Johannes Hallmann

Spectrum and Population Dynamics of Bacterial Root Endophytes

In conclusion, of the plants thus far studied, the spectrum and diversity of endophytic bacteria in the roots varies greatly. What about the endophytic bacterial spectrum of plants growing under extreme climatic conditions, such as halophytes and xerophytes? Survival mechanisms developed by those bacteria may have some interesting industrial or pharmaceutical applications. Newly developed cultivation-independent methods have made clear that there is much more diversity among endophytic bacteria than at first expected. The major factors influencing bacterial diversity and colonisation have been discussed and their potential to manage endophytic communities towards increased benefits for plants and human health have been outlined. However, the potential risks of endophytic bacteria, especially of those strains known also to be potential human pathogens, need further exploration.

Control of Plant Pathogenic Fungi with Bacterial Endophytes

Most plants are colonised by a broad spectrum of endophytic bacteria that arepotentially antagonistic towards fungal plant pathogens. This enormous potential needs to be further explored for its use in modern plant disease control strategies. This requires not only a better understanding of the underlying mechanisms and their regulation in response to environmental factors, but also amore comprehensive picture of what triggers endophytic colonisation as well as of the population dynamics of antagonistic bacterial endophytes within the plant. Continuing research in this area will hopefully lead to new and innovative concepts for biological control of fungal pathogens.

Bacillus and Streptomyces were selected as broad-spectrum antagonists against soilborne pathogens from arid areas in Egypt

Plant protection via disease-suppressive bacteria in desert farming requires specific biological control agents (BCAs) adapted to the unique arid conditions. We performed an ecological study of below-ground communities in desert farm soil and untreated desert soil, and based on these findings, selected antagonists were hierarchically evaluated. In contrast to the highly specific 16S rRNA fingerprints of bacterial communities in soil and cultivated medicinal plants, internal transcribed spacer profiles of fungal communities were less discriminative and mainly characterised by potential pathogens. Therefore, we focused on in vitro bacterial antagonists against pathogenic fungi. Based on the antifungal potential and genomic diversity, 45 unique strains were selected and characterised in detail. Bacillus/Paenibacillus were most frequently identified from agricultural soil, but antagonists from the surrounding desert soil mainly belonged to Streptomyces. All strains produced antibiotics against the nematode Meloidogyne incognita, and one-third showed additional activity against the bacterial pathogen Ralstonia solanacearum. Altogether, 13 broad-spectrum antagonists with antibacterial, antifungal and nematicidal activity were found. They belong to seven different bacterial species of the genera Bacillus and Streptomyces. These Gram-positive, spore-forming bacteria are promising drought-resistant BCAs and a potential source for antibiotics. Their rhizosphere competence was shown by fluorescence in situ hybridisation combined with laser scanning microscopy.

Bacterial Antagonists of Fungal Pathogens Also Control Root-Knot Nematodes by Induced Systemic Resistance of Tomato Plants

The potential of bacterial antagonists of fungal pathogens to control the root-knot nematode Meloidogyne incognita was investigated under greenhouse conditions. Treatment of tomato seeds with several strains significantly reduced the numbers of galls and egg masses compared with the untreated control. Best performed Bacillus subtilis isolates Sb4-23, Mc5-Re2, and Mc2-Re2, which were further studied for their mode of action with regard to direct effects by bacterial metabolites or repellents, and plant mediated effects. Drenching of soil with culture supernatants significantly reduced the number of egg masses produced by M. incognita on tomato by up to 62% compared to the control without culture supernatant. Repellence of juveniles by the antagonists was shown in a linked twin-pot set-up, where a majority of juveniles penetrated roots on the side without inoculated antagonists. All tested biocontrol strains induced systemic resistance against M. incognita in tomato, as revealed in a split-root system where the bacteria and the nematodes were inoculated at spatially separated roots of the same plant. This reduced the production of egg masses by up to 51%, while inoculation of bacteria and nematodes in the same pot had only a minor additive effect on suppression of M. incognita compared to induced systemic resistance alone. Therefore, the plant mediated effect was the major reason for antagonism rather than direct mechanisms. In conclusion, the bacteria known for their antagonistic potential against fungal pathogens also suppressed M. incognita. Such “multi-purpose” bacteria might provide new options for control strategies, especially with respect to nematode-fungus disease complexes that cause synergistic yield losses.

Importance of the O-antigen, core-region and lipid A of rhizobial lipopolysaccharides for the induction of systemic resistance in potato to Globodera pallida

Lipopolysaccharides (LPS) of the rhizobacterium Rhizobium etli strain G12 are involved in the induction of systemic resistance in potato roots towards the potato cyst nematode Globodera pallida. In the present study, the wild type strain G12 and three LPS-mutant strains with truncated O-antigens all reduced nematode infection of potato plants to about 30% that of untreated controls. Therefore, the O-antigen of the bacterial LPS cannot be the sole inducer of the resistance mechanism. In further search of the mode of action, the isolated core-region and lipid A-fraction of the LPS of one O-antigen-mutant strain R. etli #4-153 were tested for elicitor activity. Using a split-root system, which provides spatial separation of the inducing agent from the parasite, it was shown that both the entire LPS and its core-region systemically reduced G. pallida infection to 45% that of the untreated control. In contrast, the lipid A-fraction was less effective; the 20% reduction in nematode infection was not different from the control nor from the LPS and core region treatments. It is concluded that the oligosaccharides of the core-region are the main trigger of systemic resistance in potato roots towards G. pallida infection.

Effects of pyrrolizidine alkaloids on the performance of plant-parasitic and free-living nematodes.

