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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-␤-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.Streptomyces lydicus WYEC108 is a root-colonizing actinomycete originally isolated and studied for its properties as an antifungal biocontrol agent. This strain is capable of mycoparasitic colonization of fungal root pathogens and excretion of antifungal metabolites within plant rhizospheres (16,62). Recently, we demonstrated that strain WYEC108 is also a plant growth-promoting bacterium in the absence of fungal pathogen challenge. This may be due to the ability of strain WYEC108 to produce hydroxamate-type siderophores and/or other plant growth-promoting metabolites in the rhizosphere (25). Streptomyces spp. have been previously described as rhizosphere-colonizing bacteria (37, 38), antifungal biocontrol agents useful in controlling fungal root diseases (51), in vitro siderophore producers, and in vitro producers of plant growthpromoting hormones (25). Plant root exudates stimulate rhizosphere growth of actinomycetes that are strongly antagonistic to fungal pathogens, while the actinomycetes utilize root exudates for growth and synthesis of antimicrobial substances (16,62). In addition, actinomycetes synthesize an array of biodegradative enzymes, including chitinases (9, 23, 35), glucanases (18,26,29,32,59,60), peroxidases (48), and other enzymes possibly involved in mycoparasitic activity. Yet, the overall importance, physiological activities, and symbiotic roles of actinomycetes in situ within plant rhizospheres remain little studied at the biochemical or mechanistic levels. We believe that Streptomyces are far more important rhizosphere bacteria than has been generally recognized.Important to the symbiotic relationship between plants and microbes is the acquisition of iron. Although abundant in nature, under aerobic conditions at a neutral or alkaline pH, iron is found in highly insoluble forms not readil...

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Enzymatic hydrolysis of glucosinolates, a class of compounds found in Brassica species, results in a number of products with potential to inhibit seed germination. To investigate the impact of both volatile and watersoluble allelochemicals, germination bioassays using Lactuca sativa seeds were conducted with root and combined leaf and stem tissues of Brassica napus. Tissues in which glucosinolates were hydrolyzed to remove volatile glucosinolate degradation products were compared with intact tissues and water controls. Only tissues containing glucosinolates produced volatiles that inhibited germination. Volatiles were trapped and identified using GC-MS. Volatiles produced in greater quanitity from intact tissues than from tissues without glucosinolates were almost exclusively glucosinolate hydrolysis products. Water-soluble components also inhibited germination. Chemical analysis of extracts confirmed the presence of glucosinolate hydrolysis products, but indicated the involvement of additional allelochemicals, especially in leaf and stem tissues. Results support the proposal that glucosinolatecontaining plant tissues may contribute to reductions in synthetic pesticide use if weed seeds are targeted.

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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.

A nondestructive method to determine total C and N concentrations in soil size fractions is desirable when a limited sample is available. Near‐infrared reflectance spectroscopy (NIRS) was used to determine the total C and N concentrations in silt (50.0–2.0 µm) and coarse clay (2.0–0.2 µm), separated from 12 surface soils, by regressing the diffuse reflectance of near‐infrared radiation with constituent concentrations determined using combustion techniques. The correlation coefficients (R2) of the calibration equations were 0.93 for C and 0.89 for N, and the standard errors associated with NIRS predictions were 6.2 g kg−1 soil for total C and 0.6 g kg−1 for total N. Equation development with only silt samples improved the accuracy of NIRS calibration equations. Coefficients of variation [CV = (standard error of performance ÷ mean of the combustion procedure) × 100] for validation sample sets ranged from 14 to 19%, which is within the acceptable range for determining inorganic elements in plant tissues. We conclude that NIRS can be used to predict C and N concentrations in soil size fractions.