Brassicaceous seed meals as soil amendments to suppress the plant-parasitic nematodes Pratylenchus penetrans and Meloidogyne incognita.

Brassicaceous seed meals are the residual materials remaining after the extraction of oil from seeds; these seed meals contain glucosinolates that potentially degrade to nematotoxic compounds upon incorporation into soil. This study compared the nematode-suppressive ability of four seed meals obtained from Brassica juncea 'Pacific Gold', B. napus 'Dwarf Essex' and 'Sunrise', and Sinapis alba 'IdaGold', against mixed stages of Pratylenchus penetrans and Meloidogyne incognita second-stage juveniles (J2). The brassicaceous seed meals were applied to soil in laboratory assays at rates ranging from 0.5 to 10.0% dry w/w with a nonamended control included. Nematode mortality was assessed after 3 days of exposure and calculated as percentage reduction compared to a nonamended control. Across seed meals, M. incognita J2 were more sensitive to the brassicaceous seed meals compared to mixed stages of P. penetrans. Brassica juncea was the most nematode-suppressive seed meal with rates as low as 0.06% resulting in > 90% suppression of both plant-parasitic nematodes. In general B. napus 'Sunrise' was the least nematode-suppressive seed meal. Intermediate were the seed meals of S. alba and B. napus 'Dwarf Essex'; 90% suppression was achieved at 1.0% and 5.0% S. alba and 0.25% and 2.5% B. napus 'Dwarf Essex', for M. incognita and P. penetrans, respectively. For B. juncea, seed meal glucosinolate-degradation products appeared to be responsible for nematode suppression; deactivated seed meal (wetted and heated at 70 °C for 48 hr) did not result in similar P. penetrans suppression compared to active seed meal. Sinapis alba seed meal particle size also played a role in nematode suppression with ground meal resulting in 93% suppression of P. penetrans compared with 37 to 46% suppression by pelletized S. alba seed meal. This study demonstrates that all seed meals are not equally suppressive to nematodes and that care should be taken when selecting a source of brassicaceous seed meal for plant-parasitic nematode management.

Isothiocyanates Produced by Brassicaceae Species as Inhibitors of Fusarium oxysporum.

Glucosinolates contained in members of the Brassicaceae release isothiocyanates potentially useful in controlling Fusarium oxysporum pathogens in conifer seedling nursery soils. Our objective was to determine the toxicity of individual isothiocyanates to different growth stages of the fungus. Bioassays with four F. oxysporum isolates were conducted using sealed containers in which 0.3 µl of 2-propenyl, ethyl, buty, phenylethyl, benzyl, or phenyl isothiocyanate was allowed to volatilize. Propenyl and ethyl isothiocyanates were the most fungistatic of those compounds tested. The same concentrations of propenyl and ethyl isothiocyanates that inhibited mycelial growth completely suppressed conidial and chlamydospore germination of all isolates. Other isothiocyanates including ethyl, benzyl, and phenethyl were also fungitoxic to F. oxysporum conidia and chlamydospores. Reduction in pathogen populations resulting from a green-manure crop are likely achievable since chlamydospores are sensitive to isothiocyanate. Pathogenic F. oxysporum isolates infesting nursery soils would likely be most suppressed by species of plants such as Brassica carinata, B. nigra, and B. juncea, which contain glucosi-nolates that release high concentrations of propenyl isothiocyanate.

Novel plant-microbe rhizosphere interaction involving Streptomyces lydicus WYEC108 and the pea plant (Pisum sativum).

A previously undescribed plant-microbe interaction between a root-colonizing Streptomyces species, S. lydicus WYEC108, and the legume Pisum sativum is described. The interaction is potentially of great importance to the health and growth in nature of this nodulating legume. The root-colonizing soil actinomycete S. lydicus WYEC108 influences pea root nodulation by increasing root nodulation frequency, possibly at the level of infection by Rhizobium spp. S. lydicus also colonizes and then sporulates within the surface cell layers of the nodules. Colonization leads to an increase in the average size of the nodules that form and improves the vigor of bacteroids within the nodules by enhancing nodular assimilation of iron and possibly other soil nutrients. Bacteroid accumulation of the carbon storage polymer, poly-beta-hydroxybutyrate, is reduced in colonized nodules. Root nodules of peas taken from agricultural fields in the Palouse hills of northern Idaho were also found to be colonized by actinomycete hyphae. We hypothesize that root and nodule colonization is one of several mechanisms by which Streptomyces acts as a naturally occurring plant growth-promoting bacterium in pea and possibly other leguminous plants.

CoIIIEDTA- reduction by Desulfovibrio vulgaris and propagation of reactions involving dissolved sulfide and polysulfides.

