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J Postma

Publications and source records attributed to J Postma.

11 recordsLinked to original sources
PubMed2006

Effect of above-ground plant species on soil microbial community structure and its impact on suppression of Rhizoctonia solani AG3.

The extent of soil microbial diversity is seen to be critical to the maintenance of soil health and quality. Different agricultural practices are able to affect soil microbial diversity and thus the level of suppressiveness of plant diseases. In a 4-year field experiment, we investigated the microbial diversity of soil under different agricultural regimes. We studied permanent grassland, grassland turned into arable land, long-term arable land and arable land turned into grassland. The diversity of microbial communities was described by using cultivation-based and cultivation-independent methods. Both types of methods revealed differences in the diversities of soil microbial communities between different treatments. The treatments with higher above-ground biodiversity generally maintained higher levels of microbial diversity. Moreover, a positive correlation between suppression of Rhizoctonia solani AG3 and microbial diversity was observed. Permanent (species-rich) grassland and grassland turned into maize stimulated higher microbial diversities and higher levels of suppressiveness of R. solani AG3 compared with the long-term arable land. Effects of agricultural practices on Bacillus and Pseudomonas communities were also observed and clear correlations between the levels of suppressiveness and the diversities of these bacterial groups were found. This study highlighted the importance of agricultural management regime for soil microbial community structure and diversity as well as the level of soil suppressiveness.

Agriculture↗
PubMed2005

Distribution of microcystins in a lake foodweb: no evidence for biomagnification.

Microcystins, toxins produced by cyanobacteria, may play a role in fish kills, although their specific contribution remains unclear. A better understanding of the eco-toxicological effects of microcystins is hampered by a lack of analyses at different trophic levels in lake foodwebs. We present 3 years of monitoring data, and directly compare the transfer of microcystin in the foodweb starting with the uptake of (toxic) cyanobacteria by two different filter feeders: the cladoceran Daphnia galeata and the zebra mussel Dreissena polymorpha. Furthermore foodwebs are compared in years in which the colonial cyanobacterium Microcystis aeruginosa or the filamentous cyanobacterium Planktothrix agardhii dominated; there are implications in terms of the types and amount of microcystins produced and in the ingestion of cyanobacteria. Microcystin concentrations in the seston commonly reached levels where harmful effects on zooplankton are to be expected. Likewise, concentrations in zooplankton reached levels where intoxication of fish is likely. The food chain starting with Dreissena (consumed by roach and diving ducks) remained relatively free from microcystins. Liver damage, typical for exposure to microcystins, was observed in a large fraction of the populations of different fish species, although no relation with the amount of microcystin could be established. Microcystin levels were especially high in the livers of planktivorous fish, mainly smelt. This puts piscivorous birds at risk. We found no evidence for biomagnification of microcystins. Concentrations in filter feeders were always much below those in the seston, and yet vectorial transport to higher trophic levels took place. Concentrations of microcystin in smelt liver exceeded those in the diet of these fish, but it is incorrect to compare levels in a selected organ to those in a whole organism (zooplankton). The discussion focuses on the implications of detoxication and covalent binding of microcystin for the transfer of the toxin in the foodweb. It seems likely that microcystins are one, but not the sole, factor involved in fish kills during blooms of cyanobacteria.

Animals↗
PubMed2002

Inability to find consistent bacterial biocontrol agents of Pythium aphanidermatum in cucumber using screens based on ecophysiological traits.

