Modes of action of the protective strain Fo47 in controllingverticillium wilt of pepper

Abstract

The protective fungus Fusarium oxysporum Fo47 reduces the severity of wilt caused by the soilborne pathogen Verticil-lium dahliae in pepper. Modes of action responsible for the biocontrol activity were studied. Microscopic observationsof fluorescent protein-transformed strains colonizing the root surface show that the colonization patterns of Fo47 andV. dahliae were similar. Pixel counting of the images obtained by confocal microscopy showed that Fo47 reduces colo-nization of the root surface by V. dahliae, suggesting a possible role of competition for nutrients at the root surface.Besides these effects on surface colonization, the hormonal pathways activated during priming of plant defenceresponses were identified by measuring the amount of some phytohormones and their derivatives in roots and stems ofpepper. Results showed an early, slight increase of jasmonyl isoleucine, followed by a transient increase of salicylic acidduring the pre-challenged phase of priming and an increase of 12-oxo-phytodienoic acid during the challenge phase ofpriming. The caffeic, ferulic and chlorogenic acids, known to play a role in plant defence reactions, showed a strongantimicrobial activity against V. dahliae in vitro. In pepper roots, Fo47 stimulated the biosynthesis of caffeic acid andprimed that of chlorogenic acid. These results demonstrated that the effective control of V. dahliae provided by Fo47is based on different but complementary mechanisms.