3.8.1
INDUCTION OF ANTIFUNGAL PHENOLICS IN CUCUMBER ROOTS BY FLUORESCENT PSEUDOMONAS IS RELATED TO THE REDUCTION OF PYTHIUM DISEASE

M ONGENA1, F DAAYF2, P JACQUES1, P THONART1 and RR BELANGER3

1Centre Wallon de Biologie Industrielle (CWBI), Unité de Bioindustries, Faculté des Sciences Agronomiques de Gembloux, Gembloux, Belgium; 2Agriculture and Agri-Food Canada, Charlottetown Research Center, Charlottetown, PEI, Canada; 3Département de Phytologie, Faculté des Sciences de l'Agriculture et de l'Alimentation, Université Laval, Québec G1K 7P4, Canada

Background and objectives
Some fluorescent pseudomonads are referred to as plant growth-promoting rhizobacteria (PGPR) and their effectiveness in controlling a number of plant diseases caused by soilborne pathogens has been widely documented. Competition for substrate, niche exclusion, production of antifungal compounds, excretion of siderophores and more recently induction of plant resistance have been described as mechanisms involved in disease suppression [1]. Although many results support the occurrence of a systemic induction of plant resistance by PGPR, little is known about the specific mechanisms set forth by the plant upon such treatment. In this context, results presented here are based on the study of the differential ability of four PGPR strains (fluorescent Pseudomonas) to protect long English cucumber against Pythium aphanidermatum in hydroponic cultures.

Results and conclusions
In order to explain the efficacy of strain P. putida BTP1, which displayed the stronger protective effect among the isolates tested, relative incidence of pyoverdin synthesis, antifungal metabolites production and root colonization were compared. But none of these mechanisms could be associated with the protective effect observed. However, a more significant phenomenon consistent with the prophylactic properties of the tested strains is undoubtedly the production of antifungal compounds by cucumber induced as a result of a treatment with BTP1 and M3 (sid- mutant of strain BTP1). This result corroborates the hypothesis that PGPR can act indirectly by inducing resistance in plants and associates for the first time PGPR with production of antifungal compounds in cucumber. Comparative quantification of both glycosilated and aglycone forms of phenolics extracted from roots supports the concept that cucumber phytoalexins first accumulate in their inactive glycosilated form and are presumably activated by enzymatic cleavage of the sugar as postulated in recent works [2].

References
1. Glick BR, 1995. Canadian Journal of Microbiology 41, 109-17.
2. Daayf F, Schmitt A, Bélanger RR, 1997. Plant Physiology 113, 719-27.