Nitrogen translocation in a clonal dune perennial,Hydrocotyle bonariensis

Summary Hydrocotyle bonariensis, a common rhizomatous perennial of coastal North Carolina, forms extensive clones in dune systems characterized by a patchy nitrogen distribution. An experiment was conducted in which Hydrocotyle clones were grown across artificially created soil nitrogen gradients to determine: (1) the effect of soil nitrogen availability and nitrogen translocation on clonal structure and (2) the costs versus benefits of nitrogen translocation as measured by sexual and clonal reproduction. Acropetal translocation of nitrogen resulted in highly significant benefits to clones growing from areas of high N to areas of low N. Limited basipetal translocation was also demonstrated. Hydrocotyle ramets responded to increased nitrogen availability, from either intraclonal translocation or immediate uptake from the soil, by producing branches. Nitrogen level, however, had no effect on internode distances. Clonal integration of nitrogen, in tandem with a plastic morphology, allow Hydrocotyle clones to expand across a nitrogen-limited dune environment and to locally exploit nitrogen patches when they are encountered.     

The effect of local resource availability and clonal integration on ramet functional morphology in Hydrocotyle bonariensis

Summary Within a physiologically integrated clone, the structure and functioning of an individual ramet is determined by: 1) the response of that ramet to its local environment and 2) its response to resource integration within the clone. In a multifactorial experiment, Hydrocotyle bonariensis ramets were grown in limiting resource environments with and without the benefit of basipetal resource movement from another branch of the clone. Ramets were analyzed for their morphological responses to variation in local light, water and nitrogen availability and to the superimposed effect of resource integration on these conditions. The expression of ramet morphology, from induction to development, was highly plastic in response to variable local resource availability. Resource integration changed a ramet's local response in a variety of ways depending on the resource(s) being translocated and the character involved. Among leaf characteristics (leaf weight, petiole height, blade area), resource translocation into the shade resulted in an enhancement of the local response. Similarly, the translocation of nitrogen and water generally increased clonal proliferation and sexual reproduction among ramets. In contrast, the translocation of water reversed the effect of local low water conditions on ramets by inhibiting root production. Some characters such as internode distance and leaf allometry were unaffected by integration. The maintenance of connections between ramets as a Hydrocotyle clone expands allows for resource sharing among widely separated ramets and can result in an integrated morpological response to a resource environment that is patchy in time and space.     

The effect of resource integration on fitness related traits in a clonal dune perennial,Hydrocotyle bonariensis

Summary The costs and benefits of resource integration in Hydrocotyle bonariensis were examined by comparing severed and intact clones grown across multiple resource gradients. Basipetal movement of water, nitrogen and photosynthates was demonstrated to occur between two rhizome branch systems interconnecting hundreds of ramets within a clone. Hydrocotyle clones of this size and larger have been shown to span highly patchy and unpredictable resource conditions in coastal dune environments. The extensive movement of water and nitrogen to portions of a clone deficient in these resources, resulted in a significant net benefit to the clone in terms of fitness-related traits: total biomass, ramet proliferation and seed production. The translocation of photosynthates across light gradients allowed for sexual reproduction and clonal expansion in the shade although this incurred a net cost to the clone in terms of ramet and seed production.     

Growth of Chlorella sorokiniana in the presence of sulfite elevates cell content of superoxide dismutase and imparts resistance towards paraquat

1. Growth of Chlorella sorokiniana in the presence of 7.5 mM sulfite, which halved the growth rate while doubling the superoxide dismutase (SOD; EC 1.15.1.1) content per cell, rendered the cells resistant to the toxic effects of 30 M paraquat. 2. While increasing total SOD content, sulfite increased the relative amount of the H₂O₂-resistant manganese-containing SOD. 3. It appears that O₂ ^– may be involved in mediating the toxicity of SO₂ in this green alga.Abbreviations SOD superoxide, dismutase - FeSOD ironcontaining superoxide dismutase - MnSOD manganese-containing superoxide dismutase     

Mode of action of paraquat in leaves of paraquat-resistant Conyza canadensis (L.) Cronq

The mode of action of paraquat (1,1-dimethyl-4,4-bipyridinium) and the mechanism of resistance to it were studied in leaves of atrazine/paraquat co-resistant (R) and susceptible (S) biotypes of horseweed (Conyza canadensis) collected from Hungarian vineyards. The application of 0·5 mol m^?3 paraquat by spraying onto the surface of the leaves of intact plants in the light rapidly led to typical symptoms of paraquat action in the initial period in both biotypes, i.e. inhibition of CO₂ fixation, suppression of variable chlorophyll fluorescence (F_v), decrease of oxygen evolution and stimulation of ethane production. The inhibitory effect of paraquat in the S plants was irreversible, whereas it was transient in the R plants and those plants recovered gradually afterwards. The R plants recovered from the inhibitory effect of paraquat only in the light, and an increase in light intensity was found to have a pronounced effect on the recovery of F_v. The mechanism of resistance to paraquat in C. canadensis is discussed.     

