Gilles D. Leroux

The macrophage-specific membrane protein Nramp controlling natural resistance to infections in mice has homologues expressed in the root system of plants

In mice, natural resistance or susceptibility to infection with Mycobacteria, Salmonella, and Leishmania is controlled by a gene named Bcg. Bcg regulates the capacity of macrophages to limit intracellular replication of the ingested parasites, and is believed to regulate a key bactericidal mechanism of this cell. Recently, we have cloned the Bcg gene and shown that it encodes a novel macrophage-specific membrane protein designated Nramp. A routine search of the public databases for sequences homologous to Nramp identified 3 expressed sequence tags (EST) that show strong similarities to the mammalian protein.We report the identification and cloning of a full-length cDNA clone corresponding to a plant homologue (OsNramp1) of mammalian Nramp. Predicted amino acid sequence analysis of the plant protein indicates a remarkable degree of similarity (60% homology) with its mammalian counterpart, including identical number, position, and composition of transmembrane domains, glycosylation signals, and consensus transport motif, suggesting an identical overall secondary structure and membrane organization for the two proteins. This high degree of structural similarity indicates that the two proteins may be functionally related, possibly through a common mechanism of transport. RNA hybridization studies and RT-PCR analyses indicate that OsNramp1 mRNA is expressed primarily in roots and only at very low levels in leaves/stem. DNA hybridization studies indicate that OsNramp1 is not a single gene, but rather forms part of a novel gene family which has several members in all plants tested including cereals such as rice, wheat, and corn, and also in common weed species. The striking degree of conservation between the macrophage-specific mammalian Nramp and its OsNramp1 plant homologue is discussed with respect to possible implications in the metabolism of nitrate in both organisms.

Effect of crop rotations on weed control, Bidens cernua and Erigeron canadensis populations, and carrot yields in organic soils

Abstract Organic coils in Quebec are prone to infestation by nodding beggarticks ( Bidens cernua L.) and Canada fleabane ( Erigeron canadensis L.), and by the root-knot nematode ( Meloidogyne hapla Chitwood). Yield losses can be very severe in fields where carrots have been grown continuously for several years. Six 3-year crop rotations, including onion ( Allium cepa L.), barley ( Hordeum vulgare L.) or a weed fallow, were compared to continuous carrot production to establish their effect on weed control and carrot yield. Weed populations and above ground dry biomass were monitored four times each season. Total and marketable yields of carrots also were measured each year. Results show that weed populations increase when onions are part of the production cycle, and decrease with barley. Beggarticks and fleabane populations were static when cropping sequences included 2 years of carrots with or without onions. Total carrot yield increased by 35–50% and marketable yield increased 17–25-fold due to a drastic reduction in root-knot nematode populations when barley was included in the rotation. The onion-barley-carrot rotation provided good weed control, including nodding beggarticks and Canada fleabane, and yielded high quality carrots.

Atrazine action on the donor side of photosystem II in triazine-resistant and -susceptible weed biotypes

Abstract The herbicide atrazine is well known for its inhibitory action at the Q B site on the D1 polypeptide of photosystem II. In this study we demonstrate the presence of a second atrazine inhibition site localized on the donor side of photosystem II. This conclusion is based on the fact that both diphenylcarbazide and MnCl 2 overcome part of the atrazine inhibition when used as electron donors for dichlorophenolindophenol photoreduction in photosystem II submembrane fractions but not in unresolved thylakoid membranes. In the latter, this inhibitory site is only weakly accessible to atrazine. Furthermore, in photosystem II preparations, the inhibition on the donor side occurs at a lower atrazine concentration than the inhibition on the acceptor side. In resistant weed biotypes, photosystem II particles were as tolerant for atrazine as were unresolved thylakoids. Therefore, the protein modification conferring resistance to atrazine at the Q B site could also affect the lumenal portion of the PSII core complex through conformational changes of the D1 polypeptide.

Floristic diversity, size, and vertical distribution of the weed seedbank in ridge and conventional tillage systems.