The migration of 60Co, dominantly via transport of Co-EDTA complexes, into surface water and groundwater is a recognized concern at many nuclear production and storage sites. Reduction of CoIIIEDTA- to CoIIEDTA2- should decrease the mobility of 60Co in natural environments by stimulating ligand displacement with Fe(III) or Al(III) or by precipitation of CoSx in sulfidic environments. In this study, we examine direct (enzymatic) and indirect (metabolite) reduction processes of CoIIIEDTA- by the sulfate-reducing bacterium Desulfovibrio vulgaris. D. vulgaris reduces CoIIIEDTA- to CoIIEDTA2-, but growth using it as a terminal electron acceptor was not demonstrated. Rather than acting as a competing electron acceptor and limiting cobalt reduction, introducing sulfate with D. vulgaris enhances the reduction of CoIIIEDTA- as a result of sulfide production. Sulfide reduces CoIIIEDTA- in a pathway involving polysulfide formation and leads to a CoS precipitate. Thus, both direct and indirect (i.e., through the production of sulfide) microbial reduction pathways of CoIIIEDTA- may help to retard its migration within soils and waters.

Allelochemicals released in soil following incorporation of rapeseed (Brassica napus) green manures.

Plant-derived allelochemicals such as those produced by glucosinolate hydrolysis in Brassica napus, or rapeseed, are viable alternatives to synthetic compounds for the control of soil-borne plant pests. However, allelochemical production and residence times in field soils have not been determined. Soil samples were taken at 0-7.5 and 7.5-15 cm during a period of 3 weeks following plow-down of two winter rapeseed cultivars (Humus and Dwarf Essex). Soil samples were extracted with dichloromethane and analyzed using gas chromatography. Nine glucosinolate degradation products were identified-five isothiocyanates, three nitriles, and one oxazolidinethione. Maximum concentrations were observed 30 h after plow-down. Compounds derived from 2-phenylethyl glucosinolate, the principal glucosinolate in rapeseed roots, dominated the profile of degradation products. Shoot glucosinolates left few traces. This indicates that rapeseed roots may be a more important source of toxic fumigants than above-ground parts of the plant.

Toxicity of Glucosinolate Degradation Products from Brassica napus Seed Meal Toward Aphanomyces euteiches f. sp. pisi.

ABSTRACT Brassica tissues are potentially useful in the control of Aphanomyces root rot of peas (Pisum sativum), but identity of the responsible compounds and specific impacts of those compounds on the pathogen's infection potential remain uncertain. Brassica napus seed meals and water extracts from these meals were used to determine the effect of glucosinolate hydrolysis products on Aphanomyces euteiches f. sp. pisi. B. napus meal ('Dwarf Essex') containing glucosinolates and intact myrosinase, the enzyme responsible for glucosinolate hydrolysis, completely inhibited infection by A. euteiches f. sp. pisi oospores. Water extracts from this meal, likewise, severely inhibited infection by oospores, as well as mycelial growth. Extracts from autoclaved 'Dwarf Essex' meal, in which myrosinase was denatured, and a low glucosinolate B. napus variety ('Stonewall') produced little disease reduction and had less impact on mycelial growth. Gas chromatographic analysis of Brassica tissues and water extracts confirmed that glucosinolates remained in autoclaved 'Dwarf Essex' meal and that 'Stonewall' meal contained low glucosinolate concentrations. 5-Vinyloxazolidine-2-thione was identified by mass spectrometry as a dominant glucosinolate hydrolysis product in aqueous extracts of the inhibitory meal. Bioassays conducted with aqueous solutions of this compound reduced mycelial growth, but not to the extent of those from intact 'Dwarf Essex' meal. Water-soluble compounds produced from the hydrolysis of glucosinolates in B. napus tissues reduced A. euteiches oospore infection and inhibited mycelial growth, thus, demonstrating potential utility of Brassica species in the control of A. euteiches.

Inhibition of Aphanomyces euteiches f. sp. pisi by Volatiles Produced by Hydrolysis of Brassica napus Seed Meal.

Seed meal from Brassica napus (rapeseed) produced volatile fungitoxic compounds potentially of value in the control of Aphanomyces root rot of pea. Hyphal growth, germination of encysted zoospores, and oospore survival and inoculum potential, were determined in the presence of volatiles produced from B. napus seed meal. Volatile compounds from B. napus meal completely suppressed mycelial growth and germination of encysted zoospores on agar. In growth chamber bioassays, pea (Pisum sativum) seed inoculated with zoospore suspensions and incubated 24 h in the presence of volatiles from rapeseed meal had 50% lower root rot disease severity than in the absence of meal. Volatile compounds passing through soil also significantly decreased survival and inoculum potential of oospores. Gas chromatographic analysis of rapeseed tissues and the volatile compounds evolved from tissues showed that substrate glucosinolates were hydrolyzed enzymatically to produce mainly isothiocyanates. Non-autoclaved rapeseed meal produced significantly higher levels of volatile compounds than did autoclaved meal. Also, volatile compounds produced from autoclaved meal were dominated by nitriles, whereas isothiocyanates were more common volatile products from non-autoclaved meal. Our results indicate that B. napus allelochemicals responsible for toxic effects toward A. euteiches f. sp. pisi are enzymatic hydrolysis products of glucosinolates.