An investigation into the reproductive characteristics ofMicroctonus hyperodae (Hym.: braconidae), a parasitoid ofListronotus bonariensis (Col.: Curculionidae)

The reproductive tract of the parasitoidMicroctonus hyperodae was found to comprise 6.1±0.2 ovarioles containing a total of 40–60 oöcytes. After oviposition into its hostListronotus bonariensis, the parasitoid's egg volume increased by 205 times prior to hatching. At 19.1°C ovipositingM. hyperodae survived for a mean 21±4 days and laid a mean of 48±8 eggs.M. hyperodae collected from Colonia, Uruguay laid a mean of 62±15 eggs which was significantly more than the other ecotypes. Under caging conditions with an ample supply of hosts,M. hyperodae laid 51% of its eggs in the first 72 hours and on average 41% of the species' life-span occurred after the exhaustion of its egg supply. Minimum temperature forM. hyperodae oviposition was found to be c. 5°C; beyond this the rate of increase in egg-laying was approximately linear until 30°C whereafter the rate fell abruptly. There was effectively no egg-laying at 39°C. The parasitoid showed no preference for one host sex or the other either in the laboratory or under field conditions. Compared to otherMicroctonus spp.,M. hyperodae appeared to show low fecundity and high longevity. The adaptive implications of this are discussed.     

Glyphosate,paraquat and ACCase multiple herbicide resistance evolved in a <Emphasis Type="Italic">Lolium rigidum</Emphasis> biotype

Glyphosate is the world’s most widely used herbicide. A potential substitute for glyphosate in some use patterns is the herbicide paraquat. Following many years of successful use, neither glyphosate nor paraquat could control a biotype of the widespread annual ryegrass (Lolium rigidum), and here the world’s first case of multiple resistance to glyphosate and paraquat is confirmed. Dose–response experiments established that the glyphosate rate causing 50% mortality (LD₅₀) for the resistant (R) biotype is 14 times greater than for the susceptible (S) biotype. Similarly, the paraquat LD₅₀ for the R biotype is 32 times greater than for the S biotype. Thus, based on the LD₅₀ R/S ratio, this R biotype of L. rigidum is 14-fold resistant to glyphosate and 32-fold resistant to paraquat. This R biotype also has evolved resistance to the acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides. The mechanism of paraquat resistance in this biotype was determined as restricted paraquat translocation. Resistance to ACCase-inhibiting herbicides was determined as due to an insensitive ACCase. Two mechanisms endowing glyphosate resistance were established: firstly, a point mutation in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, resulting in an amino acid substitution of proline to alanine at position 106; secondly, reduced glyphosate translocation was found in this R biotype, indicating a co-occurrence of two distinct glyphosate resistance mechanisms within the R population. In total, this R biotype displays at least four co-existing resistance mechanisms, endowing multiple resistance to glyphosate, paraquat and ACCase herbicides. This alarming case in the history of herbicide resistance evolution represents a serious challenge for the sustainable use of the precious agrochemical resources such as glyphosate and paraquat.     

Systematic mass rearing and release ofMicroctonus hyperodae (Hym.: Braconidae,Euphorinae), a parasitoid of the argentine stem weevilListronotus bonariensis (Col.: Curculionidae) and records of its establishment in New Zealand

The method whereby equal numbers of seven ecotypes of the parasitoidMicroctonus hyperodae Loan (Hymenoptera: Braconidae, Euphorinae) were reared and released is described along with the reasons for doing so. This was achieved by variably intense rearing effort depending on the number of founder females in that particular ecotype. The parasitoid was released in three regions of New Zealand as a control agent of theListronotus bonariensis Kuschel (Col.: Curculionidae), a severe pest of New Zealand pastures. It was later recovered from all three regions.     

Genetic characterization of a mutation that enhances paraquat tolerance in the fern Ceratopteris richardii

Summary Three nuclear mutations that affect tolerance to the herbicide paraquat have been selected in the fern Ceratopteris richardii. Two of the mutations, pq2 and pq45 are allelic and confer low and moderate tolerance, respectively. A third mutation, pqa6, is not linked to the other two and significantly enhances the level of tolerance when in combination with either pq2 or pq45. The pqa6 mutation does not independently confer tolerance in the absence of the other mutations.     

