Simulating rhizodeposition as a function of shoot and root interactions within a new 3D Functional-Structural Plant Model

Introduction - Rhizodeposition, i.e. the release of organic materials by roots, represents a significant portion of plant's carbon (C) budget, ranging from 5% to 15% of net photosynthesized C (Pausch and Kuzyakov, 2018). Various rhizodeposits can be released by roots, e.g. soluble exudates, secreted mucilage, sloughed cells, or volatile organic compounds. Despite their short lifetime, some of these products have been shown to favor plant growth, e.g. by increasing water and nutrient uptake. Among rhizodeposition processes, exudation has been suggested to depend on the concentration of carbohydrates inside the roots (Personeni et al., 2007). However, rhizodeposition not only depends on the availability of C in the roots, but also on the architecture of the root system, and many have shown that rhizodeposits are more concentrated in specific areas, such as root tips. Consequently, a Functional-Structural Plant Model (FSPM) would theoretically represent the best framework for simulating the spatial and temporal dynamics of rhizodeposition, as it can describe the evolution of both the metabolism and the architecture of the plant. The objective of this work is to create such a framework by coupling a whole-plant FSPM, a 3D root architectural model, and a new model simulating rhizodeposition. Modelling approach - Our strategy has been to combine the FSPM CN-Wheat (Barillot et al., 2016), which describes the main processes of C and nitrogen (N) acquisition and transformation by an individual wheat plant and the 3D growth and development of its aerial organs, with the model ArchiSimple (Pagès et al., 2014) that simulates the development of the 3D root architecture for a range of plant species, and the new model RhizoDep, which calculates a full C balance in each part of a root system in order to simulate local rhizodeposition fluxes. The complementarity of the three models is illustrated in Figure 1: i) CN-Wheat is used to calculate the amount of C allocated from the shoots to the roots, ii) ArchiSimple provides the 3D structure of the root system, and iii) RhizoDep distributes the C provided by the shoots within the 3D root system and simulates the actual growth, respiration and rhizodeposition of each root element based on C availability. The major link between the three models lies in the exchange of C between aboveground and belowground tissues, which is driven by gradients of sucrose concentration in the different compartments of the plant. Preliminary results & short-term perspectives - The coupling of the three models has been started using the OpenAlea platform and its Multiscale Tree Graph formalism (Pradal et al., 2008). First simulations were done using the allocation of C to the roots simulated by CN-Wheat as an input to the root model based on the effective coupling of ArchiSimple and RhizoDep. These simulations show how rhizodeposition is intrinsically dependent on the architecture of the root system and on the total amount of available C. For completing the coupling, several issues still need to be tackled, e.g. how N uptake and metabolism should be spatialized in a 3D root system, how it may be regulated by local C and N availability, and how rhizodeposition can modify soil N availability. However, this modelling approach has already led to a first prototype able to simulate rhizodeposition processes on a dynamic, 3D root system that is fully integrated within the functioning of the whole plant. Its refinement will offer unique opportunities to study the possible link between rhizodeposition and the environmental factors affecting plant growth, e.g. atmospheric CO2 concentration or soil N availability

Implication of haematophagous arthropod salivary proteins in host-vector interactions

The saliva of haematophagous arthropods contains an array of anti-haemostatic, anti-inflammatory and immunomodulatory molecules that contribute to the success of the blood meal. The saliva of haematophagous arthropods is also involved in the transmission and the establishment of pathogens in the host and in allergic responses. This survey provides a comprehensive overview of the pharmacological activity and immunogenic properties of the main salivary proteins characterised in various haematophagous arthropod species. The potential biological and epidemiological applications of these immunogenic salivary molecules will be discussed with an emphasis on their use as biomarkers of exposure to haematophagous arthropod bites or vaccine candidates that are liable to improve host protection against vector-borne diseases

