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Aerobic cell suspensions of Nitrosomonas europaea oxidized ammonium [Formula: see text] to nitrous oxide (N2O) and nitrite [Formula: see text], and exogenous [Formula: see text] in the presence or absence of [Formula: see text] did not stimulate N2O formation. Acetylene (C2H2) inhibited the production of [Formula: see text] and N2O from [Formula: see text] but not from hydroxylamine (NH2OH). The total amount of N2O formed in air was proportional to the amount of [Formula: see text] oxidized; however, the total N2O N formed as a percentage of [Formula: see text] N formed varied very little (0.05–0.15%) over the range of [Formula: see text] concentrations examined (0.05–20.4 mM). Rates of production of N2O and [Formula: see text] showed similar response to pH over the range of 5.4–9.5, with maxima at pH 8.5. Anaerobically, five times more N2O was formed than under aerobic conditions. The highest rates of anaerobic N2O formation were observed in the presence of [Formula: see text] and [Formula: see text] combined (2 and 1 mM, respectively) and C2H2 reduced this rate of N2O formation to that observed with 1 mM[Formula: see text] alone in the presence or absence of C2H2. The presence of the [Formula: see text] oxidizer Nitrobacter winogradskyi had no effect on the formation of N2O by Ns. europaea either in liquid culture or in sterile soil. However, the presence of sterile soil as a suspending matrix increased by 10-fold the production of N2O, and broadened the range of O2 concentrations under which relatively high rates of N2O production occurred. Maximum N2O production by Ns. europaea occurred at 0.75 kPa O2 in liquid suspension and at 2.5 kPa O2 in sterile soil.

Acetylene (C2H2) strongly inhibited (Ki 0.25 μM or 0.66 Pa) the oxidation of ammonia (NH4+) to nitrite (NO2−) by Nitrosomonas europaea but did not affect the oxidation of hydroxylamine (NH2OH) to NO2− by this organism. We suggest that the C2H2-sensitive step is associated with the ammonia oxygenase. Oxidation of NH4+ was inhibited only approximately 50% by 0.4 mM (10 kPa) ethylene and by 2.1 mM (10 kPa) nitrous oxide and was unaffected by 0.1 mM (10 kPa) methane. The oxidation of NO2− to nitrate (NO3−) by Nitrobacter winogradskyi and of NH4+ to NH2OH and NO3− by the heterotrophic nitrifier, Arthrobacter sp., was not affected by 3.83 mM (10 kPa) C2H2.

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One of the proposed mechanisms by which rhizobacteria enhance plant growth is through the production of plant growth regulators. Five plant growth promoting rhizobacterial (PGPR) strains produced the cytokinin dihydrozeatin riboside (DHZR) in pure culture. Cytokinin production by Pseudomonas fluorescens G20-18, a rifampicin-resistant mutant (RIF), and two TnphoA-derived mutants (CNT1, CNT2), with reduced capacity to synthesize cytokinins, was further characterized in pure culture using immunoassay and thin layer chromatography. G20-18 produced higher amounts of three cytokinins, isopentenyl adenosine (IPA), trans-zeatin ribose (ZR), and DHZR than the three mutants during stationary phase. IPA was the major metabolite produced, but the proportion of ZR and DHZR accumulated by CNT1 and CNT2 increased with time. No differences were observed between strain G20-18 and the mutants in the amounts of indole acetic acid synthesized, nor were gibberellins detected in supernatants of any of the strains. Addition of 10(-5) M adenine increased cytokinin production in 96- and 168-h cultures of strain G20-18 by approximately 67%. G20-18 and the mutants CNT1 and CNT2 may be useful for determination of the role of cytokinin production in plant growth promotion by PGPR.

This study investigated how the timing of application of the biofungicide Serenade (Bacillus subtilis QST713) or it components (product filtrate and bacterial cell suspension) influenced infection of canola by Plasmodiophora brassicae under controlled conditions. The biofungicide and its components were applied as a soil drench at 5% concentration (vol/vol or equivalent CFU) to a planting mix infested with P. brassicae at seeding or at transplanting 7 or 14 days after seeding (DAS) to target primary and secondary zoospores of P. brassicae. Quantitative polymerase chain reaction (qPCR) was used to assess root colonization by B. subtilis as well as P. brassicae. The biofungicide was consistently more effective than the individual components in reducing infection by P. brassicae. Two applications were more effective than one, with the biofungicide suppressing infection completely and the individual components reducing clubroot severity by 62 to 83%. The biofungicide also reduced genomic DNA of P. brassicae in canola roots by 26 to 99% at 7 and 14 DAS, and the qPCR results were strongly correlated with root hair infection (%) assessed at the same time (r = 0.84 to 0.95). qPCR was also used to quantify the transcript activity of nine host-defense-related genes in inoculated plants treated with Serenade at 14 DAS for potential induced resistance. Genes encoding the jasmonic acid (BnOPR2), ethylene (BnACO), and phenylpropanoid (BnOPCL and BnCCR) pathways were upregulated by 2.2- to 23-fold in plants treated with the biofungicide relative to control plants. This induced defense response was translocated to the foliage (determined based on the inhibition of infection by Leptosphaeria maculans). It is possible that antibiosis and induced resistance are involved in clubroot suppression by Serenade. Activity against the infection from both primary and secondary zoospores of P. brassicae may be required for maximum efficacy against clubroot.

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