Effect of exogenous hydrogen peroxide on enzymatic and nonenzymatic antioxidants in leaves of young pea plants treated with paraquat

The effects of exogenously applied hydrogen peroxide on the antioxidant system of pea plants were investigated. Ten-day-old pea seedlings were sprayed with 2.5 mM H₂O₂ and 24 h later with 0.2 mM PQ. Samples were taken 0, 2 and 5 h after the start of illumination. The protective effect of H₂O₂ was evaluated by monitoring of parameters related to the damage caused by PQ. The treatment with PQ led to a severe leakage of electrolytes from leaf tissues. Malondialdehyde level increased in PQ treated plants, but remained unchanged in H₂O₂ pre-treated ones after 5 h of illumination. Increased catalase and glutathione-S-transferase activity was observed in pea plants treated with H₂O₂ and PQ. Ascorbate peroxidase activity decreased significantly after paraquat application, but pre-treatment with H₂O₂ prevented ascorbate peroxidase inhibition to some extent. Increased guaiacol peroxidase activity was detected after H₂O₂ application. PQ application caused a drastic decline in the levels of thiol-group bearing compounds, reduced glutathione and ascorbate, while the quantity of oxidized glutathione and dehydroascorbate were increased. The results presented on changes in enzymatic and nonenzymatic antioxidants suggest that preliminary H₂O₂ application to pea plants treated with PQ, alleviates the toxic effects of the herbicide.     

An in vitro whole plant selection system: paraquat tolerant mutants in the fern Ceratopteris

Summary A whole plant selection system using the haploid gametophyte generation of the fern Ceratopteris richardii has been developed to select for mutations that confer resistance or tolerance to various selection pressures. The expression of the mutations can be analyzed and characterized in both the haploid gametophyte and diploid sporophyte generations. Genetic analyses are facilitated by the fern's rapid life cycle and the ease of manipulating the gametophyte generation. Selection for tolerance to the herbicide paraquat has yielded two mutants which have an increased tolerance to the herbicide in both the gametophyte and sporophyte generations. Both mutants exhibit single nuclear gene inheritance patterns and appear to be closely linked or allelic.     

Dominant chloramphenicol-resistant bacteria and resistance genes in coastal marine waters of Jiaozhou Bay,China

Studies of abundance, diversity and distribution of antibiotic-resistant bacteria and their resistance determinants are necessary for effective prevention and control of antibiotic resistance and its dissemination, critically important for public health and environment management. In order to gain an understanding of the persistence of resistance in the absence of a specific antibiotic selective pressure, microbiological surveys were carried out to investigate chloramphenicol-resistant bacteria and the chloramphenicol acetyltransferase resistance genes in Jiaozhou Bay after chloramphenicol was banned since 1999 in China. About 0.15–6.70% cultivable bacteria were chloramphenicol resistant, and the highest abundances occurred mainly in the areas near river mouths or sewage processing plants. For the dominant resistant isolates, 14 genera and 25 species were identified, mostly being indigenous estuarine or marine bacteria. Antibiotic-resistant potential human or marine animal pathogens, such as Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis and Shewanella algae, were also identified. For the molecular resistance determinants, the cat I and cat III genes could be detected in some of the resistant strains, and they might have the same origins as those from clinical strains as determined via gene sequence analysis. Further investigation about the biological, environmental and anthropogenic mechanisms and their interactions that may contribute to the persistence of antibiotic-resistance in coastal marine waters in the absence of specific antibiotic selective pressure is necessary for tackling this complicated environmental issue.     

Drosophila Cu,Zn superoxide dismutase gene confers resistance to paraquat in Escherichia coli

Superoxide dismutase (SOD) is known to protect organisms from reactive oxygen metabolites. We tested the hypothesis that the Drosophila Cu,Zn SOD is capable of protecting Escherichia coli from oxidative damage caused by the herbicide paraquat. The Cu,Zn Sod gene of Drosophila sechellia was subcloned into pET-20b(+) expression vector. Transformation of E. coli with the constructed vector resulted in an overexpression of this eukaryotic superoxide dismutase, as evidenced by dramatically increased levels of the Cu,Zn SOD polypeptide in bacterial cytosolic extracts. As well, the E. coli transformants showed resistance to paraquat-mediated inhibition of growth and survival. Paraquat is known to promote formation of the superoxide radical anion inside cells and thus the data have been interpreted as indicating that the cloned superoxide dismutase provides protection in E. coli against damage attributable to free radicals.     