Highly focused anopheline breeding sites and malaria transmission in Dakar

<p>Abstract</p> <p>Background</p> <p>Urbanization has a great impact on the composition of the vector system and malaria transmission dynamics. In Dakar, some malaria cases are autochthonous but parasite rates and incidences of clinical malaria attacks have been recorded at low levels. Ecological heterogeneity of malaria transmission was investigated in Dakar, in order to characterize the <it>Anopheles </it>breeding sites in the city and to study the dynamics of larval density and adult aggressiveness in ten characteristically different urban areas.</p> <p>Methods</p> <p>Ten study areas were sampled in Dakar and Pikine. Mosquitoes were collected by human landing collection during four nights in each area (120 person-nights). The <it>Plasmodium falciparum </it>circumsporozoite (CSP) index was measured by ELISA and the entomological inoculation rates (EIR) were calculated. Open water collections in the study areas were monitored weekly for physico-chemical characterization and the presence of anopheline larvae. Adult mosquitoes and hatched larvae were identified morphologically and by molecular methods.</p> <p>Results</p> <p>In September-October 2007, 19,451 adult mosquitoes were caught among which, 1,101 were <it>Anopheles gambiae s.l</it>. The Human Biting Rate ranged from 0.1 bites per person per night in Yoff Village to 43.7 in Almadies. Seven out of 1,101 <it>An. gambiae s.l</it>. were found to be positive for <it>P. falciparum </it>(CSP index = 0.64%). EIR ranged from 0 infected bites per person per year in Yoff Village to 16.8 in Almadies. The <it>An</it>. <it>gambiae </it>complex population was composed of <it>Anopheles arabiensis </it>(94.8%) and <it>Anopheles melas </it>(5.2%). None of the <it>An. melas </it>were infected with <it>P. falciparum</it>. Of the 54 water collection sites monitored, 33 (61.1%) served as anopheline breeding sites on at least one observation. No <it>An</it>. <it>melas </it>was identified among the larval samples. Some physico-chemical characteristics of water bodies were associated with the presence/absence of anopheline larvae and with larval density. A very close parallel between larval and adult densities was found in six of the ten study areas.</p> <p>Conclusion</p> <p>The results provide evidence of malaria transmission in downtown Dakar and its surrounding suburbs. Spatial heterogeneity of human biting rates was very marked and malaria transmission was highly focal. In Dakar, mean figures for transmission would not provide a comprehensive picture of the entomological situation; risk evaluation should therefore be undertaken on a small scale.</p

Human leptospirosis on Reunion Island, Indian Ocean: are rodents the (only) ones to blame?

Background: Although leptospirosis is a zoonosis of major concern on tropical islands, the molecular epidemiology of the disease aiming at linking human cases to specific animal reservoirs has been rarely explored within these peculiar ecosystems. Methodology/Principal Findings: Five species of wild small mammals (n = 995) as well as domestic animals (n = 101) were screened for Leptospira infection on Reunion Island; positive samples were subsequently genotyped and compared to Leptospira from clinical cases diagnosed in 2012–2013 (n = 66), using MLST analysis. We identified two pathogenic species in human cases, namely Leptospira interrogans and Leptospira borgpetersenii. Leptospira interrogans was by far dominant both in clinical samples (96.6%) and in infected animal samples (95.8%), with Rattus spp and dogs being its exclusive carriers. The genetic diversity within L. interrogans was apparently limited to two sequence types (STs): ST02, identified among most clinical samples and in all rats with complete MLST, and ST34, identified in six humans, but not in rats. Noteworthy, L. interrogans detected in two stray dogs partially matched with ST02 and ST34. Leptospira borgpetersenii was identified in two clinical samples only (3.4%), as well as in cows and mice; four haplotypes were identified, of which two seemingly identical in clinical and animal samples. Leptospira borgpetersenii haplotypes detected in human cases were clearly distinct from the lineage detected so far in the endemic bat species Mormopterus francoismoutoui, thus excluding a role for this volant mammal in the local human epidemiology of the disease. Conclusions/Significance: Our data confirm rats as a major reservoir of Leptospira on Reunion Island, but also pinpoint a possible role of dogs, cows and mice in the local epidemiology of human leptospirosis. This study shows that a comprehensive molecular characterization of pathogenic Leptospira in both clinical and animal samples helps to gaining insight into leptospirosis epidemiology within a specific environmental setting

