Ann E. MacGuidwin

A nematode demographics assay in transgenic roots reveals no significant impacts of the Rhg1 locus LRR-Kinase on soybean cyst nematode resistance

BackgroundSoybean cyst nematode (Heterodera glycines, SCN) is the most economically damaging pathogen of soybean (Glycine max) in the U.S. The Rhg1 locus is repeatedly observed as the quantitative trait locus with the greatest impact on SCN resistance. The Glyma18g02680.1 gene at the Rhg1 locus that encodes an apparent leucine-rich repeat transmembrane receptor-kinase (LRR-kinase) has been proposed to be the SCN resistance gene, but its function has not been confirmed. Generation of fertile transgenic soybean lines is difficult but methods have been published that test SCN resistance in transgenic roots generated with Agrobacterium rhizogenes.ResultsWe report use of artificial microRNA (amiRNA) for gene silencing in soybean, refinements to transgenic root SCN resistance assays, and functional tests of the Rhg1 locus LRR-kinase gene. A nematode demographics assay monitored infecting nematode populations for their progress through developmental stages two weeks after inoculation, as a metric for SCN resistance. Significant differences were observed between resistant and susceptible control genotypes. Introduction of the Rhg1 locus LRR-kinase gene (genomic promoter/coding region/terminator; Peking/PI 437654-derived SCN-resistant source), into rhg1- SCN-susceptible plant lines carrying the resistant-source Rhg4+locus, provided no significant increases in SCN resistance. Use of amiRNA to reduce expression of the LRR-kinase gene from the Rhg1 locus of Fayette (PI 88788 source of Rhg1) also did not detectably alter resistance to SCN. However, silencing of the LRR-kinase gene did have impacts on root development.ConclusionThe nematode demographics assay can expedite testing of transgenic roots for SCN resistance. amiRNAs and the pSM103 vector that drives interchangeable amiRNA constructs through a soybean polyubiqutin promoter (Gmubi), with an intron-GFP marker for detection of transgenic roots, may have widespread use in legume biology. Studies in which expression of the Rhg1 locus LRR-kinase gene from different resistance sources was either reduced or complemented did not reveal significant impacts on SCN resistance.

Strategies to Reduce Nitrate Leaching into Groundwater in Potato Grown in Sandy Soils: Case Study from North Central USA

There is increasing public concern to reduce nitrate pollution to groundwater, especially in sandy soils. Strategies to reduce nitrate leaching are developed to increase N use efficiency, reduce groundwater pollution, and increase tuber yield. A growing interest in N management should consider management strategies for N supply, soil moisture for transport, and crop N demand that are economical and compatible with local production systems. We present a review of the literature on conventional and innovative strategies for N, irrigation, and crop management for potato production in reducing N leaching in sandy soils. The amount of fertilizer-N should be decided based on an integrated evaluation of soil organic matter content, soil texture, residual soil N, crop residues, credit to organic N sources, crops to be grown including varieties and crop physiological needs, cropping systems, yield potential, water management, and N concentrations in irrigation water. Research advances have no quick fix for controlling NO3 leaching to groundwater. However, the best combination of proven strategies can reduce leaching potential significantly.ResumenExiste una preocupación pública en aumento para reducir la contaminación de nitrato en el agua del subsuelo, especialmente en suelos arenosos. Se han desarrollado estrategias para reducir la lixiviación de nitratos para aumentar el uso eficiente de N, reducir la contaminación del agua del subsuelo, y para aumentar el rendimiento de tubérculo. Un interés en aumento en manejo de N debería de considerar estrategias de manejo para suministro de N, humedad del suelo para el transporte, y demanda de N del cultivo que sean económicas y compatibles con los sistemas locales de producción. Presentamos una revisión de la literatura en las estrategias convencionales e innovativas para el manejo de N, riego y del cultivo para producción de papa en la reducción de la lixiviación de N en suelos arenosos. La cantidad del fertilizante nitrogenado deberá decidirse con base a una evaluación integrada del contenido de materia orgánica en el suelo, textura, N residual, residuos de cosecha, reconocimiento a las fuentes de N orgánico, cultivos a sembrarse incluyendo variedades y necesidades fisiológicas del cultivo, sistemas de cultivo, potencial de rendimiento, manejo del agua, y concentraciones de N en el agua de riego. Los avances en investigación no tienen un remedio rápido para controlar la lixiviación de NO3 al agua del subsuelo. No obstante, la mejor combinación de estrategias probadas pueden reducir significativamente el potencial de lixiviación.

