Carsten Hase

Bio: Carsten Hase is an academic researcher from École Polytechnique Fédérale de Lausanne. The author has contributed to research in topics: Pseudomonas fluorescens & Rhizosphere. The author has an hindex of 6, co-authored 7 publications receiving 696 citations. Previous affiliations of Carsten Hase include ETH Zurich.

Induction of systemic resistance of tobacco to tobacco necrosis virus by the root-colonizing Pseudomonas fluorescens strain CHA0: influence of the gacA gene and of pyoverdine production

Abstract: Pseudomonas fluorescens strain CHA0, which suppresses various plant diseases caused by soilborne pathogens, also can restrict leaf disease. Plants of Nicotiana glutinosa and of two cultivars of N. tabacum were grown in autoclaved natural soil previously inoculated with strain CHA0. After 6 wk, all the plants tested showed resistance in leaves to infection with tobacco necrosis virus (TNV) to the same extent as plants previously immunized with TNV (induced resistance control). Polyacrylamide gel electrophoresis and enzyme assays showed that the same amount of PR proteins (Pr-1 group proteins, beta-1,3-glucanases, and endochitinases) was induced in the intercellular fluid of leaves of plants grown in the presence of strain CHA0 as in the intercellular fluid of leaves of plants immunized by a previous TNV inoculation on a lower leaf. Strain CHA0 was reisolated from the roots but could not be detected in stems or leaves. Strain CHA96, a gacA (global activator)-negative mutant of strain CHA0 defective in the production of antibiotics and in the suppression of black root rot of tobacco, had the same capacity to induce PR proteins and resistance against TNV as did the wild-type strain. CHA400, a pyoverdine-negative mutant of strain CHA0 with the same capacity to suppress black root rot of tobacco and take-all of wheat as the wild-type strain, was able to induce PR proteins but only partial resistance against TNV. P3, another P. fluorescens wild-type strain, does not suppress diseases caused by soilborne pathogens and induced neither resistance nor PR proteins in tobacco leaves. Root colonization of tobacco plants with strain CHA0 and its derivatives as well as leaf infection with TNV caused an increase in salicylic acid in leaves. These results show that colonization of tobacco roots by strain CHA0 reduces TNV leaf necrosis and induces physiological changes in the plant to the same extent as does induction of systemic resistance by leaf inoculation with TNV

The viable-but-nonculturable state induced by abiotic stress in the biocontrol agent Pseudomonas fluorescens CHA0 does not promote strain persistence in soil.

Abstract: The effects of oxygen limitation, low redox potential, and high NaCl stress for 7 days in vitro on the rifampin-resistant biocontrol inoculant Pseudomonas fluorescens CHA0-Rif and its subsequent persistence in natural soil for 54 days were investigated. Throughout the experiment, the strain was monitored using total cell counts (immunofluorescence microscopy), Kogure's direct viable counts, and colony counts (on rifampin-containing plates). Under in vitro conditions, viable-but-nonculturable (VBNC) cells of CHA0-Rif were obtained when the strain was exposed to a combination of low redox potential (230 mV) and oxygen limitation. This mimics a situation observed in the field, where VBNC cells of the strain were found in the water-logged soil layer above the plow pan. Here, VBNC cells were also observed in vitro when CHA0-Rif was subjected to high NaCl levels (i.e., NaCl at 1.5 M but not 0.7 M). In all treatments, cell numbers remained close to the inoculum level for the first 12 days after inoculation of soil, regardless of the cell enumeration method used, but decreased afterwards. At the last two samplings in soil, VBNC cells of CHA0-Rif were found in all treatments except the one in which log-phase cells had been used. In the two treatments that generated high numbers of VBNC cells in vitro, VBNC cells did not display enhanced persistence compared with culturable cells once introduced into soil, which suggests that this VBNC state did not represent a physiological strategy to improve survival under adverse conditions.

Nutrient deprivation and the subsequent survival of biocontrol Pseudomonas fluorescens CHA0 in soil

Abstract: The effects of deprivation of multiple nutrients or of selected single nutrients (C, S, N, or P) for 7 d in vitro on the subsequent persistence of the biocontrol agent Pseudomonas fluorescens CHA0 in natural soil were investigated. Experiments were carried out with the spontaneous rifampicin-resistant mutant CHA0-Rif and the strain was monitored in vitro and in soil using colony counts (on plates containing rifampicin), Kogure’s direct viable counts and total cell counts (by immunofluorescence microscopy). Single nutrient or multiple nutrient deprivation in vitro did not affect the colony-forming ability of CHA0-Rif cells. However, cell length of the strain was smaller in all nutrient deprivation treatments but one (P deprivation) when compared with cells from log-phase cultures. Once introduced into soil, CHA0-Rif cells from log-phase cultures persisted up to 14 d as culturable cells, and their population numbers (10 8 CFU (g soil) −1 ) had not declined. The strain was recovered at lower cell numbers at subsequent samplings, regardless of the method used for cell counts, and at 48 d about 90% of the cells had lost both their ability to respond to Kogure’s viability test and to form a colony on plate. Cells of CHA0-Rif deprived of a single nutrient persisted similarly to log-phase cells once introduced into soil. In contrast, deprivation of CHA0-Rif cells for multiple nutrients prior to their introduction into soil resulted in the early occurrence (i.e. within hours of soil inoculation) of cells that had lost their colony-forming ability and that did not respond to Kogure’s viability test. This suggests that the lack of a single nutrient (other than C, S, N or P) or deprivation of a combination of several nutrients, under in vitro conditions, had a negative effect on the subsequent survival of CHA0-Rif in soil.

