Soil Suppressiveness to Fusarium Wilt of Melon, Induced by Repeated Croppings of Resistant Varieties of Melons

A field soil, artificially infested with pathogenic isolates of Fusarium oxysporum f. sp. melonis was continuously used for screening resistant varieties of melon to Fusarium wilt. After 9–10 years of continuous cropping with resistant varieties, the soil had developed induced suppressiveness. Seven to 9 experimental replantings of the induced suppressive soil with the susceptible cultivar of melon, ‘Ein-Dor', nullified its suppressiveness. This was expressed by 90 % disease incidence. Only 2 replantings were required to obtain the same disease incidence in an adjacent field of a conducive soil. Nonpathogenic isolates of F. oxysporum, isolated from the rhizospheres of melon seedlings, induced various degrees of soil suppressiveness when added to soil at various ratios to the pathogenic isolate.     

Reduced tumour necrosis factor-induced cytotoxicity by inhibitors of the arachidonic acid metabolism

The mechanism of tumour necrosis factor-mediated cytotoxicity was investigated by using various inhibitors of arachidonic acid metabolism. Phospholipase A2 inhibitors with different modes of action interfered with the cytotoxic action of TNF, whereas phospholipase C inhibitors did not. Neither cyclooxygenase nor lipoxygenase-blockers had a significant effect on TNF action. Experiments with scavengers of toxic oxygen radicals gave ambiguous results. The data obtained suggest the involvement of phospholipase A2 and arachidonic acid in the cytotoxic mechanism of TNF, but the exact role of these molecules is, however, still to be determined.     

Oligonucleotide probes for the detection of TEM-1 and TEM-2 beta-lactamase genes and their transposons

We describe the use of molecular probes to detect the TEM-type beta-lactamase genes. As a general probe, we prepared a 656 base pair restriction fragment, entirely within the TEM structural gene. This probe was specific for the TEM family, hybridizing only with TEM-1 and TEM-2. The TEM-1 and TEM-2 beta-lactamases differ by only one amino acid. We synthesized two oligonucleotides whose central bases correspond to this difference. The use of these oligonucleotides enables us to discriminate between TEM-1 and TEM-2 genes. Using oligonucleotides homologous to parts of Tn3, we also monitored the presence of TnA-like transposons in bacteria harboring different beta-lactamase genes. Only the TEM-1 and TEM-2 genes were found to be on transposons with terminal sequences identical to those of Tn3. All hybridization experiments were performed with both dot-blot and colony-hybridization techniques, and the suitability of these two methods for epidemiological studies is compared.     

Responses of wild plants to nitrate availability

Summary Two annual species of Bromus, an invader (B. hordeaceus, ex B. mollis) and a non-invader (B. intermedius), were grown for 28 days in growth chambers, at 5 and 100 M NO ₃ ^- in flowing nutrient solution. No differences between the two species were observed at either NO ₃ ^- level, in terms of relative growth rate (RGR) or its components, dry matter partitioning, specific NO ₃ ^- absorption rate, nitrogen concentration, and other characteristics of NO ₃ ^- uptake and photosynthesis. The effects of decreasing NO ₃ ^- concentration in the solution were mainly to decrease the NO ₃ ^- concentration in the plants through decreased absorption rate, and to decrease the leaf area ratio through increased specific leaf mass and decreased leaf mass ratio. Organic nitrogen concentration varied little between the two treatments, which may be the reason why photosynthetic rates were not altered. Consequently, RGR was only slightly decreased in the 5-M treatment compared to the 100-M treatment. This is in contrast with other species, where growth is reduced at much higher NO ₃ ^- concentrations. These discrepancies may be related to differences in RGR, since a log-linear relationship was found between RGR and the NO ₃ ^- concentration at which growth is first reduced. In addition, a strong linear relationship was found between the RGR of these species and their maximum absorption rate for nitrate, suggesting that the growth of species with low maximum RGR may be partly regulated by nutrient uptake.     