Efflux Pump, the Masked Side of ß-Lactam Resistance in Klebsiella pneumoniae Clinical Isolates

International audienceBACKGROUND: Beta-lactamase production and porin decrease are the well-recognized mechanisms of acquired beta-lactam resistance in Klebsiella pneumoniae isolates. However, such mechanisms proved to be absent in K. pneumoniae isolates that are non susceptible to cefoxitin (FOX) and susceptible to amoxicillin+clavulanic acid in our hospital. Assessing the role of efflux pumps in this beta-lactam phenotype was the aim of this study. METHODOLOGY/FINDINGS: MICs of 9 beta-lactams, including cloxacillin (CLX), and other antibiotic families were tested alone and with an efflux pump inhibitor (EPI), then with both CLX (subinhibitory concentrations) and EPI against 11 unique bacteremia K. pneumoniae isolates displaying the unusual phenotype, and 2 ATCC strains. CLX and EPI-dose dependent effects were studied on 4 representatives strains. CLX MICs significantly decreased when tested with EPI. A similar phenomenon was observed with piperacillin+tazobactam whereas MICs of the other beta-lactams significantly decreased only in the presence of both EPI and CLX. Thus, FOX MICs decreased 128 fold in the K. pneumoniae isolates but also 16 fold in ATCC strain. Restoration of FOX activity was CLX dose-dependent suggesting a competitive relationship between CLX and the other beta-lactams with regard to their efflux. For chloramphenicol, erythromycin and nalidixic acid whose resistance was also due to efflux, adding CLX to EPI did not increase their activity suggesting differences between the efflux process of these molecules and that of beta-lactams. CONCLUSION: This is the first study demonstrating that efflux mechanism plays a key role in the beta-lactam susceptibility of clinical isolates of K. pneumoniae. Such data clearly evidence that the involvement of efflux pumps in beta-lactam resistance is specially underestimated in clinical isolates

Dynamics of CO2, CH4 and N2O concentrations throughout deep soil profiles in Eucalypt plantations subjected to contrasted rainfall regimes: consequences on soil effluxes

The major factors driving greenhouse gas exchanges in forest soils (substrate supply, temperature, water content) vary with soil depth. Our study aimed to assess the effects of clear-cutting and drought on the temporal variability of CO2, CH4 and N2O fluxes throughout very deep soil profiles in Brazilian eucalypt plantations conducted in coppice. Stands with 37% of through fall excluded by plastic sheets (-W) and stands without rain exclusion (+W) were compared. Every two weeks for 21 months, CO2, CH4 and N2O surface effluxes were measured using the closed-chamber method and concentrations in the soil were measured at 7 depths down to 15.5 m in -W and +W. At most measurement dates, CO2, CH4 and N2O effluxes at the soil surface were not significantly different between -W and +W. Mean CO2 and N2O concentrations in -W were 20.7% and 7.6% lower than in +W, respectively, across the sampling depths. By contrast, CH4 concentrations in -W were 44.4% higher than in +W throughout the soil profile. Across the two treatments, CO2 concentrations increased from 4446 _ 2188 ppm at 10 cm deep to 15622 _ 3523 ppm at 15.5 m, CH4 concentrations increased from 0.41 _ 0.17 ppm at 10 cm deep to 0.77 _ 0.24 ppm at 15.5 m and N2O concentrations remained roughly constant and were on average 478 _ 55 ppb from the soil surface to 15.5 m deep. A modeling approach (using the Min3P and Root Typ models) showed that the amount of water filling soil porosity accounted for a large share of the difference in gas concentration between +W and {W, and pointed out the consequences of through fall exclusion on the areas of CO2 production throughout the soil profile. Improving our understanding of the spatiotemporal dynamics of gas concentrations in deep soil layers is important to improve the current biogeochemical models predicting the effect of drought periods on greenhouse gas effluxes in eucalypt plantations established in deep tropical soils