Abstract The floristic diversity and the vertical distribution of the weed seedbank were studied in ridge tillage (RT) and conventional tillage (CT) systems in clay and clay loam soils. Viable seedbank populations were monitored during 3 yr using germination in a greenhouse. Ridge-tilled fields had a larger soil seedbank (2,992 seeds m−2) than moldboard-plowed fields (1,481 seeds m−2) in the top 15 cm. This result can be explained by the larger perennial seedbank of RT fields at both the 0- to 5-cm and 5- to 15-cm depths. Annual dicot seeds were more abundant in clay soils than in clay loams at the two soil depths. Annual grass seeds were more abundant under CT than under RT in clay soils at the two sampled depths. In clay loams, the density of annual grass seeds in RT fields was six times greater than in CT fields in the top 5 cm of soil and two times greater at the 5- to 15-cm depth. The vertical distribution of total seeds in soil did not differ between tillage systems. The top 5 cm of the 15-cm soil core contained 35 and 46% of all weed seed in CT and RT systems, respectively. However, the CT system had the highest concentration of annual dicot seeds 5 to 15 cm deep, whereas in the RT system, the same depth contained the highest concentration of perennial seeds. These results confirm that tillage systems and soil types can regulate seedbanks. Weed management programs must take this information into account. Nomenclature:Dicamba; SAN 582, 2-chloro-N-[(1-methyl-2-methoxy)ethyl]-N-(2,4-dimethyl-thien-3-yl)-acetamide; glyphosate; Abutilon theophrasti Medik ABUTH, velvetleaf; Ambrosia artemisiifolia L. AMBEL, common ragweed; Chenopodium album L. CHEAL, common lambsquarters; Echinochloa crus-galli (L.) Beauv. ECHCG, barnyard grass; Oxalis stricta L. OXAST, yellow woodsorrel; Panicum dichotomiflorum (L.) Michx. var. geniculatum (Wood) Fern. PANDI, fall panicum; Plantago major L. PLAMA, broad-leaved plantain; Setaria faberi Herrm. SETFA, giant foxtail; Setaria pumila (Poir) Roem et Schult SETLU, yellow foxtail; Setaria viridis (L.) Beauv. SETVI, green foxtail; Taraxacum officinale Weber in Wiggers TAROF, dandelion; Brassica napus L., oilseed rape; Glycine max (L.) Merr., soybean; Triticum aestivum L., wheat; Zea mays L., corn.

Interaction of the electron donor diphenylcarbazide with the herbicide-binding niche of Photosystem II

In experiments using thylakoid membranes, it was found that the inhibitory action of atrazine and 3-(3,4-dichlorophenyl)-1,1-dimethylurea is weaker in the presence of the Photosystem II electron donor sym -diphenylcarbazide. The concentration dependences of sym -diphenylcarbazide for electron donation and the relief of inhibition by the herbicides were different, higher concentrations being required for the latter. These data could not be explained by the electron donation properties of sym -diphenylcarbazide. Instead, the donor was shown to displace [ 14 C]atrazine from its binding site. The photoreduction of 2,5-dichlorobenzoquinone, monitored using oxygen evolution experiments, was also strongly reduced in the presence of sym -diphenylcarbazide. A direct interaction of sym -diphenylcarbazide with the herbicide-binding niche is demonstrated.