Soybean Cyst Nematode Reproduction in the North Central United States

An experiment was conducted in Heterodera glycines-infested fields in 40 north central U.S. environments (21 sites in 1994 and 19 sites in 1995) to assess reproduction of this nematode. Two resistant and two susceptible soybean cultivars from each of the maturity groups (MG) I through IV were grown at each site in 6.1 m by 4 row plots. Soil samples were collected from each plot at planting and harvest and processed at Iowa State University to determine H. glycines initial (Pi) and final (Pf) population densities as eggs per 100 cm3 of soil. Overall, reproduction (Pf/Pi) of H. glycines on susceptible cultivars in all MG was similar. Reproduction was higher on MG III and IV susceptible cultivars than on those in MG I and II. Resistant MG I and II cultivars reduced nematode population densities more consistently than those in MG III and IV. Reproduction of the nematode was similar among sites within the same maturity zone (MZ), defined as the areas of best adaptation of the corresponding MG. Nonetheless, careful monitoring of nematode population densities is necessary to assess changes that occur over time in individual fields.

Improving Phosphorus Use Efficiency Through Potato Rhizosphere Modification and Extension

Phosphorus (P) fertilization is essential for societal sustainability. However, plant P uptake is inefficient due to poor soil P solubility, especially for the potato (Solanum tuberosum L.) plant due to its relatively poor rooting efficiency. Phosphorus use efficiency (PUE) can be increased with rhizosphere modifications such as: (1) raising the bulk pH of acid soils, (2) placement of fertilizer in the rhizosphere by broadcast incorporation and/or, especially, as a concentrated band, (3) use of slow and controlled release P fertilizers, (4) use of organic acids to increase P solubility in alkaline soils, (5) use of an acid polymer, and (6) extending the effective rhizosphere zone via any practice that fosters root growth, such as through promotion of mycorrhizal development. Alone or in combination, these techniques offer an opportunity to increase PUE.ResumenLa fertilización con fósforo (P) es esencial para la sustentabilidad social. No obstante, la absorción de P por la planta es ineficiente debido a la pobre solubilidad del P en el suelo, especialmente para la planta de papa (Solanum tuberosum L.) debido a su relativamente pobre eficiencia radical. Se puede incrementar la eficiencia del uso del fósforo (PUE) con modificaciones a la rhizosfera, tales como: (1) aumentando el pH masivo de suelos ácidos, (2) colocando al fertilizante en la rhizosfera mediante incorporación amplia y/o, especialmente, en banda concentrada, (3) uso de liberación lenta y controlada de fertilizantes fosforados, (4) uso de ácidos orgánicos para aumentar la solubilidad del P en suelos alcalinos, (5) uso de un polímero ácido, y (6) expandiendo la zona efectiva de la rhizosfera por vía de cualquier práctica que estimule el crecimiento de la raíz, como mediante la promoción del desarrollo micorrízico. Solas o en combinación, estas técnicas ofrecen una oportunidad para aumentar la PUE.

Behavioural quiescence reduces the penetration and toxicity of exogenous compounds in second-stage juveniles of Heterodera glycines

Summary – Inactivity in nematodes is often correlated with survival of adverse environments. The non-feeding second-stage juvenile (J2) of Heterodera glycines must survive in a soil environment that may contain numerous toxins. In this report, we show that quiescent J2 of H. glycines survived higher concentrations of both ethanol and the plant-derived compound, allyl isothiocyanate, compared with actively moving nematodes. The mechanism for this quiescence-mediated resistance was investigated using fluorescein isothiocyanate (FITC). There was a reduction in the penetration of FITC in quiescent J2 of H. glycines compared with that in actively moving nonfeeding J2. Furthermore, exposure of quiescent nematodes to octopamine, an invertebrate neurotransmitter, induced activity and a subsequent increase in FITC penetration compared with quiescent nematodes exposed to FITC alone. These data demonstrate that behavioural quiescence is correlated with exclusion of the compound from the body of the nematode. Finally, the entry point of FITC into the nematode was examined by the application of a veterinary cyanoacrylate adhesive to occlude either the cephalic or caudal openings of the nematodes. Nematodes glued at the anterior end showed a significant reduction in fluorescence compared with nematodes glued on the posterior end and non-glued nematodes. Thus, the entry of FITC is primarily through openings in the cephalic region. This research is the first report of behavioural quiescence correlated with reduced sensitivity to toxins in a plant-parasitic nematode, and provides insight into how these important organisms cope with stress due to exogenous toxins.