Survival and cell culturability of biocontrol Pseudomonas fluorescens CHA0 in the rhizosphere of cucumber grown in two soils of contrasting fertility status

Abstract: The effect of cucumber roots on survival patterns of the biocontrol soil inoculant Pseudomonas fluorescens CHA0-Rif was assessed for 22 days in two non-sterile soils, using a combination of total immunofluorescence cell counts, Kogure's direct viable counts and colony counts on plates containing rifampicin. In Eschikon soil (high fertility status for cucumber), CHA0-Rif persisted as culturable cells in bulk soil and in the rhizosphere, but colony counts were lower than viable counts and total cell counts inside root tissues. The occurrence of viable but non-culturable (VBNC) cells inside root tissues (5 log cells g–1 root) was unlikely to have resulted from the hydrogen peroxide treatment used to disinfect the root surface, as hydrogen peroxide caused the death of CHA0-Rif cells in vitro. In Siglistorf soil (low fertility status for cucumber), the inoculant was found mostly as non-culturable cells. Colony counts and viable counts of CHA0-Rif were similar, both in bulk soil and inside root tissues, whereas in the rhizosphere viable counts exceeded colony counts at the last two samplings (giving about 7 log VBNC cells g–1). In conclusion, soil type had a significant influence on the occurrence of VBNC cells of CHA0-Rif, although these cells were found in root-associated habitats (i.e. rhizosphere and root tissues) and not in bulk soil.

Use of Plant Growth-Promoting Bacteria for Biocontrol of Plant Diseases: Principles, Mechanisms of Action, and Future Prospects

Abstract: Pathogenic microorganisms affecting plant health are a major and chronic threat to food production and ecosystem stability worldwide As agricultural production intensified over the past few decades, producers became more and more dependent on agrochemicals as a relatively reliable method of crop

Systemic resistance induced by rhizosphere bacteria

Abstract: Nonpathogenic rhizobacteria can induce a systemic resistance in plants that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Rhizobacteria-mediated induced systemic resistance (ISR) has been demonstrated against fungi, bacteria, and viruses in Arabidopsis, bean, carnation, cucumber, radish, tobacco, and tomato under conditions in which the inducing bacteria and the challenging pathogen remained spatially separated. Bacterial strains differ in their ability to induce resistance in different plant species, and plants show variation in the expression of ISR upon induction by specific bacterial strains. Bacterial determinants of ISR include lipopolysaccharides, siderophores, and salicylic acid (SA). Whereas some of the rhizobacteria induce resistance through the SA-dependent SAR pathway, others do not and require jasmonic acid and ethylene perception by the plant for ISR to develop. No consistent host plant alterations are associated with the induced state, but upon challenge inoculation, resistance responses are accelerated and enhanced. ISR is effective under field conditions and offers a natural mechanism for biological control of plant disease.

Microbial interactions and biocontrol in the rhizosphere

Abstract: The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

The rhizosphere microbiome: significance of plant beneficial, plant pathogenic, and human pathogenic microorganisms

Abstract: Microbial communities play a pivotal role in the functioning of plants by influencing their physiology and development. While many members of the rhizosphere microbiome are beneficial to plant growth, also plant pathogenic microorganisms colonize the rhizosphere striving to break through the protective microbial shield and to overcome the innate plant defense mechanisms in order to cause disease. A third group of microorganisms that can be found in the rhizosphere are the true and opportunistic human pathogenic bacteria, which can be carried on or in plant tissue and may cause disease when introduced into debilitated humans. Although the importance of the rhizosphere microbiome for plant growth has been widely recognized, for the vast majority of rhizosphere microorganisms no knowledge exists. To enhance plant growth and health, it is essential to know which microorganism is present in the rhizosphere microbiome and what they are doing. Here, we review the main functions of rhizosphere microorganisms and how they impact on health and disease. We discuss the mechanisms involved in the multitrophic interactions and chemical dialogues that occur in the rhizosphere. Finally, we highlight several strategies to redirect or reshape the rhizosphere microbiome in favor of microorganisms that are beneficial to plant growth and health.

The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

Abstract: The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health.