Measurement of bacterial growth rates in subsurface sediments using the incorporation of tritiated thymidine into DNA

Microbial growth rates in subsurface sediment from three sites were measured using incorporation of tritiated thymidine into DNA. Sampling sites included Lula, Oklahoma, Traverse City, Michigan, and Summit Lake, Wisconsin. Application of the thymidine method to subsurface sediments required (1) thymidine concentrations greater than 125 nM, (2) incubation periods of less than 4 hours, (3) addition of SDS and EDTA for optimum macromolecular extraction, and (4) DNA purification, in order to accurately measure the rate of thymidine incorporation into DNA. Macromolecule extraction recoveries, as well as the percentage of tritium label incorporated into the DNA fraction, were variable and largely dependent upon sediment composition. In general, sandy sediments yielded higher extraction recoveries and demonstrated a larger percentage of label incorporated into DNA than sediments that contained a high silt-clay component. Reported results also indicate that the acid-base hydrolysis procedure routinely used for macromolecular fractionation in water samples may not be routinely applicable to the modified sediment procedure where addition of SDS and EDTA are required for macromolecule extraction. Growth rates exhibited by subsurface communities are relatively slow, ranging from 5.1 to 10.2×10⁵ cells g^–1 day^–1. These rates are 2–1,000-fold lower than growth rates measured in surface sediments. These data lend support to the supposition that subsurface microbial communities are nutritionally stressed.     

Taxol-induced reorganization of the microtubule system in murine splenic lymphocytes inhibits response to allogeneic cells but not to concanavalin A

The exposure of mouse splenic lymphocytes to the microtubule assembly-promoting drug taxol (10 microM for 4 h) results in an extensive reorganization of the microtubule system to form one to a few large bundles of microtubules, which extend from the centrosome. Lymphocytes pretreated with taxol for 4 h, or cultured in the continued presence of taxol, respond normally to the mitogen concanavalin A up to, and including, the stage of DNA replication. In contrast, the induction of DNA synthesis during the alloactivation of lymphocytes is inhibited when taxol is present in the mixed leukocyte culture. If the stimulators are pretreated with this drug, the mixed leukocyte reaction occurs normally, but pretreatment of the responders inhibits the proliferative response markedly. Microscopic observations of nuclear morphologies in these populations and autoradiography indicate that taxol inhibition occurs early in alloactivation, prior to DNA replication. The responding ability of taxol-treated lymphocytes is not restored to control levels by the addition of interleukin 2, leading to the suggestion that interleukin 2 receptors do not emerge or function normally in these cells. We conclude that the capacity to respond to allogeneic cells, but not to a mitogen, is dependent on the presence of the normal submembranous organization of the microtubule system.     

Target organ-specific inactivation of drug metabolizing enzymes in kidney of hamsters treated with estradiol

Chronic treatment of hamsters with estradiol for several months has previously been shown to decrease the specific content of cytochrome P450 in the kidney, a target of hormonal carcinogenesis, but not in liver. The reason for this decrease in metabolic enzyme activity is unknown and has been examined in this investigation. We now report that the decrease in specific content of renal cytochrome P450 by 73% in response to estradiol was not affected by co-treatment with tamoxifen for 1 month. The subcutaneous infusion of 250 μg/day estradiol for 7 days lowered renal cytochrome P450 by 71% from control values and was therefore used for further mechanistic studies. This treatment decreased renal activities of estradiol 2- or 4-hydroxylase by 77 to 80%, of 7-ethoxycoumarin-O-deethylase by 66% of control values, respectively, and completely eliminated aryl hydrocarbon hydroxylase activities, whereas liver enzymes remained unaffected. After 7 days of infusion of estradiol, fluorescent products of lipid peroxidation were more than doubled in hamster kidney but remained unchanged in liver. The possibility of enzyme destruction by binding of estradiol 2,3-quinone to metabolizing enzymes was investigatedin vitro. In the presence of 2-hydroxyestradiol, cumene hydroperoxide, and microsomes, conditions known to favor the oxidation of the steroid to quinone, the binding of catechol estrogen metabolite to microsomal protein increased 60 fold over control values in the absence of cofactor. Purified rat liver cytochrome P450c also oxidized 2-hydroxyestradiol to 2,3-estradiol quinone. The rate of oxidation was linear for the first 2–3 min, but thereafter decreased with time. Under these incubation conditions, irreversible binding of catechol estrogen metabolite to cytochrome P450c increased for the first 2–3 min and then remained at this plateau level. It was concluded that enzyme destruction by a reactive estrogen metabolite or by lipid peroxides may be a major reason for the organ-specific decrease in cytochrome P450 enzymes in kidneys of estrogen-treated hamsters.