Role of Oxygen in the Limitation and Inhibition of Nitrogenase Activity and Respiration Rate in Individual Soybean Nodules

Although infected cell O2 concentration (Oi) is known to limit respiration and nitrogenase activity in legume nodules, techniques have not been available to measure both processes simultaneously in an individual legume nodule. Consequently, details of the relationship between nitrogenase activity and Oi are not fully appreciated. For the present study, a probe was designed that allowed open circuit measurements of H2 evolution (nitrogenase activity) and CO2 evolution (respiration rate) in a single attached soybean nodule while simultaneously monitoring fractional oxygenation of leghemoglobin (and thereby Oi) with a nodule oximeter. Compared to measurements of whole nodulated roots, use of the probe led to inhibition of nitrogenase activity in the single nodules. During oximetry measurements, total nitrogenase activity (TNA; peak H2 evolution in Ar/O2) in the single nodules was 16% of that in whole nodulated roots and 48% of nodulated root activity when Oi was not being measured simultaneously. This inhibition did not affect the nodules' ability to regulate Oi, because exposure to Ar/O2 (80:20, v/v) caused nitrogenase activity and respiration rate to decline, and this decline was linearly correlated with a concurrent decrease in Oi. When the nodules were subsequently exposed to a linear increase in external pO2 from 20 to 100% O2 at 2.7% O2/min, fractional leghemoglobin oxygenation first increased gradually and then more rapidly, reaching saturation at a pO2 between 76 and 100% O2. Plots of nitrogenase activity and respiration rate against Oi showed that rates increased with Oi up to a value of 57 nM, with half-maximal rates being attained at Oi values between 10 and 14 nM O2. The maximum nitrogenase activity achieved during the increase in pO2 (potential nitrogenase activity) was 30 to 57% of that measured in intact nodulated roots, showing that O2 limitation of nitrogenase activity could account for a significant proportion of the inhibition of TNA associated with the use of the probe. However, some factor(s) in addition to O2 must have limited the activity of single nodules at both subsaturating and saturating Oi. At Oi values greater than about 57 nM, nitrogenase activity and nodule respiration were inhibited, but, because this inhibition has been shown previously to be readily reversible when the Oi was lowered, it was not attributed to direct O2 inactivation of the nitrogenase protein. These results indicate that maximum nitrogenase activity in legume nodules is supported by a narrow range of Oi values. Possible biochemical mechanisms are discussed for both O2 limitation of nitrogenase activity at low Oi and inhibition of nitrogenase activity at high Oi.     

Mature Tissue and Crop Canopy Respiratory Characteristics of Rye, Triticale and Wheat

Crop dry matter and its chemical composition, together withcanopy and mature tissue respiration rates were measure at equivalentgrowth stages and temperatures for spring and winter rye, triticaleand wheat crops grown under irrigated field conditions. Canopyrespiration was partitioned into growth and maintenance respirationusing information from the chemical composition analysis ofthe crop biomass. Rates of dry matter accumulation early inthe growing season were significantly greater for rye cropsin comparison to triticale and wheat. However, when dry matterwas measured at similar ontogenetic stages, the productivityadvantage of the rye crop was no longer evident. Nevertheless,canopy respiration rates per unit ground area were significantlylower for rye than wheat over all temperatures and growth stages.Intergeneric differences in the respiration rates of matureleaf and stem tissues were consistent with those measured atcanopy scales. Differences in the chemical composition of thebiomass among genera were minimal, and insufficient to accountfor differences in canopy respiration due to synthesis respirationrequirement. Estimates of biomass maintenance requirements appearto be significantly lower for rye than wheat when calculatedat similar temperatures and ontogenetic stages. The maintenancecoefficient (m) depended on stage of development, suggestingthat m will decline earlier chronologically for rye than wheat,which implies that greater carbon retention is another aspectcontributing to the higher early-season crop growth rates ofspring and winter rye. Considering the lower respiration ratesof mature stems relative to leaves, the dependence of m on stem:leafratio was suggested as a useful approach to modelling ontogeneticeffects on maintenance respiration.Copyright 1993, 1999 AcademicPress Rye, triticale, wheat, dry matter, growth and maintenance respiration     

