Low-Temperature Isolation of Disease-Suppressive Bacteria and Characterization of a Distinctive Group of Pseudomonads

Abstract

The influence of environmental factors during isolation on the composition of potential biocontrol isolates is largely unknown. Bacterial isolates that efficiently suppressed wheat seedling blight caused by Fusarium culmorum were found by isolating psychrotrophic, root-associated bacteria and by screening them in a bioassay that mimicked field conditions. The impact of individual isolation factors on the disease-suppressive index (DSI) of almost 600 isolates was analyzed. The bacteria originated from 135 samples from 62 sites in Sweden and Switzerland. The isolation factors that increased the probability of finding isolates with high DSIs were sampling from arable land, Swiss origin of samples, and origination of isolates from plants belonging to the family Brassicaceae. The colony morphology of the isolates was characterized and compared to DSIs, which led to identification of a uniform morphological group containing 57 highly disease-suppressive isolates. Isolates in this group were identified as Pseudomonas sp.; they were fluorescent on King's medium B and had characteristic crystalline structures in their colonies. These isolates were morphologically similar to seven strains that had previously been selected for suppression of barley net blotch caused by Drechslera teres. Members of this morphological group grow at 1.5°C and produce an antifungal polyketide (2,3-deepoxy-2,3-didehydrorhizoxin [DDR]). They have similar two-dimensional polyacrylamide gel electrophoresis protein profiles, phenotypic characteristics, and in vitro inhibition spectra of pathogens. In summary, in this paper we describe some isolation factors that are important for obtaining disease-suppressive bacteria in our system, and we describe a novel group of biocontrol pseudomonads.In biological control, microorganisms isolated from plants and soils are employed to protect crop plants from diseases. In cereals, seed-and soilborne fungal diseases are often suppressed by fluorescent pseudomonads (4,6,16,17,25). These bacteria are well adapted to the rhizosphere environment (for a recent review see reference 29), and they produce several antimicrobial substances that are involved in suppression of plant diseases (7). Many Fusarium species, including Fusarium culmorum, and the related organism Microdochium nivale are well-known and widespread fungal plant pathogens which can cause different diseases at various developmental stages of a crop (36,43). This fact and the risk of triggering increased mycotoxin production by use of fungicides make these fungi difficult to control (30, 36). However, since biological control often operates through several modes of action, it provides new possibilities to address these two issues (15,19,21,25).Screening systems that approximate field conditions are more likely to result in selection of effective biocontrol isolates (12,24,50). In the temperate regions of the world, one critical environmental factor is the low temperature during winter and early spring (12). Hökeberg and et al. (13) developed an isolation...

Evaluation of non-chemical seed treatment methods for the control of Alternaria dauci and A. radicina on carrot seeds

“…Mutants impaired in production of DDR loose a substantial part of their suppressive ability . On the other hand, P. chlororaphis MA 342, the active bacterial ingredient of BA 2552, is also a producer of DDR and has a similar in vitro antimicrobial spectrum (Johansson and Wright 2003), but in our tests the activity of BA 2552 against A. dauci and A. radicina was clearly lower than that of MF 416.…”

“…Among the experimental, non-commercialized micro-organisms, the best results were obtained with P. (Johansson and Wright 2003). Mutants impaired in production of DDR loose a substantial part of their suppressive ability .…”

Isolation and Identification of Rhizoxin Analogs from Pseudomonas fluorescens Pf-5 by Using a Genomic Mining Strategy

“…Strains of Pseudomonas spp. that produce the rhizoxin analog WF-1360 F (synonym, 2,3-deepoxy-2,3-didehydro-rhizoxin) are known to suppress a number of plant-pathogenic fungi and an oomycete in culture (19). Pseudomonas chlororaphis MA 342 (20), a commercial biological-control agent (Cedomon) that has been used for management of seed-borne pathogens of barley in Europe since 1997, is known to produce WF-1360 F. This study establishes the toxicity of the compound WF-1360 F and other rhizoxin analogs produced by Pf-5 against a fungal and an oomycete phytopathogen.…”

“…nov. (formerly designated Burkholderia rhizoxina) (38), an endosymbiont of Rhizopus microsporus (39), and rhizoxin analogs have been isolated from strains of Pseudomonas spp. that inhabit the rhizosphere (19) and ocean waters (43). Rhizoxin exhibits phytotoxic (36), antifungal (17), and antitumor (49) activities by binding to ␤-tubulin (46), thereby interfering with microtubule dynamics during mitosis (14).…”

Evaluation of non-chemical seed treatment methods for the control of Phoma valerianellae on lamb’s lettuce seeds

“…JENSEN et al 2004;GILARDI et al 2005) most of these reports refer to seed-borne pathogens of cereals (e.g. KNUDSEN et al 1997;JOHNSSON et al 1998;JOHANSSON and WRIGHT 2003;AMEIN et al 2008). Other groups of agents that have only rarely been tested as seed treatments of vegetables are plant-derived products, such as essential oils (KRITZINGER et al 2002), and substances known to increase the self-defence of the plant (induced resistance).…”