Positive association between mycorrhiza and foliar endophytes in Poa bonariensis, a native grass

The interaction between mycorrhiza and leaf endophytes (Neotyphodium sp.) was studied in three Poa bonariensis populations, a native grass, differing significantly in endophyte infection. The association between endophytes and mycorrhizal fungi colonisation was assessed by analysing plant roots collected from the field. We found that roots from endophyte-infected populations showed a significantly higher frequency of colonisation by mycorrhizal fungi and that soil parameters were not related to endophyte infection or mycorrhiza colonization. In addition, we did not observe significant differences in the number of AM propagules in soils of the three populations sites. We also report the simultaneous development of Paris-type and Arum-type mycorrhiza morphology within the same root systems of P. bonariensis. The co-occurrence of both colonisation types in one and the same root system found in the three populations, which differed in Neotyphodium infection, suggests that foliar endophytes do not determine AM morphology. The percentage of root length colonised by different types of fungal structures (coils, arbuscules, longitudinal hyphae and vesicles) showed significant and positive differences in arbuscular frequency associated with endophyte infection, whereas the much smaller amounts of vesicles and hyphal coils did not differ significantly.     

Ketoconazole resistance in Torulopsis glabrata

The majority of clinical isolates of T. glabrata has been shown highly sensitive to ketoconazole, when tested with an agar dilution method, and resistant, when using a broth dilution method. The fact was accounted for the high degree of mutation associated with the haploid state present in this species. Experimental T. glabrata infection in immunocompetent and immunocompromised mice appears not respond to the oral administration of the drug. The results agree with that of broth dilution tests and not with those produced by the agar methods. It seems that agar dilution sensitivity tests are inadequate for yeast species with an high rate of mutation, as T. glabrata.     

Pea genes associated with non-host disease resistance to Fusarium are also active in race-specific disease resistance to Pseudomonas

A given plant species is able to resist most of the potentially pathogenic microorganisms with which it comes in contact. This phenomenon, known as non-host resistance, can be overcome only by a very small number of true pathogens which can use that plant as a host. In some cases, plants have developed mechanisms for overcoming infection by specific races or strains of a true pathogen. This race-specific resistance can be easily manipulated into agronomically important cultivars by plant breeders. We have previously described nine cDNA clones which represent pea genes active during non-host resistance against the fungus Fusarium solani f. sp. phaseoli. In the present work, we have used these cDNAs as probes to compare non-host resistance with race-specific responses of peas against three races of Pseudomonas syringae pv. pisi. Five of the genes most active during non-host resistance were also active in direct correlation with the phenotypic expression of resistance in race-specific reactions of five differential pea cultivars against three races of Pseudomonas syringae pv. pisi.     

Effects of root zone temperature and paraquat in the induction of oxidative stress in <Emphasis Type="Italic">Trichosanthes cucumerina</Emphasis> L.

We investigated the influence of root zone temperature (RZT) and the aerial application of paraquat on stress defence mechanisms of Trichosanthes cucumerina L. To achieve this objective, T. cucumerina cv Green was grown with roots at 25 and 30°C root zone temperature and maintained at 20 ± 1°C air temperature in a growth chamber. These RZT and air temperature had earlier been shown to favor growth and fruit production in T. cucumerina. Plants at each RZT were subjected to paraquat treatment (+P) and without paraquat treatment (−P). Paraquat (0.2 mmol/L) was applied as aerial spray. Results showed that the individual main effects of RZT and paraquat treatments significantly affected the chlorophyll fluorescence and gas exchange parameters, while the interaction of both treatments had no significant effect. Results showed that the total phenolics and ascorbic acid contents of T. cucumerina at 30°C were significantly higher than at 25°C. The T. cucumerina plants in +P treatment recorded significantly lower maximum photochemical efficiency (F _v/F _m), net photosynthesis (A), transpiration rate (E), intercellular CO₂ concentration (C _i) and stomatal conductance (g ₁) compared to untreated plants. Also, plants raised at 30°C recorded significantly higher F _v/F _m, A, E, C _i and g ₁ compared to plants raised at 25°C. Plants that were sampled at 48 h after paraquat treatment recorded a higher degree of oxidative damage compared to those sampled at 24 h after treatment. We showed that the degree of damage suffered by T. cucumerina, when treated with paraquat either at 25 or 30°C RZT was similar at 48 h after treatment. We concluded that either at 25 or 30°C, exposure of T. cucumerina to paraquat would impose the same degree of oxidative damage.