A collection of 821 rhizobacteria from cucumber, originating from different root locations and stages of plant development, was screened for potential biocontrol agents of Pythium aphanidermatum (Edson) Fitzp. The screening procedure exploited carbon source utilization profiles and growth rates of bacteria as indicators of a partial niche overlap with the pathogen. The bacteria were tested for growth on nine carbon sources (glucose, fucose, sucrose, maltose, asparagine, alanine, galacturonic acid, succinic acid, and linoleic acid), most of which are reported to be used by the zoospores of P. aphanidermatum in the infection process. The isolates were classified as fast- or slow-growing, depending on their growth rate in 1/10 strength TSB. By nonhierarchical cluster analysis, 20 clusters were generated of bacteria with similar profiles of carbon source utilization. Redundancy analysis showed that the type of root sample explained 47% of the variance found in the relative abundance of bacteria from the clusters. Bacteria from clusters using none or few of the carbon sources, e.g., maltose and linoleic acid, with many slow-growing isolates, showed a preference for plants in the vegetative or generative stage, or for old root regions (root base). Bacteria from clusters with fast-growing isolates, using many carbon sources, were relatively abundant in the seedling stage. A selection of 127 bacteria from the different clusters was tested for disease suppressive capabilities in bioassays on young cucumber plants in nutrient solution, inoculated with zoospores of P. aphanidermatum. Nine of these bacteria produced biosurfactants, and 27 showed antibiosis against mycelial growth in plate assays. For 31 isolates, significant positive effects on plant biomass were shown, as analyzed with a general linear regression model. For most isolates, these effects occurred only in one of two replicate assays and no reductions in the degree of root and crown rot were found. Of the isolates that used many of the tested carbon sources, only four had positive effects on plant biomass. The majority of the isolates that positively affected plant biomass used few to moderate numbers of carbon sources and did not produce antibiotics or biosurfactants. In conclusion, competition for the tested carbon sources with the zoospores did not play a decisive role in disease suppression, and no clear relation was found between ecophysiological traits and disease suppression. Only isolate 3.1T8, isolated from root tips in the generative stage of plant growth, significantly increased plant biomass and suppressed root and crown rot symptoms in five out of six bioassays. The isolate produced an antifungal substance in plate assays and showed biosurfactant production in several (cucumber-derived) media.

Bacteria↗
PubMed2001

Population dynamics of Pythium aphanidermatum in cucumber grown in closed systems.

Foot and root rot in cucumber, caused by Pythium aphanidermatum (Edson) Fitzp., is an economically important disease in soilless culture systems. Nevertheless, very few data are available on the populations of this pathogen. Therefore, two detection methods, nested PCR (polymerase chain reaction) and plating on a selective medium after concentration of samples, were optimised and evaluated. With both methods very low concentrations of P. aphanidermatum could be detected; i.e. the detection limits were around 0.05 CFU/ml nutrient solution. In addition, real-time quantitative PCR using a Molecular Beacon probe was designed and tested. The potential and limitations of the different detection methods are discussed. With these different detection techniques, the population dynamics of P. aphanidermatum in a cucumber crop was followed. The impact of different disinfection treatments was studied in a greenhouse experiment with a cucumber crop growing on rockwool slabs in 12 independent closed systems. The nutrient solution was recirculated without disinfection (control), after UV-irradiation (250 mJ/cm2), or after slow sand filtration treatment. Part of the crop was inoculated with an isolate of P. aphanidermatum. The non-inoculated part could only become infected through the recirculated nutrient solution. Disease symptoms (stem rot, wilt, and root rot) and the yield loss were recorded in addition to the population dynamics of the pathogen. Very clear differences in the spread of the pathogen and in disease symptoms were measured between the systems with and without disinfection. UV-irradiation and slow sand filtration were both effective in removing the pathogen and protected the crop from disease symptoms. Correlation indices between the final yield and the different measurements during the experiment were calculated.

Colony Count, Microbial↗
PubMed1991

Population Dynamics of Rhizobium leguminosarum Tn5 Mutants with Altered Cell Surface Properties Introduced into Sterile and Nonsterile Soils.