Could Weed Sensing in Corn Interrows Result in Efficient Weed Control

Abstract At the field scale, weeds generally appear aggregated rather than randomly distributed, and this aggregation is linked to the spatial heterogeneity of biotic and abiotic factors. Crop management practices shape the spatial pattern of weed infestations by modifying certain factors having an impact on weed emergence and growth. Although crop seeding is often the last in-field disturbance before crop and weed emergence, its effect on the distribution of weeds has received little attention in the literature. The purpose of this study was to assess the influence of the planting operation on weed cover and presence in corn fields using digital images to investigate the possibility of sensing the interrow to infer the presence or absence of weeds on the corn row. A total of 18 site-years under conventional tillage treated with a single POST application of herbicide were selected across seven locations. Image analysis, at the V2 to V4 growth stage of corn, was used to compare the weed cover in three zones: the undisturbed interrows, the corn rows, and the interrows compacted by tractor wheel traffic. For 61% of site-years, there was no significant difference among the zones. When there was a significant difference compared with the other two zones, the undisturbed interrow was usually less infested. Point-to-point comparisons of weed presence or absence (based on a threshold of five pixels) between the interrow and the corn row revealed 70 or 73% correspondence, depending on the type of interrow (undisturbed or tracked). However the error of inference of the corn row weed cover generated by sensing only adjacent interrows may be too high for efficient commercial weed control. Nomenclature: Corn, Zea mays L. Resumen A una escala de campo, las malezas generalmente aparecen distribuidas en forma agregada y no aleatoriamente, y este agregado está relacionado a la heterogeneidad espacial de los factores bióticos y abióticos. Las prácticas de manejo del cultivo dan forma a los patrones espaciales de las infestaciones de malezas, al modificar ciertos factores que impactan la emergencia y crecimiento de malezas. Aunque la siembra del cultivo es a menudo la última perturbación dentro del campo antes de que se de la emergencia del cultivo y de las malezas, su efecto sobre la distribución de las malezas ha recibido poca atención en la literatura. El objetivo de este estudio fue evaluar la influencia de la operación de siembra sobre la presencia y cobertura de malezas dentro de campos de maíz usando imágenes digitales para investigar la posibilidad de inferir la presencia o ausencia de malezas sobre la hilera de siembra, a partir de datos de los espacios entre-hileras del maíz. Un total de 18 sitios-años bajo labranza convencional tratados con una sola aplicación de herbicida fueron seleccionados a lo largo de siete localidades. Se usó análisis de imágenes, en los estados de crecimiento del maíz de V2 a V4, para comparar la cobertura de malezas en tres zonas: entre-hileras sin perturbación, en la hilera del maíz, y entre-hileras compactadas por el tráfico de las llantas del tractor Para el 61% de los sitios-años, no hubo diferencias significativas entre zonas. Cuando hubo una diferencia significativa en comparación con las otras dos zonas, las entre-hileras sin perturbación estuvieron usualmente menos infestadas. Comparaciones de punto-a-punto de la presencia o ausencia de malezas (con base en un umbral de cinco pixeles) entre la hilera del maíz y entre-hileras revelaron 70 ó 73% de correspondencia, dependiendo del tipo de entre-hilera (sin perturbación o con compactación por las llantas). Sin embargo, el error de la inferencia de la cobertura de malezas en la hilera del maíz, generada solamente con los datos de las entre-hileras adyacentes puede ser muy alto para un control de malezas eficiente a nivel comercial.

Validation of a Management Program Based on A Weed Cover Threshold Model: Effects on Herbicide Use and Weed Populations

Abstract Weed management decisions based on weed threshold models offer the opportunity to reduce herbicide use by allowing the possibility of forgoing treatment or lowering rates. Weed thresholds based on a relative leaf-cover model were tested during a 4-yr period at two locations. Two 1.62-ha fields, planted to conventional and glyphosate-resistant corn (2004, 2005, 2007) or soybean (2006), were divided in 900 m2 sections. Herbicides were applied postemergence to each of these sections with either variable rates based on weed thresholds, or constant full rates. Variable herbicide rates included: no application, half rate, or full rate. Relative weed cover values of 0.2 and 0.4 (corn) or 0.1 and 0.3 (soybean) served as thresholds for incremental rates. Digital images were used to evaluate the relative weed cover. Weed density was assessed before and after herbicide application. Weed seed production was estimated for two species in 2004 and 2005. No difference in crop yield, relative weed cover, weed density, or weed seed production was observed between conventional and glyphosate-resistant cropping systems. During the first year, herbicide use reduction was obtained (−85.4%) with marginal crop yield loss (5 to 15%). In the subsequent 3 yr, preherbicide weed densities increased and concomitant increases in relative weed cover values did not allow more than a 10% overall reduction in herbicide use. This threshold model designed to maintain crop yields within a given year did not allow significant reduction in herbicide use during the following 3 yr. Residual weed populations most likely replenished the seed bank to levels that allowed weed densities to increase afterward. Increased weed density over time in plots treated with full rates of herbicide every year also indicated that a single postemergence herbicide treatment was not sufficient to contain weed populations at low levels every year in this corn–soybean rotation. Nomenclature: Glyphosate; corn, Zea mays L.; soybean, Glycine max (L.) Merr