Source of an egg kairomone for Trissolcus basalis, a parasitoid of Nezara viridula

Abstract. The eggs of the southern green stink bug, Nezara viridula (L.) (Hemiptera: Pentatomidae), are successfully attacked by Trissolcus basalis (Woll.) (Hymenoptera: Scelionidae) and are recognized as hosts by a secretion applied to the egg chorion. This secretion is produced by the follicular cells in the proximal region of the ovariole of the female pentatomid and functions as an adhesive for attaching the eggs to the oviposition substrate. The adhesive and kairomone activity could be partially removed with water. This water extract elicited host recognition behaviour in T. basalis when applied to glass beads which stuck together as the extract dried. The adhesive and kairomonal activity was removed completely with acetone since acetone-washed host eggs were not recognized by T. basalis. Application of the acetone extract to glass beads stimulated ovipositional probes by T. basalis. The adhesive appeared to be composed of a mucopolysaccharide–protein complex.     

In vitro fusion of endocytic vesicles is inhibited by cyclin A-cdc2 kinase.

Receptor-mediated endocytosis and recycling are inhibited in mitotic mammalian cells, and previous studies have shown that inhibition of endocytic vesicle fusion in vitro occurs via cyclin B-cdc2 kinase. To test for the ability of cyclin A-cdc2 kinase to inhibit endocytic vesicle fusion, we employed recombinant cyclin A proteins. Addition of cyclin A to interphase extracts activated a histone kinase and markedly reduced the efficiency of endocytic vesicle fusion. By a number of criteria, inhibition of fusion was shown to be due to the action of cyclin A, via the mitosis-specific cdc2 kinase, and not an indirect effect through cyclin B. Two-stage incubations were used to demonstrate that at least one target of cyclin A-cdc2 kinase is a cytosolic component of the fusion apparatus. Reconstitution experiments showed that this component was also modified in mitotic cytosols and was unaffected by N-ethyl maleimide treatment.     

Mesozooplankton interactions with the shelf around the sub-Antarctic Prince Edward Islands archipelago

Mesozooplankton surveys were conducted in April/May for fourconsecutive years (1996–1999) in the vicinity of the PrinceEdward Islands (PEIs), Southern Ocean. The PEIs are locatedin the Polar Frontal Zone, directly in the path of the east-flowingAntarctic Circumpolar Current. Zooplankton were collected byoblique tows using a Bongo net fitted with 300 µm mesh.The abundance, biomass and average size of the mesozooplanktonin the upstream (USR), inter-island (IIR) and downstream (DSR)regions indicated that some groups and species were significantlyaffected by their interaction with the shallow shelf watersof the PEIs. Total mesozooplankton abundance and biomass weretypically highest in the DSR, but no consistent pattern wasevident in the USR and IIR. Copepods, euphausiids and fish weregenerally of a low average size in the IIR. This small sizewas largely attributed to the reduced abundance, or completeabsence, of mesopelagic species from the shelf region. Of totalbiomass, the mesopelagic species Euphausia longirostris, Euphausiasimilis, Pleuromamma abdominalis, Paraeuchaeta biloba and Oncaeaantarctica together contributed an average of 16% to the USR,2% to the IIR and 15% to the DSR. Conversely, epipelagic speciesshowed no consistent pattern of abundance and biomass distributionbetween regions. The low incidence of mesopelagic species overthe island shelf was attributed mainly to reduced advectionof deep water into the shelf region (average depth = 200 m),rather than predation, particularly during the through-flowmode between the islands. This resulted in substantial regionaldifferences in euphausiid community structure. The epipelagicspecies Euphausia vallentini and Thysanoessa vicina completelydominated the IIR, comprising on average 89% of total euphausiidbiomass in this region. However, predation may be importantduring the water-trapping mode between the islands. Advectionof zooplankton into the IIR appeared to be affected by the proximityof the Subantarctic Front (SAF). In 1996, when the SAF was farnorth of the PEIs, reduced current velocities resulted in somedegree of water retention over the shelf and an increased predationimpact. Conversely, when the SAF was close to the PEIs in 1999,more large plankton were transported over the island shelf.High current velocities and productivity associated with theSAF appear to increase the biomass and size of allochthonouszooplankton/nekton advected into the IIR, and consequently mayhave increased the availability of prey to land-based predators.The long-term southward movement of the SAF recently observedin the vicinity of the PEIs may therefore have important implicationsfor the ecosystem of these islands.     