Conditions of malaria transmission in Dakar from 2007 to 2010

Background: Previous studies in Dakar have highlighted the spatial and temporal heterogeneity of Anopheles gambiae s.l. biting rates. In order to improve the knowledge of the determinants of malaria transmission in this city, the present study reports the results of an extensive entomological survey that was conducted in 45 areas in Dakar from 2007 to 2010. Methods: Water collections were monitored for the presence of anopheline larvae. Adult mosquitoes were sampled by human landing collection. Plasmodium falciparum circumsporozoite (CSP) protein indexes were measured by ELISA (enzyme-linked immunosorbent assay), and the entomological inoculation rates were calculated. Results: The presence of anopheline larvae were recorded in 1,015 out of 2,683 observations made from 325 water collections. A water pH of equal to or above 8.0, a water temperature that was equal to or above 30 degrees C, the absence of larvivorous fishes, the wet season, the presence of surface vegetation, the persistence of water and location in a slightly urbanised area were significantly associated with the presence of anopheline larvae and/or with a higher density of anopheline larvae. Most of the larval habitats were observed in public areas, i.e., freely accessible. A total of 496,310 adult mosquitoes were caught during 3096 person-nights, and 44967 of these specimens were identified as An. gambiae s.l. The mean An. gambiae s.l. human-biting rate ranged from 0.1 to 248.9 bites per person per night during the rainy season. Anopheles arabiensis (93.14%), Anopheles melas (6.83%) and An. gambiae s.s. M form (0.03%) were the three members of the An. gambiae complex. Fifty-two An. arabiensis and two An. melas specimens were CSP-positive, and the annual CSP index was 0.64% in 2007, 0.09% in 2008-2009 and 0.12% in 2009-2010. In the studied areas, the average EIR ranged from 0 to 17.6 infected bites per person during the entire transmission season. Conclusion: The spatial and temporal heterogeneity of An. gambiae s.l. larval density, adult human-biting rate (HBR) and malaria transmission in Dakar has been confirmed, and the environmental factors associated with this heterogeneity have been identified. These results pave the way for the creation of malaria risk maps and for a focused anti-vectorial control strategy

IgG responses to the gSG6-P1 salivary peptide for evaluating human exposure to Anopheles bites in urban areas of Dakar region, Sénégal

<p>Abstract</p> <p>Background</p> <p>Urban malaria can be a serious public health problem in Africa. Human-landing catches of mosquitoes, a standard entomological method to assess human exposure to malaria vector bites, can lack sensitivity in areas where exposure is low. A simple and highly sensitive tool could be a complementary indicator for evaluating malaria exposure in such epidemiological contexts. The human antibody response to the specific <it>Anopheles </it>gSG6-P1 salivary peptide have been described as an adequate tool biomarker for a reliable assessment of human exposure level to <it>Anopheles </it>bites. The aim of this study was to use this biomarker to evaluate the human exposure to <it>Anopheles </it>mosquito bites in urban settings of Dakar (Senegal), one of the largest cities in West Africa, where <it>Anopheles </it>biting rates and malaria transmission are supposed to be low.</p> <p>Methods</p> <p>One cross-sectional study concerning 1,010 (505 households) children (n = 505) and adults (n = 505) living in 16 districts of downtown Dakar and its suburbs was performed from October to December 2008. The IgG responses to gSG6-P1 peptide have been assessed and compared to entomological data obtained in or near the same district.</p> <p>Results</p> <p>Considerable individual variations in anti-gSG6-P1 IgG levels were observed between and within districts. In spite of this individual heterogeneity, the median level of specific IgG and the percentage of immune responders differed significantly between districts. A positive and significant association was observed between the exposure levels to <it>Anopheles gambiae </it>bites, estimated by classical entomological methods, and the median IgG levels or the percentage of immune responders measuring the contact between human populations and <it>Anopheles </it>mosquitoes. Interestingly, immunological parameters seemed to better discriminate the exposure level to <it>Anopheles </it>bites between different exposure groups of districts.</p> <p>Conclusions</p> <p>Specific human IgG responses to gSG6-P1 peptide biomarker represent, at the population and individual levels, a credible new alternative tool to assess accurately the heterogeneity of exposure level to <it>Anopheles </it>bites and malaria risk in low urban transmission areas. The development of such biomarker tool would be particularly relevant for mapping and monitoring malaria risk and for measuring the efficiency of vector control strategies in these specific settings.</p