Determination of sulfur species by cathodic square wave stripping voltammetry; Compounds relevant to natural sulfur mineralization.

The feasibility of rapid analysis of a number of environmentally important sulfur compounds by cathodic square wave stripping voltammetry at a mercury electrode has been investigated. For cysteine/cystine a relatively anodic peak was identified, which is ascribed to the stripping of a mercurous species. The dependence of the peak currents on pH was found to be different for cysteine and cystine. Methionine and thioproline gave similar stripping peaks to those for cysteine. It is proposed that they arise from species deposited by oxidative hydrolysis. A complex ion is proposed to cause the stripping peak of thiosulfate and tetrathionate, while polysulfides give both a HgS stripping peak and a non-adsorptive reduction peak. Limits of detection are in the 10(-8)-10(-9)M range.

Toxicity of allyl isothiocyanate-amended soil toLimonius californicus (Mann.) (Coleoptera: Elateridae) wireworms.

Acute toxicity of soil amended with allyl isothiocyanate (AITC) to three size classes ofLimonius californicus (Mann.) wireworms was determined in the laboratory. Wireworms were exposed to AITC at initial concentrations of 120-300 nmol/g soil for one day. During this time, extractable AITC concentrations decreased by 66 to 93 %. Probit analysis estimated LC50 values of 238 and 226 nmol/g soil at one day posttreatment for medium and large wireworms, respectively. For small wireworms, LC50 values decreased from 211 to 157 nmol/g soil during 1-137 days posttreatment. Sublethal concentrations of AITC significantly reduced feeding activity of treated wireworms at three posttreatment times and over the entire 137 days. Wireworm weight was not significantly affected by AITC. The potential exists to use glucosinolate-containing plant tissue as an isothiocyanate (ITC) source to reduce crop damage caused byL. californicus wireworms.

Mechanisms of h(2)s production from cysteine and cystine by microorganisms isolated from soil by selective enrichment.

Hydrogen sulfide (H(2)S) is a major component of biogenic gaseous sulfur emissions from terrestrial environments. However, little is known concerning the pathways for H(2)S production from the likely substrates, cysteine and cystine. A mixed microbial culture obtained from cystine-enriched soils was used in assays (50 min, 37 degrees C) with 0.05 M Tris-HCl (pH 8.5), 25 mumol of l-cysteine, 25 mumol of l-cystine, and 0.04 mumol of pyridoxal 5'-phosphate. Sulfide was trapped in a center well containing zinc acetate, while pyruvate was measured by derivatization with 2,4-dinitrophenylhydrazine. Sulfide and total pyruvate production were 17.6 and 17.2 nmol mg of protein min, respectively. Dithiothreitol did not alter reaction stoichiometry or the amount of H(2)S and total pyruvate, whereas N-ethylmaleimide reduced both H(2)S and total pyruvate production equally. The amount of H(2)S produced was reduced by 96% when only l-cystine was included as the substrate in the assay and by 15% with the addition of propargylglycine, a specific suicide inhibitor of cystathionine gamma-lyase. These data indicate that the substrate for the reaction was cysteine and the enzyme responsible for H(2)S and pyruvate production was cysteine desulfhydrase (EC 4.4.1.1). The enzyme had a K(m) of 1.32 mM and was inactivated by temperatures greater than 60 degrees C. Because cysteine is present in soil and cysteine desulfhydrase is an inducible enzyme, the potential for H(2)S production by this mechanism exists in terrestrial environments. The relative importance of this mechanism compared with other processes involved in H(2)S production from soil is unknown.

Allelochemicals produced during glucosinolate degradation in soil.

A variety of plant pests are suppressed by the incorporation of cruciferous plant material into soil. Although this effect is attributed to decomposition of glucosinolates into toxic products, little is known concerning glucosinolate degradation in the soil environment. Arenas (30 × 18 × 8 cm) that contained soil amended with 30 g defatted winter rapeseed meal (Brassica napus L.)/kg soil on one half and unamended soil on the other were constructed. Isothiocyanate concentrations in the soil were measured using infrared analysis of CC14 extracts, and ionic thiocyanate (SCN(-)) using ion chromatography on aqueous extracts. Quantities were monitored during a 100-hr time period in conjunction with a wireworm bioassay. Isothiocyanate production reached a maximum of 301 nmol/g soil at 2 hr, but decreased by 90% within 24 hr. Production of SCN(-) reached a maximum of 180 nmol/g soil at 8 hr but persisted longer than isothiocyanate. Separate late instar wire-worms (Limonius infuscatus Mots.) were repelled by the presence of rapeseed meal in less than 24 hr even though the meal was shown in separate experiments not to be toxic. We propose that rapidly produced isothiocyanates are responsible for this repellency, but other products such as SCN(-) may play a role.