxyl groups on the benzene ring greatly reduced the phytotoxicity of analogues and the double bond seemed not to be necessary. Moreover, it was found that halogenated cinnamic acid analogues showed potent inhibitory activity. 4-Fluorocinnamic acid (4-FCA) displayed the strongest growth inhibition in a concentration-dependent manner, whereas it induced significantly weaker inhibition on wheat. This selectivity could help us to develop a novel herbicide. Exposure of S. viridis to 4-FCA increased levels of H₂O₂ and catalase (CAT) in roots, suggesting that the inhibitory effect of 4-FCA might be related to oxidative stress. Thus, we have found an active lead compound for a novel herbicide derived from crop allelochemicals, which may help in the development of new environmentally safe herbicides.     

Sulfhydryl Group Involvement in Plasmalemma Transport of HCO(3) and OH in Chara corallina

The effect of the sulfhydryl reagents (—SH) p-chloromercuribenzene-sulfonic acid (PCMBS), N-ethylmaleimide (NEM), and inorganic mercury on H¹⁴CO₃⁻ assimilation in Chara corallina is reported. Commercial grade PCMBS caused severe inhibition of H¹⁴CO₃⁻ assimilation. Results obtained using purified PCMBS (stock solution passed through a chelating resin) indicated that inhibition observed using unpurified PCMBS was due predominantly to the presence of inorganic mercury (as a contaminant). The inhibitory role of inorganic mercury was verified using HgCl₂. This chemical caused a dramatic inhibition of H¹⁴CO₃⁻ assimilation, while it had little effect on cellular ¹⁴CO₂ fixation. Reversal of the Hg²⁺ inhibition of H¹⁴CO₃⁻ assimilation (in presence of 1.0 millimolar dithioerythritol) was extremely slow, requiring 2 to 3 hours for the reestablishment of control rates. This slow recovery may reflect de novo synthesis of transport proteins.     

Loss of recovery capacity of plasmalemma k influx after cutting in chlorsulfuron pretreated maize roots

Active K⁺ influx was studied in apical segments from maize (Zea mays L., hybrid lines XL 342) and pea (Pisum sativum L. var Laxton superbo) seedlings pretreated with the herbicide chlorsulfuron (2-chloro-N-[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) aminocarbonyl]benzenesulfonamide).

Even though both plants were sensitive to chlorsulfuron, a strong inhibition of K⁺ uptake only was evident in maize root segments after 12 hours pretreatment with 10 micromolar chlorsulfuron. The inhibition was revealed only when maize root segments were washed for 2 hours before uptake measurements. This was done in order to recover K⁺ influx inhibited by cutting injury. Consequently, we demonstrated that roots from chlorsulfuron pretreated maize seedlings lost the capacity to recover from cutting injury by washing. By contrast, K⁺ influx in pea roots was not inhibited by chlorsulfuron because pea roots notoriously do not exhibit the `washing' effect.

     

Effects and Absorption of Sethoxydim in Cell Cycle Progression of Corn (Zea mays) and Pea (Pisum sativum)

The mechanisms of selective herbicidal action of sethoxydim were investigated by using cultured root tips of corn (Zea mays L. cv Goldencrossbantam) and pea (Pisum sativum L. cv Alaska). Meristematic cells in the cultured roots were arrested in G₁ and G₂ of the cell division cycle by sucrose starvation and resumed growth and cell division (proliferation) when sucrose was provided. Corn root growth after sucrose addition was inhibited by sethoxydim at concentrations of 0.01 micromolar and greater when roots were treated in the presence of sucrose but was not inhibited at 10 micromolar sethoxydim when they were treated during sucrose starvation. Greater absorption of [¹⁴C]sethoxydim into the meristematic region of corn roots was observed when cells were in proliferative condition but not when they were arrested by sucrose starvation, whereas no greater absorption of the herbicide into pea meristems was observed in either growth condition. In the cell cycle study, greater absorption of [¹⁴C]sethoxydim into the corn root meristem was observed at a certain limited time before S (DNA synthesis) stage. The physiological effects and the greater absorption of sethoxydim clearly depended on cell cycle progression of corn root meristem, whereas fatty acid synthesis, as well as its inhibition by sethoxydim, was not associated with either cell cycle progression or greater absorption of the herbicide.     

Haloxyfop Inhibition of the Pyruvate and the alpha-Ketoglutarate Dehydrogenase Complexes of Corn (Zea mays L.) and Soybean (Glycine max [L.] Merr.)