Benzothiadiazole induces disease resistance in Arabidopsis by activation of the systemic acquired resistance signal transduction pathway

Benzothiadiazole (BTH) is a novel chemical activator of disease resistance in tobacco, wheat and other important agricultural plants. In this report, it is shown that BTH works by activating SAR in Arabidopsis thaliana. BTH-treated plants were resistant to infection by turnip crinkle virus, Pseudomonas syringae pv ‘tomato’ DC3000 and Peronospora parasitica. Chemical treatment induced accumulation of mRNAs from the SAR-associated genes, PR-1, PR-2 and PR-5. BTH treatment induced both PR-1 mRNA accumulation and resistance against P. parasitica in the ethylene response mutants, etr1 and ein2, and in the methyl jasmonate-insensitive mutant, jar1, suggesting that BTH action is independent of these plant hormones. BTH treatment also induced both PR-1 mRNA accumulation and P. parasitica resistance in transgenic Arabidopsis plants expressing the nahG gene, suggesting that BTH action does not require salicylic acid accumulation. However, because BTH-treatment failed to induce either PR-1 mRNA accumulation or P. parasitica resistance in the non-inducible immunity mutant, nim1, it appears that BTH activates the SAR signal transduction pathway.     

Characterization of a novel plant poly(A) polymerase

We have purified and characterized poly(A) polymerases (PAPs) from Pisum sativum, Brassica juncea, and Zea mays. Through chromatography on DEAE-Sepharose and heparin-Sepharose, these PAPs copurified as a single enzyme along with RNPs that could provide RNA substrates for the enzyme. More extensive purification by chromatography on MonoQ resulted in the resolution of the PAPs into as many as three fractions. One of these (PAP-I) contained a 43-kDa polypeptide immunologically related to the yeast PAP, and two others (PAP-II and PAP-III) contained RNAs that could serve as substrates for polyadenylation. These fractions by themselves possessed little PAP activity, but mixtures containing combinations of these displayed substantial activity. Similar PAP factors (PAP-I and PAP-III) were identified after fractionation of extracts prepared from Brassica juncea and Zea mays. The factors from one plant were completely interchangeable with those from different plants. We conclude that the poly(A) polymerases present in vegetative plant tissues consist of more than one component. In this respect, they are substantially different from other reported plant, mammalian, and yeast PAPs.     

Amino acid sequence of the minor isomorph of the crustacean hyperglycemic hormone (CHH-II) of the Mexican crayfish Procambarus bouvieri (Ortmann): Presence of a d-amino acid

The primary structure of the neurohormone crustacean hyperglycemic hormone (CHH-II) was determined by means of enzymatic digestions, manual Edman degradation, and mass spectrometry. CHH-II is a 72 residue peptide (molecular mass 8388 Da), with six cysteines forming three disulfide bridges that connect residues 7–43, 23–39, and 26–52. The peptide has blocked N- and C-termini, and lacks tryptophan, histidine, and methionine. The CHH-I and CHH-II of Procambarus bouvieri have identical sequences and elicit levels of hyperglycemia that are not distinguishable. The difference between the two isomorphs consists in a posttranslational modification of a l-Phe in CHH-I to a d-Phe in CHH-II at the third position from the N-terminus.