An Imagery-Based Weed Cover Threshold Established Using Expert Knowledge

Abstract The implementation of site-specific weed management requires information about weed cover and decision support systems to determine weed cover thresholds and concomitant herbicide rates. Although it is possible to create accurate weed cover maps over large areas, weed cover thresholds have generally been evaluated using tedious weed density counts. To bridge this gap between weed cover obtained by machine vision and the concept of economic threshold, crop advisers specializing in weed scouting were asked to evaluate over 2,500 weed cover images (2 m by 3 m) and determine if a given image would require herbicide application or not. Using the area under the “receiver operating characteristic” curve method, an optimal weed cover threshold was established. The derived economic thresholds ranged from 0.06 to 0.31% weed cover contingent on the level of tolerance of the expert adviser. Although this threshold seems low, it is comparable with economic threshold values based on weed density.

Spatial Pattern of Weeds Based on Multispecies Infestation Maps Created by Imagery

Weeds are often spatially aggregated in maize fields, and the level of aggregation varies across and within fields. Several annual weed species are present in maize fields before postemergence herbicide application, and herbicides applied will control several species at a time. The goal of this study was to assess the spatial distribution of multispecies weed infestation in maize fields. Ground-based imagery was used to map weed infestations in rain-fed maize fields. Image segmentation was used to extract weed cover information from geocoded images, and an expert-based threshold of 0.102% weed cover was used to generate maps of weed presence/absence. From 19 site-years, 13 (68%) demonstrated a random spatial distribution, whereas six site-years demonstrated an aggregated spatial pattern of either monocotyledons, dicotyledons, or both groups. The results of this study indicated that monocotyledonous and dicotyledonous weed groups were not spatially segregated, but discriminating these weed groups slightly increased the chances of detecting an aggregated pattern. It was concluded that weeds were not always spatially aggregated in maize fields. These findings emphasize the need for techniques allowing the assessment of weed aggregation prior to conducting site-specific weed management. Nomenclature: Bird vetch, Vicia cracca L. VICCR; broadleaf plantain, Plantago major L. PLAMA; common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; dandelion, Taraxacum officinale G. H. Weber ex Wiggers TAROF; ladysthumb, Polygonum persicaria L. POLPE; oakleaf goosefoot, Chenopodium glacum L. CHEGL; oxeye daisy, Chrysanthemum leucanthemun L. CHYLE; redroot pigweed, Amaranthus retroflexus L. AMARE; rhombic copperleaf, Acalypha rhomboidea Raf. ACCRH; shepherds purse, Capsella bursa-pastoris (L.) Medik. CAPBP; white clover, Trifolium repens L. TRFRE.

Chemical and mechanical weed management strategies for grain pearl millet and forage pearl millet

There are limited options for controlling weeds in grain and forage pearl millet production in eastern Canada. Field studies were conducted near Quebec City in 2005 and 2006 to evaluate the effectiveness of herbicidal treatments (s-metolachlor/benoxacor and pendimethalin) and harrowing to control weeds and maximize yield in grain and forage pearl millet cultivation. s-Metolachlor/benoxacor and pendimethalin were applied either preemergence or early postemergence at the full dose recommended for corn and at half of that dose. Harrowing was evaluated at the three- and the five-leaf stage of pearl millet. In both types of pearl millet, s-metolachlor/benoxacor applied preemergence reduced plant density and yield. All other herbicidal treatments caused no visual injury to pearl millet and adequately controlled annual weeds, leading to grain and forage yields similar or slightly lower than those of the hand-weeded control. Better barnyard grass control resulted from early postemergence application of s-metolach...