The grass-specific herbicide haloxyfop, ((±)-2-[4-((3-chloro-5-(trifluoromethyl)-2-pyridinyl)oxy)-phenoxy] propionic acid) has been shown to inhibit lipid synthesis and respiration, to cause the accumulation of amino acids, and not to affect cellular sugar or ATP levels. Thus studies were carried out with enzyme activities from corn (Zea mays L.) (haloxyfop sensitive) and soybean (Glycine max [L.] Merr.) (haloxyfop tolerant) to locate the possible inhibition sites among the glycolytic and tricarboxylic acid (TCA) cycle enzymes. Following along the oxidative metabolism pathway of sugars, the pyruvate dehydrogenase complex (PDC) was the first enzyme among the glycolytic enzymes that demonstrated noticeable inhibition by 1 millimolar haloxyfop. Kinetic studies with corn and soybean PDC from both purified etioplasts and mitochondria gave K_i values of from 1 to 10 millimolar. Haloxyfop also inhibited the activity of the TCA cycle enzyme, the α-ketoglutarate dehydrogenase complex (α-KGDC) which carries out the same reaction as PDC except for the substitution of α-ketoglutarate for pyruvate as one of the substrates. The K_i values were somewhat lower in this case (near 1 millimolar). The relatively high K_i values for both enzyme complexes would indicate that these may not be the herbicidal sites of inhibition, but it is possible that the herbicide could be concentrated in compartments and/or the substrate concentrations may be well below optimal. Likewise little difference was seen in the haloxyfop inhibition of the enzyme activities from the sensitive species, corn, and from the tolerant species, soybean, so the selectivity of the herbicide is not evident from these results. The inhibition of the PDC and α-KGDC as the mode of action of haloxyfop is, however, consistent with the observed physiological effects of the herbicide, and these are the only enzymic activities so far found to be sensitive to haloxyfop.     

Influence of pH upon the Warburg Effect in Isolated Intact Spinach Chloroplasts: I. Carbon Dioxide Photoassimilation and Glycolate Synthesis

The influence of pH upon the O₂ inhibition of ¹⁴CO₂ photoassimilation (Warburg effect) was examined in intact spinach (Spinacia oleracea) chloroplasts. With conditions which favored the Warburg effect, i.e. rate-limiting CO₂ and 100% O₂, O₂ inhibition was greater at pH 8.4 to 8.5 than at pH 7.5 to 7.8. At pH 8.5, as compared with 7.8, there was an enhanced ¹⁴C-labeling of glycolate, and a decrease of isotope in some phosphorylated Calvin cycle intermediates, particularly triose-phosphate. The ¹⁴C-labeling of starch was also more inhibited by O₂ at higher pH. The enhanced synthesis of glycolate during ¹⁴CO₂ assimilation at higher pH resulted in a diminution in the level of phosphorylated intermediates of the Calvin cycle, and this was apparently a causal factor of the increased severity of the Warburg effect.     

On the Mechanism of Resistance to Paraquat in Hordeum glaucum and H. leporinum: Delayed Inhibition of Photosynthetic O(2) Evolution after Paraquat Application

The mechanism of resistance to paraquat was investigated in biotypes of Hordeum glaucum Steud. and H. leporinum Link. with high levels of resistance. Inhibition of photosynthetic O₂ evolution after herbicide application was used to monitor the presence of paraquat at the active site. Inhibition of photosynthetic O₂ evolution after paraquat application was delayed in both resistant biotypes compared with the susceptible biotypes; however, this differential was more pronounced in the case of H. glaucum than in H. leporinum. Similar results could be obtained with the related herbicide diquat. Examination of the concentration dependence of paraquat-induced inhibition of O₂ evolution showed that the resistant H. glaucum biotype was less affected by herbicide compared with the susceptible biotype 3 h after treatment at most rates. The resistant H. leporinum biotype, in contrast, was as inhibited as the susceptible biotype except at the higher rates. In all cases photosynthetic O₂ evolution was dramatically inhibited 24 h after treatment. Measurement of the amount of paraquat transported to the young tissue of these plants 24 h after treatment showed 57% and 53% reductions in the amount of herbicide transported in the case of the resistant H. glaucum and H. leporinum biotypes, respectively, compared with the susceptible biotypes. This was associated with 62% and 66% decreases in photosynthetic O₂ evolution of young leaves in the susceptible H. glaucum and H. leporinum biotypes, respectively, a 39% decrease in activity for the resistant H. leporinum biotype, but no change in the resistant H. glaucum biotype. Photosynthetic O₂ evolution of leaf slices from resistant H. glaucum was not as inhibited by paraquat compared with the susceptible biotype; however, those of resistant and susceptible biotypes of H. leporinum were equally inhibited by paraquat. Paraquat resistance in these two biotypes appears to be a consequence of reduced movement of the herbicide in the resistant plants; however, the mechanism involved is not the same in H. glaucum as in H. leporinum.     

