Biocontrol of Root Diseases byPseudomonas fluorescensCHA0: Current Concepts and Experimental Approaches

Abstract

Certain strains of fluorescent pseudomonads can effectively colonize plant roots and protect plants from diseases caused by a variety of root pathogens. Such beneficial or plant health-promoting strains are emerging as promising biocontrol agents. They are suited as soil inoculants either individually or in combination and may be compatible with current chemical pesticides (1, 2, 3, 4, 5, 6, 7, 8). In our biocontrol studies, we have focused on Pseudomonasfluorescens strain CHAO, an isolate from a suppressive soil in the western part of Switzerland (9). This strain was originally shown to colonize tobacco roots and to suppress black root rot, which is caused by the fungus Thielaviopsis basicola (9, 10). Subsequent work has established that disease suppression by strain CHAO displays little specificity with respect to the host plant and the pathogen. Protected plants include wheat, cucumber, sugar beet, cotton, flax, corn, and cress. Pathogenic action of at least the following fungal pathogens can be reduced by strain CHAO: Pythium ultimum, Gaeumannomyces graminis var. tritici (Ggt), Fusarium oxysporum f.s p. cucurbitaceae, Phomopsis sclerotioides, and Rhizoctonia solani (11, 12, 13, 14, 15; our unpublished data). Since the interactions between I! fluorescens, other organisms and the soil environment are extremely complex, it became important to develop reproducible methods that allow us to monitor the plant-beneficial effects of strain CHAO reliably and to analyze the traits that make it an effective biocontrol agent. In section 6.2 we will review some of our approaches to investigate the mechanisms by which strain CHAO achieves biological control.We are using strain CHAO as a model organism to study not only the mechanisms of disease suppression but also the ecological impact of introduced plant-beneficial bacteria (see section 6.3). In a parallel approach, we are investigating the potential applications of biological control agents to improve the yields of protected crops. We are testing a variety of strains, singly and in combination, for the development of greenhouse applications. A brief account of this work is presented in section 6.4. 68Disease Suppression by I! Fluorescens CHAO Mechanistic Studies on Biocontrol m i t s of Pseudomonas Fluorescens CHAO Chemical Identification of Extracellular MetabolitesMetabolites produced and excreted by I? fluorescens are assumed to be important biotic factors in the biological control of root diseases (2, 5, 16, 17, 18, 19, 20, 21). Until now, about a dozen low molecular weight compounds have been identified in culture supernatants of Rfluorescens CHAO ( Table 1). These products can be broadly classified into two groups: siderophores and secondary metabolites. The siderophores (e. g. , iron chelators) pyoverdine (pseudobactin), salicylate and pyochelin are all produced by I? fluorexens CHAO (22, 23 ; our unpublished results) and by other fluorescent pseudomonads when these bacteria are grown under ironlimiting conditions (24; reviewed by Loper & Buyer [20] and O'Sullivan...

The global regulator GacA of Pseudomonas fluorescens CHA0 is required for suppression of root diseases in dicotyledons but not in Gramineae

“…A mutation affecting the production of a given metabolite may influence also the biosynthesis of metabolites other than that affected by the mutation. Often, a bacterium having lost the ability to produce one or more metabolites may compensate by increased synthesis of another product with similar properties (Voisard et al, 1994). Indeed, Pvd ¹ mutants of strain CHA0 overproduce Pch and Sal (Meyer et al, 1992;this study).…”

“…Sal) could have compensated for the lack of Pvd (Meyer et al, 1992;Voisard et al, 1994). In the current work, a GacA ¹ Pvd ¹ double mutant of CHA0 was then constructed to evaluate whether Pvd production was involved in the ability of the GacA ¹ mutant to protect Gramineae.…”

“…(Duijff et al, 1993) and Erwinia carotovora (Xu & Gross, 1986). Nevertheless, in a number of Pseudomonas strains Pvd plays a minimal role in disease suppression (Hamdan et al, 1991;Lemanceau & Alabouvette, 1993;Voisard et al, 1994). Little is known about the two other siderophores pyochelin (Pch) and its precursor salicylic acid (Sal) (Ankenbauer & Cox, 1988) produced by certain Pseudomonas strains.…”

“…P. fluorescens CHA0, originally isolated from a soil suppressive to tobacco black root rot, protects different plants from root diseases caused by a variety of pathogenic fungi. Several secondary metabolites produced by strain CHA0 have been shown to play a role in disease suppression (Voisard et al, 1994;Keel & Défago, 1996. HCN and Phl help to suppress black rot of tobacco (Voisard et al, 1988;Keel et al, 1992), Phl being also involved in the suppression of take-all of wheat .…”

The global activator GacA of Pseudomonas aeruginosa PAO positively controls the production of the autoinducer N‐butyryl‐homoserine lactone and the formation of the virulence factors pyocyanin, cyanide, and lipase

“…The disrupted gacA gene was then transferred to the ColE1-based suicide vector pME3088 (Voisard et al, 1994), resulting in pME6111 (Fig. 1C).…”