The influence of cell surface properties on attachment to soil particles and on population dynamics of introduced bacteria was studied in sterilized and nonsterilized loamy sand and silt loam. Rhizobium leguminosarum RBL5523 and three Tn5 mutants (RBL5762, RBL5810, and RBL5811) with altered cell surface properties were used. Cellulose fibrils were not produced by RBL5762. Both RBL5810 and RBL5811 produced 80 to 90% less soluble exopolysaccharides and RBL5811 had, in addition, an altered lipopolysaccharide composition. In sterilized soil the total number of cells as well as the number of particle-associated cells of RBL5523 and RBL5810 were, in general, higher as compared with cell numbers of RBL5762 and RBL5811. Differences between strains in percentage of particle-associated cells in sterilized soil were only found at high inoculum densities, when populations increased little. In the nonsterilized silt loam, final population sizes, as well as numbers of particle-associated cells, of the parental strain (RBL5523) were higher than those of strains with altered cell surface properties after 56 and 112 days of incubation. But in general, differences in survival among the strains were not very marked. The importance of association with soil particles or aggregates for the survival of introduced cells was affirmed by the pronounced increase of the percentage of particle-associated cells during incubation in nonsterilized as well as sterilized soil. However, no clear relation among altered cell surface properties, particle association, and survival was found.

Journal Article↗
PubMed1990

DNA rotational positioning in a regulatory nucleosome is determined by base sequence. An algorithm to model the preferred superhelix.

MMTV-LTR sequences -190/-45 position a histone octamer both in vivo and in vitro. Experimental evidence suggested that nucleosome rotational positioning is determined by the DNA sequence itself. We developed an algorithm that is able to predict the most favorable path of a given DNA sequence over a histone octamer, based on rotational preferences of different dinucleotides. Our analysis shows that these preferences are sufficient for explaining the observed rotational positioning of the MMTV-LTR nucleosome, at one base pair accuracy level. Computer-generated 3-D models of the experimentally calculated and predicted MMTV-LTR nucleosome show that the predicted orientation is fully compatible with the currently available data in terms of accessibility of relevant sequences to regulatory proteins.

Algorithms↗
PubMed1990

Role of Microniches in Protecting Introduced Rhizobium leguminosarum biovar trifolii against Competition and Predation in Soil.

The importance of microniches for the survival of introduced Rhizobium leguminosarum biovar trifolii cells was studied in sterilized and recolonized sterilized loamy sand and silt loam. The recolonized soils contained several species of soil microorganisms but were free of protozoa. Part of these soil samples was inoculated with the flagellate Bodo saltans, precultured on rhizobial cells. The introduced organisms were enumerated in different soil fractions by washing the soil, using a standardized washing procedure. With this method, free organisms and organisms associated with soil particles or aggregates >50 mum were separated. The total number of rhizobia was influenced slightly (silt loam) or not at all (loamy sand) by the recolonization with microorganisms or by the addition of flagellates alone. However, when both flagellates and microorganisms were present, numbers of rhizobia decreased drastically. This decrease was more than the sum of both effects separately. Nevertheless, populations of rhizobia were still higher than in natural soil. In the presence of flagellates, higher percentages of rhizobia and other microorganisms were associated with soil particles or aggregates >50 mum than in the absence of flagellates. In recolonized soils, however, the percentages of particle-associated rhizobia were lower than in soils not recolonized previous to inoculation. Thus, the presence of other microorganisms hindered rhizobial colonization of sites where they are normally associated with soil particles or aggregates.

Journal Article↗
PubMed1989

A computer graphics study of multistranded DNA models.

A range of related nucleic acid models has been built using the computer graphics system at EMBL (Heidelberg). The relationship between these models is considered in terms of a simple "core" structure. One of these models is a four-strand structure in which two duplexes of the Watson-Crick kind are specifically related by a twofold rotation axis and which has already been discussed in some detail. The models fall into two principal classes. In one, each sugar phosphate chain is attached to every layer of bases in the "core" structure; in the other, each chain is attached to alternate layers of bases. Several of the models discussed have specificity both of complementarity and of identity.

Computer Graphics↗