Growth in Coculture Stimulates Metabolism of the Phenylurea Herbicide Isoproturon by Sphingomonas sp. Strain SRS2

Metabolism of the phenylurea herbicide isoproturon by Sphingomonas sp. strain SRS2 was significantly enhanced when the strain was grown in coculture with a soil bacterium (designated strain SRS1). Both members of this consortium were isolated from a highly enriched isoproturon-degrading culture derived from an agricultural soil previously treated regularly with the herbicide. Based on analysis of the 16S rRNA gene, strain SRS1 was assigned to the β-subdivision of the proteobacteria and probably represents a new genus. Strain SRS1 was unable to degrade either isoproturon or its known metabolites 3-(4-isopropylphenyl)-1-methylurea, 3-(4-isopropylphenyl)-urea, or 4-isopropyl-aniline. Pure culture studies indicate that Sphingomonas sp. SRS2 is auxotrophic and requires components supplied by association with other soil bacteria. A specific mixture of amino acids appeared to meet these requirements, and it was shown that methionine was essential for Sphingomonas sp. SRS2. This suggests that strain SRS1 supplies amino acids to Sphingomonas sp. SRS2, thereby leading to rapid metabolism of ¹⁴C-labeled isoproturon to ¹⁴CO₂ and corresponding growth of strain SRS2. Proliferation of strain SRS1 suggests that isoproturon metabolism by Sphingomonas sp. SRS2 provides unknown metabolites or cell debris that supports growth of strain SRS1. The role of strain SRS1 in the consortium was not ubiquitous among soil bacteria; however, the indigenous soil microflora and some strains from culture collections also stimulate isoproturon metabolism by Sphingomonas sp. strain SRS2 to a similar extent.     

The effect of metal ions on the amidolytic activity of human factor Xa (Activated Stuart-Prower Factor)

The effect of metal ions on human activated Factor X (Factor X_a) hydrolysis of the chromogenic substrate benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide (S2222) was studied utilizing initial rate enzyme kinetics. The divalent metal ions Ca²⁺, Mn²⁺, and Mg²⁺ enhanced Factor X_a amidolytic activity with K_m values of 30 μm, 20 μm, and 1.4 mm, respectively. Na⁺ activation of Factor X_a amidolytic activity was also found. The K_m for Na⁺ activation was 0.31 m. Both the divalent metal ions and Na⁺ increased the affinity of Factor X_a for S2222 and had no effect on the maximal velocity of the reaction. Other monovalent cations were unable to activate Factor X_a. However, K⁺ was a competitive inhibitor of the Na⁺ activation (K_i = 0.14 m). Lanthanide ions inhibited Factor X_a amidolytic activity. Gd³⁺ inhibition of Factor X_a hydrolysis of S2222 was noncompetitive and had a K_i of 3 μm. The lanthanide ion inhibition could not be reversed by Ca²⁺ even when Ca²⁺ was present in a 1000-fold excess over its K_m indicating nonidentity of the Factor X_a lanthanide and Ca²⁺ binding sites. It is concluded that the Factor X_a Ca²⁺ binding sites have characteristics different from those previously described for the Factor X molecule and that Mg²⁺, Na⁺, and K⁺ may be physiological regulators of Factor X_a activity.     

Effects of Nitrapyrin [2-Chloro-6-(Trichloromethyl) Pyridine] on the Obligate Methanotroph Methylosinus trichosporium OB3b

Nitrapyrin inhibited growth, CH₄ oxidation, and NH₄⁺ oxidation, but not the oxidation of CH₃OH, HCHO, or HCOONa, by Methylosinus trichosporium OB3b, suggesting that nitrapyrin acts against the methane monooxygenase enzyme system. The inhibition of CH₄ oxidation could be reversed by repeated washing of nitrapyrin-inhibited cells, indicating that its effect is bacteriostatic. The addition of Cu²⁺ did not release the inhibition. Methane oxidation was also inhibited by 6-chloro-2-picoline. These data suggest that the mode of action of nitrapyrin on M. trichosporium is different from that on chemoautotrophic NH₄⁺ oxidizers or methanogens.     