BACKGROUND Chemical nematicides such as methyl bromide have for decades played a significant role in the management of plant-parasitic nematodes. Their application is problematic because of negative environmental impacts, and therefore methyl bromide was phased out in Europe in 2005. A possible alternative to synthetically derived nematicides is seen in the use of plants and/or their secondary metabolites. These plants could either be used as nematicidal green manure or as a source for nematicidal extracts. This study aimed to evaluate the effects of 1,2-dehydropyrrolizidine alkaloids (PAs), a group of secondary plant metabolites found in hundreds of plant species throughout the world, on the performance of plant-parasitic and free-living nematodes. RESULTS PAs induced nematicidal, ovicidal and repellent effects on different plant-parasitic and free-living nematodes. There was no conclusive ranking in toxicity for the different structural types of PAs tested. However, the effects were often more pronounced for the tertiary than for the oxidised form of PAs. Further, large differences were observed in the susceptibility of different nematode species to PAs. CONCLUSIONS PAs do affect several performance parameters and developmental stages of nematodes. Therefore, PA-producing plants such as species of Crotalaria, Ageratum or Senecio might be promising candidates for nematode management strategies. [Correction made here after initial online publication].

Specific Microbial Attachment to Root Knot Nematodes in Suppressive Soil

ABSTRACT Understanding the interactions of plant-parasitic nematodes with antagonistic soil microbes could provide opportunities for novel crop protection strategies. Three arable soils were investigated for their suppressiveness against the root knot nematode Meloidogyne hapla. For all three soils, M. hapla developed significantly fewer galls, egg masses, and eggs on tomato plants in unsterilized than in sterilized infested soil. Egg numbers were reduced by up to 93%. This suggested suppression by soil microbial communities. The soils significantly differed in the composition of microbial communities and in the suppressiveness to M. hapla. To identify microorganisms interacting with M. hapla in soil, second-stage juveniles (J2) baited in the test soil were cultivation independently analyzed for attached microbes. PCR-denaturing gradient gel electrophoresis of fungal ITS or 16S rRNA genes of bacteria and bacterial groups from nematode and soil samples was performed, and DNA sequences from J2-associated bands were determined. The fingerprints showed many species that were abundant on J2 but not in the surrounding soil, especially in fungal profiles. Fungi associated with J2 from all three soils were related to the genera Davidiella and Rhizophydium, while the genera Eurotium, Ganoderma, and Cylindrocarpon were specific for the most suppressive soil. Among the 20 highly abundant operational taxonomic units of bacteria specific for J2 in suppressive soil, six were closely related to infectious species such as Shigella spp., whereas the most abundant were Malikia spinosa and Rothia amarae, as determined by 16S rRNA amplicon pyrosequencing. In conclusion, a diverse microflora specifically adhered to J2 of M. hapla in soil and presumably affected female fecundity.

Endophytic bacteria from Ethiopian coffee plants and their potential to antagonise Meloidogyne incognita

Summary – Endophytic bacteria were isolated from coffee roots in Ethiopia and identified by Fatty Acid Methyl Ester-Gas Chromatography (FAME-GC). A total of 201 and 114 endophytic bacteria were isolated and identified during the wet and dry seasons, respectively. The most abundant genera were Pseudomonas, Bacillus, Agrobacterium, Stenotrophomonas and Enterobacter. Population densities were higher during the wet season than the dry season ranging from 5.2 × 10 3 to 2.07 × 10 6 cfu (g fresh root weight) −1 . Culture filtrates of the bacterial isolates showed nematicidal effects of between 38 and 98%. The most active strains were Agrobacterium radiobacter, Bacillus pumilus, B. brevis, B. megaterium, B. mycoides, B. licheniformis, Chryseobacterium balustinum, Cedecea davisae, Cytophaga johnsonae, Lactobacillus paracasei, Micrococcus luteus, M. halobius, Pseudomonas syringae and Stenotrophomonas maltophilia. Bacillus pumilus and B. mycoides were most effective in reducing the number of galls and egg masses caused by M. incognita by 33 and 39%, respectively.

Pyrrolizidine alkaloids of Chromolaena odorata act as nematicidal agents and reduce infection of lettuce roots by Meloidogyne incognita

1,2-dehydropyrrolizidine alkaloids (PAs) represent a class of secondary plant compounds that are active in defence against herbivory. They are present in Chromolaena odorata, one of the most invasive weeds of Asia and Africa. In vitro studies demonstrate that pure PAs from C. odorata roots have nematicidal effects on the root-knot nematode Meloidogyne incognita, even at concentrations of 70-350 ppm. In vivo experiments show that mulch or aqueous crude extracts from C. odorata roots reduce the infection of lettuce by M. incognita. Thus, the use of PA-containing plants appears to be a valuable element for integrated nematode management.

Effects of plants containing pyrrolizidine alkaloids on the northern root-knot nematode Meloidogyne hapla

Abstract1,2-Dehydropyrrolizidine alkaloids (PAs), known to be nematotoxic in vitro, represent a class of secondary plant metabolites from hundreds of plant species worldwide. Pot experiments with the commercially available PA-containing plants Ageratum houstonianum, Borago officinalis, Senecio bicolor, and Symphytum officinalis demonstrate that Meloidogyne hapla is not per se repelled by these plants as all species were infested with nematodes. However, the development of M. hapla juveniles was completely suppressed on A. houstonianum and S. bicolor. Soil in which A. houstonianum and S. bicolor were cultivated and incorporated contained 200–400 times less nematodes than soil treated with Lycopersicon esculentum. Depending on their qualitative composition of PAs at least some of these plants thus appear to be valuable tools for integrated root-knot nematode management.