Sampling for Plant-parasitic Nematodes in Corn Strip Trials Comparing Nematode Management Products

Gregory L. Tylka, Department of Plant Pathology and Microbiology, Iowa State University, Ames, IA 50011; Timothy C. Todd, Department of Plant Pathology, Kansas State University, Manhattan, KS 66506; Terry L. Niblack, Department of Plant Pathology, Ohio State University, Columbus, OH 43210; Ann E. MacGuidwin, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706; and Tamra Jackson, Department of Plant Pathology, University of Nebraska, Lincoln, NE 68588

Incidence of Oscheius onirici (Nematoda: Rhabditidae), a potentially entomopathogenic nematode from the marshlands of Wisconsin, USA

In a search for an entomopathogenic nematode to control cranberry insect pests, three Oscheius populations (Rhabditidae) were recovered through the Galleria-bait method from one sample taken in a wild cranberry marsh in Jackson County, Wisconsin, USA. Morphological studies with light microscopy and scanning electron microscopy, as well as molecular analyses of the near-full-length small subunit rDNA gene, D2/D3 expansion segments of the large subunit rDNA gene, internal transcribed spacer, and mitochondrial cytochrome oxidase subunit 1 (CoxI) genes revealed this as Oscheius onirici, a species recently described from a karst cave soil of central Italy. The species belongs to the dolichura-group and is characterized by its DNA sequences; hermaphroditic reproduction; and males not found. A Bacillus-like bacterium appears to be associated with this nematode based on our microscopic and SEM observations; however its identity and persistent association with the nematode has not been confirmed. Nonetheless, this nematode is capable of infecting and killing the sparganothis fruitworm Sparganothis sulfureana Clemens (Lepidoptera: Tortricidae), the brown-banded cockroach Supella longipalpa Fabricius (Blattodea: Ectobiidae), and the cranberry fruitworm Acrobasis vaccinii Riley (Lepidoptera: Pyralidae), under laboratory conditions, and each in less than 72 hr. The mealworm Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae) and the greater wax moth Galleria mellonella Linnaeus (Lepidoptera: Pyralidae), are also susceptible, but take 3.5 and 5.2 days to die, respectively. This species is a new potential bio-control agent on insects.

Development of a Damage Function Model for Pratylenchus penetrans on Corn

The lesion nematode Pratylenchus penetrans is a common pest of corn in the north-central United States. There are relatively few studies documenting the impact of Pratylenchus spp. on grain yield even though they are recognized as pests of corn and the target of commercial seed treatments. We adapted a component error modeling approach to develop a damage function for P. penetrans that included the influence of year and site in the yield loss relationship. Field data from six site-years was used to derive panel data consisting of all pairwise comparisons of the difference in nematode population densities and the associated proportional yield difference. Fourteen regression models of the relationship between proportional yield loss and the difference in nematode density were developed from soil and root assays at different corn growth stages. Seven models were significant: four models based on nematode population densities in soil (initial and final samples) and three based on nematode densities in seminal roots (corn growth stages V1 to V2 and V6) and adventitious roots (corn growth stage R1 to R2). The model we consider to be the most important, that based on the initial soil assay, estimated the yield loss caused by each nematode to be 0.0142%. The grand mean of the 118 plots we sampled implied a yield loss of 3.79%. The random effects of year and field did not contribute significantly to any of the models but were close to significance for some, suggesting a benefit from larger data sets. Experimental error was the largest component of the variance for all of the models; therefore, the damage function is more useful for demonstrating impact of P. penetrans rather than for accurately predicting yield loss at the field level. All of the fields in our study were an irrigated loamy sand soil, with grain yields above the county average; therefore, it is possible that our damage function is conservative. The value of soil sampling has been questioned for P. penetrans and this study shows it to be equal to if not better than root assays for predicting yield.

First Report of Cactodera milleri in Wisconsin

During the summer of 2005, lemon-shaped cysts and second-stage juveniles of a cyst nematode were recovered from soil at the University of Wisconsin Agricultural Research Station in Hancock, WI. Samples were collected on multiple dates from a plot (61 × 12 m) in continuous potato production for 20 years with significant weed pressure. PCR-restriction fragment length polymorphism profiles of the internal transcribed spacer (ITS) 1 region using restriction enzyme HhaI indicated Cactodera spp. (2). Morphological observations and morphometrics made on cysts, males, J2s, and eggs were consistent with Cactodera milleri Graney and Bird, 1990 (1). Host range studies were conducted in a growth chamber. Soybean, potato, and beet did not support nematode development and reproduction. Common lambsquarters (Chenopodium album), a known host of C. milleri, was an excellent host. No obvious aboveground disease symptoms were evident on lambsquarters in the growth chamber assay. This detection represents the first record of C. milleri in Wisconsin. Unless detailed morphological or molecular measurements are made, C. milleri may be easily confused with the soybean cyst nematode, Heterodera glycines. The presence of lambsquarters in fields planted with glyphosate-resistant soybeans makes the recovery of both nematode species in a single soil sample possible. References: (1) L. S. O. Graney and G. W. Bird. J. Nematol. 22:457, 1990. (2) A. L. Szalanski et al. J. Nematol. 29:255, 1997.