Carbon Oxysulfide Inhibition of the CO(2)-Concentrating Process of Unicellular Green Algae

Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O₂ evolution, dissolved inorganic carbon accumulation, and photosynthetic CO₂ fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO₂-concentrating process. This COS inhibition can be attributed to inhibition of external HCO₃⁻ conversion to CO₂ and OH⁻ by the carbonic anhydrase component of an active CO₂ pump. At a low pH of 5 to 6, COS stimulated O₂ evolution during photosynthesis by algae with low CO₂ in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO₂. Although COS had less effect on HCO₃⁻ accumulation at pH 9 by a HCO₃⁻ pump in Scenedesmus, COS reduced O₂ evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO₂ and H₂S, its inhibition of the CO₂ pump activity and photosynthesis is not accurate, when measured by O₂ evolution, by NaH¹⁴CO₃ accumulation, or by ¹⁴CO₂ fixation.     

The interaction of catecholamines,Ca2+ and adenylate cyclase in the intact turkey erythrocyte

Epinephrine stimulated adenylate cyclase in turkey erythrocyte ghosts is inhibited by calcium. The inhibition of adenylate cyclase is not apparent when intact erythrocytes are incubated with calcium and epinephrine. However, in the presence of the specific cation ionophore A₂₃₁₈₇ and 5 mm Ca²⁺, a 90% inhibition of epinephrine stimulated 3′,5′-adenosine monophosphate formation is found. The effect of catecholamines on calcium transport in the intact turkey erythrocyte was studied. Epinephrine causes a small but significant increase in Ca²⁺ efflux. This effect is inhibited by propranolol. No effect of epinephrine on Ca²⁺ uptake was observed. However, a 22% increase in Ca²⁺ uptake in the presence of propranolol could be detected. The propranolol effect was found to possess high statistical significance (p < .001). The absence of an epinephrine effect on influx probably reflects the presence of endogenous catecholamines in the control samples.It is proposed that the activation of adenylate cyclase by catecholamines occurs in two phases. The first phase is the increase of net Ca²⁺ efflux from a crucial Ca²⁺ pool, thus removing Ca²⁺ from its inhibitory sites on the adenylate cyclase complex. The second phase is the activation of the deinhibited adenylate cyclase by the hormone.     

Evidence of atrazine mineralization in a soil from the Nile Delta: Isolation of Arthrobacter sp. TES6, an atrazine-degrading strain

The s-triazine herbicide atrazine was rapidly mineralized (i.e., about 60% of ¹⁴C-ring-labelled atrazine released as ¹⁴CO₂ within 21 days) by an agricultural soil from the Nile Delta (Egypt) that had been cropped with corn and periodically treated with this herbicide. Seven strains able to degrade atrazine were isolated by enrichment cultures of this soil. DNA fingerprint and phylogenetic studies based on 16S rRNA analysis showed that the seven strains were identical and belonged to the phylogeny of the genus Arthrobacter (99% similarity with Arthrobacter sp. AD38, EU710554). One strain, designated Arthrobacter sp. strain TES6, degraded atrazine and mineralized the ¹⁴C-chain-labelled atrazine. However, it was unable to mineralize the ¹⁴C-ring-labelled atrazine. Atrazine biodegradation ended in a metabolite that co-eluted with cyanuric acid in HPLC. This was consistent with its atrazine-degrading genetic potential, shown to be dependent on the trzN, atzB, and atzC gene combination. Southern blot analysis revealed that the three genes were located on a large plasmid of about 175 kb and clustered on a 22-kb SmaI fragment. These results reveal for the first time the adaptation of a North African agricultural soil to atrazine mineralization and raise interesting questions about the pandemic dispersion of the trzN, atzBC genes among atrazine-degrading bacteria worldwide.     

Simulation and experimental analysis of the influence of herbicides on soil nitrification

The effect of 35 herbicides on the nitrification process was tested both by experiment, and by simulation of possible mechanisms of inhibition in a mathematical model. The model consists of nine equations with six coordinated constant and seven measurable parameters (or initial values), depending on the specific soil. The only free parameters are the initial values of the oxidative enzyme systems, and the parameters which determine the course of possible inhibition effects. For the majority of the herbicides, the inhibitory effects on the NH₄ ⁺ or NO₂ ^- oxidation were found negligible in the range of practical application. Hypotheses of a completely reversible or partially reversible inhibition of the oxidase systems gave the best correspondence between the model and reality, while an alteration of the growth parameters of the nitrifying populations in the model (death rate, proliferation rate, initial kill) due to the application of herbicides led to strong contrasts between simulated and experimental curves. Significant inhibitory effects became evident only when the hydrogen ion concentration in the soil fell below pH 7. Results with several herbicides indicated that the process of NO₂ ^- oxidation was more sensitive than that of NH₄ ⁺ oxidation. With a number of herbicides, an accumulation of NO₂ ^- ions was noticed during the course of soil percolation. In consideration of the buffering capacity, the model is applicable to other soils.